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Why are some snakes so venomous?

Australia is world famous for its venomous critters, including its many highly venomous snakes. The snake that holds the popular title of “world’s most venomous” is the inland taipan (Oxyuranus microlepidotus…

The inland taipan is world famous for its venom. Stewart Macdonald

Australia is world famous for its venomous critters, including its many highly venomous snakes.

The snake that holds the popular title of “world’s most venomous” is the inland taipan (Oxyuranus microlepidotus), an inhabitant of Australia’s arid interior. Astonishingly, a single bite from an inland taipan is capable of delivering enough venom to kill 250,000 lab mice.

The venom of the inland taipan has attracted considerable research interest and the toxins responsible for its extreme toxicity have been identified. Effective antivenom also exists for the treatment of bites.

What we don’t know, though, is why the inland taipan needs such toxic venom. We know almost nothing about the evolutionary selection pressures that have refined and enhanced the toxins present in the venom of this iconic species of snake.

Snakes vs humans

Historically, the focus of snake venom research worldwide has been anthropocentric – examining the impact the venom has for humans. Large species of venomous snake, those that are known to be potentially dangerous to humans, have received the lion’s share of attention.

Most attention has been given to the development of antivenom and to studying the building blocks of toxic proteins found in snake venoms. This has allowed us to learn more about human physiology and to search for compounds that may be useful in drug design, such as the toxin from the venom of a pit viper from which the blood pressure medication Captopril was developed.

‘Milking’ snakes for antivenom.

These are important goals for venom research, but the result of this bias toward human interest is that we still know very little about the ways in which snakes use their venom in nature. We also do not know how diet influences its composition – the ecology of venom is an almost completely neglected area of research.

We do know that the common ancestor of all snakes possessed a rudimentary venom system. This means that all snakes had an equal evolutionary opportunity to become venomous. That not all snakes developed sophisticated venom delivery systems suggests that being highly venomous is not always the most efficient way for a snake to secure a meal.

There are no herbivorous snakes, but venom is not the only way that snakes can subdue their prey. Many snakes use constriction, as dramatically demonstrated in the recent battle between a python and crocodile in Queensland.

Spoiler: the snake wins.

Some snakes simply rely on powerful jaws while others feed on defenceless prey such as eggs, so have no need of any additional deadly method of subjugation.

Snake evolution in Australia

In Australia there is a unique opportunity to study the evolution of snake venom.

The majority of snakes in this country are members of the Elapidae family, which means they have fixed fangs at the front of their mouths, and all are venomous.

The family, which arrived in Australia some 10 million years ago, includes some of the world’s most famous snakes such as the cobras of Asia and Africa and the mambas of Africa.

Like all elapid snakes, the common ancestor of modern Australian species would have possessed a sophisticated venom system capable of delivering a complex cocktail of toxins into potential prey animals.

Elapid snakes quickly diversified in their new environment and today Australia is home to approximately 100 terrestrial species and more than 30 marine species – more than a third of the world’s elapid snake fauna.

Australian elapid snakes are extremely diverse in ecology and prey preference: some are general feeders that will tackle any prey, some prefer mammals, others frogs or reptiles. Some are marine specialists, while others prefer eggs.

Rough-scaled snakes primarily feed on frogs. Stewart Macdonald

Despite the opportunity this diversity represents to study venom ecology, the majority of venom research has focused on large species that are potentially dangerous to humans. As these species are typically generalist feeders, this research has given us little insight into what has shaped the venom in the other species.

So why so toxic?

A popular theory in the past was that snakes simply evolved the most toxic venom possible in order to kill quickly any potential prey they might come across. Occasionally the extreme toxicity of inland taipan venom is still used in support of this “nuclear bomb” theory of snake venom evolution.

But recent research is revealing a strong correlation between prey preference and venom composition. This extends to species with strong shifts in prey preference throughout their lives – if babies and adults specialise on different prey types, they may have different venoms.

The baby eastern brown snake (Pseudonaja textilis) feeds exclusively on reptiles, while its parents would have had a broader diet. Stewart Macdonald

At the molecular level, individual toxins have been identified that are 100-fold more toxic to natural prey than to laboratory organisms such as rodents. The evidence suggests that snakes evolve venom that is fine-tuned for the specific context in which it is used.

Why then is the inland taipan so toxic when it targets solely rodents? The precise answer to this question awaits further research, although it is possible to indulge in thought experiments.

The fact that inland taipans specialise on rodents may partly explain their extreme toxicity to lab mice, but there’s probably more to it that that.

In nature, taipans need to kill their relatively dangerous rodent prey quickly, before it escapes or has a chance to retaliate. Living in a harsh, arid environment also means they must conserve resources, so they likely deliver only a tiny fraction of the contents of their glands each time they bite a prey animal.

Inland taipans are also engaged in a co-evolutionary arms race with their natural prey, which may over time have evolved some resistance to the snake’s venom. The hapless laboratory mice used in toxicity testing are evolutionarily naïve and may be much more sensitive to the venom.

So the impressive figure of 250,000 mice per bite is misleading, reflective more of “laboratory reality” than evolutionary reality.

