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.

中文翻译：

---

女性投资在性军备竞赛中的作用

强迫交配在水禽物种（Ana科）中很常见，该群体有较大的卵和非常早熟的幼鸟。可以将它们视为明显的性冲突的结果，以及两性之间的进化军备竞赛。我们检查了暗示的雌性选择假说，但没有通过Briskie和Montgomerie（1997）以及Montgomerie和Briskie（2007）的正确检验，即雄鸭的阴茎大小可能与雌性对卵的投资相关，并预测在雌性产卵较大的物种中，阴茎的大小可能会更大，因为如果被迫交配，雌性会更不愿意放弃卵子。比以前的研究更大的数据集使我们能够以比较的方式检验该假设。我们的结果迫使我们拒绝女性选择的假设，因为卵的大小与阴茎长度和阴道螺旋数呈负相关，两者都被认为是对频繁强迫交配的适应。卵大小与形态防御（阴道螺旋）之间的明显折衷是很强的，特别是在一夫一妻制物种中。总的来说，我们得出的结论是，设定卵子大小下限的因素会限制雌性的形态防御以及水禽中两性之间的军备竞赛。