Abstract

Simple models of optimal territory size have most commonly been tested qualitatively by demonstrating that an increase in food abundance or intruder pressure causes a decrease in territory size. However, the most fundamental predictions of the theory have rarely been tested: i.e. the fitness of the defender changes with territory size and the optimum occurs at a territory of intermediate size. I tested these predictions by measuring the growth rate of convict cichlids (Cichlasoma nigrofasciatum) defending food patches that differed in area by more than an order of magnitude. Groups of five fish competed for a food patch of a given size over a 10-day period. I assumed one large fish would defend the food patch and that the four smaller fish would act as intruders. As predicted, the growth rate of the defender first increased and then decreased with increasing patch size and was highest on an intermediate patch size. Cost of defence (chase rate, intruder pressure) increased linearly with patch size, whereas the benefits of defence (number of pellets eaten by the defender) increased in a decelerating way with increasing patch size. The best predictors of the defender's foraging success were patch size, chase radius, and chase rate; the most successful foragers fed on large patches, defended large territories, and chased few intruders. The growth rate of defenders increased with the number of pellets eaten and decreased as the number of intruders on their patch increased. Taken together, these results provide strong support for the assumptions and predictions of the optimal territory size model.