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Scientists discuss evolutionary roots of social behavior

University of Wisconsin-Madison

Three adult male northern muriquis relax in a Brazilian coastal forest. Credit: C.B. Possamai

Researchers have long reflected on that most intriguing of evolutionary questions: what led to the emergence of social behavior? Following observations of primates, our nearest evolutionary kin, many scientists have proposed that social alliances may have evolved as a means to stave off competitors for resources or mates.

A more recent theory however, suggests that social systems - including "positive" behaviors such as cooperation and interdependency - might have arisen as a kind of group defense mechanism against predators.

The "social-living prey" hypothesis will be the focus of a symposium entitled "Man the Hunted: The Origin and Nature of Human Sociality," on February 19. Featuring two prominent primate researchers from the University of Wisconsin-Madison, the symposium is an event of the annual meeting of the American Association for the Advancement of Science on Feb. 16-20.

As primates spend less than 10 percent of their time engaging in social interactions, most scientists have focused on aggressive and competitive behaviors to explain the complex social dynamics seen in many primate societies.

Charles T. Snowdon, Hilldale professor of psychology. Credit: Michael F. Rothbart

But panelists Karen Strier, a UW-Madison professor of biological anthropology; and Charles Snowdon, a UW-Madison professor of psychology; will instead speak on the importance of "affiliative" or positive behaviors, such as grooming, physical touch and infant care.

For more than 20 years, Strier has monitored a population of Northern muriquis, a critically endangered primate from Brazil. Based on her observations, Strier knows that muriquis spend less than three percent of their time involved in direct social interactions, and more than 60 percent of their time near other individuals in their groups.

In her talk, entitled "How Social are Social Primates?," Strier will make the novel suggestion that rather than the frequency of direct social contact, it is the ways in which male muriquis network through their associations that explains their social structures.

Karen B. Strier, professor of anthropology, is a 2004 Distinguished Teaching award recipient,and recipient of a 2005 Hilldale Award.

"Though close proximity wouldn't normally count as a direct interaction, it could be a whole other biologically meaningful category of [primate] behavior," says Strier. "It could be that animals that are comfortable around each other may not necessarily need to interact."

Analyzing data on male and female muriqui behaviors, Strier found that the males in particular associate a great deal with other males, often taking part in massive "group hugs," for instance. The knowledge is important, says Strier, because such networks may one day be influenced by demographic trends. As successful conservation measures continue to boost muriqui numbers over the next ten years, scientists will have a chance to learn whether social patterns are essentially hard-wired into muriqui populations, or whether those patterns change as the population grows.

Psychologist Charles Snowdon, the other UW-Madison panelist, will meanwhile present "Affiliative Hormones in Primates: Cause or Consequence of Primate Behavior." Snowdon has long worked with marmosets and tamarins, two squirrel-sized primate species known for their monogamous lifestyles and for the unusual devotion of males in both species to being good fathers.

Snowdon will discuss recent work in which he demonstrated that affiliative behaviors such as grooming and stroking help increase levels of "social bonding" hormones such as prolactin. Such hormones may play an important role in ensuring that marmoset and tamarin fathers remain committed to their mates and infants, Snowdon says.

Human mothers, like marmoset and tamarin females, also rely on the help of their partners to rear children, Snowdon notes. So insights into marmoset and tamarin affiliation behaviors may help shed light on complex social questions, such as how males and females can maintain quality relationships.


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