PATCH USE BY GERBILS IN A RISKY ENVIRONMENT - MANIPULATING FOOD AND SAFETY TO TEST 4 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 ha ve been modeled in at least four different ways. Here, I test among th e four models for two species of gerbils, Gerbillus allenbyi and G. py ramidum using giving-up densities of resources (GUDs) left behind in a rtificial food patches. To do so, I augmented food using petri dishes full of additional seeds and exposed gerbils to the presence of barn o wls (Sro alba) in a large outdoor aviary. Gerbils harvested more food, foraged less time, and left resource patches at higher giving-up dens ities in the presence of extra food. They also had higher GUDs when ow ls were present. Also, gerbils exposed to augmentation of food respond ed 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 th e state of the animal through the marginal value of energy and margina l rate of substitution of energy for predation.