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The sunny, breezy Canary Islands are about the last place in the world where one would expect a cognitive revolution, yet this is where it all began. In 1913 the German psychologist Wolfgang K?hler came to Tenerife, off the coast of Africa, to head the Anthropoid Research Station, where he remained until after World War I. Even though rumor has it that his job was to spy on passing military vessels, K?hler devoted most of his attention to a small colony of chimpanzees.

Having eluded indoctrination in the learning theories of his day, K?hler was refreshingly open-minded about animal cognition. Instead of trying to control his animals to seek specific outcomes, he had a wait-and-see attitude. He presented them with simple challenges to find out how they’d meet them. For his most talented chimpanzee, Sultan, he would put a banana out of reach on the ground and offer him sticks that were too short to reach the fruit. Or he would hang a banana high up in the air and spread large wooden boxes around, none of which was tall enough for the purpose. Sultan would first jump or throw things at the banana or drag humans by the hand toward it in the hope that they’d help him out, or at least be willing to serve as a footstool. If this failed, he would sit around for a while without doing anything until he might hit at a sudden solution. He would jump up to put one bamboo stick inside another, making a longer stick. Or he would stack boxes on top of one another so as to build a tower that allowed him to reach the banana. K?hler described this moment as the “aha! experience,” as if a lightbulb had been switched on, not unlike the story of Archimedes, who jumped out of his bath in which he had discovered how to measure the volume of submerged objects, after which he ran naked through the streets of Syracuse, shouting “Eureka!”

Grande, a female chimpanzee, piles up four boxes to reach a banana. A century ago Wolfgang K?hler set the stage for animal cognition research by demonstrating that apes can solve problems in their heads by means of a flash of insight, before enacting the solution.

According to K?hler, a sudden insight explained how Sultan put together what he knew about bananas, boxes, and sticks to produce a brand-new action sequence that would take care of his problem. The scientist ruled out imitation and trial-and-error learning, since Sultan had had no previous experience with these solutions nor ever been rewarded for them. The outcome was “unwaveringly purposeful” action in which the ape kept trying to reach his goal despite the numerous stacking errors resulting in the collapse of his towers. A female, Grande, was an even more undeterred and patient architect who once built a wobbly tower of four boxes. K?hler remarked that once a solution was discovered, the apes found it easier to solve similar problems, as if they had learned something about the causal connections. He described his experiments in admirable detail in The Mentality of Apes in 1925, which was at first ignored and then disparaged, but that now stands as a classic in evolutionary cognition.1

The insightful solutions of Sultan and other apes hint at the kind of mental activity that we refer to as “thinking,” even though its precise nature was (and still is) barely understood. A few years later the American primate expert Robert Yerkes described similar solutions.

Frequently I have seen a young chimpanzee, after trying in vain to get its reward by one method, sit down and reexamine the situation as though taking stock of its former efforts and trying to decide what to do next…. More startling by far than the quick passage from one method to another, the definiteness of acts, or the pauses between efforts, is the sudden solution of problems…. Frequently, although not in all individuals or in all problems, correct and adequate solution is achieved without warning and almost instantly.2

Yerkes went on to note that those who only know animals that are good at trial-and-error learning “can scarcely be expected to believe” his descriptions. He thus anticipated the inevitable pushback to these revolutionary ideas. Unsurprisingly, it arrived in the form of trained pigeons shoving little boxes around in a dollhouse so that they could stand on top of them to reach a tiny plastic banana associated with grain rewards.3 How entertaining! At the same time, K?hler’s interpretations were criticized as anthropomorphic. But I heard an amusing antidote to these accusations from an American primatologist brave enough to enter the Skinnerian lion’s den in the 1970s, where he debated tool-using apes.

Without offering specifics, Emil Menzel told me that an eminent East Coast professor once invited him to speak. This professor looked down on primate research and was openly hostile to cognitive interpretations, two orientations that often go together. Perhaps he invited young Menzel to make fun of him, not realizing that the tables might be turned. Menzel treated his audience to spectacular footage of his chimpanzees putting a long pole against their enclosure’s high wall. While some individuals held the pole steady, others scaled it to reach temporary freedom. It was a complex operation since the apes needed to avoid coils of electrified wire while recruiting one another’s assistance at critical moments through hand gestures. Menzel, who had filmed all this himself, decided to run his footage without mentioning intelligence. He was going to be as neutral as possible. His narration was purely descriptive: “You now see Rock grab the pole while glancing at the others,” or “Here a chimpanzee swings over the wall.”4

After his lecture, the professor jumped up to accuse Menzel of being unscientific and anthropomorphic, of attributing plans and intentions to animals that obviously had neither. To a roar of approval, Menzel countered that he had not attributed anything. If this professor had seen plans and intentions, he must have seen them with his own eyes, because Menzel himself had refrained from suggesting any such things.

Interviewing Menzel at my home (he lived nearby) a few years before his death, I took the opportunity to ask him about K?hler. Widely recognized as a great expert on great apes himself, Menzel said it had taken him years working with chimps to fully appreciate this pioneer’s genius. Like K?hler, Menzel believed in watching over and over and thinking through what his observations might mean, even if he’d seen a certain behavior only once. He protested against labeling a single observation an “anecdote,” adding with a mischievous smile, “My definition of an anecdote is someone else’s observation.” If you have seen something yourself and followed the entire dynamic, there usually is no doubt in your mind of what to make of it. But others may be skeptical and need convincing.

