Whether they are finding love in a flock or a lab, female Gouldian finches (Erythrura gouldiae) know what they're looking for: a fit male with head feathers that match their own. And the females that don't end up with a desirable mate are slower to lay eggs and wind up more physiologically stressed, according to new research.

Animal species that bond in monogamous pairs—whether for life or just for a season—are often assumed to have found a perfectly satisfactory mate for their tastes. But, such a presumption is likely "naive," contend researchers behind the new finch work. "This logic would be akin to assuming that only women as attractive as Angelina Jolie find a man such as Brad Pitt attractive," they noted in their paper published online February 1 in Proceedings of the Royal Society B: Biological Sciences. So just as it is in the human world that "only one high-quality woman is able to socially pair with Brad Pitt" (at least as of late), pair-bonded animals also likely often have to settle for a less-than-ideal mate, the researchers point out in the study.

As a species that usually lives for less than three years and—breeds only once or twice a year—female Gouldian finches have a keen interest in picking the best mate each season. To do this, they rely in large part on the shorthand signal of a colorful male's head feathers (which are either red or black) to indicate the likely fitness of their prospective offspring. Offspring from mixed pairs have a 40 to 80 percent higher mortality rate, so males with a matching head color are most attractive. In the wild, these two color types often coexist in uneven numbers, leaving some females to "face the choice of breeding with an incompatible partner or not breeding at all," the researchers, led by Simon Griffith, of Macquarie University's biology department, noted in their study.

To assess the impact of this decision, Griffith and his team set out to try to measure "the extent of female 'satisfaction' with her social partner." To do this, they monitored several waves of pairing couples in a large aviary (with 20 to 28 birds per group) and in 100 more pairings in controlled single-pair lab cages. In both situations, females who ended up with a mate with a different head color were slower to lay eggs (by 20 days to a month) and had three- to four-times higher levels of the animal stress hormone corticosterone. And especially for females in the forced-pair cage experiments, the stress of being faced with an unattractive mate set in quickly, leading to "significantly higher conticosterone" within 12 hours of pairing. The speed of this stress response, the researchers suggest, "was not driven by indirect effects of male behavior on the female, but rather by her initial perception of him."

The similarity of response in an open-selection aviary versus a pre-selected single-pair cage (where females were each paired once with a compatible and incompatible male) suggests that the female's reaction has less to do with her option to choose than with the mate she ends up with.

So although the birds might not be consciously weighing their options and rationalizing their decision (or lot), the researchers suggest the finches were revealing some level of "internal conflict," they note. "These females are making the best of a bad situation and are dissatisfied with their partner although he does represent a better option than not breeding at all."

But these vexed female finches are not given entirely to avifaunal despair. In fact, the authors posit, the physiological stress response might be a way of prompting them to other adaptive behaviors, such as investing less in that season's offspring or looking for love outside of the nest.

Image of red- and black-headed male Gouldian finches courtesy of Wikimedia Commons/Nigel Jacques