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25 Comments sorted by

  1. John Newlands

    tree changer

    An alternative theory is that snakes wanted to weaken predators within seconds so as to avoid fatal injury to themselves. This needs 'overkill'.

    As an aside I've noticed areas of Tasmania that were dominated by the lowland copperhead are now favouring tiger snakes. This fits with my belief of a general drying trend with fewer damp soil areas. I made a similar comment to another TC article about shrinking habitat for the burrowing crayfish. A friend who keeps a menagerie of tiger snakes won't introduce copperheads into the enclosure since they will fight. The supposedly less aggressive and less venomous copperheads invariably triumph over the tiger snakes. Yet another puzzle.

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    1. George Michaelson

      Person

      In reply to John Newlands

      Do any snakes co-habit in the wild? I can understand unrelated species in different niches surviving in a common pen, but aren't these in direct competition?

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    2. William Fren

      logged in via email @gmail.com

      In reply to John Newlands

      Snake venom is a form of saliva loaded with enzymes. The primary purpose is to assist in digestion, it is also used in defence. Many years ago, it was discovered by vets that if a pet cobra had it's fangs removed, then it would develop a lump near the tail caused by constipation and could die if not treated for constipation. Snakes can precisely control the amount of venom being injected. They give each other love bites, very slowly and with affection, An angry bite can be a snap bite or a very angry…

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  2. John Brady

    logged in via Facebook

    lynn helena caporale's book Darwin in the Genome described the massive overkill of some cone snail species, which had (if I remember correctly) about 250 different poisons, compared to the ~ 50 that might have been expected from the conventional evolution of predator-prey poison wars. Apparently their DNA for poisons included a palindrome that encouraged hairpin mutations, increasing the incidence of mutations in code for poisons by a factor of 100,000. Caporale postulated 2nd order selection for such overkill, which would presumably win a poison war in a massive smackdown. I would be very interested if anyone can tell us what follow up has happened re 2nd order selection in poison wars.

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  3. John Crest

    logged in via email @live.com.au

    Fascinating.

    Related but slightly OT, I've read that the venom of the Sydney Funnel Web is only toxic to primates (amongst mammals) and other insects.

    If true, I find that a very interesting fact from an evolutionary pressure point of view.

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    1. christopher gow

      gainfully employed

      In reply to John Crest

      It is an oddity, what is the point of being venomous to primates when there are none present apart from the comparatively recently introduced feral humans?
      Perhaps marsupials had evolved to develop resistance to funnel web venom, but then why did funnel webs stay evolutionarily static? Maybe it's just a slow process and we turned up at the wrong time.
      Living as I do just south of Sydney there are no shortage of funnel webs about; they certainly command respect.

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    2. Miles Ruhl

      Thinker

      In reply to John Crest

      Fascinating indeed John! Wikipedia says the same about being toxic only to primates and humans, not to other mammals. I wonder the evolutionary reasons behind that also.

      I do find the fear shown by people in other countries about how deadly our fauna is quite odd. I had people in the US almost in tears saying they'd never visit for fear of being killed by one of our many deadly (mostly less than a 10th of the size of a human) animals, meanwhile showing me photos of the cougar that was in their front yard in Washington state, a state also known for it's bear population.

      Yeah, I'll take my chances with our critters thanks!

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    3. John Crest

      logged in via email @live.com.au

      In reply to Miles Ruhl

      Everything here might be poisonous, but nothing (on land) is gonna hunt you down and eat you.

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    4. In reply to John Crest

      Comment removed by moderator.

    5. In reply to John Holmes

      Comment removed by moderator.

    6. John Crest

      logged in via email @live.com.au

      In reply to John Holmes

      Hence my "on land" qualification (as salties are for the most part aquatic).

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    7. John Holmes

      Agronomist - semi retired consultant

      In reply to John Crest

      Spent an afternoon on the Daly River flood plain with a farmer whose hut was in 1 ha of big mango trees. He described watching salties running down and catching wallabies on his paddock next to a Billabong near the river. That was some time ago, the baby crocs of the 70's are now big.

      Wild pigs can be a bit daunting as well.

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  4. Digby Gotts

    Farmer

    I'm a little surprised that there is no mention of efficiency of injection, ie the amount injected cf the amount released. Is the fang of elapids hypodermic or a simple groove cmpared to rattlesnakes? An inefficient delivery system compensated for by extreme toxicity.

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  5. christopher gow

    gainfully employed

    It is a bit strange that we focus so much on how deadly Australian snakes are. They may have highly toxic venom but fatalities are very rare.
    World wide deaths from snake bite are hard to accurately determine but estimates range from 20,000 to around 100,000 per year (and probably mostly children working in fields). The majority are in India and SE Asia and typically occur in rice paddies and plantations.
    That is a lot of people; more than Rabies, about the same as Cholera and much more than Japanese Encephalitis and snake bite has now been recognised by WHO as a neglected tropical disease.
    And yet I have met plenty of English folk and Americans who happily traipse about India and Asia but are fearful of visiting Australia because of all our 'deadly creatures' - weird stuff.

