The penis-free system is undeniably awkward for the male – it is all but impossible to fertilize the female without her consent. A male can mount the female but he struggles to force his sperm inside her. So she retains control over her eggs without having to run the risk of a damaging fight. It could be that this new-found female power has even led to further significant changes in the male bird’s behaviour. Many Neoaves species are socially monogamous, with males sharing the load when it comes to parental care. Perhaps the female’s expanded sexual autonomy advanced her conflict with males
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