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 femal e medaka, Oryzias latipes, were allowed to compete over a 2-week perio d 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 individ ual fish was measured as the number of eggs spawned, growth, and surpl us power, the sum of the energy allocated to eggs and growth. Variatio n of within-group fitness was greater in the asynchronous treatment th an in the synchronous treatment when measured by growth and surplus po wer, but not by number of eggs. The frequency of aggression by an indi vidual was positively correlated with feeding success and ultimately t o fitness in the asynchronous treatment but was not correlated with ei ther in the synchronous treatment. These results suggest that food was economically defendable only in the asynchronous treatment.