Behavioral anti-predator strategies in newly-metamorphosed American toads (Bufo americanus) in response to predation risk by Eastern garter snakes (Thamnophis sirtalis).

Abstract

Behavioral mechanisms of predator avoidance have not been studied in detail in most predator-prey systems. This dissertation experimentally explores several aspects of behavioral anti-predator strategies within one relatively common and representative terrestrial vertebrate predator-prey system: Eastern garter snakes (Thamnophis sirtalis) and newly-metamorphosed American toads (Bufo americanus). Catchability of toads by snakes was studied as a function of substrate choice, behavioral postures and motions of toads in predation sequence trials, and color-matching to the substrate. Results showed that all are important in avoiding predation by snakes. Specifically, toads tend to choose physically complex substrates in the absence of predators, and snakes hunting on such substrates had lower success rates than those hunting on non-physically complex substrates. Toads tended to crouch and remain motionless in the presence of snakes, and those that did were much less likely to be captured than toads that moved, remained upright, or both. Toads that were color-matched to some substrates were also less likely to be captured than non-matched animals. Previous research has described the clumping behavior of newly-metamorphosed toads and frogs of several species around breeding ponds; other authors have hypothesized that this is another anti-predator strategy. Experiments reported here show that this behavior does not occur as a function of the risk of predation, but is related to the risk of desiccation. Specifically, toads kept together will clump with conspecifics on dry substrates, and lose less weight to desiccation than those kept in isolation. Toads on moist substrates, with or without a predator present, show no tendency to clump in space. The types of behavioral anti-predator strategies discussed above (substrate choice, movements, postures, and color-matching) are expected to evolve if the main predator(s) is (are) at least partially visual. Results of experiments using a Y-maze decision box showed that garter snakes will use both visual and olfactory cues when the cues are presented in isolation, but prioritize the visual over the olfactory cues when both are presented simultaneously in different arms of the decision box.