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The Role of Promiscuous Females in Evolution

Males have to make less of an effort to mate with promiscuous female fruit flies, making the quality and quantity of their semen all the more important in the competition to fertilise the females’ eggs. This also leads male flies to mate repeatedly with the same female, according to research published in Nature Communications (https://goo.gl/mUQSbP).

Over the past 50 years, biologists have realised that females in most animal species mate with multiple males during their lifetimes. However, they didn’t know how this behaviour influences how fruit flies and other species evolve.

Lead author Dr Juliano Morimoto of Macquarie University wanted to test the theory that increasing female promiscuity would reduce male competition before mating, while increasing their competition to fertilise the female’s eggs after mating. To do this, he and his collaborators first genetically manipulated female Drosophila melanogaster fruit flies to increase their promiscuity. By deleting a sex peptide receptor, they reduced the time the females weren’t sexually receptive after mating, which therefore led to them mating more frequently.

Hundreds of the more promiscuous females were marked with paint and their interactions with male flies monitored. The researchers then counted the thousands of offspring produced and identified their fathers based on eye colour.

“We found that when females mate promiscuously, male attractiveness is less important,” Morimoto says. “Instead, having a large ejaculate might be what males need to win the war.” This is the first time researchers have been able to show direct experimental proof of this theory.

The researchers also found that the male flies repeatedly mated with the same female in order to increase the chances of their sperm fertilising the female’s eggs. “This demonstrates that males adjust their sexual behaviour in response to the females’ promiscuity,” Morimoto says.

“Our findings help us understand a great deal about the remarkable diversity in mating behaviour and physiology seen in nature,” says senior author Dr Stuart Wigby of the University of Oxford. “Why in some species males show spectacular displays or fight to the death for access to females; while in other species males invest in making lots of sperm, or in pairing with one or a few females.”