Conventional sex roles in an amphidromous Rhinogobius goby in which females exhibit nuptial coloration

Operational sex ratios (OSR) are considered the key factor determining the pattern of intrasexual competition (Emlen and Oring, 1977). In situations w here males have the potential to reproduce faster than females, the OSR is biased the former, which thus will compete for females, which in turn becom e selective (i.e., conventional sex role; Vincent, 1994). On the other hand , when the potential reproductive rate of females exceeds that of males, th e OSR is biased toward females, which will thus compete more intensely than males for mates, the males being selective (i.e., reversed sex role; Vince nt, 1992). The OSR theory predicts that sexual selection will work more str ongly on an abundant rather than a scarce sex, the former consequently deve loping secondary sexual traits (Emlen and Oring, 1977). The Rhinogobius species live in almost all river courses of Japan, exhibiti ng paternal care at nests typically on the underside of rocks on the river bottom (Mizuno, 1960). Rhinogobius sp. DA (Dark type) (Kawanabe and Mizuno, 1989) inhabits short streams with relatively steep gradients. The potentia l reproductive rate of males of this species seems to be higher than that o f females (Takahashi and Yanagisawa, 1999; Takahashi, unpubl. data), with t he OSR biased toward males. Consequently, its sex role should be convention al with males developing pronounced secondary sexual traits. Nevertheless, females of this goby exhibit nuptial coloration. This inconsistency accordi ng to OSR theory, that is, the abundant sex develops sexually dimorphic tra its, suggests that study of the sex role in this goby will contribute to a further development of the theory. The OSR and sex role of Rhinogobius spec ies have never been studied. As a first step, the OSR, intrasexual competit ion and courtship behaviour in ii. sp. DA is investigated here.