Abstract

To test the predictions of optimal territory size models, we attempted to manipulate the size of area that a dominant convict cichlid fish (Archocentrus nigrofasciatus) would defend around a food patch by placing simulated vegetation at three different distances from the edge of the patch (0, 11, and 22 cm). As expected, the size of area defended against four smaller intruders increased as the vegetation was moved farther from the patch. Consistent with optimal territory size models, both the costs of defence, measured as chase radius and chase rate, and the benefits of defence, measured as the amount of food eaten by the defender, increased with the distance of the vegetation from the patch. Growth rates of the defenders, however, did not differ among the treatments, perhaps because the benefits of monopolizing food were balanced by the costs of defending a larger area. Our data support the hypothesis that the size of a guarded area around an ephemeral resource patch affects both the costs and benefits of defence