The diverse morphologies of animal sperm reflect putative adaptations to the highly variable conditions in which these cells compete. But the functional behaviors that drive sperm evolution are not well understood. Using phylogenetic analyses, Lukas Scharer et al. (pp. 1490-1495) identified a shift in mating behavior of the flatworm genus Macrostomum that likely coincided with changes in sperm design. The authors describe a "reciprocal mating syndrome" in M. lignano, in which the flatworms insert male stylets into their partners' female genitalia to exchange sperm.
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