Why are some snakes so venomous?


I don’t like snakes. Sri Lanka, being a tropical country has a fair share of snakes that are both poisonous and non-poisonous and so I have encountered them, though not too many because I always lived in urban areas and snake sightings are rare, though not unheard of. But despite having encountered quite a few of them in the wild in my lifetime, I have never overcome my desire to get the hell as far away from them as possible.

Apparently there are good evolutionary reasons as to why humans and other animals fear snakes and why we have developed a heightened ability to see them over other things, almost before we are consciously aware that we are seeing them. It is because our brains and visual systems have evolved to be able to quickly detect them by developing specialized neurons for the purpose.

As this recent paper says:

The present study shows preferential activity of neurons in the medial and dorsolateral pulvinar to images of snakes. Pulvinar neurons responded faster and stronger to snake stimuli than to monkey faces, monkey hands, and geometric shapes, and were sensitive to unmodified and low-pass filtered images but not to high-pass filtered images. These results identify a neurobiological substrate for rapid detection of threatening visual stimuli in primates. Our findings are unique in providing neuroscientific evidence in support of the Snake Detection Theory, which posits that the threat of snakes strongly influenced the evolution of the primate brain. This finding may have great impact on our understanding of the evolution of primates.

Another article looks at the question of why some snakes are so venomous, much more so than would seem to be required. Some of the most dangerous specimens can be found in Australia which can boast of hosting the world record holder for the most venomous snake. This is the inland taipan that produces enough venom in a single bite to kill 250,000 lab mice.

Clearly this is overkill. So why did it evolve the reptile equivalent of a nuclear weapon while other snakes are not venomous at all ?

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.

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.

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.

The fact that this snake can kill 250,000 lab mice with a single dose of venom may also be due to the sheltered life that lab mice lead, which does not require them to have evolved to acquire venom resistance.

Comments

  1. Pierce R. Butler says

    Desmond Morris (possibly in his Animal Days book) has a great story about the early days of British TV, when almost everything was broadcast live and he and other staff at the London Zoo would bring critters into the BBC studios to show them off.

    One fine day, a presenter left the stage carrying a monkey (which – the point of the anecdote – had never seen a snake in its cage-bound life), while another entered wrapped in a python. The monkey immediately panicked and escaped, taking to the scaffolding, ropes, light racks, and other convenient gear in the upper stage zone. It took several days for it to become hungry enough to get lured into a trap, and in the meantime – live tv! – everything the BBC broadcast from that studio – musical variety shows, sitcoms, soap operas, Shakespeare – was enhanced by monkey chatter, flung feces and other oddments.

    Morris didn’t say what that did to the ratings, but I would’ve watched nonstop.

  2. AnotherAnonymouse says

    I live in a small suburban development on land that was a farm until the 1980s. There’s a small pond at the edge of the development and sometimes there are (harmless) rat snakes around. I’m not a huge fan of snakes–I’ll look at them at zoos, but otherwise have no interest in interacting with them. One day while taking the dog for a walk, I suddenly found myself leaping backwards–there was a rat snake lying harmlessly at the side of the path. My jumping back was instinctive: my body did it before I was consciously aware that I had seen a snake.

  3. Francisco Bacopa says

    I was raised in a snake watching family and am quite used to being near venomous snakes. I finally had an experience with a rare for this area canebrake rattlesnake. In spite of good training and an ability to spot snakes at a distance, I took this snake by complete surprise. Rattlesnakes do not like being surprised. That rattle is scary in real life. The encounter put me off snakewatching for about a year.

  4. Mano Singham says

    @Francisco,

    You mean snake watchers are like bird watchers who try see how many kinds of snakes they can see? I had never heard of this hobby. So people actively seek the habitats of specific snakes?

  5. reinderdijkhuis says

    On the other hand, some of us lack that allegedly instinctive fear of snakes entirely. I’m one of those people; when I came face to face with a likely-to-be-venomous snake (black moccasin? I’m not quite sure as I’m not an expert), I knew rationally that it wasn’t a good idea to poke or threaten it, but I did not fear it at all. Fascination, yes. I grew up in a part of the world that is almost free of snakes, so I thought it was awesome to see one in the wild.

