Responses to escalating predatory threat in garter and ribbon snakes (Thamnophis).

Several models suggest that animals perform a stable sequence of responses during increasing threat from a potential predator. To test the comparative utility of such models, 45 garter and ribbon snakes from four species (genus Thamnophis) were given an antipredator behavior test in which the snakes were exposed to a series of five levels of escalating threat: (a) threat is out of view, (b) threat looms over arena, (c) threat is nonmoving human hand near snake's head, (d) threat is moving human hand, and (e) threat is experimenter's tapping the snake's body. Five different antipredator behaviors were measured. It was found that both qualitative and quantitative changes in behavior occurred between threat levels. However, species differences were also found in the occurrence and amount of every behavior recorded, which indicates that even closely related species may differ in behaviors exhibited in different phases of a predatory encounter.