Here I cannot resist telling an anecdote of my own. And I do not mean The Great Escape at Burgers’ Zoo, where the chimpanzee colony did exactly what Menzel had documented. After twenty-five apes raided the zoo’s restaurant, we found a tree trunk, far too heavy for a single ape to carry, propped against the inside wall of their enclosure. No, I mean an insightful solution to a social problem—a sort of social tool use—that is my specialty. Two female chimps were sitting in the sun, with their children rolling around in the sand in front of them. When the play turned into a screaming, hair-pulling fight, neither mother knew what to do because if one of them tried to break up the fight, it was guaranteed that the other would protect her offspring, since mothers are never impartial. It is not unusual for a juvenile quarrel to escalate into an adult fight. Both mothers nervously monitored each other as well as the fight. Noticing the alpha female, Mama, asleep nearby, one of them went over to poke her in the ribs. As the old matriarch got up, the mother pointed at the fight by swinging an arm in its direction. Mama needed only one glance to grasp what was going on and took a step forward with a threatening grunt. Her authority was such that this shut up the youngsters. The mother had found a quick and efficient solution to her problem, relying on the mutual understanding typical of chimpanzees.

Similar understanding can be seen in their altruism, such as when younger females collect water in their mouths for an aging female, who can barely walk anymore, spitting it into her open mouth so that she doesn’t have to walk all the way to the spigot. The British primatologist Jane Goodall described how Madame Bee, a wild chimpanzee, had become too old and weak to climb into fruiting trees. She would patiently wait at the bottom for her daughter to carry down fruits, upon which the two of them would contentedly munch together.5 In such cases, too, apes grasp a problem and come up with a fresh solution, but the striking part here is that they perceive another ape’s problem. Since these social perceptions have attracted much research, we’ll delve into them later on, but let me clarify one general point about problem solving. Although K?hler stressed that trial-and-error learning could not explain his observations, it was not as if learning played no role at all. In fact, his apes committed tons of “stupidities,” as K?hler called them, that showed that solutions were rarely perfectly formed in their minds and required quite a bit of tweaking.

His apes had undoubtedly learned the affordances of various items. This term from cognitive psychology refers to how objects can be used, such as the handle on a teacup (which affords holding) or the steps on a ladder (which afford climbing). Sultan must have known the affordances of sticks and boxes before he hit on his solutions. Similarly, the female chimp who activated Mama had no doubt witnessed the latter’s effectiveness as arbitrator. Insightful solutions invariably rest on prior information. What is special about apes is their capacity to flexibly weave such preexisting knowledge into new patterns, never tried before, that work to their advantage. I have speculated the same about their political strategies, such as the way chimps will isolate a rival from his supporters or encourage a truce by dragging reluctant former combatants toward each other.6 In all such cases, we see apes finding insightful solutions to everyday problems. They are so good at it that even the staunchest skeptic, as Menzel discovered, finds it impossible to watch them without being struck by their obvious intentionality and intelligence.

Wasp Mugs

There was a time when scientists thought behavior derived from either learning or biology. Human behavior was on the learning side, animal behavior on the biology side, and there was little in between. Never mind the false dichotomy (in all species, behavior is a product of both), but increasingly a third explanation had to be added: cognition. Cognition relates to the kind of information an organism gathers and how it processes and applies this information. Clark’s nutcrackers remember where they have stored thousands of nuts, beewolves make an orientation flight before leaving their burrow, and chimpanzees nonchalantly learn the affordances of play objects. Without any reward or punishment, animals accumulate knowledge that will come in handy in the future, from finding nuts in the spring, to returning to one’s burrow, to reaching a banana. The role of learning is obvious, but what is special about cognition is that it puts learning in its proper place. Learning is a mere tool. It allows animals to collect information in a world that, like the Internet, contains a staggering amount of it. It is easy to drown in the information swamp. An organism’s cognition narrows down the information flow and makes it learn those specific contingencies that it needs to know given its natural history.

Many animals have cognitive achievements in common. The more scientists discover, the more ripple effects we notice. Capacities that were once thought to be uniquely human, or at least uniquely Hominoid (the tiny primate family of humans plus apes), often turn out to be widespread. Traditionally, apes have been the first to inspire discoveries thanks to their manifest intellect. After the apes break down the dam between humans and the rest of the animal kingdom, the floodgates often open to include species after species. Cognitive ripples spread from apes to monkeys to dolphins, elephants, and dogs, followed by birds, reptiles, fish, and sometimes invertebrates. This historical progression is not to be confused with a scale with Hominoids on top. I rather view it as an ever-expanding pool of possibilities in which the cognition of, say, the octopus may be no less astonishing than that of any given mammal or bird.

Paperwasps live in small hierarchical colonies in which it pays to recognize every individual. Their black-and-yellow facial markings allow them to tell one another apart. A closely related wasp species with a less differentiated social life lacks face recognition, which shows how much cognition depends on ecology.

Consider face recognition, which was initially viewed as uniquely human. Now apes and monkeys have joined the countenance elite. Every year when I visit Burgers’ Zoo, in Arnhem, a few chimps still remember me from more than three decades before. They pick out my face from the crowd, greeting me with excited hooting. Not only do primates recognize faces, but faces are special to them. Like humans, they show an “inversion effect”: they have trouble recognizing faces that are turned upside down. This effect is specific for faces; how an image is oriented hardly matters for the recognition of other objects, such as plants, birds, or houses.

