Drosophila pseudoobscura, the flies used in the experiments, copulating. Courtesy: Science /AAAS

New Delhi, Nov. 21: A study of fruit flies bred in the laboratory has helped scientists solve a long-standing riddle of biology: why do females across the animal kingdom prefer multiple partners?

Scientists have long observed polyandry in species across the animal kingdom — from crickets to birds, from toads to chimpanzees. But it has remained a puzzle because it does not appear to confer any reproductive advantage to females.

Now, a team of British and Japanese scientists has shown that by seeking out multiple partners, females may be, unconsciously, avoiding deleterious genes carried by males. The findings appear in today’s issue of the journal Science.

“Multiple mating has puzzled biologists for decades. It’s more risky and costs precious time and energy for females,” said Nina Wedell, an associate professor of biosciences at the University of Exeter in Britain. In experiments on flies, Wedell and her colleagues in Liverpool and Japan have shown that females evolve higher mating rates — or increasingly seek more partners — when they are exposed to the risk of mating with males carrying such deleterious genes.

The researchers bred flies for 10 generations. Some flies carried a genetic sequence on the X-chromosome that causes sperm with Y — the male sex chromosome — to fail. So males carrying this sequence can produce only daughters, also carrying the bad gene.

Their study showed that female flies exposed to such males with a deleterious gene sequence began to mate faster over several generations — after an average gap of 2.75 days, compared with 3.25 days in the original population.

“Our study suggests the costs (of polyandry) are worthwhile because the female increases her chances of producing healthy offspring of both sexes that do not carry the selfish (deleterious) gene,” Wedell said.

“This is a novel explanation for polyandry that has been a mystery in evolutionary biology,” said Kavita Isvaran, a behavioural ecologist at the Indian Institute of Science, Bangalore. “The benefits of multiple mating by males has been obvious — it increases the number of offspring.”

But Isvaran cautioned it might be “dangerous” to directly interpret these ideas to humans. A number of social and cultural factors may also influence the reproductive behaviour of humans.