Female-biased dimorphism in size and age at maturity is reduced at higher latitudes in lake whitefish Coregonus clupeaformis.

Abstract

Female-biased sexual dimorphism in size at maturity is a common pattern observed in freshwater fishes with indeterminate growth, yet can vary in magnitude among populations for reasons that are not well understood. According to sex-specific optimization models, female-biased sexual size dimorphism can evolve due to sexual selection favouring earlier maturation by males, even when sexes are otherwise similar in their growth and mortality regimes. The magnitude of sexual size dimorphism is expected to depend on mortality rate. When mortality rates are low, both males and females are expected to mature at older ages and larger sizes, with size determined by the von Bertalanffy growth equation. The difference between size at maturity in males and females becomes reduced when maturing at older ages, closer to asymptotic size. This phenomenon is called von Bertalanffy buffering. The predicted relationship between the magnitude of female-biased sexual dimorphism in age and size at maturity and mortality rate was tested in a comparative analysis of lake whitefish Coregonus clupeaformis from 26 populations across a broad latitudinal range in North America. Most C. clupeaformis populations displayed female-biased sexual dimorphism in size and age at 50% maturity. As predicted, female-biased sexual size dimorphism was less extreme among lower mortality, high-latitude populations.