Size-dependent predation by snakes: selective foraging or differential prey vulnerability?

I staged replicate encounters between unrestrained lizards and snakes in outdoor enclosures to examine size-dependent predation within the common garden skink (Lampropholis guichenoti). Yellow-faced whip snakes (Demansia psammophis) forage widely for active prey and most often consumed large skinks, whereas death adders (Acanthophis antarcticus) ambush active prey and most often consumed small skinks. Small-eyed snakes (Rhinoplocephalus nigrescens) forage widely for inactive prey and consumed both small and large skinks equally often. Differential predation may reflect active choice by the predator, differential prey vulnerability, or both. To test for active choice, I presented foraging snakes with an inert small lizard versus an inert large lizard. They did not actively select lizards of a particular body size. To test for differential prey vulnerability I quantified variation between small and large lizards in behavior that is important for determining the Outcome of predator-prey interactions. Snakes did not differentiate between integumentary chemicals from small and large lizards. Large lizards tend to flee from approaching predators, thereby eliciting attack by the visually oriented whip snakes. Small lizards were more mobile than large lizards and therefore more likely to pass by sedentary death adders. Additionally, small skinks were more effectively hired by this sit-and-wait species and less likely to avoid its first capture attempt. In contrast, overnight retreat site selection (not body size) deters mined a lizard's chances of being detected by small-eyed snakes. Patterns of size-dependent predation by elapid snakes may arise not because of active choice but as a function of species-specific predator tactics and prey behavior.