broad geographical area . The diet of a snake may also change ontogenetically , and age-related shifts in venom composition are also common in such species .
In parallel , due to the evolutionary pressure exerted on animals which are heavily predated upon by venomous snakes , or indeed predate upon venomous snakes , many such species evolve resistance . For example , populations of ground squirrels which are heavily predated upon by rattlesnakes are able to tolerate significantly higher doses of venom than are populations of their conspecifics which inhabit areas absent of these viperine predators . There are multiple ways in which resistance can develop ; for example through the evolution of a molecule which is able to inhibit a toxin ’ s activity , or through mutations which render a target molecule unrecognisable to a toxin .
These selective pressures exerted reciprocally by predator and prey drive an ‘ evolutionary arms race ’: the evolution of a trait in one driving the evolution of a trait to counter it in the other . However , as something of a trump card , many snakes possess toxins in their arsenal which have evolved to target conserved molecules in their prey . A conserved molecule is one that has remained unchanged over long periods of evolutionary time – and for good reason . Typically , such molecules play an essential role in homeostasis . Therefore , if the coding gene mutates in a manner that would render the molecule unrecognisable by the toxin , it most likely also changes the molecule ’ s ability to carry out its vital function . This is a deleterious mutation , harmful to the organism , and therefore that mutation - and the resistance it may have conferred - will not persist in the population .
So , what is venom ? Ok . So , with a few tangents and caveats along the way , we have reached the most parsimonious definition agreed upon by many scientists :
‘ Venom is a toxic secretion that is produced by specialised glands or cells of one animal and delivered ( usually via a specialised delivery mechanism ) to another animal , through the infliction of a wound , which disrupts the normal physiological processes of the receiving animal and benefits the producing animal .’
Right ? Well , sorry to disappoint you so close to the finish line . Cue antimicrobial peptides ( AMPs for short ). AMPs are very small proteins ( peptides ) which are part of the defensive mucosal skin secretions of many amphibians : there is no specialised delivery system , and they are toxic if ingested , so there is little contention surrounding the classification of such frogs as poisonous . But researchers investigating the role of AMPs found that they have a mechanism of action which blurs the line between venom and poison . Once ingested , they essentially act as microscopic corkscrews , drilling tiny holes into cell walls to facilitate the entry of the other toxins into cells and tissues . Perhaps it could even be argued that the AMPs are the specialised delivery system .
So , what is venom ? Venom is a human construct ; our attempt to label a complex array of similar evolutionary strategies against the will of Mother Nature , who , in her indifference , reveals herself to be a rebellious teenager , defying our attempts to neatly define her ways .
Above : the Gaboon Viper ( Bitis gabonica ) has the longest fangs of any venomous snake – up to 5cm in length . It also has the highest venom yield , with enormous venom glands capable of delivering 5-7ml in a single bite . The venom itself , however , does not rate as exceedingly toxic , and humans are very seldom bitten , as these snakes are exceptionally docile and confined to rainforest habitat . Image by Eric Isselee .
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