1989 Volume 39 Issue 2 Pages 147-161
Males of the amphipod Gammarus pulex exhibit precopulatory guarding behaviour toward females for several days before copulation. In this species, male size is positively correlated with female size in precopula pairs. (size assortative mating), and in breeding individuals, males are larger than females (sexual size dimorphism). Based on quantitative field data and laboratory behavioural observations, two hypotheses have been proposed for the ecological and evolutionary causes of size assortative mating and sexual dimorphism in G. pulex. One is the sexual selection theory, where these two phenomena have been induced by male-male competition for large fecund females. The alternative hypothesis invokes the special loading effect theory, where the load for males in precopula determines the size ratio (male : female). Although proponents of the respective hypotheses have clearly demonstrated male-male competition and the effect of water current on pair formation, the field data was fragmentary, and population parameters such as life history traits were completely ignored. We argue that physiological and life history characteristics must not be neglected when considering the evolutionary mechanisms of both size assortative mating and sexual dimorphism.
