Testosterone turns male juncos into blustery hunks -- an...( BLOOMINGTON Ind. -- The ability to r...)

BLOOMINGTON, Ind. -- The ability to ramp up testosterone production appears to drive male dark-eyed juncos to find and win mates, but it comes with an evolutionary cost. Big fluctuations in testosterone may also cause males to lose interest in parenting their own young, scientists have learned.

In the December issue of The American Naturalist (now online), Indiana University Bloomington, University of Virginia and University of Southern Mississippi researchers report the results of the first study to examine, in the wild, the way in which natural changes in testosterone levels determine how a male spends his time.

It's a new take on the subject. IUB biologist Ellen Ketterson and other researchers had thought it might be the total amount of testosterone in a male bird that determines his tendency toward aggression and monogamy. The latest findings suggest it's a bit more complicated. It's how much and how quickly his testosterone levels can rise and fall that determines whether he's the kind to stick around and feed his young. Males whose testosterone levels were more stable were more likely to invest more time and energy in parenting.

"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 Ketterson, senior author of the study. "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."

Ketterson, lead author Joel McGlothlin (University of Virginia) and Jodie Jawor (University of Southern Mississippi) see the male Junco's plight as a classic trade-off in evolutionary biology: males have a certain amount of energy and ti
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