Some of the muriquis in the legume tree exchange little pats as they brush by each other. Two of them, on a short break from eating, sit haunch to haunch, one resting his hand on top of the other’s head. Before they resume picking pods, they hug.
Affectionate gestures, including full-body face-to-face embraces, are common. It’s not unusual to see five or more muriquis in a tangled furry cuddle. Strier says that some males become more popular as they age, and younger males seek the company of the elders and solicit hugs during times of tension. Squabbles are rare. “Maybe their drive for social cohesion and conformity is much stronger than their aggression,” says Strier.
They also tend to be easygoing about the other big activity that agitates almost all other primates: sex. Unlike chimpanzees and baboons, male muriquis don’t attack rivals to keep them from females, Strier says. There are no alphas in these societies, so muriqui twosomes don’t have to sneak off to evade punishment by jealous suitors. What’s more, female muriquis don’t need to form coalitions to protect infants from murderous males. Strier has called muriqui mating a “passive affair.” Males don’t chase down females or bully them into sexual submission. Instead, a male waits for an invitation from a female, who selects her partners and copulates openly. Instead of battling each other for access to females, males bond into extensive brotherhoods, and Strier suspects they have replaced fighting with “sperm competition.” In proportion to their slight frames, muriquis have oversized testicles. It may be that the male producing the most sperm has the most tickets in the reproductive raffle.
When Strier first observed these behaviors, she thought muriquis were anomalies in the primate world. But as research documented the behaviors of a broader range of primates, Strier realized there was actually a lot of variation—more than was generally acknowledged. In 1994 she wrote a paper titled “Myth of the Typical Primate” that urged her colleagues to reconsider the emphasis on aggression as a mediator of primate relationships, which “prevailed despite repeated efforts to demonstrate the limitations of such arguments.” She contended that the roots of primate social behavior, including that of people, might be more accurately reflected in the flexibility, tolerance, cooperation and affection that predominate among most primates, and that these qualities are at least as recognizably human as aggressiveness, competition and selfishness. Strier’s paper was pivotal in initiating a new way of thinking about primate behavior.
“We have this idea that competition is good,” says Robert Sussman, professor of anthropology at Washington University in St. Louis and co-author of Man the Hunted: Primates, Predators, and Human Evolution, “that everybody is out for themselves, and that the people at the top are by nature superior. But there’s now lots of evidence that competition among primates only occurs when the environment changes because of outside influence. The ultimate goal of evolution is to reach an ecological equilibrium and avoid competition and aggression, a very different point of view. Karen Strier has become one of the leaders in this alternative paradigm about the evolution of cooperation.”
So as not to influence the behavior of the muriquis themselves, Strier decided at the start only to observe them and not interact with them. She has never trapped or tranquilized a monkey to take a blood sample or to affix a radio collar, and she won’t use feeding stations to lure them to convenient spots for observations, as some researchers studying chimps in the wild have been known to do. For years she has collected hormone data on individual females by positioning herself to catch falling feces. She says they smell like cinnamon.
Though Strier maintains a kind of clinical detachment from the muriquis in the field, that doesn’t mean she’s uninvolved. She has in fact become their impassioned advocate. No matter how cooperative they are, they can’t by themselves overcome the forces at work to destroy them.
Once called woolly spider monkeys, muriquis occur in two closely related species that scientists didn’t officially split until 2000: northern (Brachyteles hypoxanthus) and southern (Brachyteles arachnoides). Both species live only in Brazil, in scattered remnants of the once-vast Atlantic coastal forest, now greatly reduced by clearing for pasture and agricultural land. Because of extensive habitat fragmentation, both muriqui species are classified as endangered, the northern one critically: Only 1,000 of them survive, spread across about a dozen patches of forest, one of which is Strier’s study site. Early in Strier’s career, colleagues asked her why she wanted to study monkey behavior in such an altered habitat. But Strier didn’t see the environment as an obstacle; she wanted to know how the monkeys adapt.
Born in New Jersey, Strier grew up in southern California, western New York and then Maryland. She enjoyed the outdoors, hiking and backpacking with friends, but she doesn’t trace her deep fascination with primates to any childhood “aha” moment, unlike Jane Goodall, who recalls receiving a toy chimpanzee as a youngster. As an undergraduate studying biology and anthropology at Swarthmore College, Strier actually thought she might go on to conduct research on bears in the United States. But during her junior year she was offered the opportunity to work on the Amboseli Baboon Project in Kenya. She had never taken a course in primatology.
“It was a catharsis,” she says. “Everything about who I was and what I liked came together—the outdoors, the animals, science.” It was in graduate school that her adviser connected her with Mittermeier, who connected her with the muriquis. “She’s one of the great leaders in primatology today,” says Mittermeier. “She’s had a huge influence in Brazil. She has trained some of the key people there, the richest country on earth for primates.”