Study co-author Kevin Oh measures a finch on the UA campus, where biologists are continuing to investigate mating patterns.

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Some females rush for the flashy bachelors. Other ladies are choosy, finding that opposites attract in late-season romances.

Such is mating in the world of finches.

A new study by University of Arizona biologists points to underlying complexities in picking a partner that allow competing evolutionary processes to play out in a single finch population. Based on 10 years of detailed research into the behavior of wild finches in Montana, the scientists unexpectedly found a paradox at work.

"We've known awhile that in many species, females prefer the most extravagant and flashy males," said Kevin Oh, a doctoral student in ecology and evolutionary biology. "We've also known that when individuals choose mates that are highly related to them, it's often a bad thing, leading to offspring of poor quality."

By examining seasonal changes in the finch population — and corresponding genetic diversity — the scientists are able to explain how two apparently incongruous processes are both at play in the mating season. Early in the mating season, the colorful flashy males get picked up first. Later in the season, individuals tend to choose partners that are the most genetically dissimilar.

"What we find is when you take into account seasonal changes in the population, it creates a scenario where there are two rounds of mate choice," Oh said. "A female that is choosing early has a different pool of available mates to choose from."

Since 1995, assistant professor Alexander V. Badyaev has tracked a population of house finches near Missoula, Mont. During the breeding and nesting season, Badyaev and his researchers capture the birds to photograph, measure, take DNA samples and finally band and release them.

Over 10 years, the team has examined about 12,000 finches, tracking their hormonal status throughout mating season. Most finches breed just once or twice in a lifetime, and offspring have the best survival rates when their parents were more genetically dissimilar.

"What's unique about this study is because we have such detailed information on the population, we could say on any given date who was available to any given female and how that affected how she chose," Oh said.

Lab studies in mate choice were missing a substantial part of the equation because the pool of available mates, which is ever-changing in the wild, can have a large bearing on which male any individual female chooses.

"Females are much more choosy in mate choice and pay much more attention to male quality," Badyaev said. "Males try to mate with just about any available female, while females select first the most ornamented male, second the most unfamiliar."

The researchers recorded how bright the males' feathers are in relation to one another using the photographs and a biochemical analysis. They have also genotyped 10 successive generations of the birds and tracked females' ovulation cycles.

"One of the major implications is that it shows us how these two processes, both preference for elaborate mates and preference for genetically complementary mates, can both occur in a population," Oh said. "It highlights the importance of taking into account the environment in which the animals are behaving."

The researchers will publish their paper in an upcoming issue of the Proceedings of the Royal Society B. The research was funded with a National Science Foundation grant.

Oh and Badyaev are now working on a similarly detailed study of finches on and near the UA campus to determine whether the same factors work in finch populations in a different environment.

Beyond the broad implications for evolutionary biology, the study of finch mating just might have some lessons for humans.

"Another implication for humans is the hope that there is someone for everyone," Badyaev said.