Why Venom Evolved in Mammals

April 18, 2010 at 1:48 pm (Evolution, Shrews & the Like) (, , , , , , , )

This is just something I ended up pondering earlier today.

In the early 1900’s, it was generally accepted that mammals were not venomous, and any that were were the exception to the rule. The explanation was that, since mammals were such high performance animals (compared to other vertebrates), they tended to win out over those around them with their overactive metabolisms and high-functioning brains. This made sense; why would you need venom, which kills more slowly than a bite to the back of the neck, when you have the ability and intelligence to take prey or defend yourself with the lethal efficiency of a mammal? Plus, there were no venomous birds, which are the reptiles’ attempt to make a mammal. We still have not discovered any venomous birds, and we assume that this is the reasoning.

A large proportion of snakes are venomous (I’m looking at you, colubridae!), and snakes do not need to eat nearly as often or as much as a mammal of the same mass. They can afford to wait for their venom to kill their prey, and venom will work on an animal whether injected into the leg or into the back of the neck, so snakes do not need to target a specific area on the prey. Venomous snakes can rarely outrun their prey, and tend to be ambush predators – slower, more deliberate, and seemingly more lethargic with their slower metabolisms.

The three lizards (yes it is now three: gila monsters, beaded lizards, and it has been confirmed that the komodo dragon has self-produced antigenic proteins) that are venomous all have slow metabolisms, and are not nearly as sprightly as some of their faster cousins. The komodo waits around for days for prey to die; I think the other two use venom defensivey and/or socially.

Only the male platypus is venomous, and only during mating seasons. Platypus venom causes an inordinate amount of pain to humans, but is rarely lethal. If this is the case with its own species, then perhaps the venom evolved along with the mating systems to be a tool for social dominance.

But of course we are here to talk about shrews. Shrews are the exact opposite of reptiles: they need to eat a lot, and often, just to stay alive. Their hearts beat hundreds of times per minute (~700 on a good, adrenaline pumping, fear induced day) and they are constantly on the prowl for something to eat. Its a good thing they go into a torpor in the winter or they would never survive. The point is that shrews do not have the time at all for venom to take effect before they need to eat. Any amount of waiting would make them burn out and die.

And here’s the kicker – shrew venom is convergent with the gila monster and beaded lizard. It is based on the same precursor protein and has similar properties. What would a high performance animal like the shrew and a low performance animal like the gila monster share in needs that molecularly similar venoms would evolve? What advantage is there for a shrew to have venom?

The answer I came up with is two-fold. First, some shrews satiate their endless hunger by preying on animals as large or larger than themselves, like frogs, voles, mice, and even rabbits – not an easy task for something as small and blind as Blarina brevicauda, a task made much easier with venom. For those shrews that are not blarina, though, like neomys and sorex that prey on invertebrates, what is the use of this venom? The answer lies in the caching behavior observed in many soricomorphs.

You have just eaten a juicy grasshopper, and that will do for the next hour. You come across a frog, and you bite that, then run off and find another grasshopper, but you don’t have to eat again quite yet. You bite the grasshopper and it becomes paralyzed (the venom acts differetly on inverts) and you stash it underground. Then, you eat a cricket or a centipede, and go back for the frog and eat some of that until you are scared away by a fox. You are ok for a while because the frog was a good meal, but for two hours you can’t catch any more, so you go back and eat the stashed grashopper.  In other words, with impeccable timing, a shrew can ensure that it has food whenever it needs it. Then, of course, lightning strikes a tree nearby and the shrew has a heart attack, but that’s a different story.

Anyway, that’s what I was pondering this morning.

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