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 subject 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 500 free-ranging snakes, to explore the determinants of snake response. Snakes responded to a human finger in the same way as they did to a more realistic stimulus (a model crow). A snake's response to attack depended on several factors, which interacted in complex ways. The primary influences on response were body temperature (warmer snakes tended to flee, whereas colder snakes remained cryptic or flattened and/or gaped and struck) and sex (males were more likely to flee). Responses also depended on microhabitat (i.e., inside the winter den versus in adjacent grassland) and on the snake's prior activity (e.g., courting snakes often ignored our close approach). These factors interacted in significant ways; for example, snakes outside the den were smaller and warmer than those inside, male snakes were smaller and warmer than females, and mean body temperatures were higher in larger snakes within each sex. Thus, a snake's body size and its location affected its defensive response indirectly (via their influence on body temperature). Our results 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 considered before we can identify underlying causal processes.