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    1. Ian Bolton

      Farmer

      In reply to christopher gow

      Christopher, tourists simply need to be told the truth that there are no poisonous snakes in Australia. All Australian snakes are perfectly edible but some care is required to get them on the BBQ before they bite You.

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    2. John Holmes

      Agronomist - semi retired consultant

      In reply to christopher gow

      How long have humans been here? Snakes are food, so most are quite hesitant to stay in the same place as a human, as those which did often got BBQed.

      Bites are usually in unusual situations where the snake is trapped or has been spooked by something. Very humorous afterwards to watch peoples reactions when the sheep dog has rounded up a snake and is chasing it into a group working in the paddock. Especially as you were the one who looked down and saw the snake passing between your legs and yelled a warning.

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    3. Sandor von Kontz

      farmer

      In reply to Ian Bolton

      Ian, don't forget to tell them to be sure the poison gland just behind the head has to be removed, and I suggest boiling and or smoking rather than BBQ.

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  6. Greg O'Reilly

    Environmental Science

    The Georgina River basin has other poisonous mysteries besides the inland taipan. Despite enormous research effort in the decades after WW2, scientists could not explain why Georgina gidyea (Acacia georginae) has toxic levels of sodium monofluoroacetate (1080) in seemingly random patterns only in this area. Some trees are always toxic; some are toxic seasonally, while others become toxic when safe only days earlier. One tree may be poisonous yet a neaby tree safe. Significantly this Acacia has a…

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    1. John Holmes

      Agronomist - semi retired consultant

      In reply to Greg O'Reilly

      I would strongly suggest that variable toxicities in poisonous plants be explored in the context as to what is happening in that plant community. For example giraffes graze up wind. Some trees when grazed will emit ethylene, a plant hormone, and trees of that species which have not been grazed will increase the level of phenolic compounds in their leaves. This deters grazing. Excessive consumption will damage and kill giraffes. Confine the animals in a small area, and they are forced to graze trees which have b een activated and so some deaths have been observed.

      Do not under estimate the subtleties of plants.

      As boy I picked poison - WA gastronomical species - 1080 the toxin. After a shower of rain in summer as the plants would increase the toxin levels rapidly so we lost the odd sheep in areas we thought were OK. Sheep are not native and have no tolerance to that chemical.

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  7. John Holmes

    Agronomist - semi retired consultant

    Conventional wisdom suggests that it is more energy efficient to produce a very toxic compound so that when the prey is hit does not get away and no fighting is needed - see the python and crocodile example. Onlookers suggested that the snake was exhausted after the event.

    Similar to the use of pesticides of which nature creates and uses allot, for which it also does not need to get approved for use at the APVMA.

    Why cultivate at L/ha of fuel if the equivalent of say 30 grams of crude oil converted into 15 g of metsulfuron will kill >99% of the weed sorrel per hectare where as cultivation just kills the tops, but spreads the weed via root fragments about the area to increase the density of that weed. Your wheat crop is OK, and most other broadleaved weeds are also removed. Just take care on high pH soils.

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  8. Allan Gardiner

    Dr

    Inland taipans' venom may need to be extremely toxic so that one of them can kill another if they have to compete for scare food. Does every inland taipan's venom have exactly the same toxicity and will any other snake's venom kill an inland taipan?

    Has an inland taipan ever been injected with various venoms and then monitored to see what effect if any this has on its own venom, or what the taipandoes internally to counteract any other venom's possibly killing it.?

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    1. William Fren

      logged in via email @gmail.com

      In reply to Allan Gardiner

      Taipan venom would need to be very toxic in order to be able to kill another taipan. It is possible that they compete. Venomous snakes generally have a degree of immunity to their own venom as shown by the fact that they give love bites. Interestingly, many snake handlers dismiss the concept of love bites as a myth. Even the truly great Eric Worrell did not believe in love bites. I became a believer when I received two love bites from a small-eyed snake many years ago. Some of the snake handlers who did shows in the 1950's and earlier may have actually died from love bites.

      A very large amount of tiger snake venom would surely kill a taipan, but I doubt if that has ever been tested. In NSW snakes have been protected for a long time now, and venom experiments with live captive snakes would not get approval. The question of a taipan's response to other snake venoms will remain academic.

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  9. Caroline Copley

    student

    A great lot of comments here and an interesting article.
    The stuff said about plants reminds me that when I did plant phys years ago there was a whole regime of northern hemisphere studies on allelochemicals and almost nothing on chemical competition between Australian flora.
    Supposedly all our plants and animals were benign and uncompetitive compared to those in the north, so allelochemicals etc here weren't worth the study effort. I might add I think I saw something vaguely in the literature…

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    1. John Holmes

      Agronomist - semi retired consultant

      In reply to Caroline Copley

      A recent herbicide was discovered through an observation that some plants did not do too well growing under a Western Australian plant in Calf.

      Australian plants do very well in the toxin department. I suspect that bio-prospectors have not been thorough the place carefully enough yet.

      Time we spent monies on local R&D - say a small levy on pesticides and pharmaceuticals for R&D here in this area, as well as for what happens with the products after we use them.

      Only need one or two novel actives per decade to make it profitable. Ensure that the intellectual property is nailed down first so the nation gets the benefit.

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