Sumarto B.K.A., Fujimoto S., Kobayashi H., Yamahira K.

Tropical animals are characterized by showy ornaments and conspicuous body colors as compared with their temperate relatives. Some recent studies have hypothesized that sexual selection pressures are stronger in the tropics than in the temperate zone. In this study, we test whether sexual-selection pressure varies latitudinally, using the Adrianichthyidae, a freshwater fish family having a wide geographic range, as a model system. We compared the degree of sexual dimorphism in body size, fin lengths, and body coloration among 32 species. Correlations between latitude and the degree of sexual dimorphisms revealed that lower-latitude species are sexually more dimorphic in all characters than higher-latitude species. Phylogenetic generalized least squares analyses using a mtDNA phylogeny demonstrated that the negative correlations between latitude and the degree of sexual dimorphism become non-significant when phylogeny is considered, but that the variance in the degree of sexual dimorphism is explained not only by phylogeny but also almost equally by latitude. Ancestral state reconstruction demonstrated that the evolution of these sexually dimorphic traits is not phylogenetically constrained. These findings are consistent with the view that tropical species are exposed to stronger sexual selection pressures than temperate species.