tag:theconversation.com,2011:/africa/topics/odor-42347/articlesOdor – The Conversation2023-09-20T12:26:43Ztag:theconversation.com,2011:article/2102312023-09-20T12:26:43Z2023-09-20T12:26:43ZYour unique body odor could identify who you are and provide insights into your health – all from the touch of a hand<figure><img src="https://images.theconversation.com/files/549153/original/file-20230919-31-na83ko.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2120%2C1414&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The scent emitted from your hands could offer clues about who you are.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/digital-composite-image-of-hand-amidst-geometric-royalty-free-image/1307462742">Siro Rodenas Cortes/Moment via Getty Images</a></span></figcaption></figure><p>From the aroma of fresh-cut grass to the smell of a loved one, you encounter scents in every part of your life. Not only are you constantly surrounded by odor, you’re also producing it. And it is so distinctive that it can be used to tell you apart from everyone around you.</p>
<p>Your scent is a complex product influenced by many factors, including your genetics. Researchers believe that a particular group of genes, the <a href="https://doi.org/10.1177/147470490700500206">major histocompatibility complex</a>, play a large role in scent production. These genes are involved in the body’s immune response and are believed to influence body odor by encoding the production of specific proteins and chemicals.</p>
<p>But your scent isn’t fixed once your body produces it. As sweat, oils and other secretions make it to the surface of your skin, <a href="https://asm.org/Articles/2021/December/Microbial-Origins-of-Body-Odor">microbes break down and transform</a> these compounds, changing and adding to the odors that make up your scent. This scent medley emanates from your body and settles into the environments around you. And it can be used to track, locate or identify a particular person, as well as distinguish between healthy and unhealthy people.</p>
<p><a href="https://scholar.google.com/citations?user=H4mfIRkAAAAJ&hl=en">We are</a> <a href="https://scholar.google.com/citations?user=H_xIriMAAAAJ&hl=en">researchers who</a> <a href="https://scholar.google.com/citations?user=JgBnpdkAAAAJ&hl=en">specialize in</a> studying human scent through the detection and characterization of gaseous chemicals called <a href="https://doi.org/10.1111%2Fj.1365-2133.2008.08748.x">volatile organic compounds</a>. These gases can relay an abundance of information for both forensic researchers and health care providers.</p>
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<figcaption><span class="caption">Human scent analysis breaks down body odor to its individual components.</span></figcaption>
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<h2>Science of body odor</h2>
<p>When you are near another person, you can feel their body heat without touching them. You may even be able to smell them without getting very close. The natural warmth of the human body creates a temperature differential with the air around it. You warm up the air nearest to you, while air that’s farther away remains cool, creating <a href="https://doi.org/10.1111/j.1556-4029.2009.01236.x">warm currents of air</a> that surround your body. </p>
<p>Researchers believe that this plume of air helps disperse your scent by pushing the millions of skin cells you shed over the course of a day off your body and into the environment. These skin cells <a href="https://doi.org/10.1007/978-0-387-73003-5_279">act as boats or rafts</a> carrying glandular secretions and your resident microbes – a combination of ingredients that emit your scent – and depositing them in your surroundings.</p>
<p>Your scent is composed of the volatile organic compounds present in the <a href="https://doi.org/10.1007/s10886-005-5801-4">gases emitted from your skin</a>. These gases are the combination of sweat, oils and trace elements exuded from the glands in your skin. The primary components of your odor depend on internal factors such as your race, ethnicity, biological sex and other traits. Secondary components waver based on factors like stress, diet and illness. And tertiary components from external sources like perfumes and soaps build on top of your distinguishable odor profile.</p>
<h2>Identity of scent</h2>
<p>With so many factors influencing the scent of any given person, your body odor can be used as an identifying feature. <a href="https://doi.org/10.1007/978-0-387-73003-5_279">Scent detection canines</a> searching for a suspect can look past all the other odors they encounter to follow a scent trail left behind by the person they are pursuing. This practice relies on the assumption that each person’s scent is distinct enough that it can be distinguished from other people’s.</p>
<p>Researchers have been studying the discriminating potential of human scent for over three decades. A 1988 experiment demonstrated that a dog could distinguish <a href="https://doi.org/10.1068/p170549">identical twins living apart</a> and exposed to different environmental conditions by their scent alone. This is a feat that could not be accomplished using DNA evidence, as identical twins share the same genetic code.</p>
<p>The field of human scent analysis has expanded over the years to further study the composition of human scent and how it can be used as a form of forensic evidence. Researchers have seen differences in human odor composition that can be classified based on sex, gender, race and ethnicity. Our research team’s 2017 study of 105 participants found that <a href="https://doi.org/10.1016/j.forsciint.2016.09.011">specific combinations</a> of 15 volatile organic compounds collected from people’s hands could distinguish between race and ethnicity with an accuracy of 72% for whites, 82% for East Asians and 67% for Hispanics. Based on a combination of 13 compounds, participants could be distinguished as male or female with an overall 80% accuracy.</p>
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<figcaption><span class="caption">Researchers have trained dogs to sniff out COVID-19 infections.</span></figcaption>
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<p>Researchers are also producing models to predict the characteristics of a person based on their scent. From a sample pool of 30 women and 30 men, our team built a <a href="https://doi.org/10.1371/journal.pone.0286452">machine learning model</a> that could predict a person’s biological sex with 96% accuracy based on hand odor.</p>
<h2>Scent of health</h2>
<p>Odor research continues to provide insights into illnesses. Well-known examples of using scent in medical assessments include <a href="https://theconversation.com/doctor-dog-how-our-canine-companions-can-help-us-detect-covid-and-other-diseases-204603">seizure and diabetic alert canines</a>. These dogs can give their handlers time to prepare for an impending seizure or notify them when they need to adjust their blood glucose levels.</p>
<p>While these canines often work with a single patient known to have a condition that requires close monitoring, medical detection dogs can also indicate whether someone is ill. For example, researchers have shown that dogs can be trained to <a href="https://theconversation.com/the-scent-of-sickness-5-questions-answered-about-using-dogs-and-mice-and-ferrets-to-detect-disease-151832">detect cancer</a> in people. Canines have also been trained to <a href="https://theconversation.com/dogs-can-be-trained-to-sniff-out-covid-19-a-team-of-forensic-researchers-explain-the-science-169012">detect COVID-19 infections</a> at a 90% accuracy rate.</p>
<p>Similarly, our research team found that a laboratory analysis of <a href="https://doi.org/10.3390/diagnostics13040707">hand odor samples</a> could discriminate between people who are COVID-19 positive or negative with 75% accuracy.</p>
<h2>Forensics of scent</h2>
<p>Human scent offers a noninvasive method to collect samples. While direct contact with a surface like touching a doorknob or wearing a sweater provides a clear route for your scent to transfer to that surface, simply standing still will also transfer your odor into the surrounding area.</p>
<p>Although human scent has the potential to be a critical form of forensic evidence, it is still a developing field. Imagine a law enforcement officer collecting a scent sample from a crime scene in hopes that it may match with a suspect. </p>
<p>Further research into human scent analysis can help fill the gaps in our understanding of the individuality of human scent and how to apply this information in forensic and biomedical labs.</p><img src="https://counter.theconversation.com/content/210231/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr. Kenneth G. Furton has consulted for and owns shares with VOC Health, Inc. He has received funding from the Netherlands National Police, the U.S. Department of Defense, the Federal Bureau of Investigation, Colgate-Palmolive, and the National Institutes of Health.
</span></em></p><p class="fine-print"><em><span>Dr. Vidia Gokool formerly of Florida International University, is currently an employee of the Department of Energy. The writing and preparation of this work was in part performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.</span></em></p><p class="fine-print"><em><span>Chantrell Frazier does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Human scent could one day be used as evidence in forensics and as diagnostic information in medicine.Chantrell Frazier, Assistant Professor of Chemistry and Food Science, Framingham State UniversityKenneth G. Furton, Professor of Chemistry and Biochemistry, Florida International UniversityVidia A. Gokool, Postdoctoral Researcher, Lawrence Livermore National LaboratoryLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1889562023-03-13T12:26:33Z2023-03-13T12:26:33ZSmell is the crucial sense that holds ant society together, helping the insects recognize, communicate and cooperate with one another<figure><img src="https://images.theconversation.com/files/513805/original/file-20230306-20-apwc3a.jpg?ixlib=rb-1.1.0&rect=0%2C1224%2C2696%2C1582&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ants from different colonies will fight based on smell alone.</span> <span class="attribution"><span class="source">Joseph Howell, Vanderbilt University</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>Ants can be found in <a href="https://doi.org/10.1038/s41467-018-04218-4">nearly every location on Earth</a>, with rough estimates suggesting there are <a href="https://www.bbc.com/news/magazine-29281253">over 10 quadrillion individuals</a> – that is a 1 followed by 16 zeroes, or about 1 million ants per person. Ants are among the most biologically successful animals on the planet. </p>
<p>A surprising part of their evolutionary success is the amazing sense of smell that lets them recognize, communicate and cooperate with one another.</p>
<p>Ants live in complex colonies, sometimes referred to as nests, that are home to <a href="https://wwnorton.com/books/9780393067040">a wide range of social interactions</a>. Here, one or more queens are responsible for all the reproduction within that colony. The vast majority of colony members are female workers – sisters that never mate or reproduce and live only to serve the group.</p>
<p>Ants need to <a href="https://doi.org/10.1186/s12915-022-01505-x">defend their colony</a>, <a href="https://doi.org/10.1371/journal.pone.0052219">seek food</a> and <a href="https://doi.org/10.1086/690840">take care of offspring</a>. To accomplish these tasks some ant species domesticate other insects, while others create agricultural systems, harvesting leaves from which they <a href="https://doi.org/10.1086/661128">grow edible fungal gardens</a>. Successfully coordinating all these intricate tasks requires reliable and secure communication among nestmates.</p>
<p><a href="https://scholar.google.com/citations?user=PrNrnI8AAAAJ&hl=en&oi=sra">We</a> <a href="https://scholar.google.com/citations?hl=en&user=IZUwOQ0AAAAJ">are</a> biologists who study the remarkable sensory abilities of ants. <a href="https://lab.vanderbilt.edu/zwiebel-lab/">Our recent work</a> shows how their societies depend on the exchange of reliable information which, if disrupted, spells doom for their colonies.</p>
<h2>Unique scents</h2>
<p>Human communication relies primarily on verbal and visual cues. We usually identify our friends by the sound of their voice, the appearance of their face or the clothes they wear. Ants, however, <a href="https://doi.org/10.1242/jeb.215400">rely primarily on their acute sense of smell</a>. </p>
<p>An exterior shell, known as an exoskeleton, encases an ant’s body. This greasy coat carries a unique scent that varies from individual to individual and gives each ant a <a href="https://www.hup.harvard.edu/catalog.php?isbn=9780674040755">unique odor signature that other ants can detect</a>. This odor signature can communicate important information. </p>
<p>The queen, for example, will smell slightly different from a worker, and thus receive special treatment within the colony. Importantly, ants from different colonies will smell slightly different from one another. The detection and decoding of these differences is <a href="https://doi.org/10.3389/fnbeh.2018.00191">vital for colony defense</a> and can trigger aggressive turf wars between colonies when ants catch a whiff of intruders.</p>
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<figcaption><span class="caption">Interactions between nestmates are friendly. But when ants sniff out enemy non-nestmates, there is rapid and deadly aggression. Produced by the Zwiebel Lab, Vanderbilt University, filmed by Stephen Ferguson.</span></figcaption>
