Female fitness declines with increasing female density but not male harassment in the western mosquitofish, Gambusia affinis

Sexual conflict occurs when individuals of one sex express traits that reduce the fitness of individuals of the other sex. In many poeciliid fish, males harass females for copulations, which is thought to reduce female fitness by lowering foraging efficiency and increasing predation risk, energetic expenditure and the likelihood of disease transmission. Mating may also be costly for males, who often engage in aggressive interactions with other males in addition to expending energy pursuing females. We examined the effects of three operational sex ratios on male behaviour, female fitness and male body condition in the western mosquitofish, Gambusia affinis, during a 10-week experiment. Despite a significant reduction in male harassment, female growth and reproductive success decreased as the proportion of females increased. Results suggest that increases in female density had a strong, negative effect on female fitness, overwhelming any potential costs of male harassment. Aggressive behaviour between males increased and male copulation rate decreased as the proportion of males increased, suggesting that operational sex ratio influences the number of copulations that a male attempts by altering the frequency of agonistic interactions with other males and the number of females available to mate. We did not detect a difference in male body condition between treatments. Sources of female density dependence and consequences of variation in operational sex ratio on male fitness are discussed. (c) 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.