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If you're a female and you're with an incompatible mate, beware: The stress could drive you to take drastic action.

Researchers from Macquarie University in Sydney report in this week's Proceedings of the Royal Society B that female Gouldian finches forced to pair up with poor quality partners delayed their breeding and were four times more stressed than their happier nest mates.

The study claims to be the first to measure how satisfied, or unsatisfied, females are with their partners in a monogamous species.

Dr Simon Griffiths and Dr Sarah Pryke wanted to find out what happens when female finches don't get to mate with their ideal partner.

Birds, like humans, have a monogamous system where high quality mates are quickly snapped up by females.
Best of a bad situation

"In our society there's been an assumption that you'll end up with the partner that's perfect for you. But if that was true, everyone would end up with Brad Pitt or Angelina Jolie," says co-author Dr Simon Griffiths.

Instead, birds, like humans, must make the best of the choice available to them.

The researchers wondered if this forced choice might be affecting the breeding success and stress levels of females.

To test this hypothesis, they deliberately paired females with 'incompatible' males.

Gouldian finches have two natural colour variations: a black or a red head. The two colour morphs don't interbreed very well, with sons and daughters of mixed pairs suffering 40 per cent and 80 per cent mortality respectively.

Consequently, a red headed female, for example, stuck with a black headed male would see him as a poor quality partner.

In the first experiment, equal numbers of male and female finches were released into an aviary. Gradually over the next several weeks they paired up. The researchers measured how long it took each pair to produce eggs, and also sampled the female's blood level of the stress hormone corticosterone.

In the second experiment, female finches were put individually in cages, with either a compatible or incompatible male.

After they had bred and the female had rested for a few weeks, a male of the other head colour was introduced. Researchers measured the time taken to egg laying and stress levels.
Stressful situation

"We found that the females with good partners started laying eggs straight away, while those with bad partners waited up to a month. That's one way we know they are not happy," says Griffiths.

"In their blood we found females with the wrong male were stressed within just 12 hours of him being introduced into the cage."

But the story doesn't end there. Griffiths says the high stress levels serve a practical purpose, propelling the female to take radical steps.

She may reduce the size and number of eggs and provide less care for the chicks, in the hope she'll get a better partner the following year.

She may even cheat on her partner with another bird more genetically compatible, so she'll have the best of both worlds. That is, the social benefit of a partner but not paying the costs of having the wrong genes.

"These results are telling us that female reproduction is potentially being adjusted in all kinds of ways by these stress hormones," says Griffiths.

"Infertility in humans for example could be related to these things - if you're stressed, you're less likely to become pregnant or ovulate."
Boredom and infidelity

Professor Rob Brooks who is Director of the Evolution and Ecology Research Centre at the University of New South Wales says it is well known in humans that making a bad match is a recipe for unhappiness and a good predictor for divorce.

"Some folks think (finding a partner is) this happy business, where we gravitate to someone of a similar quality and everything is hunky dory, but this paper shows that it's not," says Brooks.

"It goes against some of the current wisdom about relationships and fidelity, where infidelity is often attributed to people being bored and looking for a bit of excitement.

"But I don't think people necessarily fall into extramarital relationships out of boredom. This paper suggests they gravitate that way because of an underlying stress and that's certainly an alternative."

Brooks says the study could lead to a similar study conducted on humans.

"I think they'll try and measure corticosteroid levels in married people (along with using questionnaires) to understand the nature of those relationships," he says.

"You could look for tacit predictors early in the marriage and then measure stress levels to look at how that predicts things, like the breakdown of marriages or infidelity."

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