The Kiss of Death: How Pitvipers Use Their Venom
Hayes, William K.
Department of Earth and Biological Sciences Loma Linda University
Loma Linda, California, USA
Venomous animals usually rely on their venom to secure food and/or defend themselves, though other functions of venom exist. As a limited commodity and a costly investment, venom should be deployed judiciously. Insufficient venom expenditure will fail to achieve the desired outcome, and excessive use can result in unnecessary metabolic costs of venom regeneration and ecological costs of venom depletion. Thus, natural selection should favor strategies whereby animals modulate, or meter, the quantities of venom expended. This talk will review the assumptions, mechanisms, evidence, and clinical and evolutionary importance of venom-metering by pitvipers and other snakes. Ample studies clearly indicate that the amount of venom a snake injects into a target varies with snake size, bite context (predatory or defensive), target identity (species and size), and a host of other factors. Causes for variable quantities of venom expenditure vary, but a number of studies support the interpretation that snakes possess the cognitive (decision-making) capacity to meter venom expenditure during both predatory and defensive contexts. Although objections have been raised regarding the capacity of snakes to modulate venom expenditure, neurologically much simpler organisms (cnidarians, mollusks, and arthropods) similarly deploy variable quantities of venom in ways that appear to be adaptive. When considered in a larger context, selection should act on multiple features of the venom apparatus, including venom production and storage (venom yield), venom composition, delivery structures, and venom deployment. Although venom composition is often viewed as the critical factor for efficacious venom use, especially in light of shifting prey bases and coevolutionary counter-adaptations of prey, the behavioral capacity of snakes to meter their venom should give them opportunities to “experiment” with venom composition.