Image caption Flour beetles are a major pest in grain stores worldwide

Scientists have shown that inbreeding can lead to female promiscuity.

The team, which reports its findings in the journal Science, witnessed a change in mating behaviour when they bred female flour beetles with their close relatives.

The researchers suspect that promiscuous females are avoiding the ill-effects of inbreeding by exposing themselves to a larger pool of sperm.

The results help explain why females of some species mate with several males.

For females, sex can be traumatic. In some insects, insemination involves wounding females and infecting them with dangerous microbes. In many other species, mating reduces females' lifespans.

Given that in most species, a single mating is generally enough to fertilise all a female's eggs, she has little incentive to mate again. And yet in many species, females mate multiple times with different males.

The new results help evolutionary biologists explain this perplexing phenomenon.

Driving extinction

By driving a population of flour beetles, Tribolium castaneum, to the brink of extinction, and then allowing their numbers to recover from a few individuals, the researchers at the University of East Anglia (UEA) created highly inbred populations.

The researchers found that females in these inbred populations were more eager to mate than those that had not been forced through a bottleneck.

Attempting to explain why unbridled mating was on the increase, the researchers went on to show that inbred females left twice as many descendents as those that mated with just one male.

Image caption Highly magnified Tribolium sperm fight it out within the female reproductive tract

"It is quite easy to imagine how promiscuity could spread through the population if [promiscuous females] leave more descendents," explained UEA evolutionary biologist Matthew Gage.

When a population is inbred, the chances of mating with a genetically similar male are heightened, so hedging your bets and mating with more suitors is a sensible strategy, he explained.

Dr Gage suspects that promiscuous females amass a large pool of sperm, and select ones that are more genetically dissimilar to them to fertilise their eggs. Mating with more males gives females a larger range of sperm from which to select.

However, Dr Gage warned that he and his colleagues might not have witnessed "the evolution" of a new mating behaviour.

Rather than changing genetically, he explained, the females might simply have been adjusting their behaviour to their new environmental conditions.

Dr Gage and his team are now looking into this.