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<p>For ants and other insects, receiving chemical information begins when an odor enters the small hairs located along their antennae. These hairs are hollow and contain <a href="https://doi.org/10.1016/j.cois.2014.10.006">special receptors, called chemosensory neurons</a>, that sort and send the chemical information to the ant’s brain. </p>
<p>Odors, such as those given off from an ant’s greasy coat, <a href="https://doi.org/10.1242/jeb.215400">act like chemical “keys</a>.” Ants can smell these odor keys only if they are inserted into the correct set of chemosensory neuron “locks.” A neuronal lock remains shut to any odors except its particular key. When the correct key binds to the correct neuronal lock, though, the receptor sends a complex message to the brain. The ant’s brain is able to decode this sensory information to make decisions that ultimately lead to cooperation between nestmates – or battles between non-nestmates. </p>
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<a href="https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A Tupperware container filled with ants. Three test tubes with cotton stoppers appear to hold water." src="https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/514327/original/file-20230308-22-8r9eol.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">A colony of carpenter ants (<em>Camponotus floridanus</em>) reared in the Zwiebel Lab at Vanderbilt University.</span>
<span class="attribution"><span class="source">LJ Zwiebel, Vanderbilt University</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<h2>Changing the locks</h2>
<p>To better understand how ants detect and communicate information, we use laboratory tools such as <a href="https://doi.org/10.1242/jeb.215400">precisely targeted drugs</a> and <a href="https://doi.org/10.1016/j.cell.2017.06.051">genetic</a> <a href="https://doi.org/10.1016/j.cell.2017.07.001">engineering</a> to manipulate their sense of smell. We are especially interested in what happens when an ant’s sense of smell goes wrong. </p>
<p>For example, when we prevent an odor “key” from opening a chemosensory “lock,” it prevents the chemical information from reaching the brain. This would be like plugging your nose or standing in a completely dark room – no scents or sights would register. We can also open all the “locks” at the same time, which floods the neurons with too many messages. Both of these scenarios dramatically compromise an ant’s ability to detect and receive accurate information.</p>
<p>When we messed with ants’ sense of smell – whether shutting down or flooding their odor receptors – we found <a href="https://doi.org/10.1242/jeb.215400">they no longer attacked non-nestmates</a>. Instead, they became less aggressive. In the absence of clear information, ants exercised restraint and opted to accept rather than attack their fellow ant. Put another way, ants ask questions first and shoot later. </p>
<p>We believe this social restraint is hard-wired and gives ants an evolutionary advantage. When you live in a colony with tens of thousands of sisters, a simple case of mistaken identity or miscommunication could lead to deadly infighting and societal chaos, which is potentially very costly.</p>
<p>When ants in our experiments lose their sense of smell, and their ability to detect accurate information becomes compromised, <a href="https://doi.org/10.1016/j.cell.2017.06.051">they no longer stick together</a> <a href="https://doi.org/10.1016/j.cell.2017.07.001">in a cohesive colony</a>. </p>
<p>Not only do they fail to recognize and attack foes, they also stop cooperating with their friends. Without nurses to take care of the young or foragers to collect food, the eggs dry up and the queen goes hungry. </p>
<p>We discovered that without an accurate means of communicating and receiving chemical information, ant societies collapse and the colony quickly dies. Miscommunication or the lack of accurate information <a href="https://www.pbs.org/newshour/nation/miscommunication-blamed-deadly-u-s-mistake-afghanistan">affects other highly social animals, including humans</a>, as well. For ants, it all depends on their sense of smell.</p><img src="https://counter.theconversation.com/content/188956/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Laurence Zwiebel currently receives funding from the NIH and Vanderbilt University. </span></em></p><p class="fine-print"><em><span>Stephen Ferguson currently receives funding from the NIH and Vanderbilt University.</span></em></p>Researchers explore what happens when ants can’t properly use smell to detect friend from foe.Laurence Zwiebel, Professor of Biological Sciences and of Pharmacology, Vanderbilt UniversityStephen Ferguson, Postdoctoral Scholar in Biological Sciences, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1858332022-06-30T15:20:20Z2022-06-30T15:20:20ZViruses can change your scent to make you more attractive to mosquitoes, new research in mice finds<figure><img src="https://images.theconversation.com/files/471659/original/file-20220629-13-56a1wn.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2129%2C1402&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Mosquito-borne diseases are estimated to cause over 1 million deaths a year.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/biting-mosquito-royalty-free-image/960349766">mrs/Moment via Getty Images</a></span></figcaption></figure><p>Mosquitoes are the <a href="https://www.cdc.gov/globalhealth/stories/2019/world-deadliest-animal.html">world’s deadliest animal</a>. <a href="https://doi.org/10.1016/B978-0-12-812365-2.00003-2">Over 1 million deaths</a> per year are attributed to <a href="https://www.cdc.gov/niosh/topics/outdoor/mosquito-borne/default.html">mosquito-borne diseases</a>, including malaria, yellow fever, dengue fever, Zika and chikungunya fever.</p>
<p>How mosquitoes seek out and feed on their hosts are important factors in how a virus circulates in nature. Mosquitoes spread diseases by acting as carriers of viruses and other pathogens: A mosquito that bites a person infected with a virus can acquire the virus and pass it on to the next person it bites. </p>
<p>For immunologists and infectious disease researchers <a href="https://scholar.google.com/citations?user=6iEQIeIAAAAJ&hl=en">like me</a>, a better understanding of how a virus interacts with a host may offer new strategies for preventing and treating mosquito-borne diseases. In our <a href="https://doi.org/10.1016/j.cell.2022.05.016">recently published study</a>, my colleagues and I found that some viruses can alter a person’s body odor to be more attractive to mosquitoes, leading to more bites that allow a virus to spread. </p>
<h2>Viruses change host odors to attract mosquitoes</h2>
<p>Mosquitoes locate a potential host through <a href="https://doi.org/10.1016/j.cub.2015.07.057">different sensory cues</a>, such as your <a href="https://doi.org/10.7554/eLife.11750">body temperature</a> and the <a href="https://doi.org/10.1017/S0007485300007811">carbon dioxide</a> emitted from your breath. Odors also play a role. Previous lab research has found that mice <a href="https://doi.org/10.1073%2Fpnas.1405617111">infected with malaria</a> have changes in their scents that make them more attractive to mosquitoes. With this in mind, my colleagues and I wondered if other mosquito-borne viruses, such as dengue and Zika, can also change a person’s scent to make them more attractive to mosquitoes, and whether there is a way to prevent these changes.</p>
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<figcaption><span class="caption">A number of factors can make you more attractive to mosquitoes, including the odors you emit.</span></figcaption>
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<p>To investigate this, we placed mice infected with the dengue or Zika virus, uninfected mice and mosquitoes in one of three arms of a glass chamber. When we applied airflow through the mouse chambers to funnel their odors toward the mosquitoes, we found that more mosquitoes chose to fly toward the infected mice over the uninfected mice.</p>
<p>We ruled out carbon dioxide as a reason for why the mosquitoes were attracted to the infected mice, because while Zika-infected mice emitted less carbon dioxide than uninfected mice, dengue-infected mice did not change emission levels. Likewise, we ruled out body temperature as a potential attractive factor when mosquitoes did not differentiate between mice with elevated or normal body temperatures. </p>
<p>Then we assessed the role of body odors in the mosquitoes’ increased attraction to infected mice. After placing a filter in the glass chambers to prevent mice odors from reaching the mosquitoes, we found that the number of mosquitoes flying toward infected and uninfected mice were comparable. This suggests that there was something about the odors of the infected mice that drew the mosquitoes toward them.</p>
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<a href="https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Gloved hand holding two test tubes full of mosquitoes in a lab" src="https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/471703/original/file-20220629-17-7vpcaf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Volunteering in a mosquito study may require a few bites.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/cropped-hand-holding-mosquito-in-test-tube-at-royalty-free-image/1026135942">Panyawat Boontanom/EyeEm via Getty Images</a></span>
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</figure>
<p>To identify the odor, we isolated 20 different gaseous chemical compounds from the scent emitted by the infected mice. Of these, we found three to stimulate a significant response in mosquito antennae. When we applied these three compounds to the skin of healthy mice and the hands of human volunteers, only one, <a href="https://doi.org/10.1111/eea.12297">acetophenone</a>, attracted more mosquitoes compared to the control. We found that infected mice produced 10 times more acetophenone than uninfected mice. </p>
<p>Similarly, we found that the odors collected from the armpits of dengue fever patients contained more acetophenone than those from healthy people. When we applied the dengue fever patient odors on one hand of a volunteer and a healthy person’s odor on the other hand, mosquitoes were consistently more attracted to the hand with dengue fever odors.</p>
<p>These findings imply that the dengue and Zika viruses are capable of increasing the amount of acetophenone their hosts produce and emit, making them even more attractive to mosquitoes. When uninfected mosquitoes bite these attractive hosts, they may go on to bite other people and spread the virus even further.</p>
<h2>How viruses increase acetophenone production</h2>
<p>Next, we wanted to figure out how viruses were increasing the amount of mosquito-attracting acetophenone their hosts produce. <a href="https://doi.org/10.1111/eea.12297">Acetophenone</a>, along with being a chemical commonly used as a <a href="https://pubchem.ncbi.nlm.nih.gov/compound/Acetophenone">fragrance</a> in perfumes, is also a metabolic byproduct commonly produced by certain bacteria living on the skin and in the intestines of both people and mice. So we wondered if it had something to do with changes in the type of bacteria on the skin.</p>
<p>To test this idea, we removed either the skin or intestinal bacteria from infected mice before exposing them to mosquitoes. While mosquitoes were still more attracted to infected mice with depleted intestinal bacteria compared to uninfected mice, they were significantly less attracted to infected mice with depleted skin bacteria. These results suggest that skin microbes are an essential source of acetophenone. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Long chains of Bacillus megatherium under a microscope" src="https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=429&fit=crop&dpr=1 600w, https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=429&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=429&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=539&fit=crop&dpr=1 754w, https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=539&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/471714/original/file-20220629-20-3iu2bq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=539&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Viruses can alter the skin microbiome to increase the presence of bacteria like <em>Bacillus</em>, which produce mosquito-attracting odors.</span>
<span class="attribution"><a class="source" href="https://flic.kr/p/aLPmBV">Marc Perkins/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>When we compared the skin bacteria compositions of infected and uninfected mice, we identified that a common type of rod-shaped bacteria, <a href="https://www.britannica.com/science/bacillus-bacteria"><em>Bacillus</em></a>, was a major acetophenone producer and had significantly increased numbers on infected mice. This meant that the dengue and Zika viruses were able to change their host’s odor by altering the microbiome of the skin.</p>
<h2>Reducing mosquito-attracting odors</h2>
<p>Finally, we wondered if there was a way to prevent this change in odors. </p>
<p>We found one potential option when we observed that infected mice had decreased levels of an important microbe-fighting molecule produced by skin cells, called RELMα. This suggested that the dengue and Zika viruses suppressed production of this molecule, making the mice more vulnerable to infection.</p>
<p><a href="https://doi.org/10.1016/j.chom.2019.04.004">Vitamin A</a> and its related chemical compounds are known to strongly boost production of RELMα. So we fed a vitamin A derivative to infected mice over the course of a few days and measured the amount of RELMα and <em>Bacillus</em> bacteria present on their skin, then exposed them to mosquitoes.</p>
<p>We found that infected mice treated with the vitamin A derivative were able to restore their RELMα levels back to those of uninfected mice, as well as reduce the amount of <em>Bacillus</em> bacteria on their skin. Mosquitoes were also no more attracted to these treated, infected mice than uninfected mice. </p>
