Capuchin nut cracking thoroughly upset the evolutionary narrative that had been woven around humans and apes. According to this story, we are not the only ones who knew a Stone Age: our closest relatives still live in one. To stress this point, a “percussive stone technology” site (including stone assemblies and the remains of smashed nuts) was excavated in a tropical forest in Ivory Coast, where chimpanzees must have been opening nuts for at least four thousand years.31 These discoveries led to a human-ape lithic culture story that fit together nicely, tying us to our close relatives.
This is why the discovery of similar behavior in a more distant relative, such as the capuchin monkey—equipped with tails by which they can hang!—was met with surprise and initial grumbling. The monkeys didn’t fit. The more we learned, however, the more the nut cracking by capuchins in Brazil began to resemble that of chimpanzees in West Africa. Yet capuchins belong to the neotropical monkeys, a distant group that split off 30 to 40 million years ago from the rest of the primate order. Perhaps the similar tool use was a case of convergent evolution, since both chimps and capuchins are extractive foragers. They break things open, destroy outer shells, and smash things to pulp in order to eat, which might be the context in which their high intelligence evolved. On the other hand, since both are large-brained primates with binocular vision and manipulative hands, there is an undeniably evolutionary connection. The distinction between homology and analogy is not always as clear as we’d like it to be.
To complicate matters, the tool use of capuchins and chimpanzees may not be cognitively at the same level. Over many years of working with both species, I have formed a distinct impression of how they go about their business, which I’ll offer here in everyday language. Chimpanzees, like all the apes, think before they act. The most deliberate ape is perhaps the orangutan, but chimps and bonobos, despite their emotional excitability, also judge a situation before tackling it, weighing the effect of their actions. They often find solutions in their heads rather than having to try things out. Sometimes we see a combination of both, as when they start acting on a plan before it is completely formed, which is of course not unusual in our species either. In contrast, the capuchin monkey is a frenzied trial-and-error machine. These monkeys are hyperactive, hypermanipulative, and afraid of nothing. They try out a great variety of manipulations and possibilities, and once they discover something that works, they instantly learn from it. They don’t mind making tons of mistakes and rarely give up. There is not much pondering and thinking behind their behavior: they are overwhelmingly action-driven. Even if these monkeys often end up with the same solutions as the apes, they seem to get there in an entirely different way.
While all this may be a gross simplification, it is not without experimental support. An Italian primatologist, Elisabetta Visalberghi, has spent a lifetime studying the tool use of brown capuchins at her facility adjoining the Rome Zoo. In one illuminating experiment, a monkey faced a horizontal transparent tube with a peanut visible in the center. The plastic tube was mounted so that the peanut was at monkey eye level. The monkey couldn’t get to it, though, since the tube was too narrow and long. Many objects were available to push the food out, ranging from the most suitable (a long stick) to the least (short sticks, soft flexible rubber). The capuchins made an astonishing number of errors, such as hitting the tube with the stick, vigorously shaking the tube, pushing the wrong material into one end, or pushing short sticks into both ends so that the peanut couldn’t budge. The monkeys learned over time, however, and began to prefer the long stick.
A brown capuchin monkey (top) inserts a long stick into a transparent tube to push out a peanut. Placed in a regular tube, the peanut may be pushed in either direction to solve the problem. The trap-tube (bottom), by contrast, requires the peanut to be pushed in only one direction, otherwise it will drop into the trap and be lost to the monkey. Monkeys can learn to avoid the trap after many errors, but apes show cause-effect understanding and recognize the solution right away.
At this point, Visalberghi added an ingenious twist by making a hole in the tube. Now it suddenly mattered which way the peanut was pushed. Pushed toward the hole, the peanut would fall into a small plastic container and be lost to the monkey. Would capuchin monkeys understand the need to stay clear of the trap, and would they do so right away or only after many failed attempts?
Handing four monkeys a long stick to work on the trap tube, three performed at random, being successful half the time, which they seemed perfectly happy with. But not Roberta, a slender young female, who kept trying and trying. She’d push the stick into the left end of the tube, then race around to see how it and the peanut looked from the right end. Then she’d switch sides, inserting the stick into the right end, only to race around to peek into the tube from the left. She kept going back and forth, sometimes failing, sometimes succeeding, but in the end becoming quite successful.
How had Roberta solved the problem? The investigators concluded that she followed a simple rule of thumb: insert the stick into the end of the tube farthest away from the reward. This way the peanut could be pushed out without having to cross the trap. They tested it out in several ways, one of which was to offer Roberta a new plastic tube without any trap at all. Now she could push the stick whichever way she wanted and be successful. She kept racing around the tube, however, looking for the longest distance from the peanut, insisting on the rule that had been the key to her success. Since Roberta acted as if the trap were still there, she clearly had not paid much attention to how it worked. Visalberghi concluded that monkeys are able to solve the trap-tube task without actually understanding it.32
This task may look simple, yet is harder than it seems: human children solve it reliably only when they are over three years old. Testing five chimpanzees on the same problem, two of them grasped the cause-effect relation and learned to specifically avoid the trap.33 While Roberta had merely learned which actions led to success, the apes recognized how the trap worked. They were representing the connections between actions, tools, and outcomes in their heads. This is known as a representational mental strategy, which allows solutions before action. This difference may seem minor, since both monkeys and apes solved the problem, but it is actually huge. The level at which apes understand the purpose of tools affords them incredible flexibility. The richness of their technology, the toolkits, and the frequent toolmaking all prove that higher cognition helps. The American primate expert William Mason concluded in the 1970s that evolution has endowed the Hominoids’ with a cognition that sets them apart from the other primates, so that an ape is best described as a thinking being.
The ape structures the world in which it lives, giving order and meaning to its environment, which is clearly reflected in its actions. It is not very illuminating, perhaps, to describe a chimpanzee as “figuring out” how to proceed, while it sits and stares at the problem before it. Certainly such an assertion lacks originality, as well as precision. But we cannot escape the inference that some such process is at work, and that it has a significant effect on the ape’s performance. It seems better to be vaguely correct than positively wrong.34
Here Come the Crows!