When we tested capuchin monkeys using touchscreens, we noticed that they freely tapped all sorts of images, but they freaked out at the first face that appeared. They clutched themselves and whined, reluctant to touch the picture. Did they treat it with more respect because putting a hand on a face violates a social taboo? Once they got over their hesitation, we showed them portraits of group mates and unknown monkeys. All these portraits look alike to naïve humans since they concern the same species, but our monkeys had no trouble telling them apart, indicating with a little tap on the screen which ones they knew and which ones they didn’t.7 We humans take this ability for granted, but the monkeys had to link a two-dimensional pattern of pixels to a live individual in the real world, which they did. Face recognition, science concluded, is a specialized cognitive skill of primates. But no sooner had it done so than the first cognitive ripples arrived. Face recognition has been found in crows, sheep, even wasps.

It is unclear what faces mean to crows. In their natural lives, they have so many other ways of recognizing one another by calls, flight patterns, size, and so on, that faces may not be relevant. But crows have incredibly sharp eyes, so they likely notice that humans are easiest recognized by their faces. Lorenz reported harassment of certain people by crows and was so convinced of their ability to hold a grudge that he disguised himself with a costume whenever he captured and banded his jackdaws. (Both jackdaws and crows are corvids, a brainy bird family that also includes jays, magpies, and ravens.) Wildlife biologist John Marzluff at the University of Washington, in Seattle, has captured so many crows that these birds take his name in vain whenever he walks around, scolding and dive-bombing him, doing justice to the “murder” label used for a whole bunch of them.

We don’t know how they pick us out of the forty thousand folk scurrying like two-legged ants over well-worn trails. But single us out they do, and nearby crows flee while uttering a call that sounds to us like vocal disgust. In contrast, they calmly walk among our students and colleagues who have never captured, measured, banded, or otherwise humiliated them.8

Marzluff set out to test this recognition with rubber face masks like those we put on at Halloween. After all, crows may recognize certain people by their bodies, hair, or clothes, but with masks you can move a human “face” around from one body to the next, isolating its specific role. His angry birds experiment involved capturing crows while wearing a particular mask, then have coworkers walk around with either this mask or a neutral one. The crows easily remembered the mask of the capturer, far from fondly. Funny enough, the neutral mask was Vice President Dick Cheney’s face, which elicited more negative reactions from the students on campus than from the crows. Not only did birds that had never been captured recognize the “predator” mask, but years later they still harassed its wearers. They must have picked up on the hateful response of their fellows resulting in massive distrust of specific humans. As Marzluff explains, “It would be a rare hawk that would be nice to a crow, but with humans you have to classify us as individuals. Clearly, they’re able to do that.”9

While corvids always impress us, sheep seem to go a step further in that they remember one another’s faces. British scientists led by Keith Kendrick taught sheep the difference between twenty-five pairs of their own species’ faces by rewarding a choice for one face and not for the other. To us, all these faces look eerily alike, but the sheep learned and retained the twenty-five differences for up to two years. In doing so, they used the same brain regions and neural circuits as humans, with some neurons responding specifically to faces and not to other stimuli. These special neurons were activated if the sheep saw pictures of companions that they remembered—they actually called out to these pictures as if the individuals were present. Publishing their study under the subtitle “sheep are not so stupid after all”—a title to which I object, since I don’t believe in stupid animals—the investigators put the face-recognition ability of sheep on a par with that of primates and speculated that a flock, which to us looks like an anonymous mass, is in fact quite differentiated. This also means that mixing flocks, as is sometimes done, may cause more distress than we realize.

Having made primate chauvinists look sheepish, science piled it on with wasps. The northern paperwasp, common in the American Midwest, has a highly structured society with a hierarchy among its founding queens, who are dominant over all workers. Given the intense competition, each wasp needs to know her place. The alpha queen lays most eggs, followed by the beta queen, and so on. Members of the small colony are aggressive to outsiders as well as to females whose facial markings have been altered by experimenters. They recognize one another by strikingly different patterns of yellow and black on every female’s face. The American scientists Michael Sheehan and Elizabeth Tibbetts tested individual recognition and found it to be as specialized as that of primates and sheep. The wasps distinguish their own species’ mugs far better than other visual stimuli, and they also outperform a closely related wasp that lives in colonies founded by a single queen. These wasps hardly have a hierarchy and have far more homogeneous faces. They don’t need individual recognition.10

If face recognition has evolved in such disparate pockets of the animal kingdom, one wonders how these capacities connect. Wasps do not have the big brains of primates and sheep—they have minuscule sets of neural ganglia—hence they must be doing it in a different manner. Biologists never tire of stressing the distinction between mechanism and function: it is very common for animals to achieve the same end (function) by different means (mechanism). Yet with respect to cognition, this distinction is sometimes forgotten when the mental achievements of large-brained animals are questioned by pointing at “lower” animals doing something similar. Skeptics delight in asking “If wasps can do it, what’s the big deal?” This race to the bottom has given us trained pigeons hopping onto little boxes to disparage K?hler’s experiments on apes and the holding up of intelligence outside the primate order to cast doubt on mental continuity between humans and other Hominoids.11 The underlying thought is that of a linear cognitive scale, and the argument that since we rarely assume complex cognition in “lower” animals, there is no reason to do so in “higher” ones.12 As if there were only one way to achieve a given outcome!

