Why female birds seek extra mates

Washington, May 01 (ANI): A new study has added fuel to the debate about why female birds seek extra mates.

According to the study, when female birds mate with males other than their social partners and have broods of mixed paternity, the offspring sired by these 'extra-pair' fathers may often get a head start in life.

"A diverse range of explanations have been proposed to account for female participation in extra-pair copulations. The explanations that have received most attention suggest that females stand to gain genetically superior offspring by having their eggs fertilized by males that are of higher genetic quality or that are genetically more compatible." said Michael Magrath of University of Groningen, The Netherlands

Perhaps the most convincing evidence for this idea comes from the numerous studies reporting the superior performance of offspring sired by extra-pair males over their half-siblings sired by the social partner.

However, the new study in blue tits suggests that the superiority of extra-pair offspring might have little to do with their genes.

They found that eggs fertilized by males other than the mating partner tend to be laid and to hatch earlier. Indeed, they report, nearly 75 percent of extra-pair offspring were produced in the first half of the clutch.

"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.

"After correcting for the effects of hatching time, we found that the differences between extra-pair offspring and their within-pair half-sibs were reduced or absent, indicating that non-genetic laying order effects largely accounted for the observed superiority of extra-pair offspring," Magrath added.

Magrath said they still don't know why there would be a connection between paternity and hatching order.

However, the result may nonetheless lend support to alternative explanations for the birds' promiscuous behaviour.

The study has been published online on April 30th in Current Biology, a Cell Press publication. (ANI)