Titrating food and safety in a heterogeneous environment: When are the risky and safe patches of equal value?

A forager should exploit a depletable resource patch until the marginal benefits of patch exploitation fall to equal the marginal costs, and it should allocate effort to different patches in order to equalize marginal value across patches. Foraging decisions are therefore titrations of marginal costs and marginal benefits within patches and of marginal value across patches. Often, a forager must balance conflicting demands for food acquisition and safety. Thus, insights into foraging can be gained through titration experiments involving food and safety. Two types of behavioral titrations are possible: (1) equalizing marginal costs and marginal benefits within patches and marginal value across patches and (2) equalizing time, energy harvested, or some other quantity across patches and habitats. The first type of titration is performed by the animal, and the second type can be performed by the experimenter; the first type of titration makes the second possible. We conducted titration experiments involving food and safety with gerbils subjected to predation by owls in a large aviary. We examined patch use by gerbils in manipulated resource patches (seed trays) placed in both the bush and the open microhabitats. Rodents took both food and safety into consideration when deciding how long to stay in a patch: giving-up densities of resources in seed trays (GUDs) were higher in the open than in the bush microhabitat. Also, enrichment of resource patches in the open microhabitat revealed that resource patches in the open needed to be 4 to 8 times richer than patches in the bush microhabitat in order to be equally valuable. These results are consistent with field data collected elsewhere indicating that foraging costs are comprised mostly of costs arising from the risk of predation.