Watch them fight. Seeing two other males square off hiked testosterone levels in male tilapia fish.

Almost Like Fighting Yourself

Ever wonder why some sports fans get so riled up? Now a study of fish provides the first solid evidence that simply watching an aggressive interaction can rapidly trigger a surge in hormone levels. Researchers believe the boost may prepare the animals in case they become involved in a fight of their own.

Countless episodes of Wild Kingdom demonstrate that males--from rutting deer to bickering baboons--will fight. The levels of testosterone, a hormone tied to aggression, should go up whenever males' status is threatened, according to an idea called the "challenge hypothesis." Indeed, testosterone levels shoot up in territorial male birds that both see and hear a challenger, and the elevated level seems to keep them aggressive. But it's not clear what effect a conflict would have on bystanders.

To find out, Rui Oliveira, a behavioral endocrinologist at the Instituto Superior de Psicologia Aplicada in Lisbon, Portugal, and his colleagues turned to a small fish, the Mozambique tilapia (Oreochromis mossambicus). In the wild, tilapia males congregate at special mating sites in rivers. Each male defends a small part of the sandy bottom from his competition and attempts to lure a female.

In the lab, the scientists let bystander fish watch through a one-way mirror as pairs of other males had hour-long fights. At several intervals after the fight ended, the researchers gently squeezed the bystanders to collect their urine. It contained elevated levels of the main fish testosterone, 11-ketotestosterone, as long as 6 hours after the fight ended, the team reports in the 25 January issue of Nature. In the wild, such pumped up hormone levels most likely prepare bystanders for fighting should one of the combatants come their way, Oliveira says. "The probability of getting [attacked by] an intruder increases when neighbors are fighting."

"I like this experiment," says John Wingfield, an environmental endocrinologist at the University of Washington in Seattle who developed the challenge hypothesis. "They look at the very rapid effect of suddenly being confronted with an aggressive interaction. So far as I know, no one has ever done that in such a controlled way."

Related sites

More about the Mozambique tilapia (Oreochromis mossambicus)

The American Tilapia Association