Density-dependent foraging effort of Deer Mice (Peromyscus maniculatus)

1. Little is known about how population density affects the foraging behavi our of individuals. Simple models are developed to predict the net effect o f density on the quitting-harvest rates of optimal foragers. The theory was tested with experiments that measured the foraging behaviour of free-rangi ng Deer Mice under control and reduced densities. 2. An increased density of conspecifics may (a) reduce the costs of foragin g by increasing competition for resources (reduces the energetic state of t he forager; competition hypothesis) or (b) increase the costs of foraging b y increasing the value of time spent on social activities (social benefits hypothesis). 3. A reduction in the costs of foraging caused by competition will reduce t he quitting-harvest rate of an optimal forager, whereas an increase in the value of alternative activities will increase the quitting-harvest rate. Bo th hypotheses predict a reduction in optimal foraging time with increased d ensity. 4. The hypothesis that applies to Deer Mice (Peromyscus maniculatus, Wagner ) was assessed by measuring their foraging activity and quitting-harvest ra tes at control and reduced population densities on four study plots located in boreal forest in northwestern Ontario, Canada. 5. Deer Mice increased their per capita foraging activity and their quittin g-harvest rates when population densities were reduced. The results confirm the very important role of competition in the behaviour of optimal forager s.