New research shows that mating with multiple partners brings benefits for females. In a study published this week in the journal BMC Evolutionary Biology, scientists show that promiscuous female guppies are more fertile than singly mated females.

For evolutionary biologists, it is obvious why male promiscuity has selective advantages: mating with several females gives males more chances to fertilize eggs and produce more viable descendants. However, female promiscuity, or polyandry (poly- many, andras- male), still stirs a debate in the scientific community, because it doesn't bring any apparent benefit for the females. On the contrary, multiple mating can come at a high cost. Besides consuming time and energy, multiple mating exposes females to predation, disease and physical harm from males. Female polyandry is nonetheless widespread in nature, and growing evidence shows that choosing to mate with several partners seems to be the rule, rather than the exception in a wide number of species, from invertebrates to birds, reptiles and even some mammals.

So why do females prefer to mate with multiple males? Scientists believe that polyandry might have indirect genetic benefits for the females, ensuring the 'good genes' pass on to the next generation. For instance, in some species the offspring of promiscuous females is better adapted, and hence produces more grandchildren for the female, than ofspring from single mated females. However, in the study performed by Anne Magurran's team at the University of St Andrews in the UK, the researchers found that, unexpectedly, multiple mating brings direct benefits for the females.

The scientists performed controlled experiments in the laboratory with wild caught guppies from the Lower Tacarigua River in Trinidad. They placed about 80 females in individual tanks and then allowed them to mate either with a single male, or with multiple males. They carefully followed these guppies for two generations, keeping count of the number of children and grandchildren they produced. They found that promiscuous females had more offspring, but there was no difference in their size, growth rate or viability, when compared to offspring of single mated females. Miguel Barbosa who led the study says 'The surprise came when the results showed that the benefits of multiple mating were achieved through an increase in female fecundity rather than by increasing offspring viability/attractiveness, as expected.'

Previous research showed a similar increase in fertility in promiscuous females of other species, but this is the first study where both direct and indirect benefits of multiple mating are investigated over two generations. But why is there an increase in fertility in multiply mated females? Barbosa explains 'The presence of sperm from multiple sources/fathers reduces the risk of genetic incompatibility, but also promotes sperm competition. Both can contribute to the increase in fecundity reported in our study.'

Another surprising finding in this study was that promiscuous female guppies had more sons, and scientists believe this accounts for the larger number of grandchildren. 'There was 60% more sons produced by multiple mated females than produced by single mated ones' says Barbosa. This is the first evidence that female multiple mating influences the offspring sex-ratio in guppies, but the scientists, however, still don't understand what causes this overproduction of males.

Tommaso Pizzari, an evolutionary biologist from the University of Oxford in the UK, says 'The present study offers experimental evidence suggesting that female promiscuity might be associated with some net fitness benefits to the female (...) These results contribute to shed light into a major evolutionary puzzle: namely, why do females mate with multiple males when often one insemination is sufficient for fertilization and mating is costly.'