In socially monogamous species, from birds to humans, most individuals find partners.
A large proportion of females, however, wind up with unattractive males of below-average quality, according to a new study that also found such less-than-ideal relationships raise female stress levels.
The findings negate prior theories that, in monogamous species throughout the animal kingdom, each female has a good chance of pairing with a male that matches her ideal choice of partner.
"In socially monogamous animals, very few individuals end up with the perfect partner because, of course, he or she is likely to be paired to someone else. That is, lots of men would like to be married to, say, Angelina Jolie, and lots of women would love to be married to Brad Pitt. But the reality is that they can't and only someone like Brad Pitt is able to marry someone like Angelina Jolie," lead author Simon Griffith told Discovery News.
"So how does a female respond to her real partner?" Griffith, an associate professor in Macquarie University's Department of Biological Sciences, asked.
"Work over the past few decades has shown that females can actually make a number of subtle strategies to improve their own fitness," he added, explaining that these include sleeping with other males that could improve the genetic fitness of any potential offspring.
To determine what might underlie such behavior, Griffith and colleagues Sarah Pryke and William Buttemer observed partnerships and mating in Gouldian finches.
In these birds, red- and black-headed individuals are partially genetically incompatible with each other. Red-headed Gouldian finches additionally are more aggressive than black-headed males and are not as good at providing parental care.
In one experiment, both types of birds were placed in an aviary where they had the freedom to select the partners of their choice. In a second forced-pairing experiment, 50 red females were individually paired with either a red or black male.
When females paired with males of the same head color, eggs were laid nearly a month earlier than those for mismatched couples. Blood tests determined that females matched with a different colored male had elevated levels of the stress hormone corticosterone that were three to four times higher than levels in females paired with preferred mates.
It's now thought that these hormones may help to drive everything from cheating to break-ups.
The findings, published in the latest issue of Proceedings of the Royal Society B, are believed to apply to humans as well.
"If a female is stressed by her partner's attractiveness, then it is quite possible that the speed of becoming pregnant and the number of children she has may vary as a result," Griffith explained. "In humans, we can't do these experiments to prove this, but it is completely plausible."
Attractiveness itself is likely then tied to genetic compatibility, which can include behavior factors in addition to those linked to physical appearance. A certain amount of genetic diversity is desirable, to avoid inbreeding, but too much risks outbreeding, according to the researchers.
So long as a couple is not too genetically incompatible, however, the best advice is to "invest all of your energies into working well as a team."
"We have also shown recently that individuals reproduce better with partners that they have been with for a long time," Griffith added.
Robert Brooks, a professor in the University of New South Wales School of Biological, Earth and Environmental Sciences, told Discovery News that the new findings "are robust and incredibly important."
"For too long we have looked at monogamous relationships as mostly happy cooperative ventures, but the authors have shown that females who are forced by circumstance into unsuitable pairings suffer ongoing stress," said Brooks. He hopes the study will inspire new ways of examining the evolution of stress within relationships.
While monogamy clearly has its drawbacks, one of nature's alternatives is polygynous mating, such as in peacocks, where females compete to mate with the same top male.
"A consequence of this is that most males can live a whole life without getting any copulations," Griffith said.