Fighting costs stabilize aggressive behavior in intersexual con ̄icts

We analyze the evolution of aggressive behavior in intersexual con ̄icts, with a special reference to mate guarding behavior in crustaceans. An analysis of a discrete-strategy game shows that an ESS with only one of the sexes being aggressive prevail if ®ghting costs or ®tness values of winning are asymmetric. Non-aggressiveness of both sexes is stable if ®ghting behavior is very costly for females and if the cost is at least partly paid independent of the strategy of the opponent. Most interestingly, the solutions of both sexes being aggressive prevails only if both sexes have some probability of winning, and if ®ghting costs are small. Second, we solve for the expected levels of aggressiveness in a game with continuous strategies. The form of the ®ghting cost function largely determines the stability of the solution. When ®ghting cost increases linearly with aggressiveness, mutual aggressiveness ̄uctuates cyclically instead of stabilizing at an ESS. However, if there is an asymmetry in ®tness payo€s, a solution with only the sex having most to lose being aggressive alone is possible. With quadratically increasing ®ghting costs an ES combination of mutual aggressiveness may exist. It is predicted that ®ghts between the sexes should be hardest when payo€s are symmetric, and that an overt behavioral con ̄ict will always take place as long as there is a ®tness loss to each of the sexes if losing the con ̄ict and both sexes have a chance to win. We discuss the models in the context of ®ghts preceding precopulatory guarding, but the models o€er a general frame for analyzing any intersexual con ̄ict.