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
Contact: Jim Barlow, Life Sciences Editor
University of Illinois at Urbana-Champaign