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 aggres sion by competing males increase with decreasing synchrony of female a rrival were experimentally tested. Groups of three male Japanese medak a, Oryzias latipes (Pisces, Oryziidae) were allowed to compete for fem ales that were placed in the tank either simultaneously (synchronous t reatment, male-to-female operational sex ratio=0.5) or sequentially (a synchronous 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 an d the rate of aggression by males was higher in the asynchronous than in the synchronous treatment. In addition, the percentage of matings i n which a sneaker participated was also higher in the asynchronous tha n in the synchronous treatment. Operational sex ratio, mediated by fem ale synchrony, seems to be an important proximate factor influencing t he intensity of male-male competition. These results suggest that diff erences between males in their ability to scramble for females can gen erate important variance in mating success, a mechanism that is often overlooked in the literature on mating systems.