<p>Our next step is to replicate these results in people and eventually apply what we learn to patients. <a href="https://www.who.int/data/nutrition/nlis/info/vitamin-a-deficiency">Vitamin A deficiency</a> is common in developing countries. This is especially the case in sub-Saharan Africa and Southeast Asia, where mosquito-transmitted viral diseases are prevalent. Our next steps are to investigate whether dietary vitamin A or its derivatives could reduce mosquito attraction to people infected with Zika and dengue, and subsequently reduce mosquito-borne diseases in the long term.</p><img src="https://counter.theconversation.com/content/185833/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Penghua Wang receives funding from the National Institutes of Health, Department of Health and Human Services, United States organization. He is a regular member of American Association of Immunologists, American Society for Virology, The Society of Chinese Bioscientists in America. </span></em></p>Certain viruses like dengue and Zika can make their hosts smell tastier to mosquitoes. Luckily, vitamin A and its derivatives may help combat these odor changes.Penghua Wang, Assistant Professor of Immunology, University of ConnecticutLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1738112022-01-25T13:27:01Z2022-01-25T13:27:01ZFrom odor to action – how smells are processed in the brain and influence behavior<figure><img src="https://images.theconversation.com/files/441580/original/file-20220119-23-1y38fqx.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2734%2C1342&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The compact olfactory system provides a more accessible way to study the brain as a whole.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/close-up-of-a-dogs-nose-royalty-free-image/603137803">Esther Kok/EyeEm via Getty Images</a></span></figcaption></figure><p>A dog raises its nose in the air before chasing after a scent. A mosquito zigzags back and forth before it lands on your arm for its next meal. What these behaviors have in common is that they help these animals “see” their world through their noses.</p>
<p>While humans primarily use their vision to navigate their environment, the vast majority of organisms on Earth communicate and experience the world through <a href="https://doi.org/10.1016/j.neuron.2005.10.022">olfaction</a> – their sense of smell.</p>
<p><a href="https://scholar.google.com/citations?user=wn_f7y0AAAAJ&hl=en">We</a> <a href="https://scholar.google.com/citations?user=JEi-fdoAAAAJ&hl=en">are</a> <a href="https://www.bbe.caltech.edu/people/elizabeth-j-hong">members</a> <a href="https://scholar.google.com/citations?user=GpkJjVUAAAAJ&hl=en">of</a> <a href="https://www.odor2action.org">Odor2Action</a>, an international network of over 50 scientists and students using olfaction to study brain function in animals. Our goal is to understand a fundamental question in neuroscience: How do animal brains translate information from their environments to changes in their behaviors?</p>
<p>Here, we trace the interconnections between smells and behaviors – looking at how behavior influences odor detection, how the brain processes sensory information from smells and how this information triggers new behaviors.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/58U52lDTuvk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Visualizing what smells look like helps researchers design technologies that detect odors as well as a dog can.</span></figcaption>
</figure>
<h2>Detecting odors in the environment</h2>
<p>When the odor of a flower is released into the air, it takes the shape of a wind-borne <a href="https://doi.org/10.1007/s003480000263">cloud of molecules called a plume</a>. It encounters physical obstacles and temperature differences as it flows through space. These interactions create turbulence that splits the odor plume into thin threads that spread out as the scent moves away from its source. These filaments eventually reach an animal’s nose or an insect’s antenna.</p>
<p>Odors that are broken up into filaments present a challenge to animals using them to find food or mates or avoid threats. It becomes difficult to predict precisely where the odor is coming from. Is the source directly ahead, to the left or right, above or below?</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/jQaHbHMrqmE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">This video by the Crimaldi Laboratory of the University of Colorado Boulder shows an odor plume developing behind a moving source over time. The source moves up and down from the left side, and the odor flows from left to right.</span></figcaption>
</figure>
<p>To work around this, animals have evolved what are called <a href="https://doi.org/10.1007/s10827-021-00798-1">active sensing</a> behaviors that improve their ability to detect and find odors in the environment.</p>
<p>When a fly detects the smell of fruit or a mosquito detects carbon dioxide from a possible host, for example, both insects first move upwind to get closer to the odor of the food source. They then move in a meandering, back-and-forth motion called casting to find more odor threads before surging upwind again. If they lose the scent, they’ll start casting again until they find the scent. Larger animals, such as mice and dogs, also alternate between more directed movements and more exploratory searching actions. </p>
<p>Animals also move their noses and antennae to improve the chances that they’ll encounter an odor. This is why dogs raise their noses in the air to increase the amount of odor they can sniff, and why insects move their antennae to stir up and penetrate the air to make better contact with odor molecules. </p>
<p>Once information from odors tell the animal that they’re close to the source, visual searching then comes into play.</p>
<h2>Making sense of odors</h2>
<p>When an animal comes into contact with an odor plume, it detects the presence of these odor molecules through tiny proteins called <a href="https://www.nobelprize.org/prizes/medicine/2004/summary/">odorant receptors</a>. These receptors are embedded in the sensory neurons lining its nasal cavity or antennae.</p>
<p>Each sensory neuron contains only one type of odorant receptor. And each type of odorant receptor has a different shape and set of chemical properties that determine which odors can bind to and activate it. Most of these receptors recognize multiple odors, and most odors can bind to multiple different receptors. What encodes the identity of a specific odor in the brain is determined by which combination of receptors are activated, and their relative strength of activation.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/MyHR6a-zJM0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">This video from the Wachowiak Lab at the University of Utah shows the activity of the olfactory bulb in a mouse brain as the mouse is exposed to different odors. Different odors make different combinations of neurons in the olfactory bulb light up.</span></figcaption>
</figure>
<p>An animal like a mouse has about a <a href="https://doi.org/10.1016/j.neuron.2005.10.022">thousand types</a> of odorant receptors. Having a large number of these receptors with diverse shapes allows the system to detect and distinguish between a very large number of chemically unique odors, including ones the animal has never encountered before. Most odors in the environment are often a mix of many different types of molecules. The smell of some <a href="https://doi.org/10.1146/annurev.ecolsys.38.091206.095601">flowers</a> can be a blend of over 100 different chemical compounds.</p>
<p>Once an odor molecule binds to a receptor, sensory neurons send specific <a href="https://nba.uth.tmc.edu/neuroscience/m/s2/chapter09.html">electrical signals</a> into compartments of the brain called <a href="https://doi.org/10.3389/fncir.2014.00098">olfactory glomeruli</a>. Different odors elicit distinct patterns of electrical activity across these regions, and this generates a specific neural representation of the odor in the brain.</p>
<p>An important step toward understanding olfaction is figuring out how different classes of odors map to different patterns of electrical signals in the brain.</p>
<p>Neuroscientists hypothesize that as these signals undergo successive stages of processing deep in the brain, sensory representations of odor are <a href="https://doi.org/10.1146/annurev-neuro-071013-013941">reformatted</a> in ways that extract information most useful to survival. This could be whether the smell is coming from something nutritious, indicating a potential source of food, or it could help the animal identify whether the smell is coming from a potential competitor or predator.</p>
<p>These reformatted sensory representations form the basis for how animals perceive smell and determine what actions they take in response to this information.</p>
<h2>From odor to action</h2>
<p>Once information about a particular odor reaches the brain, it often elicits both instinctual and learned <a href="https://doi.org/10.1523/JNEUROSCI.1668-18.2018">behaviors</a>. Odors that signal danger may trigger the animal to freeze or run away, while odors from a member of the same species may trigger the animal to mark its territory or initiate courtship. </p>
<p>In many cases, animals perform these tasks with incredible <a href="https://www.pbs.org/wgbh/nova/article/dogs-sense-of-smell/">precision and effectiveness</a>. It’s still common to use search dogs to find lost people and pigs to find truffles because available technologies aren’t capable of performing as well.</p>
<p>Animals achieve this level of performance not just because they’re able to detect and identify an odor. They’re also able to integrate odor features, like how intense the odor smells, with environmental clues, like wind direction, and internal cues, like hunger. All this information comes together to generate specific sequences of behaviors such as “face into the wind and then walk forward.”</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/FLH36ML8IEU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Dogs rely on smells to provide long-distance information. Humans, on the other hand, use smells for short distances.</span></figcaption>
</figure>
<p>To understand how odor guides these behaviors, scientists measure or manipulate an animal’s brain activity as they perform specific actions. This is done using imaging, electrophysiology or <a href="https://doi.org/10.1038/nn.4091">optogenetics</a>, which selectively activates specific neurons by shining a light on them. These approaches allow researchers to understand how patterns of brain activity shift when an animal changes its behavior to chase after an odor, or how environmental and internal cues combine to produce a best guess on the location of its next meal. </p>
<h2>Leading science and technology by the nose</h2>
<p>The olfactory system offers a unique opportunity to understand how the brain processes environmental information and translates it to behavior. Compared to other areas of the brain, the olfactory circuit is simpler in structure and uses fewer stages of processing. Its relative simplicity is what allows scientists like us to study it from end to end and learn how the brain works as a whole.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A rescue worker with a service dog goes through the ruins of a residential house to search for survivors" src="https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=521&fit=crop&dpr=1 754w, https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=521&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/441622/original/file-20220119-15-1atg4u1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=521&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Robots may one day be able to replace dogs in search and rescue situations.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/rescue-worker-with-a-service-dog-goes-through-the-ruins-of-news-photo/1229115883">Valery Sharifulin/TASS via Getty Images</a></span>
</figcaption>
</figure>
<p>Understanding brain function through the lens of olfaction could also pave the way for transformative developments in engineering, neuroscience and public health. Our research should accelerate the development of robots with <a href="https://doi.org/10.1177%2F0278364908095118">electronic noses</a> that can use odors to search for <a href="https://doi.org/10.1016/j.sbsr.2019.100305">chemical weapons</a>,
<a href="https://www.reuters.com/world/us/divers-try-locate-source-reported-oil-spill-gulf-coast-guard-2021-09-05/">underwater oil spills</a>
and <a href="https://doi.org/10.3390/inventions5030028">natural gas</a> leaking from pipelines in environments where it may be tedious or dangerous for humans or animals to go. Robots might also be able to search for missing people or disaster victims, something typically done with <a href="https://www.popsci.com/scientists-want-to-build-robotic-sniffer-that-outperforms-search-dogs/">trained dogs</a>.</p>
<p>An exciting future in scientific and medical development, we believe, is right under our noses.</p>
