Of the 21 species of macaque monkeys, pigtailed macaques live in societies that fall somewhere between the despotic and egalitarian extremes. A dominant male and female run the show but conflict among other monkeys is common, though rarely extreme. Most such conflicts end with a third party intervening, usually in favor of one opponent. But sometimes the most powerful monkeys literally stand between the two combatants and, occasionally without even threatening them, impartially resolve the conflict. It is this occasional policing that allows pigtailed macaque society to be more diverse instead of breaking down into warring cliques, according to new research.

Jessica Flack and her colleagues at the Yerkes National Primate Research Center studied a group of 84 pigtailed macaques both under normal conditions and in the absence of the three high-powered males, including the leader of the troop, who did most of the policing. The researchers recorded the amount of grooming, play, sitting while touching and sitting close to one another under each of these conditions. Without the police, the remaining monkeys predominantly interacted with their most familiar fellows, forming small groups and sitting apart from each other in order to avoid conflicts.

"Our study is the first to show that these interventions not only control conflict, despite being infrequent, but also have a systemic effect on social organization by changing how individuals build their social networks," Flack says. "For example, we find that in the presence of policing, individuals have more grooming partners."

The researchers only removed the dominant monkeys for short periods of time, because otherwise dominance struggles might have broken out, Flack explains. They also were not able to remove the dominant female--who also plays a policing role from time to time--because to do so would have entirely disrupted the monkeys' social structure. No other monkeys stepped up to fill the policing void, and, in another variation, the removal of a lower ranking monkey had no effect on the overall social network, according to the findings published today in Nature.

Large primate social networks have been shown by previous research to cut down on infant mortality, among other things, which means that conflict management such as policing is crucial not just for reducing fighting but also in promoting species survival. This kind of interaction between individuals and their social milieu provides a more accurate picture of how monkeys' physical characteristics and behavior evolve in tandem with the overall environment, Flack argues. "Phenotypes do not evolve to fit a static environment," she says. "Both the environment and the phenotype dynamically co-construct one another."