RESOURCE DEFENSE, MONOPOLIZATION AND VARIATION OF FITNESS IN GROUPS OF FEMALE JAPANESE MEDAKA DEPEND ON THE SYNCHRONY OF FOOD ARRIVAL

The predictions that a decrease in the synchrony of food arrival leads to an increase in aggression, resource monopolization and, ultimately, variation in fitness within groups were tested. Groups of four female medaka, Oryzias latipes, were allowed to compete over a 2-week period for prey (Artemia sp.) that arrived either synchronously (every 5 s) or asynchronously (every 60 s). As predicted, both the monopolization of food and the frequency of aggression were higher when food arrived asynchronously than when it arrived synchronously. Fitness of individual fish was measured as the number of eggs spawned, growth, and surplus power, the sum of the energy allocated to eggs and growth. Variation of within-group fitness was greater in the asynchronous treatment than in the synchronous treatment when measured by growth and surplus power, but not by number of eggs. The frequency of aggression by an individual was positively correlated with feeding success and ultimately to fitness in the asynchronous treatment but was not correlated with either in the synchronous treatment. These results suggest that food was economically defendable only in the asynchronous treatment.