EVOLUTIONARILY STABLE FORAGING SPEEDS IN FEEDING SCRAMBLES - A MODEL AND AN EXPERIMENTAL TEST

An evolutionarily stable strategy (ESS) model of scramble effort (for example, foraging speed) in competition for food or mates, where payof f may depend on competitor density, is described. An individual can in crease speed (at some energetic cost) to gain a greater share of resou rces. The predictions are that if food input per competitor is constan t (density independence), the Ess foraging speed should increase with density, but if food input per patch is constant (density dependence), then the ESS foraging speed should decrease with density. The predict ions of this model are tested in an experiment using different densiti es of cichlid fish, Aequidens portalegrensis, scrambling for food at a discrete resource patch. The results provide support for the model in a situation where payoffs are density dependent, but fit less well wh ere they are density independent.