Meaningless sex? Male mounting reduces the sexual promiscuity of hens

Why does a male copulate without delivering semen? In a study that sheds new light on the evolution of sexual behaviour, researchers have utilised a novel technique to reveal that in feral chickens, the simple stimulus generated by male mounting reduces the sexual promiscuity of a hen, indicating that even copulations that do not result in semen transfer, a puzzling male behaviour frequently observed in the chicken and many other species, may be crucial to defend the paternity of a male.

Females of many species often copulate with multiple males, leading to the competition between the ejaculates of different males over fertilisation (sperm competition). Males are therefore selected to avoid sperm competition by preventing females from mating with other males. In a number of species, mostly insects, female promiscuity is temporarily reduced following copulation and this has been associated with highly specialised male insemination products. However, these paternity defence mechanisms typically require costly male investment in a female, and are at odds with the fact that males often copulate without delivering semen. Importantly, previous studies have not considered that female promiscuity may be inhibited by stimuli generated by the simple act of male mounting.

In a new study, a team of researchers from Stockholm University (Sweden), University of Sheffield and University of Oxford (UK) tested this idea in the sexually promiscuous chicken, using a novel technique that separates the effect of insemination products from that of mounting alone. The researchers detected the response of hens to the mounting of a rooster, by fitting some hens with a light plastic harness covering their cloaca, thus preventing insemination. The use of this technique demonstrated that mounting alone (independently of insemination) not only drastically inhibits the propensity of a hen to mate with a new rooster, but also reduces the number of sperm that she obtains from a new rooster in the 2-4 days following mounting. Therefore, roosters can defend their paternity by reducing the promiscuity of the hens that they have inseminated, by exploiting the hen’s response to the simple stimulus of mounting. Consistent with this idea, roosters often mount hens that they previously inseminated without delivering additional semen. Therefore, this study demonstrates that even though the ultimate function of sex is fertilisation, copulations resulting in the delivery of little or no sperm are not necessarily functionally meaningless but may have an important evolutionary significance.