Evolutionary science distinguishes between homology (the traits of two species derive from a common ancestor) and analogy (similar traits evolved independently in two species). The human hand is homologous with the bat’s wing since both derive from the vertebrate forelimb, as is recognizable by the shared arm bones and five phalanges. The wings of insects, on the other hand, are analogous to those of bats. As products of convergent evolution, they serve the same function but have a different origin.

This is not the case. Nature abounds with illustrations to the contrary. One that I know firsthand is a pair-bonding Amazonian cichlid, the discus fish, that has achieved the equivalent of mammalian nursing. Once the fry have absorbed the egg yolk, they gather along the flanks of Mom and Dad to nibble mucus off their bodies. The breeding pair secretes extra mucus for this purpose. The young enjoy both nutrition and protection for about a month until they are “weaned” by parents who now turn away each time they approach.13 No one would use these fish to make a point about the complexity or simplicity of mammalian nursing for the obvious reason that the mechanisms are radically different. All that they share is the function of feeding and raising the young. Mechanism and function are the eternal yin and yang of biology: they interact and intertwine, yet there is no greater sin than confusing the two.

To understand how evolution works its magic across the evolutionary tree, we often invoke the twin concepts of homology and analogy. Homology refers to shared traits derived from a common ancestor. Thus, the human hand is homologous with the wing of a bat, since both derive from an ancestral forelimb and carry the exact same number of bones to prove it. Analogies, on the other hand, arise when distant animals independently evolve in the same direction, known as convergent evolution. The parental care of the discus fish is analogous to mammalian nursing but certainly not homologous, since fish and mammals do not share an ancestor that did the same. Another example is how dolphins, ichthyosaurs (extinct marine reptiles), and fish all have strikingly similar shapes owing to an environment in which a streamlined body with fins serves speed and maneuverability. Since dolphins, ichthyosaurs, and fish did not share an aquatic ancestor, their shapes are analogous. We can apply the same line of thought to behavior. The sensitivity to faces in wasps and primates came about independently, as a striking analogy, based on the need to recognize individual group mates.

Convergent evolution is incredibly powerful. It has equipped both bats and whales with echolocation, both insects and birds with wings, and both primates and opossums with opposable thumbs. It has also produced spectacularly similar species in distant geographic regions, such as the armored bodies of armadillos and pangolins, the prickly defense of hedgehogs and porcupines, and the predatory weaponry of the Tasmanian tiger and the coyote. There is even a primate, the aye-aye of Madagascar, that looks like E.T. with an extremely elongated middle finger (to tap for hollow spots and extract grubs from wood), a trait that it shares with a small marsupial, the long-fingered triok of New Guinea. These species are genetically miles apart, yet they have evolved the same functional solution. We should not be surprised therefore to find similar cognitive and behavioral traits in species that are eons and continents apart. Cognitive rippling is common precisely because it isn’t bound by the evolutionary tree: the same capacity may pop up almost anywhere it is needed. Instead of taking this as an argument against cognitive evolution, as some have done, it perfectly fits the way evolution works through either common descent or adaptation to similar circumstances.

A prime example of convergent evolution is the use of tools.

Redefining Man

As soon as an ape sees something attractive yet out of reach, he starts to cast about for a bodily extension. An apple floats in the moat around the zoo island: the ape takes one glance at the fruit before racing around in search of a suitable stick or a few stones that he can throw behind it so that it will float toward him. He distances himself from his goal in order to reach it—an illogical thing to do—while carrying a search image of what tool might work best. He is in a hurry, because if he doesn’t return fast enough, someone else will beat him to the prize. If, on the other hand, his goal is to eat fresh green leaves from a tree, the required tool is quite different: something sturdy to climb on. He may work for half an hour to drag and roll a heavy loose tree stump in the direction of the one tree on the island that has a low side branch. The whole reason he needs a tool is to get across the electric wire around the tree. Before making the actual attempt, he has figured out that the low branch will come in handy. I have even seen apes check the hot wires with the hair on the back of their wrist, hand bent inward, barely touching it, but enough to know if the power is on. If it is off, obviously no tool will be needed, and the foliage is fair game.

Apes do not just search for tools for specific occasions; they actually fabricate them. When the British anthropologist Kenneth Oakley, in 1957, wrote Man the Toolmaker, which claimed that only humans make tools, he was well aware of K?hler’s observations of Sultan fitting sticks together. But Oakley refused to count this as tool manufacture, since it was done in reaction to a given situation rather than in anticipation of an imagined future. Even today some scholars dismiss ape tools by stressing how human technology is embedded in social roles, symbols, production, and education. A chimpanzee cracking nuts with rocks doesn’t qualify; nor, I suspect, does a farmer picking his teeth with a twig. One philosopher even felt that since chimpanzees don’t need their so-called tools, it remains a feeble comparison.14

One of the most complex tool skills is the cracking of tough nuts with rocks. A wild female chimpanzee selects an anvil stone and finds a hammer that fits her hand to open a nut, while her son watches and learns. Only by the age of six will he reach adult proficiency.

