Anim. Behav., 1995, 49, 367–375 Operational sex ratio, mediated by synchrony of female arrival, alters the variance of male mating success in Japanese medaka JAMES W. A. GRANT, MICHAEL J. BRYANT* & CATHERINE E. SOOS Department of Biology, Concordia University, 1455 de M aisonneuve Blvd W est, M ontreal, Quebec, H3G 1M8 Canada ( R eceived 2 June 1993; initial acceptance 30 A ugust 1993; final acceptance 16 January 1994; M S. number: 6724) Abstract. 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. The temporal dispersion of resources is thought to influence both the way animals compete and the intensity of resource monopolization (Emlen & Oring 1977; Wells 1977; Grant 1993). When resources arrive asynchronously, individuals with high competitive ability can potentially compete for each arriving unit of resource and, thus, acquire a disproportionately large share of the total. When resources arrive synchronously, less competitive individuals are able to acquire resources when the more competitive individuals are busy exploiting other resource units. Hence, the degree of resource monopolization, the uneven distribution of resources among individuals, is predicted to decrease as the synchrony of resource arrival increases. Because the potential variance in resource gain among individuals is high in asynchronous en- vironments, the potential benefits of using aggres- sion during competition are also high. Dominant individuals can use the time between the arrival of resources to intimidate or exclude others from favourable sites as a means of increasing their share of the resource. In contrast, time spent on aggression in synchronous environments is poten- tial time lost from resource exploitation. Hence, animals are predicted to scramble for resources in synchronous environments and to engage in con- test competition in asynchronous environments (Emlen & Oring 1977; Wells 1977). The synchrony hypothesis was originally devel- oped to predict the type and outcome of compe- tition by males for access to females (Trivers 1972; Emlen & Oring 1977; Wells 1977), but has subse- quently been applied to foraging systems (e.g. Milinski & Parker 1991; Blanckenhorn & Caraco 1992). Surprisingly, the hypothesis has only been tested experimentally in foraging systems, where it has been shown that defence and monopolization of food increase as temporal clumping of food arrival decreases (Blanckenhorn 1991; Grant & Kramer 1992). The aim of our study was to test experimentally the hypothesis that the degree of monopolization of females and the use of aggression by competing males increase as the synchrony of female arrival decreases. We chose the Japanese medaka, *Present address: Department of Biology, University of California, Riverside, CA 92521, U.S.A. 0003–3472/95/020367+ 09 $08.00/0  1995 The Association for the Study of Animal Behaviour 367