Size and pairing success in Gammarus duebeni: can females be too big?

The crustacean Gammarus duebeni exhibits precopula mate guarding and size-assortative pairing, in which larger males tend to pair with larger females. Size-assortative pairing may result from sexual selection or natural selection (mechanical or loading constraints limiting the size of female that can be carried by the male). If loading constraints are important, large females should have lower pairing success than females of intermediate size as they will be less likely to encounter sufficiently large males capable of carrying them in precopula. We tested this hypothesis in a laboratory study. Female pairing success was dependent on size; however, the relationship was curvilinear: pairing success increased with size up to a point, but larger females suffered decreased pairing success. This supports the hypothesis that loading constraints play a part in structuring size-assortative pairing in this species. We found no evidence for size-related female resistance in structuring the pattern of pairing. We considered size-related pairing success with regard to environmental sex determination and parasitic sex-ratio distortion in G. duebeni1997 The Association for the Study of Animal BehaviourCopyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour