Oxytocin is the hormone of sex, connection and love. It combines with dopamine in our brains to make those kinds of connections feel good -- and to reinforce the idea that it's one specific partner who makes us feel so good.
Of course, sex and love are only a couple of life's many pleasures. Well, it looks like aggression feels just as good as connection.
Maria Coupis of Vanderbilt University designed an ingenious experiment that showed that male mice would, if they could, choose to engage in aggressive interactions with others.
She housed a male and female mouse together, and then introduced another male mouse, the intruder. The resident male would attempt to drive him away with tail rattling, boxing and biting. Next, she trained the house males to poke at a target with their noses to get the intruders to return. She made the target available once a day and the males consistently poked at it. This shows that they experienced the opportunity to display aggression as a reward. They "wanted" to fight.
When Coupis suppressed activity of the dopamine receptors in the males' brains, their interest in poking the target and mixing it up with other males decreased. (She also proved that this wasn't due to general lethargy as a result of suppressed dopamine.)
According to the news article:
“We learned from these experiments that an individual will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it,” [Craig Kennedy, professor of special education and pediatrics and Coupis' graduate advisor] said. “This shows for the first time that aggression, on its own, is motivating, and that the well-known positive reinforcer dopamine plays a critical role.”
It would be interesting to see whether female rats also enjoyed aggression. Female mammals tend to be more receptive to the effects of oxytocin, due to their higher estrogen and lower testosterone levels. It also would be interesting to see whether the males of a monogamous species were as enthusiastic about aggression.
In humans, mating seems to lower a man's testosterone a bit -- another example of the civilizing effects of marriage.