<p>[<em>Over 140,000 readers rely on The Conversation’s newsletters to understand the world.</em> <a href="https://memberservices.theconversation.com/newsletters/?source=inline-140ksignup">Sign up today</a>.]</p><img src="https://counter.theconversation.com/content/173811/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Crimaldi receives funding from the National Science Foundation, the National Institutes of Health, and the Department of Defense.</span></em></p><p class="fine-print"><em><span>Brian H Smith receives funding from the National Science Foundation.</span></em></p><p class="fine-print"><em><span>Elizabeth Hong receives funding from the National Science Foundation, the National Institutes of Health, the Curci Research Foundation, and the Luce Foundation</span></em></p><p class="fine-print"><em><span>Nathan Urban receives funding from the National Science Foundation and the National Institutes of Health. </span></em></p>Understanding how the brain translates smells into behavior change can help advance search and rescue technology and treatments for neurological conditions.John Crimaldi, Professor of Civil, Environmental and Architectural Engineering, University of Colorado BoulderBrian H. Smith, Trustees of ASU Professor, Arizona State UniversityElizabeth Hong, Assistant Professor of Neuroscience, California Institute of TechnologyNathan Urban, Provost and Senior Vice President, Lehigh University Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1423632021-06-16T12:35:58Z2021-06-16T12:35:58ZSmelling in stereo – the real reason snakes have flicking, forked tongues<figure><img src="https://images.theconversation.com/files/403812/original/file-20210601-15-15hx7un.jpg?ixlib=rb-1.1.0&rect=7%2C0%2C5121%2C3427&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Long misunderstood, snake tongues have fascinated naturalists for centuries.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/red-tailed-green-rat-snake-gonyosoma-oxycephalum-royalty-free-image/499911945">reptiles4all/iStock via Getty Images Plus</a></span></figcaption></figure><p>As dinosaurs lumbered through the <a href="https://www.britannica.com/science/Jurassic-Period">humid cycad forests of ancient South America</a> 180 million years ago, primeval lizards scurried, unnoticed, beneath their feet. Perhaps to avoid being trampled by their giant kin, some of these <a href="https://doi.org/10.1126/sciadv.1500743">early lizards sought refuge underground</a>. </p>
<p>Here they <a href="https://doi.org/10.1093/icb/15.2.455">evolved long, slender bodies and reduced limbs</a> to negotiate the narrow nooks and crevices beneath the surface. Without light, <a href="https://doi.org/10.2307/1439015">their vision faded</a>, but to take its place, an especially acute sense of smell evolved.</p>
<p>It was during this period that these proto-snakes evolved one of their most iconic traits – a <a href="https://theconversation.com/explainer-why-do-snakes-flick-their-tongues-29935">long, flicking, forked tongue</a>. These reptiles eventually returned to the surface, but it wasn’t until the extinction of dinosaurs many millions of years later that they <a href="https://doi.org/10.1186/s12862-015-0358-5">diversified into myriad types of modern snakes</a>.</p>
<p>As an <a href="https://scholar.google.com/citations?hl=en&user=G-2IS0oAAAAJ">evolutionary biologist</a>, I am fascinated by these bizarre tongues – and <a href="https://doi.org/10.1126/science.263.5153.1573">the role they have played in snakes’ success</a>.</p>
<h2>A puzzle for the ages</h2>
<p>Snake tongues are so peculiar they have fascinated naturalists for centuries. Aristotle believed the forked tips provided snakes a <a href="https://archive.org/details/worksofaristotle512aris">“twofold pleasure” from taste</a> – a view mirrored centuries later by French naturalist Bernard Germain de Lacépède, who suggested the twin tips could adhere more closely to “<a href="https://www.biodiversitylibrary.org/item/54321#page/9/mode/1up">the tasty body</a>” of the soon-to-be snack.</p>
<p>A 17th-century astronomer and naturalist, Giovanni Battista Hodierna, thought snakes used their tongues for “<a href="https://doi.org/10.1098/rstl.1683.0010">picking the dirt out of their noses … since they are always grovelling on the ground</a>.” Others contended the tongue captured flies “<a href="http://hdl.loc.gov/loc.gdc/lhbcb.27239">with wonderful nimbleness … betwixt the forks</a>,” or <a href="https://www.biodiversitylibrary.org/item/24612#page/7/mode/1up">gathered air for sustenance</a>.</p>
<p>One of the most persistent beliefs has been that the <a href="https://www.biodiversitylibrary.org/item/31811#page/5/mode/1u">darting tongue is a venomous stinger</a>, a misconception perpetuated by Shakespeare with his many references to “stinging” serpents and adders, “<a href="https://shakespeare.folger.edu/shakespeares-works/richard-ii/act-3-scene-2/?search=adder/#line-3.2.4">Whose double tongue may with mortal touch throw death upon thy … enemies</a>.”</p>
<p>According to the French naturalist and early evolutionist Jean-Baptiste Lamarck, snakes’ limited vision obliged them to use their forked tongues “<a href="https://www.journals.uchicago.edu/doi/pdfplus/10.1086/274830">to feel several objects at once</a>.” Lamarck’s belief that the <a href="https://www.biodiversitylibrary.org/item/31811#page/5/mode/1up">tongue functioned as an organ of touch</a> was the prevailing scientific view by the end of the 19th century.</p>
<h2>Smelling with tongues</h2>
<p>Clues to the true significance of snake tongues began to emerge in the early 1900s when scientists turned their attention to two bulblike organs located just above the snake’s palate, below its nose. Known as Jacobson’s, or vomeronasal, organs, <a href="https://doi.org/10.1016/S0169-5347(00)88953-3">each opens to the mouth through a tiny hole in the palate</a>. Vomeronasal organs are found in a variety of land animals, including mammals, but not in most primates, so humans don’t experience whatever sensation they provide.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing the location of the vomeronasal organ on a snake." src="https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=263&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=263&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=263&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=330&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=330&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406283/original/file-20210614-126997-r2t1cy.PNG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=330&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tongue tips deliver odor molecules to the vomeronasal organ.</span>
<span class="attribution"><span class="source">Kurt Schwenk</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Scientists found that vomeronasal organs are, in fact, an offshoot of the nose, lined with similar sensory cells that <a href="https://doi.org/10.1177/000348943404300311">send impulses to the same part of the brain as the nose</a>, and discovered that tiny particles picked up by the tongue tips ended up inside the vomeronasal organ. These breakthroughs led to the realization that snakes use their tongues to collect and transport molecules to their vomeronasal organs – not to taste them, but to smell them.</p>
<p>In 1994, I used film and photo evidence to show that when snakes sample chemicals on the ground, they separate their tongues tips far apart just as they touch the ground. This action allows them to sample odor molecules from <a href="https://doi.org/10.1126/science.263.5153.1573">two widely separated points simultaneously</a>. </p>
<figure>
<img src="https://cdn.theconversation.com/static_files/files/1596/ezgif.com-gif-maker.gif?1622555314">
<figcaption><span class="caption">Sampling two points at once. <i>Credit: Kurt Schwenk</i></span></figcaption>
</figure>
<p>Each tip delivers to its own vomeronasal organ separately, allowing the snake’s brain to assess instantly which side has the stronger smell. Snakes have two tongue tips for the same reason you have two ears – it provides them with directional or “stereo” smell with every flick – a skill that turns out to be extremely useful when following scent trails left by potential prey or mates.</p>
<p>Fork-tongued lizards, the legged cousins of snakes, do something very similar. But snakes take it one step farther.</p>
<h2>Swirls of odor</h2>
<p>Unlike lizards, when snakes collect odor molecules in the air to smell, they oscillate their forked tongues up and down in a blur of rapid motion. To visualize how this affects air movement, graduate student <a href="https://scholar.google.com/citations?hl=en&user=8jJWMBgAAAAJ">Bill Ryerson</a> and I used a laser focused into a thin sheet of light to illuminate tiny particles suspended in the air.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A snake flicking its toungue through a veil of smoke creating two swirls." src="https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/404121/original/file-20210602-21-1tj11os.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tongue-flicking creates small eddies in the air, condensing the molecules floating within it.</span>
<span class="attribution"><span class="source">Kurt Schwenk</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>We discovered that the flickering snake tongue generates two pairs of small, swirling masses of air, or vortices, that act like tiny fans, pulling odors in from each side and <a href="https://doi.org/10.1002/jez.725">jetting them directly into the path of each tongue tip</a>. </p>
<p>Since odor molecules in the air are few and far between, we believe snakes’ unique form of tongue-flicking serves to concentrate the molecules and accelerate their collection onto the tongue tips. Preliminary data also suggests that the airflow on each side remains separate enough for snakes to benefit from the same “stereo” smell they get from odors on the ground.</p>
<p>Owing to history, genetics and other factors, natural selection often falls short in creating optimally designed animal parts. But when it comes to the snake tongue, evolution seems to have hit one out of the park. I doubt any engineer could do better.</p>
<p>[<em>You’re smart and curious about the world. So are The Conversation’s authors and editors.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=youresmart">You can read us daily by subscribing to our newsletter</a>.]</p><img src="https://counter.theconversation.com/content/142363/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kurt Schwenk receives funding from The National Science Foundation and The University of Connecticut</span></em></p>Two tongue tips are better than one – an evolutionary biologist explains why snakes have forked tongues.Kurt Schwenk, Professor of Ecology and Evolutionary Biology, University of ConnecticutLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1514472021-01-26T13:27:09Z2021-01-26T13:27:09ZBeetle parents manipulate information broadcast from bacteria in a rotting corpse<figure><img src="https://images.theconversation.com/files/380306/original/file-20210123-19-bzmvih.jpg?ixlib=rb-1.1.0&rect=0%2C2%2C1363%2C901&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Parent beetle regurgitating liquefied carrion to its young.</span> <span class="attribution"><span class="source">Stephen Trumbo</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Biologists are accustomed to hearing stories of microbes manipulating their host – <a href="https://doi.org/10.1016/S0007-1536(82)80037-5">a fungus that turns ants into suicidal zombies</a>, a <a href="https://doi.org/10.1098/rspb.2000.1182">protozoan that makes rats seek out cat urine</a> – but there are few examples of hosts turning the tables on their microbes. </p>
<p>My colleagues and <a href="http://hydrodictyon.eeb.uconn.edu/eebedia/index.php/Stephen_Trumbo">I</a> <a href="https://doi.org//10.1086/712602">just published a paper</a> that demonstrated that the burying beetle, <em>Nicrophorus orbicollis</em>, found in eastern North America, alters the odors produced by microbes from their subterranean nest to thwart competitors that would steal the beetles’ cache. </p>
<h2>A series of unpleasant odors</h2>
<p>I have studied burying beetles for over 30 years, at first to understand their parental behavior and physiology, but more recently their role in the community of carrion insects that recycle vital nutrients into the soil. The olfactory environment of burying beetles is one that disgusts many humans but has fascinated me because it is the context in which beetles find their food, advertise for a mate and compete with rivals. </p>
<p>The volatile chemicals that microbes produce as they flourish on a corpse change as the animal decomposes. This changing bouquet of molecules attracts a succession of different insect species. The different mixes of odors <a href="https://doi.org/10.1007/s00253-011-3417-x">represent specific stages of decay</a> that will cue insects that specialize on a fresh corpse or the remains at the end of decomposition, or something in between. Such information may be useful in criminal cases <a href="https://link.springer.com/article/10.1007/s00253-011-3417-x">to determine the post-mortem interval</a>. </p>
<p>The focus of a burying beetle nest is a small dead animal that a male-female pair moves underground to prepare as food for its young. Microbes living on a fresh mouse carcass begin to metabolize proteins, emitting sulfurous byproducts that waft in the breeze. These odors attract <a href="https://doi.org/10.1007/s00049-020-00318-0">a flying burying beetle searching for a breeding opportunity</a>. </p>
<p>Working with <a href="http://hydrodictyon.eeb.uconn.edu/eebedia/index.php/Paula_Philbrick">Paula Philbrick</a>, a microbiologist, I began with field trials to identify the chemicals that burying beetles respond to, so we could discover which ones they might want to manipulate. We tested two chemicals – dimethyl disulfide and dimethyl trisulfide – known to attract carrion insects. These chemicals are used by corpse-mimicking plants in their own manipulation – fooling carrion-seeking flies and beetles into <a href="https://doi.org/10.1038/420625a">pollinating their putrid flowers</a>. </p>
<p>When we tried these compounds as supplements next to a fresh mouse carcass, however, free-flying burying beetles showed little interest. With our best guess off the mark, we were overwhelmed at the thought of randomly testing each of the more than <a href="https://doi.org/10.1371/journal.pone.0095107">500 chemicals associated with a rotting carcass</a>. </p>
