In most studies of sexually reproducing animals it is assumed that inter male competition results in selection of larger males. In higher vertebrates there appears to be a correlation between the type of mating system and the degree of sexual size dimorphism. Amongst the lower vertebrates and invertebrates, however, this correlation is less obvious; in these groups the females are usually larger than the male, despite considerable conflict between males for mates. The hypothesis that differential loading or mechanical constraints, operating on males and females during mate guarding, are important factors influencing the relative sizes of sexes was investigated for two species; a pondskater, (Gerris sp.) and the common toad ( Bufo bufo ). In both species, the females carry the male prior to mating, the female being the larger sex. Previous work on the common toad has shown that assortative mating is operating, this study, however, shows that the converse is true, ie that mating is random. Several suggestions have been proposed to explain these results. In contrast, pondskaters show assortative mating, the females paired with males according to body size and possibly on midleg length. Both sexes, however, appear to be adapted to the loading constraints to which they are subjected during pairing
