Anna Gosline in New Scientist:
Possessive cockerels use fake sex to keep their hens faithful. By merely mounting females – without bothering to waste precious sperm – cocks ensure their partners will not go looking for male competitors to fertilise them, a new study suggests. The finding may explain why males of many species – from insects to mammals – engage in seemingly meaningless sperm-free sex.
“Copulations that appear to be successful, but with no semen transferred, are almost ubiquitous,” says Tommaso Pizzari at the University of Oxford, UK, co-author of the study. “It suggests that this behaviour may be rather more than an accident or a by-product of males running out of sperm.”
While sperm was always thought of as a cheaper investment than eggs, in the past few years, researchers have begun to realise that sperm also carries a hefty biological price tag. In 2003, Pizzari and his colleagues showed that male chickens allocated their precious seed according to the likelihood of fathering children. Unfamiliar females always received a fulsome dose, while hens with which the cock had already mated several times ended up receiving little more than ruffled feathers.
More here.