<h2>Tell us what you know</h2>
<p>Rather than playing a chemical guessing game, we decided to take another approach, to see whether the beetles could show us what was important to them. </p>
<p>Our colleagues <a href="http://www.eae.uni-bayreuth.de/eae/en/mitarbeiter/mit/mitarbeiter_detail.php?id_obj=147213">Sandra Steiger</a> and <a href="http://www.eae.uni-bayreuth.de/eae/en/mitarbeiter/mit/mitarbeiter_detail.php?id_obj=147247">Johannes Stökl</a> at the University of Bayreuth used a technique called <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095107">gas chromatography-mass spectroscopy</a> to compare the volatile molecules emitted from carcasses prepared by a pair of <em>N. orbicollis</em> with those emitted from carcasses that had not been touched by beetles. </p>
<p>Surprisingly, two sulfur compounds that were not known to be an important cue for for any insect – methyl thiocyanate and methyl thiolacetate – were both reduced more than twentyfold by the beetles’ labor on the carcass. Why would they do this, and how? </p>
<p>Methyl thiocyanate turns out to be a great cue for burying beetles searching for a carcass. When we went back to the field and placed methyl thiocyanate next to carcasses, over 90% were discovered by burying beetles the first night, compared with a discovery rate of 0% to 20% for fresh carcasses without the chemical supplement. </p>
<p>Methyl thiocyanate appears to be heaven-scent for a beetle searching for that rare, newly deceased mouse or bird somewhere in the forest that is unclaimed by a vertebrate predator or scavenger. Once a carcass is discovered, however, the resident beetles face a problem. The same odors that alerted them could also reveal their carrion prize to competitors. Burying beetles are excellent at detecting and responding to information, but do they control this information as well? </p>
<h2>A disinformation campaign</h2>
<p>The transformation of a mouse carcass into beetle food is astonishing. After burying the carcass, the pair works day and night to remove the hair, round the carcass into a ball, and apply anal secretions to the exposed skin, dragging their abdomens in <a href="https://youtu.be/klpnnUENbxA">a zigzag pattern while circling the carcass</a>. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=950&fit=crop&dpr=1 600w, https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=950&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=950&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1194&fit=crop&dpr=1 754w, https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1194&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/374558/original/file-20201212-13-z0j844.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1194&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A mouse carcass before (left) and after (right) being prepared by a pair of burying beetles. After the carcass is shaved and rounded, the beetles apply anal secretions to control the carrion microbiota.</span>
<span class="attribution"><span class="source">Stephen Trumbo</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Scientists used to believe that the resident pair of burying beetles might be sterilizing the carcass, eliminating the microbes that release the telltale odors from the carcass hidden beneath the forest floor. While the secretions do contain antimicrobials, they also <a href="https://doi.org/10.1111/1365-2656.12725">contain microbes from the beetles’ gut</a>. The result is a microbial community where the microbes are just as numerous as on an unprepared carcass, but <a href="https://doi.org/10.1371/journal.pone.0225711">with fewer microbial species than in the normal mix</a>. </p>
<p>This manipulated microbiota emits far less methyl thiocyanate, and surprisingly, much greater amounts of dimethyl trisulfide – the aforementioned compound that is associated with the middle stages of decomposition where competing blowfly larvae make the carcass worthless to a burying beetle. </p>
<p>When we placed dimethyl trisulfide next to a fresh mouse carcass, free-flying beetles were not likely to land, apparently deterred by an odor that indicates a carcass is too far decomposed for breeding burying beetles. A resident pair of beetles makes it difficult for beetle competitors to use odors to find their carcass in two ways: by decreasing chemical attractants and by disinforming rivals by increasing chemical deterrents. </p>
<p>[<em>Understand new developments in science, health and technology, each week.</em> <a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-understand">Subscribe to The Conversation’s science newsletter</a>.]</p>
<p>When we took beetle-prepared carcasses from the lab and buried them in the field, they were much less likely to be discovered than similar-aged carcasses that had not been prepared by breeding beetles. Although resident burying beetles will fight to the death if an intruder shows up, the beetles prefer to avoid combat altogether.</p>
<p>Complex adaptations of animals with their microbiota are most often associated with gut microbes that aid host digestion, or cultured microbes that provide food. It makes sense, however, for resource specialists like burying beetles that consistently encounter an external microbiota to evolve similar levels of complexity. </p>
<p>Odors emitted by microbes are essential components of <a href="https://link.springer.com/article/10.1007/s10886-013-0306-z">animal communication, social interactions, sexual selection, predator-prey interactions</a> and plant-fungi symbioses. While the control of microbially derived odors by burying beetles might be one of the better examples, the ubiquity of microbes and their chemical products suggest that similar host manipulations will be common, even though humans have been oblivious to these adaptations and their importance.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/klpnnUENbxA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Burying beetle parents rounding a carcass and applying anal secretions.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/151447/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stephen Trumbo receives funding from the National Science Foundation and the National Geographic Society. </span></em></p>If you think only humans engage in disinformation, think again. Here is a stunning example of a beetle manipulating the odors emitted from a rotting corpse to keep it hidden from competitors.Stephen Trumbo, Professor of Ecology and Evolutionary Biology, University of ConnecticutLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1518322021-01-13T13:20:18Z2021-01-13T13:20:18ZThe scent of sickness: 5 questions answered about using dogs – and mice and ferrets – to detect disease<figure><img src="https://images.theconversation.com/files/378085/original/file-20210111-15-1b5qelp.jpg?ixlib=rb-1.1.0&rect=18%2C6%2C2026%2C1355&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Moose, a mixed-breed dog from the Nebraska Humane Society, trains in odor-detection work. </span> <span class="attribution"><span class="source">Bill Cotton/CSU</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p><em>Editor’s note: As COVID-19 continues to spread worldwide, scientists are analyzing new ways to track it. One promising approach is <a href="https://doi.org/10.1038/d41586-020-03149-9">training dogs to detect people who are infected</a> by smelling samples of human urine or sweat. Research scientist Glen Golden, who has trained dogs and ferrets to detect avian flu in birds, explains why certain animals are well suited to sniff out sickness.</em></p>
<h2>1. Which species have a nose for disease?</h2>
<p>Some animals have highly developed senses of smell. They include rodents; dogs and their wild relatives, like wolves and coyotes; and <a href="https://www.britannica.com/topic/list-of-mustelids-2058294">mustelids</a> – carnivorous mammals such as weasels, otters and ferrets. These species’ brains have three or more times more functional olfactory receptor neurons – nerve cells that respond to odors – than species with less keen smelling abilities, including humans and other primates. </p>
<p>These neurons are responsible for detecting and identifying volatile olfactory compounds that send meaningful signals, like smoke from a fire or the aroma of fresh meat. A substance is volatile if it changes readily from liquid to gas at low temperatures, like the acetone that gives nail polish remover its fruity smell. Once it vaporizes, it can spread rapidly through the air.</p>
<p>When one of these animals detects a meaningful odor, the chemical signal is translated into messages and transported throughout its brain. The messages go simultaneously to the olfactory cortex, which is responsible for identifying, localizing and remembering odor, and to other brain regions responsible for decision-making and emotion. So these animals can detect many chemical signals over great distances and can make rapid and accurate mental associations about them. </p>
<h2>2. How do researchers choose a target scent?</h2>
<p>In most studies that have used dogs to detect cancer, the dogs have identified physical samples, such as skin, urine or breath, from patients who either have been diagnosed with cancer or have <a href="https://doi.org/10.1016/j.applanim.2004.04.008">undiagnosed cancer at an early stage</a>. Scientists don’t know what odor cue the dogs use or whether it varies by type of cancer.</p>
<p>The U.S. Department of Agriculture’s <a href="https://www.aphis.usda.gov/aphis/ourfocus/wildlifedamage/programs/nwrc">National Wildlife Research Center</a> in Colorado and the <a href="https://monell.org/">Monell Chemical Senses Center</a> in Pennsylvania have trained mice to detect <a href="https://doi.org/10.1371/journal.pone.0075411">avian influenza in fecal samples from infected ducks</a>. Bird flu is hard to detect in wild flocks, and it <a href="https://www.cdc.gov/flu/avianflu/avian-in-humans.htm">can spread to humans</a>, so this work is designed to help wildlife biologists monitor for outbreaks. </p>
<p>The Kimball lab at Monell taught the mice to get a reward when they smelled a confirmed positive sample from an infected animal. For example, mice would get a drink of water when they traveled down the arm of a Y-shaped maze that contained feces from a duck infected with avian influenza virus. </p>
<p>By chemically analyzing the fecal samples, researchers found that the concentration of volatile chemical compounds in them changed when a duck became infected with bird flu. So they inferred that this altered smell profile was what the mice recognized.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/0UxLt3yugUA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Members of the mustelid family, such as ferrets, badgers and otters, have highly developed senses of smell. Here a wolverine sniffs out frozen meat buried deep in the snow.</span></figcaption>
</figure>
<p>Building on that work, we’ve trained ferrets and dogs to detect avian influenza in fowl, such as wild ducks and domestic chickens, in a collaborative study between Colorado State University and the National Wildlife Research Center that is currently under review for publication.</p>
<p>With ferrets, we started by training them to alert, or signal that they had detected the target odor, by scratching on a box that contained high ratios of those volatile compounds and to ignore boxes that contained low ratios. Next we showed the ferrets fecal samples from both infected and noninfected ducks, and the ferrets immediately began alerting to the box containing the fecal sample from an infected duck. </p>
<p>This approach is similar to the way that dogs are trained to detect known volatile odors in explosives or illegal drugs. Sometimes, though, we have to let the detector animal determine the odor profile that it will respond to.</p>
<h2>3. Can animals be trained to detect more than one target?</h2>
<p>Yes. To avoid confusion about what a trained animal is detecting, we can teach it a different behavioral response for each target odor. </p>
<p>For example, the dogs in the U.S. Department of Agriculture’s <a href="https://www.aphis.usda.gov/aphis/ourfocus/wildlifedamage/programs/nwrc/sa_spotlight/sniffing+out+disease">Wildlife Services Canine Disease Detection Program</a> respond with an aggressive alert, such as scratching, when they detect a sample from a duck infected with bird flu. When they detect a sample from a white-tailed deer infected by the prion that causes <a href="https://www.cdc.gov/prions/cwd/index.html">chronic wasting disease</a>, they respond with a passive alert such as sitting down.</p>
<p>Research at the University of Auburn has shown that dogs can remember and respond to <a href="https://doi.org/10.1007/s10071-020-01362-7">72 odors during an odor memory task</a>. The only limitation is how many ways a dog can communicate about different odor cues. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Sign showing images of a dog and the SARS-CoV-2 virus." src="https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378089/original/file-20210111-21-8ff0si.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A sign notifies travelers about a pilot study at Helsinki airport that offers free coronavirus tests using dogs to detect infections by smell.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/airport-signs-illustrate-the-new-covid-19-canine-test-news-photo/1228704590">Shoja Lak/Getty Images</a></span>
</figcaption>
</figure>
<h2>4. What kinds of factors can complicate this process?</h2>
<p>First, any organization that trains animals to detect disease needs the right type of laboratory and equipment. Depending on the disease, that could include personal protection equipment and air filtering. </p>
<p>Another concern is whether the pathogen might infect the detection animals. If that’s a risk, researchers may need to inactivate the samples before they expose the animals. Then they need to see whether that process has altered the volatiles that they are teaching the animals to associate with infection. </p>
