Apprehension and time allocation in gerbils: The effects of predatory risk and energetic state

Apprehension is a behavioral response of a forager to risk of predation involving the allocation of the forager's attention away from foraging-related tasks and toward predator detection. It, along with time allocation, can be used by a forager to manage risk. Apprehension should increase, and time allocation should decrease with a forager's energetic state (marginal value of energy). We tested these predictions on Allenby's gerbils (Gerbillus andersoni allenbyi) by manipulating the energetic state of the gerbils through augmentations of millet seeds. We also manipulated two risk factors, the microhabitat of the foraging patch (bush or open) and the presence of barn owls (Tyto alba). We measured apprehension of gerbils using the selectivity of gerbils for trays that differed in the distribution of seeds that they contained and time allocation using the amount of seeds left behind in trays, the giving-up density (GUD). Gerbils harvested 4.6 times more seeds when supplemental seeds were available, 17.4% fewer seeds when owls were present, 25.8% fewer seeds from the open than the bush microhabitat, and 19.3% fewer seeds from trays when seed augmentations were also present. With regard to time allocation, gerbils had higher GUDs (allocated less time) when risk was higher: in the open microhabitat and in the presence of owls. Also, the response of gerbils to owls was affected both by the seed augmentation and by microhabitat. With regard to apprehension, the gerbils were more apprehensive when extra seeds were added. Their level of apprehension changed little in the bush microhabitat but increased greatly in the open microhabitat when energetic state was manipulated. Thus, gerbils use both time allocation and apprehension in response to predators. Changes in the energetic state of a forager led to the predicted increase in apprehension accordance with Clark's asset protection principle.