The foraging response of gerbils to a gradient of owl numbers

Background: While many studies have addressed a prey's behavioural responses to predators, very few have tested how the prey's anti-predator behaviour changes as a function of predator number. Hypotheses: Encounter rate with predators should increase with increasing numbers of predators, thus increasing the predation risk (a cost of foraging) for prey individuals. With increased predation risk, prey animals should quit foraging sooner, and leave more resources behind. Increased predation risk should also cause prey to devote more attention to predator detection and less to foraging. This redirection of attention should result in lower harvest rates, and a higher quitting harvest rate for the prey. Organisms: Prey: Allenbyi's gerbil, Gerbillus andersoni allenbyi, a psammophilic, 25-g desert rodent. Predator: barn owl, Tyto alba. Methods: We allowed gerbils to forage in a large outdoor aviary in Sede Boker, Israel, subject to various risks of predation (i.e. in the presence of 0, 1, 2, or 3 barn owls). We measured gerbil giving-up densities (GUDs), the amount of food left behind by gerbils foraging in artificial resource patches. In each trial, resource patches were set up in different microhabitats with different arrangements of seeds. Comparing GUDs between these resource patches provided a gauge of the gerbils' perceived risk of predation and apprehension (a forager's redirection of attention from foraging to predator detection). Results: Gerbils had higher GUDs when owls were present. Furthermore, gerbils increased their apprehensiveness when more owls were present in the aviary. The increase in gerbil GUD with each additional owl was less than additive.