Resource defense in a group-foraging context

When foraging in groups, animals frequently use either scramble or contest tactics to obtain food at clumps found by others. The question of which competitive tactic should be used has been addressed from two different perspectives: a simple optimality approach and a game theoretic approach. Surprisingly both approaches make strikingly different predictions about how per-capita frequency of aggression within groups should change as a function of food abundance and competitor density. Resource defense theory typically predicts dome-shaped relationships between the per-capita frequency of aggression and both food abundance and competitor density, whereas game theoretic models predict an increase in aggression with competitor density and a decline in aggression with increased food abundance. We developed a game theoretic model to explore whether the predictions of resource defense theory and the game theoretic approach can be reconciled. Our model assumes that players have different competitive abilities and can adopt roles of either finder or joiner that affect the quantity of food that can be gained front a food clump. In accordance with earlier game theoretic models, we predict an increase in aggression with competitor density when animals compete by pair-wise contests. However, when food clumps can be challenged by more than one competitor, both the costs and benefits of defending increase with competitor density, which results in a dome-shaped relationship between the two variables. Our model predicts that aggression should always decrease as the density of food clumps increases.