How do Allenby’s gerbils titrate risk and reward in response to different predators?

Abstract

In heterogenous environments, predation risk from multiple predators and the availability of resources fluctuate both spatially and temporally. The various predators may include both aerial and terrestrial species that can facilitate each other and present qualitatively different risks to prey. Animals therefore forage across a complex landscape of fear, with areas of risks and relative safety where resources are generally asymmetrically distributed. Therefore, a trade-off exists between remaining safe and locating food. Animals make foraging decisions regarding where, when and for how long to forage by titrating marginal costs and benefits of foraging within and the marginal value of foraging across depletable resource patches. We conducted a series of titration experiments to determine how Allenby’s gerbils (Gerbillus andersoni allenbyi) titrated food and safety when presented with predation risk from owls, vipers and the joint risk from both predators. We manipulated bush and open microhabitats by increasing food availability in the riskier patches. In response to the different levels of enrichment, gerbils titrated food and safety. Riskier open microhabitats needed to be four times as rich in food as bush patches to be of equal value when subjected to predation from owls and the joint risk from owls and vipers. In response to vipers alone, riskier bush patches needed to be 2–4 times as rich in food as safer open patches for the marginal value of foraging to equalize across microhabitats. Overall, predation risk from owls and the joint risk from owls and vipers resulted in the greatest foraging costs for gerbils in risky microhabitats. Thus, the combined overall risk from multiple predator species was equivalent to the risk presented by the gerbils’ most dangerous predator (owls alone). Animals trade-off remaining safe with locating food by titrating the marginal costs and benefits of foraging. We assessed how foragers titrate these costs and benefits through behavioural titration experiments. In our study, gerbils titrated food and safety in response to owls and vipers, aerial and terrestrial predators known to facilitate one another. Riskier patches needed to be 2–4 times as rich in food as safer patches to be of equal value to foraging gerbils. From the gerbils’ perspective, owls are more formidable predators than vipers; despite the facilitation that occurs between these predators, the combination of the two predators had little effect on the titration beyond that of the owls alone. Behavioural titrations are important for quantifying differences in foraging costs between microhabitats and in response to multiple predators with different hunting strategies. As single-predator systems are relatively rare, titration experiments provide the opportunity to gain insights into the foraging/safety trade-offs made by animals in response to multiple predators on the landscape.