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.