THE INFLUENCE OF SNAKES ON THE FORAGING BEHAVIOR OF GERBILS

Predation may have profound effects on the behavior of prey individuals with consequences for population dynamics and community structure. Here, we report on an experiment in which we examine the response of two 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 of predation from owls). We measured rodent foraging behavior by the number of seed trays foraged in each microhabitat and amount of seed resources left in patches following exploitation by gerbils (giving-up densities; GUDs). Fewer seed trays foraged and higher GUDs are indicative of higher perceived predatory risk. Accordingly, both species foraged fewer seed trays in response to illumination, and G. pyramidum foraged fewer trays in the bush microhabitat. In response to the presence of snakes, G. pyramidum foraged fewer resource patches in the absence of added illumination. Overall, this species preferred the open microhabitat and may have intensified its use of the open in the presence of snakes. In contrast, G. allenbyi intensified its use of the bush microhabitat in the presence of added illumination. Both gerbil species left resource patches at a higher GUD in response to illumination and to snakes. While the GUD of G. allenbyi did not differ between microhabitats, G. pyramidum had a higher GUD in the bush than open microhabitat. Thus, gerbils altered their use of resource patches in response to the risk of predation. Predator facilitation is a higher order interaction in which the presence of one predator species makes it easier for another predator to capture prey. Predator facilitation may result when different predators place conflicting demands on the appropriate behavioral response of their prey. This requires that prey can distinguish among predator species and respond accordingly. We have previously shown experimentally that predation by owls causes G. allenbyi and G. pyramidum to alter their foraging activity; they shift their foraging effort away from risky patches in the open microhabitat and towards safer patches in the bush. Our current work shows that the gerbils also respond to snakes, and their response to snakes is qualitatively different than their response to owls. In accord with predator facilitation, our results support the hypothesis that owls exert a greater risk in the open microhabitat, and snakes may pose a greater threat in the bush microhabitat.