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 male in a group of males. In this study we examined the relationship between body size and the success of gonopodial thrusting, a mating tactic characterisitc of the poeciliids that enables a male to achieve copulation without the female's cooperation. Without competitors the probability that a mating attempt was successful decreased exponentially with male length while it increased with the size of the female. When tested in a group, males competed for access to females. The largest individual 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 mechanisms of sexual selection may interact under natural condition. The results of computer simulations indicated that the size of successful males always ranged well below the average length of females. Small male advantage in copulation was generally more effective than intrasexual competition and female choice in determining male mating success. A simulation which accounted for seasonal variation in body size and in the pattern of male aggregation indicated that small males had a reproductive. advantage for most of the time, and that the correlation between body size and mating success became positive only at the end of the reproductive season, when population density peaks and the sex-ratio becomes male-biased. We suggest that the mating advantage of small males may be the cause of the reverse sexual size dimorphism of the poeciliids.