Conflict and assessment in a predator–prey system: ground squirrels versus rattlesnakes

Abstract

Adult California ground squirrels,Spermophilus beecheyi beecheyi, actively confront and harass northern Pacific rattlesnakes,Crotalus viridis oreganus, which are the principal predator of ground squirrel pups. In this report we examine the roles of risk (snake size) and context (location of encounter and squirrel reproductive category) in rattlesnake assessment by ground squirrels. In interpreting the results, we borrow heavily from the well-developed conceptual framework applied in the analogous case of intraspecific conflict. Large and small snakes were tethered near the home burrows of male 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 results suggest that ground squirrels exercise greater caution when dealing with large snakes and invest more in monitoring the snake from a safe distance. 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 their pups to larger snakes. Finally, ground squirrels discriminated between snakes found close to their home burrow and those encountered further abroad. At the home burrow, squirrels monitored the snake from a closer distance, displayed a greater willingness to confront the snake, and escalated to more dangerous levels of harassment. This assessment strategy may reflect a higher payoff to squirrels that persist in driving snakes out of the home area, thereby reducing the risk of future ambush.