After winning a fight, don't be surprised if you feel the desire to pick another one.

Research in mice and other animals clearly shows that winning a competition increases their motivation to engage in future disputes.

This urge to seek out additional altercations, which researchers refer to as the "winner effect," may not only enhance an animal's desire to fight, but increase their chances of winning future brawls, said Sabrina Burmeister, a biology professor at the University of North Carolina. "However, we understand very little about what brain regions or genes mediate this desire to win," Burmeister noted.

Now, researchers at the University of Wisconsin at Madison have shown that, for mice, winning fights may rewire circuitry in the brain's reward centers, prompting the desire for additional bouts.

Lead investigators Matthew Fuxjager and Catherine Marler discovered this by having sexually experienced and territorial male mice battle against smaller, sexually inexperienced opponents. They also had the fight occur in either the bigger mouse's home cage or in an unfamiliar cage.

After the more formidable mice had won three consecutive matches the researchers compared their brains to those of mice not set up for fights.

Compared to their opponents, the mice that won fights -- either at home or away -- exhibited significantly more activity in brain receptors that bind testosterone. These receptors, called androgen receptors, are located in regions of the brain that modulate social aggression, motivation and reward.

The mice that engaged in territorial disputes were also much more sensitive to testosterone.

"These results are very interesting," said Burmeister, who was not involved in the study. "They give us a sense of the brain circuitry involved in social aggression."

The neurological changes lasted several days after the first three fights had ended, suggesting that winning may cause long-term changes in the brain and may motivate the mice to seek out additional contests, said Fuxjager.

Perhaps the most notable finding, according to the researchers, was that the fight's location mattered. Although mice who won at home and away showed similar levels of aggression, only those who won at home exhibited more activity in two brain regions that govern motivation and reward-the nucleus accumbens and ventral tegmental area- possibly bolstering their confidence and excitement after a win.

"It is not entirely clear why winning experiences in familiar and unfamiliar cages should induce different changes in the brain," said Yuying Hsu, an associate professor at the National Taiwan Normal University in Taiwan who was not involved in the study.

Fuxjager suggested that the ability to successfully guard one's home territory might be at the root of these additional neural changes. "Home field advantage may play a role in how good we feel after a win."

In addition, Burmeister pointed out that androgen receptors don't only regulate testosterone, but they also regulate other genes. "Androgen receptors can alter how active certain genes are or whether they are expressed at all," she said. "The fact that androgen receptors influence other genes could explain the long-lasting effects of winning."

Although both Burmeister and Fuxjager were cautious about extrapolating their findings to people, "I would not be surprised at all if similar neurological changes do occur in humans," Burmeister said.

Fuxjager and his colleagues are now exploring other factors that may regulate social aggression, including estrogen.

The results of this study were printed online July 6 in the journal Proceedings of the National Academy of Sciences.