Thinking Like a Monkey

Adapting methods used to study thinking in human infants, a pioneering researcher peers into the minds of our fellow creatures

12-01-2004 // Michael Lipske

When Marc Hauser’s experiment in a Ugandan rain forest suggested that wild chimpanzees can count to three, the Harvard psychologist didn’t hang around to celebrate. He took to his heels, fleeing his outraged research subjects.

An authority on animal thinking, Hauser knew that primate experts such as Jane Goodall had observed male chimpanzees in their home territories attack and kill chimp intruders from other communities—but only when the odds favored them at least three-to-one. These observations implied that chimpanzees knew how to count. To find out if the animals truly were heeding the numbers, Hauser and two Harvard colleagues traveled to Uganda’s Kibale Forest and set up a loudspeaker inside a known chimp territory. The researchers then broadcast a recording of a foreign male’s “pant-hoot” vocalization, simulating a newcomer in the neighborhood.

When female chimps heard the recording, they remained silent and even moved away. If one or two males heard the interloper, they also kept quiet. But when a group of three or more males heard the call, the balance tipped, and the neighborhood vigilantes erupted in fierce screams, crashing through the jungle toward the source of the sound. “They came with their hair bristling, ready to kill,” recalls Hauser. For the scientist, this was “concrete evidence that the numbers seem to matter.

” Hauser’s jungle experiment is just one of many empirical tests he has devised over the years to pry open windows into the minds of our fellow creatures. “We share the planet with thinking animals,” the researcher wrote in his book Wild Minds: What Animals Really Think. The rub is finding ways to reveal the mental workings of thinkers that cannot speak.

To overcome the challenge, Hauser has become a leader in applying to animal studies the same nonverbal techniques developmental psychologists use to study thinking in human infants. One of these is the “looking-time” technique, in which a researcher shows an infant two toys being placed behind a screen. If the screen is then removed, revealing only one toy or perhaps three, the amount of time the baby stares at the scene can reveal his or her comprehension that the numbers are not correct. “We look longer at events that intrigue us,” explains Hauser.

Hauser and his colleagues conduct similar experiments with rhesus monkeys and eggplants. The researchers set an eggplant on the ground in front of a monkey, cover the vegetable with a screen and, as the animal watches, add another eggplant behind the screen. When the screen is raised, a monkey looks briefly if there are two eggplants. But if only one eggplant is present, the animal usually looks at the scene for a longer time. “What these results tell us is that when animals see an addition experiment—one plus one—they expect two, not roughly two, precisely two” says Hauser.

“Marc has made the study of animal minds a science,” says Steven Pinker, a colleague in Harvard University’s Department of Psychology. Both scientists are exponents of the flourishing field of evolutionary psychology, which holds that the same process of Darwinian natural selection that has shaped animal bodies has also sculpted brains, creating in each species complex systems of mental organs tailored for understanding and interacting with the world. “All brains, human brains included, consist of specialized tools,” says Hauser. “Those tools have been shaped by the pressures of evolution to meet the problems that the animal confronts in its environment.

” Hauser’s efforts to determine which cognitive tools are found in which animals’ toolkits has taken him down some curious experimental paths. Consider his pink-haired tamarins. Previous researchers had demonstrated that, like people, chimpanzees can recognize their reflections in a mirror. In one famous experiment, for example, chimpanzees’ faces were marked with red dots. When the apes were given a mirror, they immediately touched the painted spots on their faces, suggesting they knew something was different.

Hauser wondered if captive cotton-top tamarins would respond similarly. He knew, however, that tamarins have virtually no facial expressions and, compared to chimps, pay little attention to their faces. Discussing the problem in class one day, Hauser noticed a student’s orange-and-green dyed hair. Maybe the cotton-top’s spectacular shock of white head hair—a feature that distinguishes it from other tamarin species—was the way to go.

