The role of female investment in a sexual arms race

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Forced copulations are common among waterfowl species (Anatidae), a group with relatively large eggs and very precocial young. They can be viewed as the outcome of an overt sexual conflict, and an evolutionary arms race between the sexes. We examined the female choice hypothesis suggested, but not properly tested by Briskie and Montgomerie (1997) and Montgomerie and Briskie (2007), that penis size in male ducks might correlate with female investment in eggs, predicting that in species where females lay larger eggs, penis size might be larger, because females would be more reluctant to abandon their eggs if forced to copulate. A larger data set than in earlier studies enabled us to test that hypothesis in a comparative way. Our results compelled us to reject the female choice hypothesis since egg size is negatively correlated with penis length and the number of vaginal spirals, both being seen as adaptations to frequent forced copulations. The apparent trade‐off between egg size and morphological defences (vaginal spirals) is strong particularly among monogamous species. Overall, we conclude that factors that set a lower limit for egg size constrain the morphological defences of females and the arms race between the sexes in waterfowl.