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1. Little is known about how population density affects the foraging behaviour of individuals. Simple models are developed to predict the net effect of density on the quitting-harvest rates of optimal foragers. The theory was tested with experiments that measured the foraging behaviour of free-ranging Deer Mice under control and reduced densities.

2. An increased density of conspecifics may (a) reduce the costs of foraging by increasing competition for resources (reduces the energetic state of the forager; competition hypothesis) or (b) increase the costs of foraging by 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 the quitting-harvest rate of an optimal forager, whereas an increase in the value of alternative activities will increase the quitting-harvest rate. Both hypotheses predict a reduction in optimal foraging time with increased density.

4. The hypothesis that applies to Deer Mice (Peromyscus maniculatus, Wagner) was assessed by measuring their foraging activity and quitting-harvest rates 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 quitting-harvest rates when population densities were reduced. The results confirm the very important role of competition in the behaviour of optimal foragers.