Patch Use by Gerbils in a Risky Environment: Manipulating Food and Safety to Test Four Models

Previous experimental work has demonstrated that patch use in gerbils is sensitive to the amounts of both food and predatory risk. Inputs of predatory risk and feeding rates in determining patch use behavior have been modeled in at least four different ways. Here, I test among the four models for two species of gerbils, Gerbillus allenbyi and G. pyramidum using giving-up densities of resources (GUDs) left behind in artificial food patches. To do so, I augmented food using petri dishes full of additional seeds and exposed gerbils to the presence of barn owls (Tyto alba) in a large outdoor aviary. Gerbils harvested more food, foraged less time, and left resource patches at higher giving-up densities in the presence of extra food. They also had higher GUDs when owls were present. Also, gerbils exposed to augmentation of food responded more strongly to microhabitat on the night of the food augmentation and to the presence of owls on the following night than those lacking the extra food. The results of these experiments best support Brown's patch use model. They also reconfirm that the risk of predation is a foraging cost. Furthermore, this cost is complex and is affected by the state of the animal through the marginal value of energy and marginal rate of substitution of energy for predation.