OPERATIONAL SEX-RATIO, MEDIATED BY SYNCHRONY OF FEMALE ARRIVAL, ALTERS THE VARIANCE OF MALE MATING SUCCESS IN JAPANESE MEDAKA

The hypotheses that variation in male mating success and use of aggression by competing males increase with decreasing synchrony of female arrival were experimentally tested. Groups of three male Japanese medaka, Oryzias latipes (Pisces, Oryziidae) were allowed to compete for females that were placed in the tank either simultaneously (synchronous treatment, male-to-female operational sex ratio=0.5) or sequentially (asynchronous treatment, operational sex ratio=3). In these experiments, the mating system of medaka was scramble-competition polygyny because male mating success was primarily determined by their persistence in following and courting females rather than by dominance and aggression. As predicted, the coefficient of variation of male mating success and the rate of aggression by males was higher in the asynchronous than in the synchronous treatment. In addition, the percentage of matings in which a sneaker participated was also higher in the asynchronous than in the synchronous treatment. Operational sex ratio, mediated by female synchrony, seems to be an important proximate factor influencing the intensity of male-male competition. These results suggest that differences between males in their ability to scramble for females can generate important variance in mating success, a mechanism that is often overlooked in the literature on mating systems.