Why are males bigger than females in pre-copula pairs of Gammarus pulex?
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In natural streams, males are bigger than females in precopula pairs of the freshwater amphipod Gammarus pulex. A set of experiments is described, the results of which are consistent with the hypothesis that males are larger as a result of a mechanical constraint and not intrasexual competition for mates. Where the male in a pair is relatively larger than the female the swimming performance is superior to those pairs in which the male and female are of similar sizes. This minimises the risk of being washed downstream by the current.
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