Female birds' promiscuous behaviour breeds genetically better offspring that are also likely to do better in life, according to a new report.
The discovery adds fuel to the debate about why some female birds seek those extra mates in the first place.
"A diverse range of explanations has been proposed to account for female participation in extra-pair copulations," said Michael Magrath of the University of Groningen, The Netherlands.
"The explanations that have received most attention suggest that females stand to gain genetically superior offspring by having their eggs fertilised by males that are of higher genetic quality or that are genetically more compatible" said Magrath.
Perhaps the most convincing evidence for this idea comes from the numerous studies that have reported the superior performance of offspring sired by extra-pair males compared to their half-siblings sired by the social partner.
But new findings suggest that the superiority of such offspring might have little to do with their genes. It was found that eggs fertilised by males other than the mating partner tend to hatch earlier.
"Generally, earlier hatching chicks perform better than their later hatching siblings because they gain an initial size advantage, giving them the edge in competition for food during the nestling period," Magrath said.
{{/usCountry}}"Generally, earlier hatching chicks perform better than their later hatching siblings because they gain an initial size advantage, giving them the edge in competition for food during the nestling period," Magrath said.
{{/usCountry}}"After correcting for the effects of hatching time, we found that the differences between extra-pair offspring and their within-pair half-siblings were reduced or absent, indicating that non-genetic laying order effects largely accounted for the observed superiority of extra-pair offspring."
Magrath said they still don't know why there would be a connection between paternity and hatching order. But the result may nonetheless lend support to alternative explanations for the birds' promiscuous behaviour.
Plenty of ideas have been put forth. Females may choose to mate with extra-pair males to guard against the possibility that their social partner is infertile, said a Groningen release.
They might also engage in sex with other males primarily to avoid being harassed by them. In other words, it might simply be easier and less risky just to give in.
The study was published online in Thursday's edition of Current Biology.