While cleaning out the old library at the Yerkes Primate Center, we unearthed forgotten treasures. One was the old wooden desk of Robert Yerkes, which is now my personal desk. The other was a film that probably had not been looked at for half a century. It took us a while to find the right projector, but it was worth the trouble. Lacking sound, the film had written titles inserted in between poor-quality black-and-white scenes. It featured two young chimpanzees working together on a task. In true slapstick style, befitting the movie’s flickering format, one of the chimps would slap the other on her back every time her dedication flagged. I have shown a digitized version to many audiences, causing much laughter in recognition of the humanlike encouragements. People are quick to grasp the movie’s essence: apes have a solid understanding of the advantages of cooperation.
The experiment was run in the 1930s by Meredith Crawford, a student of Yerkes.27 We see two juveniles, Bula and Bimba, pulling at ropes attached to a heavy box outside their cage. Food has been placed on the box, which is too heavy for one of them to pull in alone. The synchronized pulling by Bula and Bimba is remarkable. They do so in four or five bursts, so well coordinated that you’d almost think they were counting—“one, two, three … pull!”—but of course they are not. In a second phase, Bula has been fed so much that her motivation has evaporated, and her performance is lackluster. Bimba solicits her every now and then, poking her or pushing her hand toward the rope. Once they have successfully brought the box within reach, Bula barely collects any food, leaving it all to Bimba. Why did Bula work so hard with so little interest in the payoff? The likely answer is reciprocity. These two chimps know each other and probably live together, so that every favor they do for each other will likely be repaid. They are buddies, and buddies help each other out.
This pioneering study contains all the ingredients later expanded upon by more rigorous research. The cooperative pulling paradigm, as it is known, has been applied to monkeys, hyenas, parrots, rooks, elephants, and so on. The pulling is less successful if the partners are prevented from seeing each other, so success rests on true coordination. It is not as if the two individuals pull at random and, by luck, happen to pull together.28 Furthermore, primates prefer partners who cooperate eagerly and are tolerant enough to share the prize.29 They also understand that a partner’s labor requires repayment. Capuchin monkeys, for example, seem to appreciate each other’s effort in that they share more food with a partner who has helped them obtain it than with one whose help went unneeded.30 Given all this evidence, one wonders why the social sciences in recent years have settled on the curious idea that human cooperation represents a “huge anomaly” in the natural realm.31
It has become commonplace to assert that only humans truly understand how cooperation works or know how to handle competition and freeloading. Animal cooperation is presented as mostly based on kinship, as if mammals were social insects. This idea was quickly disproven when fieldworkers analyzed DNA extracted from the feces of wild chimpanzees, which allowed them to determine genetic relatedness. They concluded that the vast majority of mutual aid in the forest occurs between unrelated apes.32 Captive studies have shown that even strangers—primates who didn’t know each other before they were put together—can be enticed to share food or exchange favors.33
Despite these findings, the human uniqueness meme keeps stubbornly replicating. Are its proponents oblivious to the rampant, varied, and massive cooperation found in nature? I just attended a conference on Collective Behavior: From Cells to Societies, which addressed the extraordinary ways in which single cells, organisms, and entire species realize goals together.34 Our best theories about the evolution of cooperation stem from the study of animal behavior. Summarizing these ideas in his 1975 book Sociobiology, E. O. Wilson helped launch the evolutionary approach to human behavior.35
Excitement about Wilson’s grand synthesis seems to have faded, though. Perhaps it was too sweeping and inclusive for disciplines that consider humans in isolation. Chimpanzees in particular are nowadays often depicted as so aggressive and competitive that they can’t be truly cooperative. If this applies to our closest relatives, so the thinking goes, we can justifiably ignore the rest of the animal kingdom. One prominent advocate of this position, the American psychologist Michael Tomasello, extensively compared children and apes, which has led him to conclude that our species is the only one capable of shared intentions in relation to common goals. He once condensed his view in the catchy statement “It is inconceivable that you would ever see two chimpanzees carrying a log together.”36
At Burgers’ Zoo, live trees are surrounded by electrified wire, yet the chimps manage to get into them anyway. They break long branches out of dead trees and carry them to a live one, where one of them holds the branch steady while another scales it.
This is quite an assertion, given Emil Menzel’s photographed and filmed sequences of juvenile apes recruiting one another to collectively prop a heavy pole up against the wall of their enclosure in order to get out.37 I have regularly seen chimps use long sticks as ladders to get across hot wire surrounding live beech trees; one chimp holds the stick while another scales it to reach fresh leaves without getting shocked. We have also videotaped two adolescent females who regularly tried to reach the window of my office, which overlooks the chimp compound at the Yerkes Field Station. Both females would exchange hand gestures while moving a heavy plastic drum right underneath my window. One ape would jump onto the drum, after which the other would climb on top of her and stand on her shoulders. The two females would then synchronously bob up and down like a giant spring; the one standing on top would reach for my window every time she came close. Well synchronized and clearly of the same mind, these females played this game often in alternating roles. Since they never succeeded, their common goal was largely imaginary.
Literally carrying a log together may not be part of these efforts, but this behavior is trained for all the time in Asian elephants. Until recently, the forest industry in Southeast Asia employed elephants as beasts of burden; now they are rarely used for this purpose anymore, but they still demonstrate their skills for tourists. At the Elephant Conservation Center near Chiang Mai, in Thailand, two tall adolescent bulls will effortlessly pick up a long log with their tusks, each standing on one end, draping their trunks over the log to keep it from rolling off. Then they will walk in perfect unison several meters apart, with the log between them, while the two mahouts on their necks sit chatting and laughing and looking around. They are most certainly not directing every move.