Latitudinal Variation in Male Competitiveness and Female Choosiness in a Fish: Are Sexual Selection Pressures Stronger at Lower Latitudes?

Tropical animals are characterized by their showy ornaments and conspicuous colors, and numerous studies on sexual selection have been conducted using tropical species as a model system. In this study, hypothesizing that sexual selection pressures are stronger at lower latitudes, we examined the latitudinal variation in the degree of (1) male aggressiveness in male–male competitions, (2) male eagerness in courtship, and (3) female choosiness to mates in the medaka fish (Oryzias latipes complex). Laboratory mating experiments revealed that males from lower-latitude populations fight each other and court females more frequently than males from higher-latitude populations, supporting the hypothesis that both intra- and intersexual selection pressures are stronger at lower latitudes. In addition, females from lower-latitude populations, where males are more conspicuous, did not accept males easily, especially when mated with higher-latitude males, suggesting that female choosiness has also coevolved across latitudes through Fisherian runaway process. We suggest that latitudinal cline in reproductive seasonality, which is the case not only in the medaka but also among a variety of taxa, leads to latitudinal cline in operational sex ratios of local populations, which ultimately results in latitudinal cline in sexual selection pressures.