A new study on monogamous animals has shown that most pairings are less than ideal, leading to stressful relationships, according to Discovery News.

The findings raise serious doubts about previous theories that monogamous pairs stay monogamous because they form ideal matches. Instead, it appears that the majority of monogamous pairs stay together because they prefer to settle for less rather than settle for nothing.

One of the advantages of social monogamy is that it allows most individuals in a species to find a mate. By comparison, among many polygamous animals, such as peacocks, females compete fiercely to mate with one top male, which leaves the unattractive individuals out of luck. 

But allowing most individuals in a species to mate comes with the tradeoff that a large proportion of those matings are between mutually unattractive partners.

"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," Simon Griffith, lead author of the study, told Discovery News.

Griffith's study, which was performed with the help of colleagues Sarah Pryke and William Buttemer, focused on partnerships and mating among monogamous Gouldian finches. The finches come in two varieties, red-headed and black-headed. Although the two varieties are partially genetically incompatible with one another, they do form monogamous pairs with individuals of the other color when their options are limited.

Furthermore, while both the red- and black-headed finches have their own redeeming qualities, the red-headed ones find redheads more attractive and the black-headed ones prefer the dark and mysterious look.

The study found that when finches with different colors had to settle for one another because of forced circumstance, eggs were laid more reluctantly and females had elevated levels of the stress hormone corticosterone — levels three to four times higher than that of females paired with preferred mates.

In other words, females with unattractive partners not only settled for less, but they also experienced more stressful, unhappy relationships. Corticosterone has also been linked to increased cheating behavior.

Though this particular study was restricted to finches, researchers said they believe the findings likely apply to most species that practice monogamy, including humans.

"In humans, we can't [ethically] do these experiments to prove this, but it is completely plausible," said Griffith.

Robert Brooks, a professor in the University of New South Wales School of Biological, Earth and Environmental Sciences, said the 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," he added.