<p>Finally, handlers have to think about how to reinforce the desired response from detection animals in the field. If they are working in a population of mostly noninfected people – for example, in an airport – and an animal doesn’t get a chance to earn a reward, it may lose interest and stop working. We look for animals that have a strong drive to work without stopping, but working for a long time without reward can be challenging for even the most motivated animal. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1254696791003541505"}"></div></p>
<h2>5. Why not build a machine that can do this?</h2>
<p>Right now we don’t have devices that are as sensitive as animals with well-developed senses of smell. For example, a dog’s sense of smell is <a href="https://doi.org/10.1016/j.applanim.2005.07.009">at least 1,000 times more sensitive than any mechanical device</a>. This could explain why dogs have detected cancer in tissue samples that have been <a href="https://doi.org/10.1016/j.applanim.2004.04.008">medically cleared as not cancerous</a> </p>
<p>We also know that ferrets can detect avian flu infection in fecal samples before and after laboratory analysis shows that the virus has stopped shedding. This suggests that for some pathogens, there may be changes in volatiles in individuals who are infected but are asymptomatic. </p>
<p>As scientists learn more about how mammals’ sense of smell works, they’ll have a better chance of creating devices that are as sensitive and reliable in sniffing out disease.</p><img src="https://counter.theconversation.com/content/151832/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glen Golden receives funding from the United Stated Department of Agriculture.</span></em></p>Scientists are experimenting with using dogs to sniff out people infected with COVID-19. But dogs aren’t the only animals with a nose for disease.Glen J. Golden, Research Scientist/Scholar I, Colorado State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1414372020-07-08T12:13:26Z2020-07-08T12:13:26ZSynthetic odors created by activating brain cells help neuroscientists understand how smell works<figure><img src="https://images.theconversation.com/files/346129/original/file-20200707-194405-awzgsl.jpg?ixlib=rb-1.1.0&rect=767%2C8%2C4838%2C3242&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">When you sniff a particular scent, your brain cells fire in a recognizable pattern.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/young-woman-smelling-perfume-from-bottle-at-royalty-free-image/953961844">Maskot via Getty Images</a></span></figcaption></figure><p>When you experience something with your senses, it evokes complex patterns of activity in your brain. One important goal in neuroscience is to decipher how these neural patterns drive the sensory experience.</p>
<p>For example, can the smell of chocolate be represented by a single brain cell, groups of cells firing all at the same time or cells firing in some precise symphony? The answers to these questions will lead to a broader understanding of how our brains represent the external world. They also have implications for treating disorders where the brain fails in representing the external world: for example, in the loss of sight of smell.</p>
<p>To understand how the brain drives sensory experience, <a href="https://rinberglab.com">my colleagues and I</a> focus on the sense of smell in mice. We directly control a mouse’s neural activity, <a href="https://doi.org/10.1126/science.aba2357">generating “synthetic smells”</a> in the olfactory part of its brain in order to learn more about how the sense of smell works.</p>
<p>Our latest experiments discovered that scents are represented by very specific patterns of activity in the brain. Like the notes of a melody, the cells fire in a unique sequence with particular timing to represent the sensation of smelling a unique odor.</p>
<h2>Scents produced by light projections</h2>
<p>Using mice to study smell is appealing to researchers because the <a href="https://doi.org/10.1016/j.conb.2018.04.008">relevant brain circuits have been mapped out</a>, and modern tools allow us to directly manipulate these brain connections.</p>
<p>The mice we use are genetically engineered so we can activate individual brain cells simply by shining light of specific wavelengths onto them – <a href="https://doi.org/10.1038/nn1525">a technique known as optogenetics</a>. Early uses of optogenetics involved light delivered through implanted optical fibers, letting researchers control coarse patches of brain cells. More recent uses of optogenetics allow <a href="https://doi.org/10.1126/science.aaw5202">more sophisticated control</a> of <a href="https://doi.org/10.1016/j.cell.2019.05.045">precise patterns of brain activity</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/346080/original/file-20200707-22-1rfavl2.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A simplified image of a mouse brain, looking down from the top. The olfactory bulb (left) is at the front of the brain and receives connections from receptor cells in the nose.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Mouse_brain_top_view.png">Database Center for Life Science/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>For our study, we projected light patterns onto the surface of the brain, targeting a region known as the olfactory bulb. Previous research has found that when mice sniff different scents, cells in the olfactory bulb appear to fire in a sort of patterned symphony, with a <a href="https://doi.org/10.1152/jn.90902.2008">unique pattern formed in response to each distinct smell</a>.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=617&fit=crop&dpr=1 600w, https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=617&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=617&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=776&fit=crop&dpr=1 754w, https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=776&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/346101/original/file-20200707-194405-1ojhb4o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=776&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Rather than receiving sensory signals from the nose, the olfactory bulb was activated by light projections.</span>
<span class="attribution"><span class="source">Edmund Chong</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>When we shined light patterns onto a mouse’s olfactory bulb, it generated corresponding patterns of cellular activity. We called these patterns “synthetic smells.” As opposed to a pattern of activity triggered by a mouse sniffing a real odor, we directly triggered the neural activity of a “synthetic smell” with our light projections.</p>
<p>Next we trained each individual mouse to recognize a randomly generated synthetic smell. Since they can’t describe to us in words what they’re perceiving, we rewarded each mouse with water if it licked a water spout whenever it detected its assigned smell. Over weeks of training, mice learned to lick when their assigned smell was activated, and not to lick for other randomly generated synthetic smells. </p>
<p>[<em><a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=experts">Expertise in your inbox. Sign up for The Conversation’s newsletter and get expert takes on today’s news, every day.</a></em>]</p>
<p>We cannot say for sure that these synthetic smells correspond to any known odor in the world, nor do we know what they smell like to the mouse. But we did calibrate our synthetic patterns to broadly resemble olfactory bulb patterns observed when actual scents are present. Furthermore, mice learn to discriminate synthetic smells about as quickly as they did real smells.</p>
<h2>Tweaking the pattern of a synthetic smell</h2>
<p>Once each mouse learned to recognize its assigned synthetic smell, we measured how much they still licked when we modified the assigned smell. Within each synthetic pattern, we altered which cells were activated or when they activated.</p>
<p>Imagine taking a familiar song, changing individual notes in the song, and asking whether you still recognized the song after each change. By testing many different changes, one can eventually understand which precise composition of notes is essential to the song’s identity and which tweaks are extreme enough to make the song unrecognizable.</p>
<p>Likewise, by measuring how mice changed their licking as we modified their projected light patterns, we were able to understand which combinations of cells within the pattern were important for identifying the synthetic smell.</p>
<p>The precise combination of cells activated was crucial. But just as important was when they are activated in an ordered sequence, akin to timed notes in a melody. For example, changing the order of cells in the sequence would render the smell unrecognizable.</p>
<p>It turned out that the cells activated earlier in the sequence were more important for recognition – changing the sequences later in the pattern seemed to have negligible effects.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/lJ2bof_fWgM?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Watch an animation of how these sequences in the brain work.</span></figcaption>
</figure>
<p>Changes in recognition were graded, and not drastic: When we changed small parts of the pattern, the smell did not become completely unrecognizable. In fact, the degree to which the smell was recognized was proportional to the degree of change in the pattern. This implies that if I slightly modify the brain activity that represents an orange, you would still smell something similar – maybe recognizing it as citrus, or fruity.</p>
<p>So while the brain has huge capacity to store many different smells in unique timed sequences of cell activity, you can still recognize similar smells by the similarity in their patterns: The smell of coffee is still distinctly recognizable even with a splash of vanilla added to it. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=506&fit=crop&dpr=1 754w, https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=506&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/346134/original/file-20200707-194418-1oc455r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=506&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">You know the smell of coffee even if it’s served with a dash of fragrant vanilla.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/hot-espresso-royalty-free-image/981402468">Roland Beerli/500px Prime via Getty Images</a></span>
</figcaption>
</figure>
<p>The next step in this research is to bring the synthetic approach to real smells. To do so, we would need to record brain activity in response to a real smell, then reactivate the very same cells using optogenetics. The synthetic re-creation of real objects in the brain is the current focus of research in <a href="https://doi.org/10.1126/science.aaw5202">multiple</a> <a href="https://doi.org/10.1016/j.cell.2019.05.045">labs</a> <a href="https://doi.org/10.1364/BRAIN.2019.BM3A.2">including ours</a>.</p>
<p>Addressing this issue is exciting because it opens up possibilities not just for understanding how the brain works, but also for developing brain implants that may one day restore the loss of sensory experiences.</p><img src="https://counter.theconversation.com/content/141437/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Edmund Chong does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Brains recognize a smell based on which cells fire, in what order – the same way you recognize a song based on its pattern of notes. How much can you change the ‘tune’ and still know the smell?Edmund Chong, Ph.D. Student in Neuroscience, New York UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1369292020-06-01T12:16:54Z2020-06-01T12:16:54ZWhat makes something smell good or bad?<figure><img src="https://images.theconversation.com/files/334484/original/file-20200512-82403-1wqng6q.jpg?ixlib=rb-1.1.0&rect=184%2C1062%2C1956%2C1423&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Cookies taste so good. Smell tells us that before we even take a bite. How?</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/OfdDiqx8Cz8">Jennifer Pallian/Unsplash</a></span></figcaption></figure><hr>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>What makes something smell bad or good? – Taylor, Atlanta, Georgia</strong></p>
</blockquote>
<hr>
<p>Pee-yew! Your old socks <a href="https://theconversation.com/why-do-feet-stink-by-the-end-of-the-day-125037">smell soooo bad</a>.</p>
<p>But why?</p>
<p>Maybe you’ve learned to dislike the smell. Maybe your socks are full of gross bacteria. Or maybe, it’s both. Our team studies the brain and sense of smell – it’s one of our favorite topics. But first, how do you smell?</p>
<h2>What is that smell?</h2>
<p>The air is filled with many small odor molecules which are released from “smelly” things like perfume or food. Your nose has the astonishing ability to smell thousands of different scents because in your nose are millions of <a href="https://youtu.be/snJnO6OpjCs">smell receptors</a> – cells that can recognize odor molecules. When you sniff the air, these special cells are alerted. </p>
<p>These receptor cells then send a signal to your brain. Your brain recognizes many scents when different types of odors enter your nose. The smell of baking cookies, for instance, is composed of <a href="https://www.brainfacts.org/thinking-sensing-and-behaving/smell/2015/making-sense-of-scents-smell-and-the-brain">many odor molecules</a>. Your brain can piece together all this information and let you know there are cookies baking in the oven.</p>
<h2>Smells that make memories</h2>
