The risk and intensity of sperm ejection in female birds

The way females utilise the gametes of different males has important consequences for sexual selection, sexual conflict and intersexual coevolution in natural populations. However, patterns of sperm utilisation by females are difficult to demonstrate, and their functional significance remains unclear. Here, we experimentally study sperm ejection in the fowl, Gallus gallus domesticus, where females eject preferentially the sperm of socially subordinate males. We study two measures of sperm ejection: (i) the probability that an ejaculate is ejected ('risk'), and (ii) the proportion of semen ejected ('intensity'), and show that both measures are strongly non-random with respect to characteristics of the ejaculate, the male and the female. Sperm ejection neutralised on average 80% of an ejaculate, and while larger ejaculates suffered a higher ejection risk, smaller ejaculates suffered more intense ejection. After controlling for ejaculate volume, socially subdominant males suffered higher e...