I feel like recalling my know-thy-animal rule here, according to which we can safely dismiss a philosopher who thinks that wild chimpanzees sit there pounding and pounding hard nuts with rocks, an average of thirty-three blows per consumed kernel, for generation after generation, for no good reason at all. During peak season, chimpanzees at some field sites spend close to 20 percent of their waking hours fishing with twigs for termites or cracking nuts between rocks. It is estimated that they gain nine times as many kilocalories of energy from this activity as they put into it.15 Moreover, the Japanese primatologist Gen Yamakoshi found that nuts serve as fallback foods when the apes’ main nutrition—seasonal fruits—is scarce.16 Another fallback is palm pith, which is obtained through “pestle pounding.” High up in a tree, a chimpanzee stands bipedally at the edge of the tree crown, pounding the top with a leaf stalk, thus creating a deep hole from which fiber and sap can be collected. In other words, the survival of chimpanzees is quite dependent on tools.

Ben Beck gave us the best-known definition of tool use, of which the short version goes as follows: “the external deployment of an unattached environmental object to alter more efficiently the form, position, or condition of another object.”17 Though imperfect, this definition has served the field of animal behavior for decades.18 Tool manufacture can then be defined as the active modification of an unattached object to make it more effective in relation to one’s goal. Note that intentionality matters a great deal. Tools are brought in from a distance and modified with a goal in mind, which is the reason traditional learning scenarios, which revolve around accidentally discovered benefits, have such trouble explaining this behavior. If you see a chimpanzee strip the side branches off a twig to make it right for ant fishing, or collect a fistful of fresh leaves and chew them into a spongelike clump to absorb water from a tree hole, it is hard to miss the purposefulness. By making suitable tools out of raw materials, chimpanzees are exhibiting the very behavior that once defined Homo faber, man the creator. This is why the British paleontologist Louis Leakey, when he first heard about such behavior from Goodall, wrote her back, “I feel that scientists holding to this definition are faced with three choices: They must accept chimpanzees as man, they must redefine man, or they must redefine tools.”19

After the many observations of chimpanzee tool use in captivity, seeing tool use in the wild by the same species did perhaps not come as a surprise, yet its discovery was crucial since it could not be explained away by human influence. Moreover, wild chimps not only use and make tools, but they learn from one another, which allows them to refine their tools over generations. The result is more sophisticated than anything we know in zoo chimps. A good example are the toolkits, which can be so complex that it is hard to imagine that they were invented in a single step. A typical one was found by the American primatologist Crickette Sanz in the Goualougo Triangle, Republic of Congo, where a chimpanzee may arrive with two different sticks at a particular open spot in the forest. It is always the same combination: one is a stout woody sapling of about a meter long, while the other is a flexible slender herb stem. The chimp then proceeds to deliberately drive the first stick into the ground, working it with both hands and feet the way we do with a shovel. Having made a sizable hole to perforate an army ant nest deep under the surface, she pulls out the stick and smells it, then carefully inserts her second tool. The flexible stem captures bite-happy insects that she pulls up and eats, dipping regularly into the nest below. Apes often climb off the ground, moving onto tree buttresses, to avoid the nasty bites of colony defenders. Sanz collected more than one thousand such tools, which shows how common the perforator-dipping combination is.20

More elaborate toolkits are known for chimpanzees in Gabon hunting for honey. In yet another dangerous activity, these chimps raid bee nests using a five-piece toolkit, which includes a pounder (a heavy stick to break open the hive’s entrance), a perforator (a stick to perforate the ground to get to the honey chamber), an enlarger (to enlarge an opening through sideways action), a collector (a stick with a frayed end to dip into honey and slurp it off), and swabs (strips of bark to scoop up honey).21 This tool use is complicated since the tools are prepared and carried to the hive before most of the work begins, and they will need to be kept nearby until the chimp is forced to quit due to aggressive bees. Their use takes foresight and planning of sequential steps, exactly the sort of organization of activities often emphasized for our human ancestors. At one level chimpanzee tool use may seem primitive, as it is based on sticks and stones, but on another level it is extremely advanced.22 Sticks and stones are all they have in the forest, and we should keep in mind that also for the Bushmen the most ubiquitous instrument is the digging stick (a sharpened stick to break open anthills and dig up roots). The tool use of wild chimpanzees by far exceeds what was ever held possible.

Chimpanzees use between fifteen and twenty-five different tools per community, and the precise tools vary with cultural and ecological circumstances. One savanna community, for example, uses pointed sticks to hunt. This came as a shock, since hunting weapons were thought to be another uniquely human advance. The chimpanzees jab their “spears” into a tree cavity to kill a sleeping bush baby, a small primate that serves as a protein source for female apes unable to run down monkeys the way males do.23 It is also well known that chimpanzee communities in West Africa crack nuts with stones, a behavior unheard of in East African communities. Human novices have trouble cracking the same tough nuts, partly because they do not have the same muscle strength as an adult chimpanzee, but also because they lack the required coordination. It takes years of practice to place one of the hardest nuts in the world on a level surface, find a good-sized hammer stone, and hit the nut with the right speed while keeping one’s fingers out of the way.