<p>Your brain is very good at memorizing good and bad experiences and associating particular smells with them. Scientists call these “<a href="https://www.livescience.com/why-smells-trigger-memories.html">olfaction-associated memories</a>.”</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/BjBOel3A6n4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">What’s that smell? Now I remember.</span></figcaption>
</figure>
<p>One example of this is when you smell a favorite meal. It might remind you of someone who makes it for you, which triggers your brain to release chemicals that make you feel good and comforted.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/334495/original/file-20200512-82375-1bvdibn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Memory can signal a smell tied to happiness.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/China-Food/92f09e881a5849fdae1ea71b95735ee4/15/0">AP Photo/Ng Han Guan</a></span>
</figcaption>
</figure>
<p>Of course, smell can also be associated with unpleasant experiences. You have probably eaten some food that went bad, and you might find that you hate that food now. This is your brain associating getting sick with a certain smell, which stops you from eating something that could be bad for you. Memories linked to smells can form because of good and bad feelings. </p>
<h2>Smells to warn you</h2>
<p>But what about things that you know smell good or bad even if you’ve never experienced them? Scientists have found that although a lot of the smells people like come from past experiences, <a href="https://www.fredhutch.org/en/news/center-news/2015/04/instinctive-reactions-to-smells-linked-to-olfactory-neurons.html">instincts</a> play a big role.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=800&fit=crop&dpr=1 600w, https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=800&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=800&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1005&fit=crop&dpr=1 754w, https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1005&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/334491/original/file-20200512-82379-1vnpyql.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1005&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Skunks are cute, but wow, that smell!</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/jG8eaA5Iq3A">Bryan Padron/Unsplash</a></span>
</figcaption>
</figure>
<p>Scent tells you a lot about your environment, and your instincts help to decide what is safe or dangerous. For example, <a href="https://www.livescience.com/60827-blood-molecule-attracts-and-repels.html">blood</a> has been shown to repel humans and many prey species, like deer, but attract predators, like wolves. This guides people away from predators that might want to eat us, but lets the predator get its meal. </p>
<p>Smell can warn you when something could make you sick. When eggs rot, bacteria multiply like crazy inside them, <a href="https://www.thedailymeal.com/eat/why-do-rotten-eggs-smell-sulfur">breaking down proteins</a> that release a toxic chemical called hydrogen sulfide. This produces a stench that makes you want to stay far away, stopping you from eating the egg and becoming ill.</p>
<p>As for your socks… if they smell bad now, don’t wait. Wash them with soap and water! The bacteria growing on your socks will be <a href="https://youtu.be/RZc09wD5wYQ">killed</a>, which will stop that nasty smell. </p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/136929/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Weihong Lin receives research funding from NIH. </span></em></p><p class="fine-print"><em><span>Kayla Lemons and Rakaia Kenney do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Mmmmmmm. That smells delicious. Wait, how do you know that?Rakaia Kenney, Research Assistant, University of Maryland, Baltimore CountyKayla Lemons, Research Associate, Ph.D., University of Maryland, Baltimore CountyWeihong Lin, Professor of Biological Sciences, University of Maryland, Baltimore CountyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1295192020-01-16T14:00:29Z2020-01-16T14:00:29ZWhy do onions make you cry?<figure><img src="https://images.theconversation.com/files/308901/original/file-20200107-123411-g5cu2l.jpg?ixlib=rb-1.1.0&rect=46%2C0%2C5184%2C3453&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Need a handkerchief?</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/onion-slices-on-wooden-cutting-board-753583324">Num LP Photo/Shutterstock</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>Why do onions make you cry? – Dana L., age 12, Belmont, Massachusetts</strong></p>
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<hr>
<p>Onions are grown and used all over the world, and anyone who has cut into one knows that it can make you cry. This happens because onions release an irritating chemical that makes your eyes sting. </p>
<p>Onions are mostly water, plus some vitamins and sugar compounds. They also contain compounds that include <a href="https://en.wikipedia.org/wiki/Sulfur">sulfur</a>, a natural chemical found in many smelly substances, such as <a href="https://io9.gizmodo.com/the-chemistry-of-skunk-spray-1567355548">skunk spray and garlic</a>. This is one way that plants <a href="https://www.britannica.com/list/botanical-barbarity-9-plant-defense-mechanisms">defend themselves</a> – producing substances that repel creatures who might eat them. Other plants have thorns or stinging leaves, or are made of special cells that make them hard to chew. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=882&fit=crop&dpr=1 600w, https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=882&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=882&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1109&fit=crop&dpr=1 754w, https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1109&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/308906/original/file-20200107-123368-khod9n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1109&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Red onion plants, showing their roots, stems and developing bulbs.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Onion#/media/File:ARS_red_onion.jpg">USDA ARS/Stephen Ausmus</a></span>
</figcaption>
</figure>
<p>One sulfur compound in onions, called propyl sulfoxide, escapes into the air when you slice an onion. When it comes into contact with moisture, such as water vapor in the air or the natural moisture around your eyes, it changes into sulfuric acid. Sulfuric acid has a strong smell and irritates your eyes, so they make tears to wash it away.</p>
<p>There are some tricks to avoid this “emotional” onion experience. Next time you’re getting ready to dice an onion, start by cutting off and throwing away a little bit of the root end, which has lots of stringy little roots hanging from it. This lets most of the noxious sulfuric compounds, which are found in the root, escape. Then you can remove the pointy tip of the onion, peel its skin and slice it with fewer tears. </p>
<p>Some cooks chill onions for 30 minutes before they cut them, which helps because the sulfur compounds don’t escape into the air as easily when they’re cold.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/308905/original/file-20200107-123377-1ueywze.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ornamental alliums (related to onions) are a popular flower for sunny gardens.</span>
<span class="attribution"><a class="source" href="https://www.pexels.com/photo/beautiful-bloom-blooming-blossom-109828/">Mike/Pexels</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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</figure>
<p>Onions add flavor to lots of our favorite foods, from spaghetti sauce to
tuna salad, so don’t let the smell drive you away. And gardeners love to grow <a href="https://lee.ces.ncsu.edu/2017/12/add-oomph-to-your-garden-with-ornamental-onions/">ornamental alliums</a> – members of the onion family that are bred for their looks. Many are very attractive, with blooms that make balls of color on long straight stalks. And their onion-y smell helps fend off rabbits, deer and other animals looking for a tasty garden meal.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/129519/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Minda Daughtry is affiliated with Harvesting Hope NC., a not-for-profit community garden in Smithfield, NC.</span></em></p>Like many plants, onions have defenses to ward off creatures that may want to eat them. Their secret weapon is a kind of natural tear gas.Minda Daughtry, Extension Agency, Agriculture – Horticulture, North Carolina State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1015072018-08-27T10:47:25Z2018-08-27T10:47:25ZWhy you can smell rain<figure><img src="https://images.theconversation.com/files/233335/original/file-20180823-149463-1y5q7lw.jpg?ixlib=rb-1.1.0&rect=290%2C231%2C3661%2C2393&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Your nose knows what's on the way.</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/LYq7W1lRal4">Lucy Chian/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p><em><a href="https://theconversation.com/por-que-sentimos-el-olor-de-la-lluvia-102529">Leer in español</a></em>.</p>
<p>When those first fat drops of summer rain fall to the hot, dry ground, have you ever noticed a distinctive odor? I have childhood memories of family members who were farmers describing how they could always “smell rain” right before a storm.</p>
<p>Of course rain itself has no scent. But moments before a rain event, an “earthy” smell known as petrichor does permeate the air. People call it musky, fresh – generally pleasant.</p>
<p>This smell actually comes from the moistening of the ground. Australian scientists first documented the <a href="https://doi.org/10.1038/201993a0">process of petrichor formation in 1964</a> and scientists from the Massachusetts Institute of Technology further <a href="https://doi.org/10.1038/ncomms7083">studied the mechanics of the process</a> in the 2010s.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233537/original/file-20180824-149484-mciivu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Petrichor’s main ingredients are made by plants and bacteria that live in the ground.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/green-seedling-growing-on-ground-rain-265568735">vovan/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>Petrichor is a combination of fragrant chemical compounds. Some are from oils made by plants. The main contributor to petrichor are <a href="http://eol.org/pages/7861/overview">actinobacteria</a>. These tiny microorganisms can be found in rural and urban areas as well as in marine environments. They decompose dead or decaying organic matter into simple chemical compounds which can then become nutrients for developing plants and other organisms.</p>
<p>A byproduct of their activity is an organic compound <a href="https://www.thoughtco.com/can-you-smell-rain-geosmin-and-petrichor-607587">called geosmin</a> which contributes to the petrichor scent. Geosmin is a type of alcohol, like rubbing alcohol. Alcohol molecules tend to have a strong scent, but the complex chemical structure of geosmin makes it especially noticeable to people even at extremely low levels. Our noses can detect just a few parts of geosmin per trillion of air molecules.</p>
<p>During a prolonged period of dryness when it has not rained for several days, the decomposition activity rate of the actinobacteria slows down. Just before a rain event, the air becomes more humid and the ground begins to moisten. This process helps to speed up the activity of the actinobacteria and more geosmin is formed.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233541/original/file-20180824-149481-14ys448.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Before you see it, do you smell it?</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/starting-rain-1094204522">elisa galceran garcia/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>When raindrops fall on the ground, especially porous surfaces such as loose soil or rough concrete, they will splatter and <a href="https://youtu.be/Waqmq_GTyjA">eject tiny particles called aerosols</a>. The geosmin and other petrichor compounds that may be present on the ground or dissolved within the raindrop are released in aerosol form and carried by the wind to surrounding areas. If the rainfall is heavy enough, the petrichor scent can travel rapidly downwind and alert people that rain is soon on the way.</p>
<p>The scent eventually goes away after the storm has passed and the ground begins to dry. This leaves the actinobacteria lying in wait – ready to help us know when it might rain again.</p><img src="https://counter.theconversation.com/content/101507/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tim Logan does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A weather expert explains where petrichor – that pleasant, earthy scent that accompanies a storm’s first raindrops – comes from.Tim Logan, Instructional Assistant Professor of Atmospheric Sciences, Texas A&M UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/946962018-05-04T10:47:32Z2018-05-04T10:47:32ZThat distinctive springtime smell: Asparagus pee<figure><img src="https://images.theconversation.com/files/217076/original/file-20180501-135837-1nmkwlg.jpg?ixlib=rb-1.1.0&rect=178%2C0%2C2693%2C1781&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">One of the signature fragrances of spring comes after the consumption of asparagus.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/s7stem/5169230033">Anton G</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span></figcaption></figure><p>Along with many other delights, springtime brings the beginning of the asparagus growing season. Regardless of whether you prefer the green, purple or white variety, asparagus provides a rich source of vitamins and minerals, and its consumption as part of a healthy diet may <a href="https://doi.org/10.3390/nu8030156">reduce risk of cancer</a> and <a href="https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/vitamins/vitamin-b/">cardiovascular-related diseases</a>.</p>
