On a hot morning in early August, the primate census of Cayo Santiago, a 38-acre island just off the coast of Puerto Rico, numbers approximately 875. Of those, 861 are resident Macaca mulatta, commonly known as rhesus macaques, the descendants of a colony transported here from Calcutta in 1938 to provide a permanent breeding stock for medical researchers. The rest are Homo sapiens who have made the trip in a motorboat, including workers stocking the feeding bins with dun-colored biscuits of monkey chow, and researchers for whom the island provides a rare opportunity to study free-ranging primates without the drudgery of having to locate them deep in some remote forest.
The researchers comprise two distinct disciplines, with widely divergent interests and approaches. Ever since E. O. Wilson visited here in 1956 and came away with the ideas that would eventually become the foundation of a whole new field of research he called sociobiology, the island has been a mecca for ethologists, who study the monkeys' social hierarchies and interactions. It has also been discovered by experimental psychologists, who study the animals' thinking processes. Since the former try to stay as unobtrusive as possible, while the latter employ attention-getting constructions of colored posterboard and bags of fruit, there's a certain unavoidable tension between the disciplines. Trailed by three undergraduates toting armloads of gear up a path sodden with monkey droppings, Laurie Santos, a psychologist at Yale, is in the latter camp. "This is what we do," she says, "hike around looking for monkeys by themselves who are hungry and want to play. It's hard to find social creatures by themselves," she adds as she backs out of the field of view of a primatologist's video camera, "and even harder to find ones that aren't being followed by other researchers."
Santos has been coming to Cayo every year since 1993, when she was a freshman at Harvard and volunteered to work here with her psychology professor, Marc Hauser. She keeps that tradition alive with her own undergraduates. With her bright smile and mass of curly dark hair, Santos, 32, could pass for an undergraduate herself. Her boyfriend, Mark Maxwell, actually is an undergrad—albeit one who dropped out of MIT and supported himself for years by playing poker before returning this year to finish his degree at Yale. Santos teaches a class, "Sex, Evolution and Human Nature" with a course description ("Topics include...human mating strategies, the biology of warfare, sex differences in behavior, love and lust...") that all but guaranteed it would have to be held in the largest classroom on campus, the law school auditorium. She was embarrassed last year when her mother attended one of her lectures and by happenstance chose the day that she was discussing the female orgasm. "I had to cover it, but my mom was in the auditorium, so I kind of rushed through it," Santos says. "I hope the students didn't notice."
She has built a growing and impressive list of publications in cognitive neuroscience (mostly having to do with how primates understand physical objects and relations) and evolutionary psychology, the field that grew out of sociobiology. "If you see something in a primate," Santos reasons, "you can use it as a window into the evolutionary past of human beings."
On this summer day, if her undergraduate volunteers expected to be investigating the exuberant and promiscuous sex life of the rhesus macaque, they must be disappointed. Santos' interest here is in what psychologists call "theory of mind," the ability to impute thoughts and intentions to another individual, one of the cornerstones of human cognition. "Sitting here talking with you," Santos explains, "all I can see is your behavior, but I draw inferences about your desires and thoughts. The interesting question is, how far back in evolutionary time does that ability extend? Can it exist without language?" As recently as a decade ago, the conventional wisdom doubted that even chimpanzees, which are more closely related to human beings than are monkeys, possessed theory of mind. This view is changing, in large measure because of the work of Santos and her collaborators. With her students in tow and a small bag of grapes in her pocket, Santos is now out to demonstrate the phenomenon—if a Macaca mulatta can be induced to cooperate.
Trial 1: The experiment relies on one of the most predictable traits of rhesus monkeys: their tendency to steal food at every opportunity. Santos discovered this a few years ago when she and her colleagues were running experiments in cognition and tool use involving lemons, and frequently had to quit early because the animals stole all the fruit. The island's monkeys are supplied with food, of course, and they also forage, but to leave so much as a raisin unguarded is to invite larceny; the researchers eat their own lunches inside a locked cage of cyclone fencing.
The theory-of-mind experiment is designed to test whether the monkeys, who obsessively guard their own food, assume that people do the same. If so, Santos reasons, they should prefer to steal from people who are looking away. So Santos enlists Olivia Scheck and Katharine Jan, Yale student volunteers here for the month. They are dressed alike in blue slacks and white shirts to minimize any confounding effect from their appearance—although there are differences Santos cannot do anything about, because Olivia is several inches shorter than Katharine, and blond, where Katharine is dark-haired. In general, Santos has found, rhesus macaques prefer to steal from the shorter person, although top-ranking dominant males sometimes do the opposite, apparently just to show off.
The goal is to locate a monkey that isn't busy doing something else and isn't distracted by other monkeys. That's not always easy on this crowded island; monkeys who seem to be off by themselves are often low-ranking males skulking around a female in hopes of getting a quick copulation—out of sight of the dominant males. Once Santos has a monkey's attention, she holds up two grapes for it to see and impales each on a stick placed a few feet apart on the ground. Each student stands behind one of the grapes. Then Katharine turns her back on the monkey, while Olivia stares straight ahead. If the monkey doesn't fall asleep, wander off or lose interest, it will scamper, saunter or nervously edge over to one grape or the other and snatch it up. Based on published results, says Santos, nine times out of ten the person whose back is turned is the one who gets robbed.
This time, the monkey, who apparently hadn't read the literature, heads straight for Olivia's grape, grabs it from right under her nose and runs off.
Santos has traveled a long and (to her) unexpected path to this patch of tropical forest. She grew up in New Bedford, Massachusetts, the daughter of a high-school guidance counselor mother and a computer programmer father. She's French-Canadian on her mother's side, and on her father's is descended from Cape Verdean fishermen who settled in New England generations ago. In high school, all she knew about college was that she wanted to attend one in Boston; she chose Harvard because, taking financial aid into account, it was the least expensive. She enrolled in Hauser's psychology class, on which her own is modeled, because she was closed out of a course she'd needed for her intended career as a lawyer, and was won over by the charismatic professor and the intellectual challenge of a rapidly evolving field.