June 2, 1999 CHAMPAIGN, Ill. -- Birds that stay in one place year-round may reproduce seasonally like migrating birds, but they are one up when it comes to testosterone. In a Panama rainforest, spotted antbirds (Hylophylax n. naevioides) raise their testosterone levels when necessary in the "off-season" to boost their aggressiveness against invaders.
A team of researchers, led by Martin Wikelski of the University of Illinois, studied the neotropical antbirds, measuring testosterone levels by blood tests taken at different times over two years in the 10-acre natural habitat of a bird species that rarely migrates. The findings were detailed recently in the Proceedings of the Royal Society of London B.
It has been shown that in temperate-zone birds, testosterone in males rises in the spring as birds settle into new territories and seek mates. Afterward, testosterone levels drop off and remain low. Wikelski's team found that testosterone levels in tropical birds may remain at baseline year round. But when confronted by other birds in their territory - or even by a prolonged playback of tape-recorded sounds made by potential enemies - male antbirds increased their testosterone and became more aggressive, even in this sexually inactive period when their gonads remained entirely regressed.
"It makes sense for birds to maintain a baseline level of aggression without testosterone, because testosterone has costs, such as higher mortality rates," said Wikelski, a professor of ecology, ethology and evolution. "But when challenged, sometimes you need to boost your body so hard that it doesn't matter if you suffer. You'll risk everything to stay in your territory. These birds have a mechanism, which we don't entirely understand, to increase their testosterone at will for short periods of time."
Could the same mechanism exist in humans? Wikelski points to human studies in which testosterone levels rose among die-hard fans of winning basketball and soccer teams. Researchers also have been looking at the effect of testosterone on aggression and sexual desire. Another study involving birds found that male dark-eyed juncos -- a temperate, migrating species -- stray from their mates and care-giving duties when given large doses of the hormone.
"We want to find out how aggression is regulated and what factors influence aggressiveness," Wikelski said. "Birds are good models, because they are easy to observe, as opposed to mammals who display most of their aggression at night or out of sight. We need to understand aggression in natural systems to really understand how aggressiveness is regulated in humans, and what kinds of physiological parameters are important. We know from this study that social challenge plays a role."
Co-authors of the paper were U. of I. colleague Michaela Hau and John G. Wingfield of the University of Washington. Funding came from the National Science Foundation, Alexander Von Humboldt Society, Smithsonian Tropical Research Institute, Deutsche Forschungsgemeinschaft, Deutsche Ornitholgen-Gesellschaft and the Max Planck Gesellschaft.
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