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Curr Biol. 2009 Mar 10;19(5):404-7. doi: 10.1016/j.cub.2009.01.045. Epub 2009 Feb 19.

Sperm competition favors harmful males in seed beetles.

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Evolutionary Biology Centre, Department of Ecology and Evolution, Animal Ecology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden.


One of the most enigmatic observations in evolutionary biology is the evolution of morphological or physiological traits in one sex that physically injure members of the other sex. Such traits occur in a wide range of taxa and range from toxic ejaculate substances to genital or external spines that wound females during copulation. Current hypotheses for the adaptive evolution of such injurious traits rest entirely on the assumption that they are beneficial to their bearer by aiding in reproductive competition. Here, we assess this key assumption in seed beetles where genital spines in males physically injure females. We demonstrate that male spine length is positively correlated with harm to females during mating but also that males with longer spines are more successful in sperm competition. This is the first complete support for the proposal that sexual selection by sperm competition can favor morphological traits in males that inflict injury upon females. However, our results suggest that harm to females is a pleiotropic by-product, such that genital spines in males elevate success in sperm competition by means other than by causing harm.

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