Pre-copulatory guarding and the evolution of sexual size dimorphism in asellus aquaticus L

Sexual selection and assortative mating are examined in two field populations of the fresh-water isopod Asellus aquaticus. The ability of males to carry the female in the pre-copula pair under adverse conditions was investigated experimentally, this ability was related to the weight of the female in the pair. The implications of female size as a loading constraint on males, the evolution of sexual size dimorphism and the significance of assortative mating are discussed.