Stimulus control of defensive behaviors of garter snakes (Thamnophis sirtalis): Effects of eye spots and movement.

Abstract

The purpose of these experiments was to examine the effects of eye spots and stimulus movement as mediators of antipredator responses in garter snakes. In Experiment 1, 13 Eastern garter snakes (Thamnophis sirtalis) at 6-8 weeks of age were confronted with a series of threatening models that varied in the configuration of the eyes (artificial glass eyes, circular black spots, elongated black bars, or no eyespots). There were significant differences in the number of strikes elicited by the models, and the model with realistic glass eyes elicited the most strikes. In Experiment 2, we examined the effects of erratic and consistent stimulus movement on defensive behavior in 12 garter snakes at 12-16 weeks of age. The snakes delivered significantly more strikes to an erratically moving model than to a model oscillated at a regular rate. Animals of most species are prey for other animals, and it is critical that they can identify potential predators at a distance. Although a wide variety of cues are used in predator recognition (e.g., Edmunds, 1974; Endler, 1986), the involvement of visual cues has received most attention. (See Curio, 1993, for a comprehensive review.) Eyes and eyelike markings play a particularly important role in interactions between predator and prey. For example, eyelike stimuli have been shown to prolong tonic immobility in a number of species under laboratory conditions (e.g., Gagliardi, Gallup, & Boren, 1976; Gallup, 1971; Hennig, 1977) and produce increased flight distances in animals tested in nature (Burger, Gochfeld & Murray, 1991, 1992). Despite the apparent importance of the eyes of predators as a trigger of defensive responses, there has been little research on the role of eye markings as releasers of defensive behavior in snakes. Burghardt and Greene (1988) found that direct gaze by a human experimenter potentiated deathfeigning response in neonate hog-nosed snakes. Herzog and Bern (1992) recently reported that juvenile garter snakes direct a disproportionate number of strikes toward the eyes of predator models. Herzog, Bowers, and Burghardt (1989) found that the size and movement of a threat stimulus affect antipredator responses of neonate and juvenile garter snakes and that realistic models elicited higher levels of defensive strikes than nonrealistic models. In this study we examine the role of eye markings in mediating antipredator responses. In Experiment 1, we tested the hypothesis that eyespots would enhance a model's ability to