Interactions between the spatial and temporal clumping of food affect the intensity of aggression in Japanese medaka

Previous studies have largely supported the predictions that an animal's aggressiveness increases as the clumping of resources increases in space but decreases in time. Such tests of resource defence theory, however, have manipulated only one aspect of resource distribution at a time. We tested for an interaction between the effects of the spatial and temporal clumping of food by allowing groups of 10 Japanese medaka fish, Oryzias latipes, to compete for food in a 2x2 factorial design. There was a significant interaction between the effects of the spatial and temporal clumping of food on both the total rate of aggression in tanks and on the per capita rate of aggression by fish in the feeding patch. Despite these interactions, all trends in per capita rate of aggression in the feeding patch were consistent with the predictions. In contrast, total rate of aggression in the tank, which included fish that were not competing for food, apparently contradicted a key prediction. Our results suggest that powerful tests of resource defence theory should take into account an animal's opportunity to be aggressive. In addition, it may be difficult to extrapolate the results of single-factor tests of resource defence theory to multi-factor or field situations.Copyright 1998 The Association for the Study of Animal Behaviour.