The student directed the professor to a local shop that stocked Manic Panic hair coloring in shades such as lagoon blue and flamingo pink. Under anesthesia, the tamarins’ cotton-tops were dyed a range of unnatural colors. After waking, most of the animals touched their suddenly vivid hair when they saw themselves in a mirror. (A control group of tamarins, whose hair was dyed white with the same brand of coloring, did not touch their hair.)

“These animals went from barely ever looking in the mirror to sitting in front of it peacefully, quietly, with no aggressive display,” notes Hauser. To the psychologist, the experiment suggests that, like chimpanzees, some monkeys “may have a sense of self in terms of self-recognition.” Whether or not that turns out to be true, the experiment also seems to provide incontestable proof that for tamarins, as well as for people, there is nothing quite like a new hairdo for soothing one’s spirits.

Writer Michael Lipske lives in Washington, D.C.


Some Universal, Some Unique
Mental Tool Kits

Some mental tools, like the ability to navigate, are universal. “Although humans will navigate to a restaurant to eat French cuisine and then leave a 20 percent tip while honeybees will navigate to a field of flowers and then return to the hive to waggle out the distance and direction to food, the underlying mental tools are generally similar,” writes Marc Hauser in Wild Minds: What Animals Really Think. Other mental tools are unique. Bats evolved a brain that processes high-frequency sounds, enabling the animals to navigate in darkness. Humans recognize hundreds of people by their faces, while social insects such as honeybees cannot recognize the faces of nest mates . “For humans,” notes Hauser, “the face is a special object because it has a unique configuration of features and because it represents a crucial window into a person’s identity, beliefs and feelings. Consequently, humans evolved a brain that is specially designed to process faces.”


Web Exclusive
Scientists Discover New Tool-Use Talents in Chimps

Beyond the abilities to count and recognize faces, scientists recently discovered that the “mental tool kits” of chimpanzees also include an aptitude for assembling tool kits—combinations of more than than one kind of tool, which the primates use to collect termites.

In the Republic of Congo’s Goualougo Triangle, a remote 100-square-mile rain forest, a research team sponsored by the Wildlife Conservation Society (WCS) and National Geographic Society set up unmanned video cameras that recorded the behavior of a group of 54 chimps over the course of six months. Such remote monitoring allowed the scientists to watch the animals without disturbance, as well as collect in a relatively short time information that would have taken years to gather through direct observation. Their most startling discovery, reported in the November 2004 issue of The American Naturalist, is that the chimps make and use tools in innovative ways never seen before—a finding that bolsters the notion that different chimpanzee groups have their own unique cultures.

While tool use by chimps collecting termites has been observed previously elsewhere in Africa, the scientists say the Goualougo chimps are the first known example of these primates employing two different tools to accomplish the task. In the videos, they observed the animals selecting, first, heavy sticks to punch holes in the tops of termite mounds, then switching to lighter sticks, or “fishing tools,” to extract the insects. When the chimps wanted access to an underground termite mound, they used yet another kind of stick to perforate the top of the nest.

When choosing tools, the chimps were quite particular, frequently traveling long distances to find the right sticks, then transporting them back to the termite mounds. The primates also modified the tools, stripping sticks of leaves and shortening them to uniform lengths. The tips of sticks that were used as fishing probes were adapted as well. One video segment “shows a female chimp taking a stick and pulling it through her teeth, intentionally modifying the end like a paint brush,” says coauthor Crickette Sanz of the Max Planck Institute and Washington University. “This is a very effective way of gathering termites.”

The chimps’ inventive tool-use behavior nearly disappeared before it was discovered. Four years ago a Swiss timber company, CIB, was planning to establish a logging operation in the. Goualougo Triangle. But collaboration among WCS, the Republic of Congo and CIB led to the forest’s protection instead. Coauthor and WCS researcher David Morgan of Cambridge University hopes for similar rescue efforts elsewhere: “We still have a chance to preserve some of the intact forests in the Congo Basin and the unique chimpanzee cultures that reside within them,” he says—Laura Tangley

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