THE INFLUENCE OF SNAKES ON THE FORAGING BEHAVIOR OF GERBILS

Predation may have profound effects on the behavior of prey individual s with consequences for population dynamics and community structure. H ere, we report on an experiment in which we examine the response of tw o gerbil species (Gerbillus allenbyi and G. pyramidum) to the presence of a snake predator, the desert diadema snake (Spalerosophus diadema) , and to added illumination (a factor associated with increased risk o f predation from owls). We measured rodent foraging behavior by the nu mber of seed trays foraged in each microhabitat and amount of seed res ources left in patches following exploitation by gerbils (giving-up de nsities; GUDs). Fewer seed trays foraged and higher GUDs are indicativ e of higher perceived predatory risk. Accordingly, both species forage d fewer seed trays in response to illumination, and G. pyramidum forag ed fewer trays in the bush microhabitat. In response to the presence o f snakes, G. pyramidum foraged fewer resource patches in the absence o f added illumination. Overall, this species preferred the open microha bitat and may have intensified its use of the open in the presence of snakes. In contrast, G. allenbyi intensified its use of the bush micro habitat in the presence of added illumination. Both gerbil species lef t resource patches at a higher GUD in response to illumination and to snakes. While the GUD of G. allenbyi did not differ between microhabit ats, G. pyramidum had a higher GUD in the bush than open microhabitat. Thus, gerbils altered their use of resource patches in response to th e risk of predation.Predator facilitation is a higher order interactio n in which the presence of one predator species makes it easier for an other predator to capture prey. Predator facilitation may result when different predators place conflicting demands on the appropriate behav ioral response of their prey. This requires that prey can distinguish among predator species and respond accordingly. We have previously sho wn experimentally that predation by owls causes G. allenbyi and G. pyr amidum to alter their foraging activity; they shift their foraging eff ort away from risky patches in the open microhabitat and towards safer patches in the bush. Our current work shows that the gerbils also res pond to snakes, and their response to snakes is qualitatively differen t than their response to owls. In accord with predator facilitation, o ur results support the hypothesis that owls exert a greater risk in th e open microhabitat, and snakes may pose a greater threat in the bush microhabitat.