Plasticity and selection of male sexual traits across operational sex ratios in Japanese medaka (Oryzias latipes)

Abstract

In many species, the mating advantage associated with sexually-selected traits may vary with operational sex ratio (OSR; the number of sexually active males to fertilizable females in a population), and this metric is often used as a proxy for the intensity of competition for mates. The goal of this research was to assess whether OSR influenced male mating behaviour and morphology of sexually-selected traits. I examined these effects both intra- and inter-generationally by housing Japanese medaka (Oryzias latipes) in four different OSRs ranging from female-biased to highly male-biased. I found that: (1) courtship behaviour decreased and aggression tended to increase with increasing OSR, and male body size was the best predictor of aggression; (2) there is plasticity in mating behaviour, particularly courtship; (3) within males, larger primary and secondary sexual structures occurred in female-biased OSRs; and (4) overall levels of courtship and aggression were higher after one generation within OSRs.