Monkeys defy crowding-aggression link - rhesus monkeys react to crowding with coping behaviors

Science News, July 9, 1994 by Bruce Bower

For more than 30 years, an influential theory has held that crowding brings out beastly behavior in people and many other animals. But the largest primate study of crowding to date finds that rhesus monkeys maintain a remarkably stable level of aggression across a spectrum of population densities, from confinement in small pens to open living on a small island.

Although generally considered the most fight-prone of monkeys, rhesus individuals reacted to crowded conditions with a surge of "coping" behaviors that kept aggression in check, assert psychologist Peter G. Judge and ethologist Frans B.M. de Waal, both of the Yerkes Regional Primate Research Center in Atlanta. Coping responses included grooming each other, making submissive gestures, avoiding dominant animals, and huddling with relatives after a dispute rather than starting a fight.

"Rhesus are the primate species we would least expect to display effective coping responses under high population density," says de Waal. "The human capacity to adapt to crowding has evolutionary parallels in other primates."

The new findings, submitted for publication, contrast with the tendency of overcrowded mice and rats to attack one another and rapidly die out (SN: 5/31/86, p.346). In close quarters, rodents may rage, but primates try to keep their cool, within the limits of species-typical behavior, Judge and de Waal argue.

The researchers studied nine social groups, containing 413 monkeys, that had lived for at least several years in small indoor pens, in medium-size in-door-outdoor cages, in large outdoor corrals, or on a small island off the coast of South Carolina. Detailed observations of interactions with other group members were gathered for 145 adult males and females in the nine groups.

The frequency of aggressive acts rose slightly in more congested quarters; openly hostile behavior occurred 1.5 times as often in indoor pens as on the island. Yet that disparity was quite small considering that island monkeys had 6,000 times more available space than those restricted to pens, Judge says.

In contrast, crowded conditions brought out large increases in coping behaviors. For instance, monkeys took steps to defuse potential fights more than twice as often in indoor pens as in any of the other living situations.

In previous research, the Yerkes scientists found that when rhesus monkeys are moved suddenly to a much smaller living area, they tend to huddle together with relatives and stay still. The researchers compare this to the tendency of people to avoid eye contact or talking on a crowded subway or elevator.

Rhesus monkeys housed with more conciliatory, stub-tailed monkeys also show boosts in coping behaviors, de Waal says.

This category of responses, as well as social "pecking orders" that influence access to food and other resources, may have evolved to rein in aggression and violence in primate groups, he contends.

In a related study, psychologists find that the willingness of city dwellers to help strangers in a variety of ways increases as population density (the number of people per square mile) drops. Population size shows a much weaker link to helpful responses, they contend.

High population density may contribute to sensory overload and a tendency to avoid others' requests, suggest Robert V. Levine of California State University, Fresno, and his coworkers. Larger groups of bystanders may also lessen the sense of individual responsibility toward strangers, they argue.

Experimenters presented six "helping scenarios" to pedestrians in 36 U.S. cities. The situations included asking for change for a quarter and pretending to be unable to pick up a pile of magazines because of an injured leg.

Greater helping tended to occur in cities with lower violent crime rates, the scientists report in the July JOURNAL OF PERSONALITY AND SOCIAL PSYCHOLOGY. Wide-spread violence may lead to avoidance of direct confrontations with strangers, they contend. Or, as Judge and de Waal might put it, human coping behavior includes avoiding strangers in threatening situations.