    What did worry me later on was realizing that I might step on another one of those creatures by accident and that I would have a problem if that happened. That spooked me and made me very cautious… Actually seeing it, not so much.

  6. kraut says

    I always like snakes. But then – in Germany we do not really have any venomous ones. My reaction to snakes always on of interest.
    Spiders scared the crap out of me. As an antidote I learned to handle tarantulas and still have one pet left. She must be close to seven years old now.

  7. says

    We took care of a friend’s kitten for a few days once, and I bought it one of those toy spiders that jumps when you squeeze a rubber bulb. I thought it would be a great cat toy — lots of wiggly things. But the kitten was terrified of it. Which was strange — he was such a holy terror that his name was “Abu Nidal”, a famous terrorist at the time. But he obviously had an instinctive fear of large spiders.

  8. Francisco Bacopa says

    Yes, there are people who document how many different snakes they can see. Snakes are not as diverse as birds, and they don’t migrate, so there’s no seasonal variation, but it’s still fun to document a rare species. Best time to look for snakes is in Spring and Fall. patches of morning sun attract snakes.

    I was lucky to find a canebrake rattler in a fairly urban area. And lucky that canebrakes almost never bite. They are one of the deadliest snakes in North America. They are a subspecies of the Timber rattlesnake that has a neurotoxic venom in addition to the usual mix for the larger rattlesnakes.

  9. baryogenesis says

    Nearly every summer I do handiwork on island property on Lake Huron. There are increasing numbers of small Massasauga rattlers. Lately I have had to leap over or jump sideways when one is on a path. They are well camouflaged against a background of pine needles, dried leaves and sand. The cottage owner is also trained to remove rattler from others’ property. I went with her on such a call once, armed with a paint pole with a bent wire or crook on the end and a plastic garbage bin. We never found the intruder, but she explained that you scoop up the snake midsection. It can’t muscle its way off of such a narrow surface. It’s dumped into the steep-sided bin with a lid, put in the boat and taken to another island.

    She told me she trained with a lazy old snake who was used to being handled, on a carpet in an office in downtown Toronto!

  10. says

    I had a copperhead living in my basement under the freezer chest (I guess the motor is warm) we had an arrangement that I left him alone and he kept the lower floor of the house free of mice. It worked great until he left and now I have a large black rat snake that lives down there and does the same thing, without being so poisonous. The black rat likes to curl up on my router (a big steel-cased Cisco unit) because it’s always warm.

    Mice are much more irritating and probably more likely to carry something I’d get sick from.

  11. Matt G says

    There is a great quote in one of Darwin’s books (Descent of Man?) in which he says that it might be as difficult for (some) humans to shed their belief in God as for a monkey to shed its fear of snakes.

  12. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    PPS. Mind you, I’e been bush walking for many years – and with dogs – and only ever seen a couple of snakes in the wild. One swimming in the creek and a couple on the path.

    Make enough noise (vibration) and they’ll scarper pretty quick – they don’t want to waste their venom on us and aren’t really a big problem.

    One of my brothers keeps a few pet snakes although of the non-venomous variety – they are cool critters.

  13. Lassi Hippeläinen says

    “Clearly this is overkill. So why did it evolve the reptile equivalent of a nuclear weapon while other snakes are not venomous at all ?”

    Maybe it has big enemies that it has to scare off. Venom works also as self-defense.

  14. says

    The lethality of snakes is comparable to the speed of cheetahs versus the manoeuvrability of gazelle. It’s an ever increasing “arms race” between predators and prey, between defense and dinner. And what about the mongoose, how it can survive the venom?

