The answer could be that they have more offspring and that those young were better at escaping predators.
But the researchers behind the study say the explanation probably does not apply to mammals, and certainly not to humans.
Biological puzzle
Biologists have already come up with an explanation for why some male animals are promiscuous - they sire more offspring, preserving the diversity of the gene pool.
But the reasons for females exhibiting the same behaviour were not clear, particularly as they might risk being eaten by predators.
"The benefits of promiscuity for males are very clear," Dr Anne Magurran, of the school of Environmental and Evolutionary Biology at St Andrews University told BBC News Online.
"But the advantages for females are less well demonstrated."
The team studied guppies caught in the wild in Trinidad.
'More and fitter'
They found females that had mated with several males gave birth to more and fitter offspring compared with those that had mated with a single partner.
The young were also born sooner and were better at escaping predators, giving them an evolutionary advantage.
She said this explanation for female promiscuity could extend to other members of the animal kingdom, such as birds and reptiles.
But it probably did not apply to mammals, she said.
"Clearly it doesn't apply to humans because humans don't have multiple broods," added Dr Magurran.
The work is published in Monday's edition of the Proceedings of the National Academy of Sciences of the United States of America.