Conflict and assessment in a predator-prey system: ground squirrels versusrattlesnakes

Adult California ground squirrels, Spermophilus beecheyi beecheyi, actively confront and harass northern Pacific rattlesnakes, Crotalus viridis oregan us, which are the principal predator of ground squirrel pups. In this repor t we examine the roles of risk (snake size) and context (location of encoun ter and squirrel reproductive category) in rattlesnake assessment by ground squirrels. In interpreting the results, we. borrow heavily from the well-d eveloped conceptual framework applied in the analogous case of intraspecifi c conflict. Large and small snakes were tethered near the home burrows of m ale ground squirrels, and maternal and nonmaternal female ground squirrels. Ground squirrels appeared to employ assessment strategies which served to mediate the level of risk associated with confronting larger snakes. The re sults suggest that ground squirrels exercise greater caution when dealing w ith large snakes and invest more in monitoring the snake from a safe distan ce. Maternal squirrels, which have more at stake reproductively, spent more time and effort in snake-directed activities than did squirrels from other reproductive categories. Mothers also differentiated more strongly between large and small snakes, perhaps reflecting the greater vulnerability of th eir pups to larger snakes. Finally, ground squirrels discriminated between snakes found close to their home burrow and those encountered further abroa d. At the home burrow, squirrels monitored the snake from a closer distance , displayed a greater willingness to confront the snake, and escalated to m ore dangerous levels of harassment. This assessment strategy may reflect a higher payoff to squirrels that persist in driving snakes out of the home a rea, thereby reducing the risk of future ambush. (C) 1999 The Association f or the Study of Animal Behaviour.