    You mention Australia and the lethality of venom, which is not limited to snakes. The Australian funnel web spider is lethal, but only to some species, especially humans and primates. Dogs and cats can survive doses a hundred times what would kill a person. No doubt it’s an evolutionary trait, because of which (related) species are native to Australia.

    http://www.abc.net.au/science/slab/shorter/story.htm

    Here in Taiwan, there are a lot of poisonous snakes. People I know are surprised by the variety and number of snakes, and how venomous they are, but they are pretty much the same species as found in mainland China. I’ve seen close to a dozen (from half metre cobras up to two metre long who-knows-what snakes) on my bike rides through parks and roads near rivers and the ocean.

    http://www.snakesoftaiwan.com/Start_Page_EN.htm

    http://eip.ymsnp.gov.tw/html/eng/01information/inf_d02_list.asp

    Growing up in British Columbia, I was used to seeing garter snakes which can survive freezing temperatures in winter. Several garter snake species in BC have live young, no doubt a better survival than eggs in cold climates.

    http://www.bcreptiles.ca/reptiles_north.htm

    In Manitoba, many live in burrows of hundreds of snakes, sharing body heat to keep warm through the winter. Garters are non-venomous and eat frogs, bugs and other small animals.

  15. lorn says

    In many ways snakes are very delicate. Given half a chance even a small rodent can inflict fatal damage. Mice and rats can chew through wood. The overkill may be a need to rapidly incapacitate the prey. A lesser poison may inflict pain, possibly kill if given time but the snake can’t afford to take that chance.

  16. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    Some of the most dangerous specimens can be found in Australia which can boast of hosting the world record holder for the most venomous snake.

    D’oh! I missed this before I wrote my first comment on this thread the other day. That’s what iget for skim reading and not taking it all in carefully enough I guess. Sorry.

  17. Wylann says

    I don’t fear snakes, but I have a healthy respect for them.

    I grew up mostly in the desert Southwest were rattlesnakes (most often Western Diamondbacks) and many non-venomous species are fairly common. I’ve been known to stop my car or bicycle and shoo them off the road to keep them from getting run over.

    I’ve been lucky enough to see a Gila Monster in the wild once as well. That made my year. :)

    The most venomous snake I’ve encountered in the wild, though, is probably the sea-snake that lives around the island of Okinawa. I did a lot of scuba diving there, and we saw one or more about every third dive. They are extremely toxic (I recall reading some it’s because their prey is cold blooded, and they don’t swim very fast, so it has to paralyze/kill quickly), but they don’t have what are considered traditional fangs, and are not likely to bite even in defense unless they are really irritated.

  18. Timothy says

    Perhaps the fear changes as we age, too? When I was a teen, I was fascinated by snakes and even thought of becoming a herpetologist. Now, in my late 40’s, I cannot stand to be near snakes.

    A final aside — one of the things I love (love!) about living in Northeast Ohio is that, in addition to the absence of hurricanes, major earthquakes, landslides, droughts, and forest fires that other parts of the US enjoys — we have no indigenous poisonous snakes. So I can tramp about our woods with abandon.

    I always have to resent my internal snake-o-meter whenever I tramp the woods of other states.

  19. Mano Singham says

    @Timothy,

    I don’t know about any age effect. But one of the things that I like most about New Zealand, a country that I have visited many times, is that there are absolutely no animals of any kind that pose any danger to humans. So you would find it appealing too!

  20. NitricAcid says

    When I was a youth, I spent some time wandering Dinosaur Provincial Park in Alberta by myself. The first day there, I unexpectedly encountered a) a rattlesnake, and b) a statue of a stegosaurus. The latter startled the hell out of me.

  21. Mary Jo says

    My dad and brothers belonged to the Northern Ohio Association of Herpetologists in the 1970s which was started by Marty Rosenberg, a biology professor at Case. The group would go on camp-outs to southern Ohio with the sole purpose of looking for snakes. The group still exists.

  22. Mano Singham says

    @Mary Jo,

    I knew Marty well before he retired but would not go near his snake collection.

  23. Mary Jo says

    I dreamed last night that a snake bit my finger and I had to get a friend to cut my arm off!

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