The Japanese primatologist Tetsuro Matsuzawa tracked the development of this skill at the “factory,” an open space where apes bring their nuts to anvil stones and fill the jungle with a steady rhythm of banging noise. Youngsters hang around the hardworking adults, occasionally pilfering kernels from their mothers. This way they learn the taste of nuts as well as the connection with stones. They make hundreds of futile attempts, hitting the nuts with their hands and feet, or aimlessly pushing nuts and stones around. That they still learn the skill is a great testament to the irrelevance of reinforcement, because none of these activities is ever rewarded until, by about three years of age, the juvenile starts to coordinate to the point that a nut is occasionally cracked. It is only by the age of six or seven that their skill reaches adult level.24

When it comes to tool use, chimps always catch the limelight, but there are three other great apes—bonobos, gorillas, and orangutans—that, together with chimps, us, and the gibbons, make up the Hominoid family. Not to be confused with monkeys, Hominoids are large, flat-chested primates without tails. Within this family, we are closest to chimps and bonobos, both of which are genetically nearly identical to us. Naturally, there is heated debate about what the minuscule-sounding 1.2 percent DNA difference between us and them exactly means, but that we are close family is not in doubt. In captivity, the orangutan is an absolute master tool user, dexterous enough to tie knots into loose shoelaces, and to construct instruments. One young male was seen to join three sticks, which he had first sharpened, into two tubes to build a five-section pole to knock down suspended food.25 Being notorious escape artists, orangs may dismantle their cage so patiently, from day to day and week to week, while keeping dislodged screws and bolts out of sight, that keepers fail to notice what they are doing until it’s too late. In contrast, until recently all we knew about wild orangs was that they sometimes scratched their butt with a stick or held a leafy branch over their head during rain. How could a species that is so talented offer so little evidence of tool use in the wild? The inconsistency was resolved when, in 1999, the tool technology of orangutans in a Sumatran peat swamp came to light. These orangs extract honey from bee nests with twigs and use short sticks to remove the seeds embedded in the stinging hairs of neesia fruits.26

The other ape species, too, are perfectly capable of tool use, and we have already laid to rest the view that gibbons lack this capacity.27 But reports from the wild remain meager to nonexistent, sometimes suggesting that only chimps are proficient tool users. We see glimpses, such as when gorillas preventively disarm poacher snares, which requires a grasp of basic mechanics, or traverse deep water. When elephants had dug a new water hole in a swampy forest in the Republic of Congo, the German primatologist Thomas Breuer saw a female gorilla, Leah, try to wade across. She stopped when she was waist-deep into it, however—apes hate swimming. Leah returned to shore to pick up a long branch to gauge the water’s depth. Feeling around with her stick, she walked bipedally far into the pool before retracing her steps to return to her wailing infant. This example highlights the shortcomings of Beck’s classical definition, because even though Leah’s stick altered neither anything in the environment nor her own position, it did serve as a tool.28

Chimpanzees are recognized as the most versatile primate tool user apart from us, but this heralded position has been challenged. The challenge did not come from any Hominoid but from a small South American monkey. Brown capuchin monkeys have been known for centuries as organ-grinders and more recently as trained helping hands for quadriplegics. They are extremely manipulative and particularly good at tasks that tap into their tendency to smash and bang things. Having had a colony of these monkeys for decades, I know that almost anything you hand them (a piece of carrot, an onion) is going to be pounded to mush on the floor or against the wall. In the wild, they pound oysters for a long time until the mollusk relaxes its muscle so they can pry it open. During the fall, our monkeys in Atlanta collected so many fallen hickory nuts from nearby trees that we’d hear frantic banging sounds the whole day in our office adjacent to the monkey area. It was a happy sound, because capuchins seem to be in their best mood when they are doing things. Not only did they try to break open the nuts, they also employed hard objects (a plastic toy, a block of wood) to smash them with. About half the members of one group learned to do so, whereas the second group never invented the technique despite having the same nuts and tools. This group obviously consumed fewer nuts.

Capuchins’ natural predisposition to be persistent pounders sets them up for nut cracking in the field. A Spanish naturalist first reported it five centuries ago, and more recently an international team of scientists found dozens of cracking sites in the Tietê Ecological Park and other sites in Brazil.29 At one site, capuchins eat the pulp of a large fruit, after which they drop its seeds to the ground. They return a couple of days later to collect those seeds, which by then have dried out and are often infested with larvae, which the monkeys are fond of. Traveling with the seeds stuffed in their hands, mouth, and (prehensile) tail in search of a hard surface, such as a large rock, the monkeys would get a smaller stone to pound the seeds with. While these stones are about the same size as those used by chimps, the capuchins are only about the size of a small cat, so their hammers weigh about one-third of their body! Literally acting as heavy equipment operators, they lift them high above their heads to get a good hit. When the tough seeds are cracked, the larvae are there for the picking.30

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!

I first encountered the tube task during a visit to Jigokudani Monkey Park, in Japan, in one of the world’s coldest habitats with native primates. Tourist guides use the task to demonstrate monkey intelligence. At the feeding site next to the river, which attracts snow monkeys from the surrounding montane forest, a horizontal transparent tube was baited with a piece of sweet potato. Rather than wielding a stick like the capuchins, one female snow monkey pushed her small infant into the tube while firmly holding on to its tail. The baby crawled toward the food and grabbed it, only to be quickly withdrawn by its loving mom, who pried the prize from its resistant clutch. Another female collected rocks to throw into one end of the tube, so that the food came out the other end.

These are macaques, monkeys much closer to us than capuchins. The most spectacular evidence for macaque tool use has been collected by Michael Gumert, an American primatologist. On Piak Nam Yai Island off the coast of Thailand, Gumert found an entire population of long-tailed macaques using stone tools. I am very familiar with this species, having done my dissertation on them. Also known as crab-eating macaques, these smart monkeys are rumored to hang their long tails in water to pull up crabs. I have seen them use their tail almost like a stick to obtain food. Unable to control it as South American primates do—a macaque’s tail is nonprehensile—they grab their tail with one hand and swap food from outside to inside their cage with it.

