Bayu K. A. Sumarto , Shingo Fujimoto , Hirozumi Kobayashi, Kazunori Yamahira

Although some recent studies hypothesized that sexual-selection pressures vary geographically, empirical evidence is scarce. Because sexual selection leads to the evolution of secondary sexual characteristics, the degree of secondary sexual characters or the resultant sexual dimorphism has been used as an index of the strength of sexual-selection pressures. In this study, we compared the degree of sexual dimorphisms in body size, fin lengths, and body coloration among 32 species of medaka fishes (family Adrianichthyidae), collected from all over their geographic range. We found that lower-latitude, tropical species are sexually more dimorphic in all of the characters examined than higher-latitude, temperate species. Although this latitudinal variation did not persist when the phylogenetic relationship among the species is considered, ancestral state reconstruction revealed that sexual dimorphisms evolved independently even among closely-related species, indicating that the evolution of sexual dimorphisms are not phylogenetically constrained. We conclude that tropical species are exposed to stronger sexual-selection pressures than temperate species. In the presentation, we will discuss possible causes of the latitudinal variation in sexual-selection pressures from the viewpoint of reproductive seasonality.