Good husbands cannot be that masculine. A mixed team from Indiana University Bloomington, University of Virginia and University of Southern Mississippi investigating a natural population of Carolina dark-eyed juncos (Junco hyemalis carolinensis) found that natural fluctuations in testosterone levels are linked to a male's couple behavior.

The capacity to produce large amounts of testosterone turns male dark-eyed juncos more aggressive; they mate with more females, but with an evolutionary trade-off: they lose interest in parenting their own offspring. The amount and speed of testosterone fluctuations dictated male's affinity toward monogamy. Males with steadier testosterone levels were more prone to invest more time and energy making a "good husband".

"This study is one of the first to show for a songbird living in the field under natural conditions that individual variation in the hormone testosterone maps onto variation in aggression and parental behavior," said senior author Ellen Ketterson, IUB biologist.

"Our data also suggest that there is more than one way to be successful at reproduction. Some males may seek mates at the expense of parental behavior, but other males are doing the opposite. They are being more parental at the expense of aggression. And apparently both ways of being in the world work."

This is a classic evolutionary trade-off: males posses a limited amount of energy and time they can invest in getting mates and parenting. Sometimes it may be beneficial for males to be polygamous jerks, increasing the number of partners during a mating season, but in other cases monogamy is the best solution.

"The results are exciting because they show us how animals that make different choices might differ from each other on a physiological level. On an evolutionary level, they suggest that there may often be more than one right choice, depending on the circumstances," said lead author Joel McGlothlin (University of Virginia).

"It is surely more complicated than that. The link between testosterone and aggressive and sexual behavior is probably more direct than the link between testosterone and parental behavior. The latter needs much more study." said Ketterson.

Researchers found a continuum in the behavior of male dark-eyed juncos: acting like Casanovas or being good husbands.

"One of the interesting things is that all males stick around and help. If they have higher testosterone they help less. If they have lower testosterone they help more." said Ketterson.

The question is: why do the two types of behavior coexist?

"Variation in behavior may persist because the environment varies. In cold, wet years, or years when lots of predators are attacking young in the nest, good fathers may be more successful at leaving offspring. When food is plentiful and predators are few, males that focus on mating may be more successful. The balance between the two is probably dynamic." said Ketterson.

"Another possibility is that male "quality" is why males divide up their time differently. There are only so many eggs out there to fertilize, so every male can't be successful getting extra-pair fertilizations," said McGlothlin.

"High-quality males -- those who have more energy to invest -- ought to spend more of it on trying to attract mates. These males may be less likely to survive, but they are more likely to be successful at getting extra-pair fertilizations. The low-quality males don't have as much energy to invest, so they play it safe."

Besides observing birds' behavior, the team assessed circulating testosterone levels, then injected the males with gonadotropin-releasing hormone (GnRH) to see how much testosterone each male produced in response. Males able to produce more testosterone when injected not only showed more aggressive behavior, but were also found to invest less time parenting.

"The next essential step is to relate variation in testosterone to actual measures of fitness, namely lifespan and offspring actually produced. Is it true that individuals who resolve the trade-off in different ways have equal fitness. Can a good parent be just as successful in an evolutionary sense as a good mater?", said Ketterson.