<p>Despite the nutritional benefits of asparagus, many are opposed to eating the vegetable due to its pungent aftereffects. As <a href="https://www.penguinrandomhouse.com/books/86757/fart-proudly-by-benjamin-franklin/9781583940792/">Benjamin Franklin wrote in 1781</a>, “A few stems of asparagus eaten, shall give our urine a disagreable odour.” This odor has become so well known that post-consumption urine is now often referred to as “asparagus pee.”</p>
<p>Scientists believe the odor in question is <a href="https://doi.org/10.1126/science.1162354">due to two chemicals</a>: methanethiol and S-methyl thioester. When enzymes in the human digestive tract break down the asparagusic acid that’s naturally present in the vegetable, these volatile compounds are created. When voided from the body, they become foul-smelling gas, wafting up from your asparagus pee.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=896&fit=crop&dpr=1 600w, https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=896&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=896&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1126&fit=crop&dpr=1 754w, https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1126&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/217342/original/file-20180502-153891-1nq8o8k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1126&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ready for harvest.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/fresh-green-asparagus-growing-on-garden-12506143">DUSAN ZIDAR/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>And <a href="https://doi.org/10.1093/chemse/bjq081">just because you don’t smell it</a> <a href="https://doi.org/10.1136/bmj.281.6256.1676">doesn’t mean you’re not making it</a>. Two studies have shown that people who are unable to smell the odor in their own urine also don’t detect it in the urine of known producers. Yes, volunteers sniffed samples of other people’s asparagus pee. Though most everyone probably produces the scent to some degree, it seems <a href="https://doi.org/10.1136/bmj.281.6256.1676">not everyone’s noses pick up on it</a>.</p>
<p>In fact, a study my colleagues and I conducted in 2017 <a href="https://doi.org/10.1136/bmj.i6071">found that only 40 percent</a> of those surveyed reported detecting the odor in their urine. A lower proportion of women were able to detect the odor, compared to men, despite <a href="https://doi.org/10.1016/0028-3932(89)90055-9">women being thought to have</a> a <a href="https://doi.org/10.1016/0028-3932(85)90067-3">more keen sense of smell</a>.</p>
<p>We asked almost 7,000 participants from two large cohort studies to respond to the prompt “After eating asparagus, you notice a strong characteristic odor in your urine.” By linking the questionnaire data with genetic data, we were able to show that the ability to smell or not to smell depends on a person’s genetic makeup. Hundreds of variants in the DNA sequence across multiple genes involved in sense of smell are strongly associated with the ability to detect asparagus metabolites in urine.</p>
<p>Asparagus isn’t the only food that has genetically linked controversial smell or taste effects. Some people avoid eating cilantro because they claim it has a “soapy” aftertaste. A study using data from almost 30,000 users of 23andMe found <a href="https://doi.org/10.1186/2044-7248-1-22">genetic variants in olfactory receptors</a> linked to people’s perception of this adverse taste.</p>
<p>Maybe you can conduct your own survey at the next family meal that includes a platter of asparagus – or soon after.</p><img src="https://counter.theconversation.com/content/94696/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sarah Coseo Markt receives funding from the National Institutes of Health. </span></em></p>Perhaps you’ve noticed something unusual in the bathroom after you consume this healthy spring vegetable. A Speed Read explains there’s two parts to the stinky puzzle: production and perception.Sarah Coseo Markt, Research Scientist in Epidemiology, T.H. Chan School of Public Health, Harvard UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/826812017-08-25T01:21:29Z2017-08-25T01:21:29ZCan you pass this smell test?<figure><img src="https://images.theconversation.com/files/183335/original/file-20170824-18715-mn7b09.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The smell of daffodils is a treat for most people, but some cannot experience the joy because they have lost their sense of smell. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/handsome-man-smelling-flowers-670487011?src=hFOsW29bjol477j6y-m5pQ-1-0">Mila Supinskaya Glashchenko/Shutterstock.com</a></span></figcaption></figure><p>Each of our senses gives us a unique view of our world. Our visual system detects parts of the electromagnetic spectrum, revealing movement, brightness and color, but also a smile or a tear. Our auditory system registers changes in pressure, but also allows us to hear the crash of ocean waves or the smoky contralto of Billie Holiday. To appreciate the flavor of food and drink, recognize the perfume of the first spring flowers or detect the danger of a gas leak, we rely upon our olfactory (smell) and gustatory (taste) systems.</p>
<p>Smell and taste are intimately linked to each other. They are collectively known as chemical senses because each system functions to detect chemicals in our external environment. Our brains also process aspects of smell and taste together, especially when it comes to perceiving the flavor of food. </p>
<p>The chemical senses play hugely important roles in the lives of all animals. For example, a recent pair of studies showed that social order breaks down dramatically in <a href="http://www.sciencedirect.com/science/article/pii/S0092867417307729">ants with a disrupted olfactory system</a>. These animals, which rely on odors to communicate with each other the way humans rely on language, could no longer perceive important social messages sent between individuals.</p>
<p>Anosmia, which is the complete loss of smell, and other smell or taste disorders can be life-changing for humans, too. Because your brain combines smell and taste to create a perception of flavor, impairments in either of these senses can make food seem bland or even unpalatable. And while we don’t rely on odors to communicate the way ants do, <a href="http://www.fifthsense.org.uk/the-impact-of-smell-and-taste-disorders/">those who can’t smell</a> their new baby, their partner or the freshly mown grass can feel isolated from others and from the world around them.</p>
<p>Unfortunately, there are no effective treatments for most patients with smell and taste disorders. Additionally, patients often find it nearly impossible to be properly diagnosed or to receive support that can help to improve their quality of life – critical needs that we are helping to address in the laboratory and the clinic at the University of Florida <a href="http://cst.ufl.edu/">Center for Smell and Taste</a>.</p>
<h2>Test your knowledge</h2>
<p>How well do you know your chemical senses? Answer these true-or-false questions below to find out more. </p>
<p><strong>Humans can track scent on the ground the way dogs do.</strong></p>
<p>True.</p>
<p>The human sense of smell is better than you were told.
In fact, it <a href="http://science.sciencemag.org/content/356/6338/eaam7263.long">rivals that of other animals</a>. Humans can even follow a <a href="http://www.nature.com/neuro/journal/v10/n1/full/nn1819.html">scent track on the ground</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=397&fit=crop&dpr=1 600w, https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=397&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=397&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=499&fit=crop&dpr=1 754w, https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=499&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/183336/original/file-20170824-18715-1srqfoi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=499&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dogs in a field chase a scent.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/dogs-chasing-scent-654210652?src=WvOHXtn3h07kXsLQu1aXeQ-1-2">CG3/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>However, dogs, rodents and many other mammals are attuned to parts of the chemical world of which we are unaware: They can detect specialized odors such as <a href="http://www.annualreviews.org/doi/full/10.1146/annurev.physiol.70.113006.100608?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed">pheromones</a> that may influence social interactions, mating behaviors or aggression responses. There is little evidence supporting a role for pheromones in humans, no matter what those internet ads might say!</p>
<p><strong>Some people smell odors that aren’t there.</strong></p>
<p>True.</p>
<p>People with <a href="https://en.wikipedia.org/wiki/Phantosmia">phantosmia</a> perceive an odor even when there is not one present. These smell phantoms (there are taste phantoms, too) can have many causes, including nasal infections, neurological damage accompanying surgery or conditions such as epilepsy. Phantosmia is but one type of smell and taste disorder, the most common of which are hyposmia, which is diminished ability to smell, and anosmia, which is the inability to detect smells.</p>
<p><strong>Sweet taste can be an illusion.</strong></p>
<p>True.</p>
<p>Artichokes contain chemicals that don’t taste sweet on their own, but leave behind sweetness when washed away by a <a href="http://science.sciencemag.org/content/178/4064/988.long">drink of water</a>. These chemicals act by locking the sweet taste receptor in your mouth into the “off” position; when the chemical is washed away, the receptors all snap to the “on” position simultaneously, evoking a <a href="https://www.nature.com/nature/journal/v441/n7091/full/nature04765.html">sweet “water taste”</a> even in the absence of sugar or other sweeteners. The sweet taste inhibitor lactisole – a compound added to jams and jellies to dampen the intense sweetness from high concentrations of sugar used as a preservative – can elicit a similar perception.</p>
<p><strong>Mosquitoes (and other insects) have noses.</strong></p>
<p>False (technically). </p>
<p>But they do have have antennae and other structures that function much like the human nose to detect odors. And they use their sense of smell to find you when they want a meal. The insect repellent DEET works in part by <a href="http://www.biorxiv.org/content/biorxiv/early/2016/06/22/060178.full.pdf">disrupting the mosquito’s sense of smell</a>.</p>
<p><strong>Sour can taste sweet.</strong></p>
<p>True.</p>
<p>An unusual protein called miraculin, found in the “miracle fruit” of the tropical plant <em>Synsepalum dulcificum</em>, doesn’t taste sweet on its own but becomes a potent <a href="http://www.pnas.org/content/108/40/16819.long">sweetener when exposed to acids</a>. Suck on a lemon after eating a miracle fruit, and it will taste like lemon candy.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=504&fit=crop&dpr=1 754w, https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=504&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/183337/original/file-20170824-18715-iwsdo3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=504&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A lemon will taste like candy after you eat miracle fruit.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/lemon-peel-448491604?src=dml1aqPGgH5D9vXiAthCng-1-54">grey_and/Shutterstock.com</a></span>
</figcaption>
</figure>
<p><strong>Smell and taste aren’t the only chemical senses.</strong></p>
<p>True.</p>
<p>While you can tell the difference between basil and garlic by their smell, your sense of taste is largely indifferent to herbs and spices. Then how do you sense of cooling of the menthol in mint, the heat of capsaicin in a habanero, or the tingling of sanshools in Szechuan peppercorns? Compounds in many herbs and spices trick temperature, pain and vibration sensors associated with the trigeminal nerve in the mouth and nose to give you these sensations. This third chemical sense is known as <a href="http://pubs.acs.org/doi/abs/10.1021/cn100102c">chemesthesis</a>. </p>
<p><strong>Mirrors can change smells.</strong></p>
<p>False. </p>
<p>A mirror won’t alter a smell. But some mirror image molecules (known as stereoisomers – think of comparing your right and left hands) have very different smells. The best-known stereoisomers that evoke very different aromas are <a href="https://en.wikipedia.org/wiki/Carvone">D-carvone and L-carvone</a>, which smell like caraway and spearmint, respectively.</p>
<p>Few people may appreciate the biology and chemistry that allows us to experience our chemical world. But those of us who study the chemical senses hope that our research will lead to tastier and healthier food, reduce the spread of insect-borne disease, improve the lives of people with smell or taste disorders and create a better understanding of the importance of smell and taste.</p>
<p><em>This article was written before anosmia became a common symptom of COVID-19.</em></p><img src="https://counter.theconversation.com/content/82681/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Steven D Munger receives funding from the University of Florida and from the National Institute on Deafness and Other Communication Disorders. He currently serves as the Past President of the Association for Chemoreception Sciences.</span></em></p>Our senses of taste and smell are linked to one another in ways that experts are continuing to explore. See if you can answer some questions for which experts have discovered some surprising answers.Steven D. Munger, Professor of Pharmacology and Therapeutics, University of FloridaLicensed as Creative Commons – attribution, no derivatives.