In reality, as Strier’s work would underscore, the primates are a varied group, with diverse social structures and far more complex behavior. Descended from a tree-dwelling ancestor living some 55 million years ago in Africa or Asia, the group includes tarsiers, lemurs, lorises, monkeys, apes (such as gorillas, chimps, bonobos, gibbons) and hominids. Monkeys, characterized by long tails and flat, hairless faces, are generally divided into two types: Old World monkeys, such as baboons and macaques, live in Asia and Africa. New World monkeys, including muriquis, are descended from ancestors that found their way from Africa to South America perhaps 35 million years ago.
For a long time, New World monkeys were the second-class citizens of primatology. “New World primates were considered not so smart, not so interesting, and not so relevant to human evolution,” says Frans de Waal, director of the Living Links Center at Emory University’s Yerkes National Primate Research Center. “They were sidelined—totally inappropriately, as Karen has demonstrated.”
Strier’s research introduced the world to an alternative primate lifestyle. Female muriquis mate with a lot of males and males don’t often fight. Though bonobos, known for their casual sex, are often called the “hippie” primates, the muriquis in Strier’s study site are equally deserving of that reputation. They are peace-loving and tolerant. Strier also showed that the muriquis turn out to be incredibly cooperative, a characteristic that may be just as important in primate societies as vicious rivalry.
Strier’s ideas shook up primatology, making her an influential figure in the field. Her widely used textbook, Primate Behavioral Ecology, is in its fourth edition and “has no peers,” according to the American Society of Primatologists. In 2005, at age 45, Strier was elected to the National Academy of Sciences, a rare honor. The University of Wisconsin recently recognized her with an endowed professorship. The money is being used to support her research in Brazil, where the muriquis she knows so well continue to surprise her.
Lately, they’ve been doing something arboreal primates aren’t supposed to do. In an unusual behavioral twist, they’re coming down out of the trees.
Muriquis are acrobats, spending much of the day swinging through the treetops in search of food. They ride branches down and scurry across vines like tightrope walkers. Hanging fully extended, muriquis appear five feet tall but weigh only 20 pounds, an elongated physique allowing for quick and astonishingly nimble movement.
As Strier and I walk through the forest, the muriquis sound like a herd of horses flying overhead. They neigh to maintain long-distance contact. A staccato hnk hnk hnk keeps them out of one another’s way, and an excited chirp summons the others when a monkey has found a fruiting tree.
Muriquis’ cooperative behaviors are often on display when they’re eating. A few days into my visit, Strier and I watch nine males demonstrate their manners as they eat pods in a legume tree. When one monkey scoots past another on a branch, it pauses to hug its neighbor, as if to say, “Pardon, so sorry.”
Muriquis almost never fight over food with members of their own group. They will chase howler monkeys or capuchins out of fruiting trees, and they loudly protest incursions by muriquis from other parts of the forest. But males and females, young and old, behave toward members of their own group in ways that can fairly be described as considerate.