LONDON: Male flies are not the most romantic lovers in the animal world: they struggle to prolong sex, while female flies would prefer they didn’t, says a new study.
They do so to prevent competing males from mating with the same female, says the study in the journal BMC Evolutionary Biology.
“The study clearly shows that the males and the females have conflicting desires over the length of copulation in Drosophila montana,” says Anneli Hoikkala study co-author and evolutionary geneticist from the University of Jyvaskyla in Finland.
Biologists have long known that sexual selection is a force that can mould the evolution of a species. Occasionally adaptations developed by males and females are at conflict with one another.
Males are likely to have adaptations to ensure their paternity. Females, on the other hand, are likely to have adaptations that minimise costs incurred during mating and allow them to seek out the mates with the best genes.
To document a case where these goals come into conflict researchers including Hoikkala and Dominique Mazzi, at the Swiss Federal Institute of Technology in Zurich, observed Drosophila montana flies copulating.
They found that after a period of harmonious intercourse, the females vigorously kick with their hind legs and flick their wings to dislodge the males, who struggle to stay on top and continue with the copulation.
“Copulation is by no means a harmonious cooperative enterprise in this species, but rather a battlefield fueling the evolution of male adaptations that harm females, and, in turn, the evolution of female adaptations meant to alleviate the imposed harm,” said Mazzi.
The researchers found that prolonged copulation didn’t improve the chances of insemination nor increase the number of offspring that males had, but it did reduce the chance of other competitors inseminating the female.
Males were more likely to attempt longer copulation in the presence of potential competitors with whom the female could mate immediately afterwards.
But, perhaps the most novel aspect of these results is “that females have the upper hand on a trait that is often assumed to be under significant male control,” said Mazzi.
Further tests showed that when females were allowed to interact freely with males, copulations last much less than when females were prevented from resisting. In fact copulations lasted 50% longer, the researchers found. This “implies that females gain from copulations that do not exceed a certain optimum duration,” she said.
Gordon Gallup, an evolutionary psychologist from the University at Albany, in New York, USA, says this study is “very interesting” and consistent with a variety of other strategies males adopt to ensure paternity in those species with promiscuous females.
He adds that there is some evidence that the “human penis evolved to displace rival male semen from the woman’s reproductive tract and enable men to substitute their semen for those of their competitors.”
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BMC Evolutionary Biology