Sex ratio and density alter male mating strategies in the coercive livebearing mosquitofish (Gambusia affinis)

Authors: James C. II Cureton, Rachel E. Martin, Raelynn Deaton

Year: 2008 (xl)

Abstract

Both operational sex ratio (OSR) and density fluctuations can cause sexual conflict to arise in many systems. The western mosquitofish, Gambusia affinis, is an ideal organism for investigating sexual conflict because males of all sizes coerce (force copulate with females). In this experiment, we hypothesized that both OSR and density would influence male mating and aggression. Specifically, we predicted that (1) males will attempt to mate with all available females as OSR becomes more female-biased and density increases, and 2) male aggression will increase as the OSR becomes more male-biased and as density increases. In this study, we used two approaches: first, we observed one focal male (N=40) across all densities (two and four) and all sex ratios (4:0, 3:1, 1:1, and 1:3) to assess shifts in individual male behavior; and second, we observed focal males, across all sex ratios at two different densities (two and four) in a repeated measures design (N=20 males per sex ratio treatment). When one male was observed across all treatments, copulation attempts and female harassment did not vary across sex ratio or density, but the male mated with more females as the number of available females increased. When different males were observed across the two densities, copulation attempts and the number of females mated with increased as the sex ratio became more female-biased and as density increased. In both analyses, males became more aggressive as the sex ratio became more male-biased. Both approaches suggest that males attempt to fertilize more females as the sex ratio becomes more female-biased. In female-biased sex ratios, males have less aggressive interactions and spread their copulations across several females, thereby maximizing their reproductive success.