Primates are social animals. But why did they become social, and what are the causes for the differences in social structure among various primate species?
Robert W. Sussman, Ph.D., professor of anthropology in Arts & Sciences, addressed those questions and more in his talk, “A Comparative Overview of Primate Social Organization,” during the 2009 annual meeting of the American Association for the Advancement of Science (AAAS) Feb. 15 in Chicago.
A commonly held view is that primates are social because it protects them from predation or from infanticide within the species. Because of these pressures, they are forced to be social, but, due to competition for food resources, they must be competitive and aggressive as well.
“Many theories about the evolution of primate sociality and social behavior are related to this negative idea that primates must be aggressive because they are forced to be social,” Sussman said. “The evidence, however, does not support this theory.”
Sussman has found through his examination of primates that while the animals are very close as a group, active social interaction takes up a mere 5 percent to 10 percent of their average day.
“So 90 percent or more of primate behavior is maintenance behavior in a social context,” Sussman said. “In examining what percentage of that social behavior is actually antagonistic, we find it’s actually very little. In fact, in some species it can be as infrequent as once every 175 days.
“Obviously a major aggressive incident in which an individual is injured or dies is a factor that affects that individual, but how that affects the evolution of a species is questionable,” he said.
Another theory on the evolution of primate social behavior, the ecological constraints model, suggests that as group size increases, so do competition and fighting within the group. The theory purports a direct correlation between the number of animals in the group and the energy efficiency of those animals.
Sussman questions how this model can explain cooperative social behavior.
“Sociobiologists would explain it in three ways,” he said. “The first is kin selection. Animals aren’t really altruistic, they’re just social so they can pass along their genes. The second is reciprocal altruism. Animals only help each other if they know they’ll be helped in the future. The third is social reconciliation. Because animals are forced to compete, in order to live in social groups, they must also reconcile with each other.”
In each of these views, Sussman said, the animal is forced to live socially. It’s not a choice.
“That’s a terrible way to think of the world,” he said.
He proposes an alternative theory. “None of the concepts in the previous example are actually seen in nature,” Sussman said. “There is more altruism and cooperation in humans and primates than there is any kind of aggression.”
He said animals — and humans, for that matter — benefit from being social. And if that’s true, he argued, there should be evolutionary evidence to back it up. And there is.
“There are two areas of the primate and human brain that are stimulated when we cooperate. We’ve evolved to get pleasure from cooperation,” he said.
The hormones serotonin and oxytocin also play a role in social recognition and trust.
Sussman agrees that predatory pressure is one of the reasons for group living to be evolutionarily advantageous.
But, he said, “through evolutionary time, certain animals who benefit more by living in groups have developed — through natural selection — natural biological mechanisms that enhance sociality.”
“Today, it is possible for animals to be social and even altruistic without the pressures of kin selection and others,” Sussman said. “Cooperation is a biological mechanism.”