Manipulating one’s own body appendage is yet another example that stretches the definition of tool use, but there is no doubt that what Gumert discovered is a well-developed technology. His monkeys on the coast collect stones everyday for two purposes. Bigger stones serve as hammers to pound oysters with blunt force until they break open, revealing a delicious rich food source. Smaller stones are used rather like axes, applying a precision grip and more rapid movements, in order to dislodge shellfish from rocks. During the few hours of ebb tide, both food and tools are abundant, an ideal situation for the invention of this seafood technology. It is testimony to the generalized intelligence of primates, because obviously they evolved in the trees, eating fruits and leaves, but here they were surviving on the beach. After humans, chimps, and capuchins, a fourth primate has entered the Stone Age.35

Beyond the primates, however, there are quite a few tool-using mammals and birds. Coastal Californians can watch their own floating technology every day among the kelp. The popular furry sea otter swims on his back while using both front paws to smash shellfish against a stone anvil on his chest. He also hammers abalones with a large rock to dislodge them, taking multiple dives to finish this underwater job. A close relative of the otter may possess even more spectacular talents. The honey badger is the star of a viral YouTube video full of expletives to indicate how “badass” this Chuck Norris of the animal kingdom is. The species is even featured on T-shirts emblazoned with “Honey badger don’t care.” This so-called badger is a small carnivore, which actually—like the otter—belongs to the weasel family. While I know of no official reports about their skills, a recent PBS documentary features a rescued honey badger named Stoffel who invents multiple ways to escape from his enclosure at a South African rehabilitation center.36 Assuming that what we see is not a trained trick, he outwits his human caretakers at every turn and displays the sort of insight for his Houdini act that one might expect from an ape, not a honey badger. The documentary shows Stoffel leaning a rake against the wall and claims that he once piled up large stones against it to escape. After all the stones were removed from his enclosure, he apparently constructed a heap of mud balls for the same purpose.

Even though all this is most impressive and begs for further investigation, the greatest challenge to the supremacy of primates has come, not from other mammals, but from a flock of squawking and cawing birds that landed right in the midst of the tool debate. They caused about as much mayhem as they did in that Hitchcock movie.

During the quiet hours in his pet store, my grandfather patiently trained goldfinches to pull a string. This particular finch is known in Dutch as a puttertje, a name that refers to the drawing of water from a well. Males that could both sing and draw would fetch a high price. For centuries, these little colorful birds were kept in homes with a chain around their leg, pulling a thimble up from a glass so as to fetch their own drinking water. One such finch is featured in the seventeenth-century Dutch painting central to Donna Tartt’s novel The Goldfinch. Of course, we don’t keep these birds anymore, at least not in this cruel fashion, but their traditional trick is very similar to the one that, in 2002, gave us Betty the crow.

In an aviary at Oxford University, Betty was trying to pull a little bucket out of a transparent vertical pipe. In the bucket was a small piece of meat, and next to the pipe were two tools for her to choose from. One was a straight wire, the other a hooked one. Only with the latter could Betty get a hold of the bucket’s handle. After her companion stole the hooked wire, however, she faced the task with an inappropriate tool. Undeterred, Betty used her beak to bend the straight wire into a hook so as to pull the bucket from the tube. This remarkable feat was a mere anecdote until perceptive scientists systematically investigated it with new tools. In subsequent tests, Betty received only straight wires, which she kept subjecting to her remarkable bending act.37 Apart from dispelling the “birdbrain” notion with which birds are unfairly saddled, Betty achieved instant fame by giving us the first laboratory proof of toolmaking outside the primate order. I add “laboratory,” because Betty’s species in the wild, in the Southwest Pacific, was already known for tool crafting. New Caledonian crows spontaneously modify branches until they have a little wooden hook to fish grubs out of crevices.38

Inspired by an Aesop fable, crows have been tested to see if they will throw stones into a tube filled with water to bring floating rewards within reach. They do.

The Ancient Greek poet Aesop may have had an inkling of these talents given his fable The Crow and the Pitcher. “A Crow, half-dead with thirst,” so the fabulist went, “came upon a Pitcher.” There wasn’t enough water in the pitcher for the crow to be able to drink it. He tried to reach in with his beak, but the water level was too low. “Then a thought came to him,” as Aesop put it, “and he took a pebble and dropped it into the Pitcher.” Many more pebbles followed until the water had risen enough for a drink. It seems an unlikely feat, but it has now been replicated in the lab. The first was an experiment on rooks, a corvid that in the wild does not use any tools. The rooks were presented with a vertical water-filled tube with a floating mealworm just out of reach. The water level would have to be raised if the rook were to reach the delicacy. The same experiment was carried out with New Caledonian crows, known as real tool experts. True to the dictum that necessity is the mother of invention, and confirming Aesop’s story several millennia later, both crow species successfully solved the floating worm puzzle by using pebbles to raise the water level in the tube.39

