SIZE AND PAIRING SUCCESS IN GAMMARUS-DUEBENI - CAN FEMALES BE TOO BIG

The crustacean Gammarus duebeni exhibits precopula mate guarding and s ize-assortative pairing, in which larger males tend to pair with large r females. Size-assortative pairing may result from sexual selection o r natural selection (mechanical or loading constraints limiting the si ze of female that can be carried by the male). If loading constraints are important, large females should have lower pairing success than fe males of intermediate size as they will be less likely to encounter su fficiently large males capable of carrying them in precopula. We teste d this hypothesis in a laboratory study. Female pairing success was de pendent on size; however, the relationship was curvilinear: pairing su ccess increased with size up to a point, but larger females suffered d ecreased pairing success. This supports the hypothesis that loading co nstraints play a part in structuring size-assortative pairing in this species. We found no evidence for size-related female resistance in st ructuring the pattern of pairing. We considered size-related pairing s uccess with regard to environmental sex determination and parasitic se x-ratio distortion in G. duebeni. (C) 1997 The Association for the Stu dy of Animal Behaviour.