Effects of sex, body size, temperature, and location on the antipredator tactics of free-ranging gartersnakes (Thamnophis sirtalis, Colubridae)

Gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba are subj ect to intense predation (primarily by crows) during their spring breeding season. The huge numbers of snakes provide a unique opportunity to quantify behavioral traits. We simulated predator attacks by "pecking" more than 50 0 free-ranging snakes, to explore the determinants of snake response. Snake s responded to a human finger in the same way as they did to a more realist ic stimulus (a model crow). A snake's response to attack depended on severa l factors, which interacted in complex ways. The primary influences on resp onse were body temperature (warmer snakes tended to flee, whereas colder sn akes remained cryptic or flattened and/or gaped and struck) and sex (males were more likely to flee). Responses also depended on microhabitat (i.e., i nside the winter den versus in adjacent grassland) and on the snake's prior activity (e.g., courting snakes often ignored our close approach). These f actors interacted in significant ways; for example, snakes outside the den were smaller and warmer than those inside, male snakes were smaller and war mer than females, and mean body temperatures were higher in larger snakes w ithin each sex. Thus, a snake's body size and its location affected its def ensive response indirectly (via their influence on body temperature). Our r esults differ from those of previous studies and suggest that antipredator responses in these animals depend in a flexible and complex way upon biotic and abiotic variables. Interactions among these variables also must be con sidered before we can identify underlying causal processes.