Flexible defense: Context-dependent antipredator responses of two species of Australian elapid snakes

Abstract

Snakes exhibit a complex repertoire of defensive responses, shifting from one tactic to another depending upon conditions at the time of encounter with a potential predator. Standardized laboratory trials, controlling factors such as time of day, temperature, and type of encounter, can be used to test predictions about the form of this context dependency. We studied two sympatric species of elapid snakes from southeastern Australia. Although similar in body sizes, habitat use, and nocturnality, small-eyed snakes (Cryptophis nigrescens) are wide foragers (and hence, encounter predators under cool conditions at night in relatively open areas), whereas broad-headed snakes (Hoplocephalus bungaroides) lie in ambush for long periods within sun-warmed rocky retreat sites (and hence, encounter predators mostly when the snake is warm, and within its retreat). Based on those differences, we predicted that small-eyed snakes would be thermal generalists; that is, temperature would exert relatively little effect on the snakes' locomotor abilities, alertness, and preparedness to flee from threat. Broad-headed snakes would be likely to utilize tactics other than fleeing, and to increase response intensity substantially at higher temperatures. Both species were predicted to exhibit more intense responses at night, and when warm. Data from our laboratory trials supported these predictions: (1) antipredator behaviors of small-eyed snakes were less sensitive to thermal variation than were those of broad-headed snakes, (2) both species relied primarily on fleeing, but broad-headed snakes were more likely to adopt retaliatory behavior than were small-eyed snakes, and (3) both species responded more intensely at night and when warm. These results are consistent with the broader theory that snakes flexibly adjust their antipredator tactics to local conditions. © 2010 The Herpetologists' League, Inc.