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Why females with many mates can save a species

Promiscuous females could prevent their species becoming extinct, scientists have found.

According to research by the Universities of Exeter and Liverpool, females that have multiple mates reduce the risk of producing a brood of offspring without males.

The study, published in Current Biology, says an all-female brood could occur when all the 'male' Y chromosome sperm are killed before fertilisation, because of a sex-ratio distortion (SR) chromosome.

Scientists believe all-female broods will pass the chromosome on to their sons, which will in turn produce more female-only broods and eventually there will be no males and the population will die out.

Known as 'polyandry' among scientists, the phenomenon of females having multiple mates is shared across most animal species.

For this study, the scientists worked with the fruitfly Drosophila pseudoobscura.

They gave some populations the opportunity to mate naturally, meaning that the females had multiple partners. The others were restricted to having one mate each.

Over 15 generations, five of the 12 populations that had been monogamous became extinct as a result of males dying out. The SR chromosome was far less prevalent in the populations in which females had the opportunity to have multiple mates and none of these populations became extinct.

Having multiple mates can suppress the spread of the SR chromosome, making all-female broods a rarity, the researchers suggest.

Lead author Professor Nina Wedell, of the University of Exeter, said: "We were surprised by how quickly - within nine generations - a population could die out as a result of females only mating with one partner.

"This study is the first to suggest that it could actually save a population from extinction."