Loading constraints sexual selection and assortative mating in peracarid Crustacea

In precopula pairs of amphipod and isopod Crustacea in which males carry females, the males are larger than their mates and mating is size-assortative. Mate-guarding is a product of sexual selection. Size dimorphism and assortative mating have also been attributed to sexual selection but the supporting evidence for amphipods is equivocal. We describe a series of experiments confirming that relatively large male Gammarus pulex L. have an advantage because they can swim against stronger currents when carrying a mate. At higher current speeds, the male/female size ratio which forms is significantly greater, and in field collections size ratios of pairs are higher in streams than in lakes for a number of species. In a simulation we show that a size-assortative pattern inevitably develops if the observed size restriction is used as a rule for pairing. The results are discussed with respect to size-assortative mating, which has been attributed to male selectivity and male-male competition for access to large, fecund females.