Study fuels debate about why female birds seek extra mates

Apr 30, 2009

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, according to a new report published online on April 30th in Current Biology, a Cell Press publication. 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 have been proposed to account for female participation in extra-pair copulations," said Michael Magrath of University of Groningen, The Netherlands. "The explanations that have received most attention suggest that females stand to gain genetically superior by having their eggs fertilized by males that are of higher genetic quality or that are genetically more compatible."

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.

But the new findings in blue tits suggest 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 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 behavior.

And plenty of ideas have been put forth. Females may elect to mate with extra-pair males to guard against the possibility that their social partner is infertile. 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.

Although the new findings don't rule out any of the possibilities, the researchers said they are "perhaps most consistent with the fertility insurance hypothesis," which predicts that females should seek extra-pair copulations before laying starts so that all eggs could still be fertilized in the event of pair-male infertility.

"Because birds can store sperm for an extended period in specialized storage tubules, females may have little need to continue engaging in extra-pair copulation after laying starts, and this would rather neatly explain the decline in extra-pair offspring that we observed with laying order," Magrath said.

Alternatively, he added, if extra-pair young are indeed genetically superior, placing them early in the laying order may be the mother's strategy to promote their chances of survival through the most risky period of their life.

Source: Cell Press (news : web)

Explore further: Insect mating behavior has lessons for drones

Related Stories

Study reveals why starling females cheat

Jun 20, 2007

While women may cheat on men for personal reasons, superb starling females appear to stray from their mates for the sake of their chicks, according to recent Cornell research published in the Proceedings of the Royal Society of ...

Males have adapted to battle with competing sperm

Feb 09, 2007

In the context of sexual reproduction, natural selection is generally thought of as a pre-copulation mechanism. We are drawn to features of the human body that tell us our partner is healthy and will provide us a fighting ...

Females do best if they wait a while

Apr 05, 2007

Starting to breed late in life is a bad idea if you want to maximise the number of offspring that you produce - or so the theory goes. But doubt has now been cast on this hypothesis - one of the biggest assumptions ...

Females compensate for unattractive partners

Nov 10, 2008

(PhysOrg.com) -- Attractive males promise quality offspring. Most female birds therefore invest a lot of energy in their attempts to breed with attractive partners. Not so the female zebra finch. If they have ...

Seagulls: Are males the weaker sex?

May 07, 2008

Male seagulls may be more vulnerable to their environment during embryonic development than females, according to Maria Bogdanova and Ruedi Nager from the University of Glasgow in the UK. Until now, the sex differences in ...

Recommended for you

Insect mating behavior has lessons for drones

9 hours ago

Male moths locate females by navigating along the latter's pheromone (odor) plume, often flying hundreds of meters to do so. Two strategies are involved to accomplish this: males must find the outer envelope ...

Godwits are flexible... when they get the chance

May 29, 2015

Black-tailed godwits are able to cope with unpredictable weather. This was revealed by a thorough analysis of the extraordinary spring of 2013 by ecologist Nathan Senner of the University of Groningen and ...

Do you have the time? Flies sure do

May 28, 2015

Flies might be smarter than you think. According to research reported in the Cell Press journal Current Biology on May 28, fruit flies know what time of day it is. What's more, the insects can learn to con ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.