Venom injection by rattlesnakes (Crotalus atrox): peripheral resistance and the pressure-balance hypothesis

Abstract

Differential venom injection by snakes, between two size classes of prey for example, has typically been explained within the rubric of the venom-metering hypothesis, which claims that snakes decide how much venom to inject in a given strike. Recently, an alternative, the pressure-balance hypothesis, was advanced, which attributes differential venom flow to the balance of internal forces acting at the venom gland and venom chambers and external forces acting at the exit orifice of the fang. This study tests these competing hypotheses. High-speed digital videos of predatory and defensive strikes by western diamondback rattlesnakes, Crotalus atrox, revealed considerable variation in the trajectory of the fang relative to the target, which would yield wounds with potentially different levels of peripheral resistance. The importance of peripheral resistance was also suggested by the expulsion of venom from the fang after withdrawal from the target (in 7% of strikes) and by the forceful ejection of fluid from the target around the embedded fang (in 2.8% of strikes). Experimental milking chambers were constructed that exposed the right and left venom-delivery systems to different levels of peripheral resistance; with increased peripheral resistance significantly less venom was injected into the chamber and significantly more venom was released on the chamber's surface.