SEXUAL SELECTION AND SEXUAL SIZE DIMORPHISM IN THE EASTERN MOSQUITOFISH GAMBUSIA-HOLBROOKI (PISCES POECILIIDAE)

We have previously shown (BISAZZA and MARIN 1991) that female eastern mosquitofish, Gambusia holbrooki, did not discriminate among males of different size, although they showed a preference for the dominant mal e in a group of males. In this study we examined the relationship betw een body size and the success of gonopodial thrusting, a mating tactic characterisitc of the poeciliids that enables a male to achieve copul ation without the female's cooperation. Without competitors the probab ility that a mating attempt was successful decreased exponentially wit h male length while it increased with the size of the female. When tes ted in a group, males competed for access to females. The largest indi vidual monopolized access to females and accomplished the majority of mating attempts, whatever the number of its competitors. We used these results and demographic data (length distributions and composition of groups) of a natural population to simulate how the different mechani sms of sexual selection may interact under natural condition. The resu lts of computer simulations indicated that the size of successful male s always ranged well below the average length of females. Small male a dvantage in copulation was generally more effective than intrasexual c ompetition and female choice in determining male mating success. A sim ulation which accounted for seasonal variation in body size and in the pattern of male aggregation indicated that small males had a reproduc tive. advantage for most of the time, and that the correlation between body size and mating success became positive only at the end of the r eproductive season, when population density peaks and the sex-ratio be comes male-biased. We suggest that the mating advantage of small males may be the cause of the reverse sexual size dimorphism of the poecili ids.