The relationship between sexually selected morphological and behavioural traits in male Japanese medaka (Oryzias latipes) and their variation across social settings

Abstract

Sexual selection can lead to the development of secondary sex traits and mating behaviours. Morphological and behavioural traits may increase individual reproductive success, however, under varying sexual selection types and intensities the relative importance of morphology and behaviour on reproductive success will vary. This study examines the relationship between the morphology of a sexually selected trait (anal fin size) and three mating behaviours of male Japanese medaka (Oryzias latipes), and how their frequency varies across four different social settings. To determine if male mating behaviours varied across social settings, I manipulated the intensity of sexual selection in the laboratory by subjecting the medaka to four different operational sex ratios (defined as the ratio of sexually mature males to fertilizable females). Anal fin size, and aggression, following and courtship behaviours were quantified for all treatments. Based on previous literature, I predicted a positive relationship between mating behaviours and anal fin size in male biased operational sex ratios, as well as plasticity in the frequency of mating behaviours performed across varying social settings. I found a significant relationship between standard length (mm) and anal fin surface area (mm2) in male Japanese medaka. In highly male biased operational sex ratios, there was a positive relationship between anal fin size and aggressive behaviour, and male medakas show plasticity in their mating behaviours across varying social settings (measured by OSR), as they react to varying intensities of sexual selection pressures.