THE EFFECT OF COMPETITION ON FORAGING ACTIVITY IN DESERT RODENTS - THEORY AND EXPERIMENTS

Considered 2 models, one for a time-minimizer satisfying an energy requirement, the other for an animal maximizing fitness as a function of multiple inputs subject to a time constraint. The goal of satisfying an energy constraint predicts that foraging effort should increase with increased competition. The goal of maximizing fitness subject to a time constraint on multiple inputs may also predict that foraging effort should increase with increased competition because of the missed opportunity cost that results when different inputs are complementary. However, if the fitness-maximizer with multiple inputs incurs an energy cost of foraging (in addition to missed opportunity costs), then it should often reduce foraging effort in response to an increase in competition. The foraging response to increased competition by Gerbillus allenbyi and G. pyramidum was tested over a range of manipulated population densities in field enclosures in the Negev Desert of Israel. Results support the cost-benefit model when the additional energy cost of foraging is important. Per capita activity declined as a function of intraspecific density for each species and as a function of interspecific density for G. allenbyi. No interspecific effect, however was noted on G. pyramidum. -from Authors