The beak of the snake: fang length evolution in vipers is predicted by diet

Fangs, stingers, spines, and harpoons are used by diverse animal taxa to inject venom into their prey. Strong selection on venom composition has been repeatedly documented, and we might expect the venom injection apparatus to be under similarly strong selection to meet specific functional demands. Snakes in the family Viperidae (true vipers and pitvipers) consist of ~320 species widely studied by both ecologists and evolutionary biologists. Vipers provide an opportunity to determine how venom injection systems evolve in response to functional demands of prey killing. Utilizing museum collections, we obtained measurements of fang length in >2000 individual specimens representing 200 viper species. We then leverage data collected from over 100 published diet studies to test the hypothesis that longer fangs evolved in response to demands associated with feeding on mammalian prey. We find support for this hypothesis, where the percentage of mammals in viper diets is positively correlated with relative fang length. Finally, when controlling for head size, the Gaboon Viper is dethroned as the snake species with the longest fangs, and overtaken instead by the Speckled Forest Pitviper of South America. Venom and the venom delivery system merit further work to determine if they are part of a broader functional and evolutionary module that facilitates feeding in venomous animals.