Have you ever met a really tall man who has a high-pitched voice? Did it seem odd?
We intuitively expect large mammals to have low, booming voices, while small mammals should have high-pitched, squeaky voices. The feathertail glider is a tiny marsupial weighing only as much as three teaspoons of sugar, and as you would expect, it squeaks.
At the other extreme a lion prowling the African plains, intimidating his competition, has a deep booming roar. Other mammals that are intermediate in size, such as the Tasmanian devil, have a suitably intermediate tone to their calls.
The frequency of the calls made by animals tend to correlate with their body mass. This is because the larger the animal’s body, the larger their vocal apparatus.
It’s similar to musical instruments in an orchestra: the larger the instrument, the lower the pitch of the sound. Think about the difference between a tuba and a piccolo.
Although some animals cheat – such as the howler monkey and the koala, which have specialised chambers in their larnyx to produce a lower tone – for most terrestrial mammals, their size correlates with the tone of their voice.
Aquatic mammals break the rules
But it is not always that simple. Our research shows that aquatic mammals do not produce calls at the frequency we would expect based on their size.
Rather than having a deep roar like a tiger, dolphins and toothed whales make high frequency whistles and squeaks. To put this in perspective, the largest terrestrial mammal, the African elephant, weighs almost 4,000kg and produces calls at frequencies up to 8kHz. In contrast, the Arnoux beaked whale, which is more than twice the size of an elephant, whistles like a bird at frequencies up to 11kHz.
Listen to the whistles of the Arnoux beaked whale; the whistles warble up and down in a haunting tone.
It’s not only the dolphins and whales that possess this ability to produce such high frequencies for their size. The semi-aquatic seals also produce strange and wonderful calls underwater.
Weddell seals sound like something out of your favourite sci-fi movie rather than their canine cousins.
Leopard seals grow to be the size of a large bull, yet when they sing underwater they produce sounds like a cricket. What is even stranger, rather than having a deeper voice as they get bigger as most mammals do, the larger seals have a higher pitched voice.
Why the difference?
We recently set out to find out what causes larger marine mammals to have higher pitched voices. Previous studies have found that body mass or the environment in which an animal lives tends to influence vocalisation pitch.
For example, in a forest, very high and very low pitched sounds tend to drop in volume rapidly – or attenuate – as you move away from the source. So birds in these forests tend to produce sounds in a relatively narrow frequency band that can propagate farther.
Likewise, with marine mammals, we found that it’s the environment that matters most.
Sound travels faster through water than air, and sound waves attenuate less quickly and are less distorted. This means that higher frequency sounds are able to travel farther through water than air, without the loss of important information. It is this propagation efficiency that enables aquatic mammals to use calls of high frequencies.
But mammals also face challenges when it comes to making sound underwater.
As you would know, you can’t breathe and vocalise while underwater. So aquatic and semi-aquatic mammals have adapted complex respiratory systems to recycle air while vocalising; they move air back and forth between the lungs and the upper respiratory organs. This means that they do not need to resurface to get another lung full of air in order to keep singing.
But there is a second problem for mammals that call underwater.
When sound waves travel from one acoustic medium to another, such as from air in the vocal tract to water, a large amount of the sound is lost to scattering and attenuation. This is called an impedance mismatch.
So, in aquatic mammals, the fatty tissue around the larynx is a similar density to water. High frequency sounds are produced by recycling air back and forth through organs such as the larynx, which consequently pass through the fatty tissue surrounding them and into the surrounding water, mitigating sound loss due to scattering and attenuation.
The most well-known case of aquatic mammals producing ultra-high frequencies is the ability of some dolphin species to echolocate to navigate and find prey. They have fatty melons on their heads which their calls pass through and are received through. These appendages are also similar in density to water and are therefore very effective at transferring the ultrasonic frequencies into and from the water.
While body size still drives the frequencies produced by terrestrial mammals, the acoustic properties of the aquatic environment mean that bigger doesn’t necessarily mean deeper when it comes to aquatic mammals. In fact, you’re far more likely to hear a dolphin squeak like a mouse than roar like a lion.