From the February 2004 Anthropology News

Mechanisms for Maintaining Peace in Primate Societies

New Perspectives on Primate Sociality

KAREN B STRIER
U WISCONSIN-MADISON

ROBERT W SUSSMAN
WASHINGTON U

Most nonhuman primates are highly social, and many are members of groups throughout all or nearly all of their long lives. Such continuous group living imposes inevitable social challenges, which have typically been viewed as trade-offs between the conflicts that arise from competition for resources such as food and mates, and the benefits that come from cooperating with familiar allies against common opponents.

Investigations into the evolution of primate sociality have tended to focus on aggression and the processes by which primates reconcile their conflicts with one another to retain their allies’ support, emphasizing competition instead of cooperation. In an Invited Session on “Mechanisms for Maintaining Peace in Primate Societies” at the AAA Annual Meeting this past November, six experts presented results from behavioral, hormonal, and brain imaging studies that offer new perspectives about primates and their proclivities for peace.

Hard-Wired to Help
The evolution of cooperation has been interpreted as a product of the mutual benefits or opportunities for future reciprocity that participants gain. Yet, as James Rilling (anthropology, Emory) explained, there is also evidence that the basic principals of reciprocity may be embedded in our psyches. Rilling described functional MRI (fMRI) studies of brain activation in 36 women playing a simulation of the iterated Prisoner’s Dilemma, in which participants have the option of behaving selfishly or altruistically and the outcome of the interaction depends upon the choices both participants make. Mutual cooperation requires that both participants opt for altruism, and when this happens, the fMRI data demonstrate activity in the reward centers of the brain. Even more compelling, the strength of the neural response increases with the persistence of mutual cooperation over successive trials. This activation of our brains’ reward centers may account for why we tend to feel good when we cooperate, and provides a neural mechanism for understanding the positive reinforcement that sustains cooperative social relationships in other primates as well.

Hormonal mechanisms are also critical to cooperation and other manifestations of affiliative behavior. Charles Snowdon (psychology, U Wisconsin-Madison) described results from both behavioral endocrinological and fMRI studies on tamarin and marmoset monkeys, respectively. In these primates, males and other nonmaternal helpers provide essential infant care, and elevated levels of the hormone, prolactin, usually associated with lactation, have also been implicated in paternal care. New findings indicate that prolactin can also be considered to be a “relationship hormone,” because its levels fluctuate in response to the quality of a male’s relationship with his mate. Moreover, the same regions of the brain associated with positive emotional states (such as mutual cooperation) in humans are also activated in male monkeys when exposed to cues from familiar mates and unfamiliar, but ovulating, females.

Neuropeptides, such as oxytocin and its antagonist, vasopressin, are also involved in male parental care and the maintenance of pair-bonds in socially monogamous mammals, as demonstrated by Sue Carter (psychology, U Illinois-Chicago). Oxytocin levels rise during positive social and sexual interactions, simultaneously suppressing the production of adrenal hormones, such as corticosterone, generally associated with fear, anxiety and other forms of stress. Positive social experiences early in life can affect the brain’s sensitivity to these neuropeptides, and therefore have life-long consequences on an individual’s social behavior.

Affinities for Affiliations
Armed with these neuro-endocrine mechanisms, it is not surprising that positive social interactions, whether among humans or other primates, can feel so good. And, as the speakers in the second half of the session revealed, there is ample evidence that many primates have an affinity for affiliative relationships. Paul Garber (anthropology, U Illinois-Champagne Urbana) and Robert Sussman (anthropology, Washington U) have shown that contrary to the emphasis on aggression and competition, some 80-90% of all primate social interactions are affiliative. Garber extended the implications of this finding by critically considering long-held assumptions about the high costs of group living. Among other findings, he found that most primate groups fission before they become large enough to incur the costs of high levels of feeding competition. As he reasoned, if the costs of group living are actually so low, then the benefits of affiliation must play a greater role in primate social evolution than they have traditionally been granted.

More specific illustrations of the role of affiliative behavior were provided in papers by Deborah Overdorff (anthropology, U Texas-Austin) and colleagues (Thomas Mutschler, UT-Austin, and Elizabeth Erhard, anthropology, Southwestern Texas State) for lemurs, and by Mary Glenn (anthropology, Humbolt State) and colleagues (Marissa Sousa, anthropology, UC Santa Cruz) and Keith Bensen (Windward Islands Research and Education Foundation) for Mona monkeys. In their comparisons of rates of social interactions among four species of lemurs, Overdorff and colleagues challenged the long-standing view that the core of lemur societies revolves around relationships among females that affect their contests over food. Instead, they found the dynamics and affiliative nature of male-female interactions to be more important than female-female interactions related to competition or cooperation for competitive advantages.

Male Mona monkeys also diverge from long-held views about the competitive nature of male-male interactions. As the paper by Glenn and colleagues revealed, males in this species seek out one another and engage in high levels of affiliative behaviors instead of avoiding or challenging one another when avoidance is not possible. Males in these two to five member groups are tolerant toward newcomers, and males associating with one another exhibit strongly synchronized behavior patterns with frequent friendly contact, including grooming and play, and food sharing.

A New Paradigm for Peace
Collectively, the papers presented in this session make a strong case for the role of cooperation and affiliation in primate societies. The brains of humans and other primates are hard-wired to recognize cooperation and affiliative social relationships as positive and rewarding, and sensitivity to positive social stimuli is enhanced by experience. With these mechanisms in place, it is not so surprising that primates engage in affiliative interactions much more often than aggression. There may still be sound evolutionary explanations for why affiliation and cooperation are so common among primates, but it is clear that these new perspectives on primate sociality have the potential to help us understand that our heritage as primates may include a preference for peace.

Mona monkey all-male group members spend a large part of their day participating in affilitive contact behavior, as demonstrated by these adults engaged in a lengthy grooming bout. Photo by Marissa Sousa-Ramsiern

Karen Strier and Robert Sussman organized the AAA Executive Program Committee Invited Session “Mechanisms for Maintaining Peace in Primate Societies” held November 20, 2003, at the AAA Annual Meeting in Chicago.