Effects of competitor-to-resource ratio on aggression and size variation within groups of convict cichlids

Resource defence theory predicts that the intensity of competitive aggression, degree of resource monopolization, and variation in fitness will be highest at intermediate levels of the spatial clumping of resources. We tested for this predicted dome-shaped relationship by manipulating the spatial clumping of food via competitor-to-resource ratio (CRR), the number of potential competitors divided by the number of resource units. Groups of 10 convict cichlids, Archocentrus nigrofasciatus, were allowed to compete for a fixed amount of food in one of five CRR treatments (1, 1.43, 2, 5 and 10), created by varying the number of patches in which the food appeared (i.e. 10, 7, 5, 2 and 1 patch, respectively). As predicted, both the frequency of aggression and the coefficient of variation of body mass (a measure of the consequences of food monopolization and variation in a component of fitness within groups) increased significantly as CRR increased from 1 to 2, and then decreased significantly at CRRs of 5 and 10. In addition, mean growth rate decreased in groups with high rates of aggression, suggesting an important cost of aggression. Our study provides the first quantitative support for the predicted dome-shaped relationship and suggests that CRR is a useful measure of the degree of spatial clumping of resources.