Abstract

We compared the patterns of female–female and male–male competition in Japanese medaka (Oryzias latipes) in response to changes in the operational sex ratio (OSR), the ratio of sexually active members of one sex to sexually active members of the opposite sex. As expected, courtship behaviour and intrasexual aggression were more frequent in males than in females. However, the overall patterns of female–female and male–male aggression were similar: intrasexual aggression increased with the OSR (female/male OSR for females and male/female OSR for males), consistent with predictions of mating-systems theory. A comparative analysis of our data and those from three other studies indicated that the rate of intrasexual aggression was greater in males than in females, but the patterns of intrasexual aggression were similar: the rate increased linearly and with a common slope with increasing OSR over a range of 0.4–3.