Body shape varies more between sexes than environments in Icelandic sticklebacks

Abstract Body shape, a key trait in fishes, often varies among geographic regions and may reflect adaptation to variation in the local environment. In addition, differences in sexes may arise not only through sexual selection, but also natural selection to meet specific ecological requirements for each sex. We studied body-shape variation among three-spine sticklebacks (Gasterosteus aculeatus), examining the effects of both environmental and sex differentiation. We compared fish from marine habitats to those from clear or turbid freshwater lakes of low- or high-altitude. We hypothesized that there would be body-shape differences between marine and freshwater fish, and also between clear and turbid environments. We found the strongest shape differences reflected sexual dimorphism, with males having a deeper body shape than females. However, there were also shape differences between environments, most significantly between marine and freshwater populations and, to a lesser extent, between altitudes and turbidity. Additionally, we found that the degree of sexual dimorphism varied between the different environments, suggesting a complex interaction between natural and sexual selection pressures mediating body shape. Our results indicate the significance of sexual dimorphism in evolution and adaptation when studying organismal response to changing environments.