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A common mechanism through which males can enhance their success in postcopulatory contests over paternity is to inseminate more sperm than their rivals. However, ejaculate production is costly and the evolution of prudent sperm allocation strategies sensitive to variation in local levels of sperm competition has now been demonstrated in diverse taxa, including mammals. Theory predicts an increased sperm allocation in response to an elevated risk of sperm competition, but here we show that male house mice (Mus musculus domesticus) instead ejaculate fewer sperm per ejaculate when mating in the presence of a rival male. This surprising sperm allocation pattern may be a necessary consequence of adaptive changes in copulatory behavior, enabling males to achieve more rapid sperm transfer and/or to ejaculate repeatedly under risk of sexual competition. The size of a second ejaculate component, the copulatory plug, is unaffected by sperm competition risk. Our results highlight how the often complex interplay between different reproductive traits can affect the evolution of sperm competition phenotypes. Copyright 2007, Oxford University Press.

Ejaculate allocation under varying sperm competition risk in the house mouse, Mus musculus domesticus