Into the Mind of a Monkey
Primates can teach us a lot about what it means to be human—if we're careful.
Deep in the tropical rain forest, white-faced capuchin monkeys hungrily eyed bananas that sat on shelves inside an open Plexiglas box. Capuchins, considered by some to be the smartest of the New World monkeys of Central and South America, had been seen using tools in captivity, so some scientists had proposed studying them to understand how tool use evolved in ancestral humans.
To see if wild capuchins could use tools, Paul Garber, an LAS anthropology professor who studies primates, speared bananas with wooden dowels and set out the dowels inside the Plexiglas box. If the monkeys were tool users, the idea went, they would learn to reach into the box, pick up a dowel (a primitive tool), and slide off the banana. In the end, six of the 15 monkeys succeeded. "If you looked at the video, you'd say they knew exactly what they were doing," says Garber, who videotaped the monkeys for 55 days from a nearby blind. But it turned out that they didn't, and therein lies a lesson: When it comes to understanding monkeys and apes, it's easy to assume incorrectly that they think as we do.
We see monkeys using tools and imagine they know how the toolworks. We see chimps fight and assume their society is as riven by war as ours. We see them hunting together and think they planned it in advance. "A lot of times in popular culture they're treated as little furry humans," says primatologist Melissa Panger of George Washington University in Washington, D.C.
Even trained primate researchers have been led astray by this sort of anthropomorphism. But it's possible, using careful experiments and rigorous analysis, to learn why nonhuman primates act the way they do. Then something valuable emerges. As our closest living relatives, primates like monkeys and apes are intelligent, social animals that share a common heritage with humans. By understanding how they think and behave, Garber says, we can "get a better sense of how we came to be the way we are."
Some scientists have proposed that primates needed to get smarter to remember where to find food in a complex, ever-changing tropical forest. Others say that primates needed intelligence to develop the social skills necessary to get along in a troop. Because all primates are social foragers, Garber believes both foraging and social skills led to smarter monkeys and apes.
But humans have much larger brains, pound for pound, than even our nearest primate relatives. According to one popular theory, human ancestors needed to get even smarter than other primates to make and use tools. Constructing even the simplest tools, such as the chipped rocks used more than two million years ago by Homo habilis, requires an ability to imagine an object in a different shape, much as a sculptor sees his subject's face in an uncarved block of wood. It also requires an ability to imagine how using the tool could change the environment. That ability "may have been built upon for things like language, complex planning, and problem solving," Garber explains.
To see how tool use evolved in ancestral hominids, Garber and other anthropologists sought to study a primate that not only used tools, but also understood how they functioned. Only a few primates have that ability: humans, chimps, some orangutans—and, some believed, white-faced capuchin monkeys. To see if capuchins could really understand how tools function—and if anthropologists could learn anything about human evolution from studying them—Garber spent nearly two months at the La Suerte Biological Research Station in northeastern Costa Rica, watching the monkeys try to figure out how to get bananas from a box.
The six capuchins that learned to retrieve the bananas, including an alpha male named Mr. Cool and a beta male named Fuzzy, had clearly learned to use a dowel as a tool. So Garber changed the rules to see if the monkeys really understood how the tool worked.
Now, a hungry monkey that wanted a banana would have to pick up a dowel or a stick from the forest, poke it through a hole in the Plexiglas, and use it to knock a banana off a shelf. Garber and Ellen Brown, a former U of I undergrad now working on her PhD at Yale, monitored the site each day to see if any of the monkeys would succeed in retrieving a banana.
When Garber explains the experiment, he picks up a pen and uses it to push a paper cup along his desk. It's a task any two-year-old could do. But the vast majority of monkeys would use their finger instead. That's because the cognitive abilities that enable humans to use an object such as a pen for a new purpose appear to be absent in most monkeys. In other words, most monkeys don't understand how tools work. Some scientists had assumed that capuchins were an exception.
But after 266 visits over 15 days, none of the capuchins at La Suerte had retrieved a banana under the new rules. They'd hang around the box, try to push it over, and bark at it, demonstrating that they craved the fruit. But they never poked a stick or dowel through a hole to get their reward.
What capuchins are doing is trial and error, as opposed to understanding how the tool works," Panger says.
The results show that studying capuchins won't tell scientists much about how human ancestors learned to use tools, Garber says. Instead, they'll have to focus on the few animals that do seem to understand how tools work, such as orangutans and chimpanzees. More broadly, the study reveals how easy it is to get off track by assuming that animals think like we do.
Researchers have also ventured off course by assuming that primates and humans are innately warlike and aggressive. That's the upshot of another study Garber did with anthropologist Robert Sussman of Washington University in St. Louis. Since the early days of primate research, many anthropologists have believed that competition for food and other resources bred aggression and shaped primate society, laying a template for human violence and warfare.
Garber and Sussman suspected instead that cooperation might be more common and more important than aggression. So they pored over 80 published field studies of 50 species of primates, including chimps, gorillas, orangutans, and monkeys, and synthesized the results. It turned out that primates actually spend less than 1 percent of their day fighting or otherwise being aggressive. About 90 percent of their social interactions, on average, were friendly acts like grooming and feeding together. Such cooperation offers many advantages, including better food and protection from predators. Cooperation also induces the body to produce endorphins and other hormones that make us feel good.
Understanding [cooperation] will get us further along in understanding human behavior than simply relying on a model of competition and aggression, Garber concludes.
The heritage we have from nonhumans is one of incredible ability to form bonds.
By Dan Ferber, who did postdoctoral research in microbiology in LAS