Let me add some caution, though, because it is unclear how insightful this solution was. For one thing, all the birds had been pretrained using a slightly different task. They had received ample rewards for plunging stones into a tube. Moreover, while they were facing the tube with the mealworm, stones had been conveniently placed right next to it. The experimental setup strongly suggested the solution, therefore. Imagine that K?hler had taught his chimps to stack boxes! We would never have heard of him, as it would have undermined any claims of insight. In the course of testing, the crows did learn that large stones work better than small ones, and that there is no point dropping stones into a pipe filled with sawdust. Rather than working these answers out in their minds, however, it may have been a matter of fast learning. Perhaps they noticed that adding stones to water brought the mealworm closer, which led them to persist.40

When we recently presented our chimpanzees with a floating peanut task, a female named Liza solved it right away, adding water to a plastic tube. After some vigorous but ineffective kicking and shaking of the tube, Liza abruptly turned around, went to the drinker to fill her mouth, and returned to the tube to add water. She made several more trips to the drinker before she got the peanut at the right level to reach it with her fingers. Other chimps were less successful, but one female tried to pee into the tube! She had the right idea even though the execution was flawed. I have known Liza all her life and am sure that this problem was brand new to her.

Our experiment was inspired by a floating peanut task conducted on a large number of orangutans and chimpanzees, a subset of which cracked the puzzle at first sight.41 This is especially remarkable, since—unlike the crows—the apes had no pretraining; nor did they find any tools nearby. Rather, they must have conjured the effectiveness of water in their heads before going out of their way to collect it. Water doesn’t even look like a tool. How hard this task is became clear from tests on children, many of which never found the solution. Only 58 percent of eight-year-olds came up with it, and only 8 percent of four-year-olds. Most children frantically try to reach the prize with their fingers, then give up.42

These studies have set up a friendly rivalry between primate chauvinists and corvid aficionados. I sometimes teasingly accuse the latter of “ape envy,” because in every publication they draw a contrast with the primates, saying the corvids are either doing better or at least equally well. Calling their birds “feathered apes,” they make outrageous claims such as “The only credible evidence of technological evolution in nonhumans to date comes from New Caledonian crows.”43 Primatologists, on the other hand, wonder how generalizable corvid tool skills are, and if “feathered monkeys” isn’t a better moniker for the birds. Are crows one-trick ponies, like the clam-smashing otters or the Egyptian vultures that throw rocks at ostrich eggs? Or do they have the intelligence to take on a broad array of problems?44 This issue is far from settled, because even though ape intelligence has been studied for over a century, corvid tool studies have come up only in the last decade.

An intriguing new entry is the use of metatools by New Caledonian crows. A crow is presented with a piece of meat that it can retrieve only by using a long stick, but this stick is behind bars wide enough for the crow’s beak but not its head. The crow is unable to reach the tool. In a nearby box, however, lies a short stick suitable for retrieving the long one. To solve this problem, the right order is to pick up the short stick, use it to fetch the long one, and then apply the latter stick for the meat. The crow needs to understand that tools can be used on nonfood objects and to take steps in the right order. Alex Taylor and coworkers used wild New Caledonian crows on Maré Island, placed temporarily in an aviary. They tested seven crows, all of which managed metatool use; three followed the right sequence on the first attempt.45 Presently, Taylor is trying out tasks with even more steps, and the crows are keeping up with the challenge. This is most impressive, and considerably better than monkeys, which have trouble with stepwise tasks.

Given the evolutionary gulf between primates and corvids, and the many ancestral species of mammals and birds in between that don’t use tools, we are dealing with a typical example of convergent evolution. Independently, both taxonomic groups must have faced a need for complex manipulations of items in their environment, or other challenges that stimulated brain growth, which led them to evolve strikingly similar cognitive skills.46 The arrival of corvids on the scene illustrates how discoveries of mental life ripple across the animal kingdom, a process best summarized by what I’ll call my cognitive ripple rule: Every cognitive capacity that we discover is going to be older and more widespread than initially thought. This is rapidly becoming one of the best-supported tenets of evolutionary cognition.

As a case in point, we now have evidence of tool use outside mammals and birds. Primates and corvids may well show the most sophisticated use of technology, but what to think of partially submerged crocodiles and alligators balancing large sticks on their snouts? Crocodilians do so especially in pools and swamps near rookeries during the nesting season, when herons and other wading birds are in desperate need of sticks and twigs. You can imagine the scene: a heron lands on a log in the water from which it wants to pick up an attractive branch, but suddenly the log comes to life and grabs the bird. Perhaps crocs initially learn that birds land on them when branches float nearby and then extend this association by making sure to be near branches when herons are nesting. From there, it may be a small step to cover oneself with objects that attract birds. The problem with this idea, however, is that there are actually very few free-floating branches and twigs around. There is too much demand for them. Is it possible that the crocs—which the scientists lament are historically taken to be “lethargic, stupid, and boring”—bring their stick-lures with them from far away? This would be another spectacular cognitive ripple, one that extends deliberate tool use to the reptiles.47

The final example, which may again stretch the definition of a tool, concerns the veined octopus in the seas around Indonesia. Here we are dealing with an invertebrate: a mollusk! It has been seen collecting coconut shells. Since octopuses are a favorite food of many predators, camouflage is one of their main goals in life. Initially, the coconut shells yield no benefit, however, because they have to be transported, which only draws unwanted attention. Stretching its arms into rigid limbs, the octopus tiptoes over the sea floor while holding its prize in some of its other arms. Awkwardly walking to a safe lair, it can then use the shells to hide underneath.48 A mollusk collecting tools for future protection, however simple, goes to show how far we have come since the days when technology was thought to be the defining characteristic of our species.

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