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Chimp Cognition

Chimpanzees and humans are "more closely related to each other than either is to any other living primate" (Begun, 1992). Perhaps you have heard that humans and chimps are "99% identical" in their genetic structure. That is true, but it is a bit misleading.

As Plomin and Kuse (1979) point out, the average stretch of human DNA is almost 99% identical to the corresponding stretch of chimp DNA. However, small differences in DNA can lead to numerous differences in the proteins generated by the DNA.

Also, epigenetic effects (not visible in DNA) can influence which portions of the DNA sequence are expressed. Identical stretches of DNA do not necessarily imply identical genetic expressions.

How is the claim that chimps are 99% genetically identical to humans somewhat misleading?

Comparing proteins from chimps and humans, only about 75% are identical. Still, that is a lot of overlap, so it is not surprising that chimps come the closest to human intelligence of all non-human animal species.

old photos of chimp assembling stick and using it to knock down banana
Kohler said Sultan showed "insight" in solving a problem. First he assembled a stick, then piled up boxes, then used the stick to reach a banana.

For a long time, psychologists have been impressed by the intelligence of chim­panzees. Perhaps the most famous example comes from Wolfgang Kohler in his book The Mentality of Apes (1925).

Kohler suspended a banana out of the reach of a chimpanzee named Sultan. The chimp assembled two sticks together to make a single instrument, then piled up boxes to reach the bananas.

What was Kohler's classic study of "insight"?

Kohler described Sultan as showing insight, coming up with the idea on his own. Epstein, one of the behavioral psychologists who showed pigeons how to use mirror images of their bodies, decided to see if he could teach a pigeon to do something similar.

Epstein could not completely imitate Kohler's experiment, because a pigeon cannot lift a box. Instead, Epstein had the pigeon retrieve a box from another compartment, climb on it, and peck a small plastic banana in order to get grain reinforcement.

The experiment worked. First Epstein taught components of the goal behavior. He taught the pigeon to peck at the plastic banana for grain reinforcement.

Then he taught the pigeon to push the box around. Finally, as predicted, the pigeon had the insight to put these components together when the banana was hoisted out of reach. The pigeon retrieved the box, climbed on it, and pecked the banana.

How did Epstein get a pigeon to imitate Sultan's feat?

Both Kohler's ape and Epstein's pigeon were combining previously familiar actions to reach a goal. Creativity always involves combining pre-existing components into new combinations.

Kohler claimed that Sultan came up with the solution on his own, based on past experience with sticks, climbing, and bananas. Epstein's pigeon needed long and careful training.

The differences between the two per­formances may be just as important as the similarities. It depends what point you want to make.

Epstein showed that no mysterious, magical abilities were required to explain insight, and that was Epstein's main goal. Kohler showed that apes were could come up with creative solutions to problems without special training, which was significant in itself.

What were similarities and differences between the two performances?

Menzel (1973) studied spatial memory organization in chimpanzees by letting the chimp observe food being hidden in 18 randomly chosen places within a yard. Released from its cage, the chimp quickly gathered the food, following an optimum route that minimized the distance traveled.

How did a chimp optimize its hunt for food?

This indicated that it was not merely imitating the route that the humans had taken. It had formed a mental map of where the food was hidden and was following its own map.

What did Sands and Wright demonstrate?

Sands and Wright (1980) demonstrated that a rhesus monkey could memorize a list of items. Like humans doing such a task, it showed primacy and recency effects, meaning it showed superior recall of early and late items in the list.

The researchers also found that picture memory in monkeys is almost identical to that of humans (Sands and Wright, 1982). Both chimps and humans are exceptionally good at recognizing pictures they have seen the day before.

Ape Language

Robert Yerkes, a pioneering comparative psychologist who studied primates, wrote about the communicative sounds of chimpanzees in 1925:

...Their vocalizations do not consti­tute true language... Apparently the sounds are primarily innate emotional expressions. This is surprising in view of the evidence that they have ideas, and may on occasion act with insight...

Perhaps they can be taught to use their fingers, somewhat as does the deaf...person, and thus helped to acquire a simple, non-vocal sign language. (Yerkes, 1925)

What did Yerkes propose in 1925? What was Yerkish?

Yerkes might have been surprised to learn that 50 years later chimps would be pressing plastic buttons on a keyboard, using a language called Yerkish at the Yerkes Primate Research Center in Atlanta. Yerkes was right that chim­panzees do better with motor movements than with sounds.

The earliest attempts to teach chim­panzees language used spoken words. Results were discouraging. As Yerkes reported in 1925, chimps have "no gift for the use of sounds."

What project did the Kelloggs undertake?

In the 1930s Winthrop and Luella Kellogg raised a female chimp named Gua along with their own baby. Gua learned to respond appropriately to 100 words but never learned to speak.

However, in their own defense, the Kelloggs later pointed out they were not trying to teach her to speak. They were more interested in the experiment of raising a chimpanzee alongside a human baby and comparing their development.

In the 1940s Keith and Cathy Hayes also raised a chimpanzee from infancy. They succeeded in teaching the chimp to mouth some words by using his hand to push his lips into unfamiliar positions.

The words that the chimp spoke were Mama, Papa, and cup, all pronounced in a breathy whisper barely recognizable as speech. The chimp mouth is simply not designed for human speech. (By con­trast, Neanderthal throat structure is similar to humans, revealed by the shape of the hyoid bone, so scientists believe they could speak.

How did the Gardners finally make progress?

The solution, as Yerkes suggested, was to use signs. A chimp named Washoe was raised from one year of age by Allen and Beatrice Gardner, with the express purpose of teaching her sign language.

The Gardners used only American Sign Language around Washoe. Experienced signers visited Washoe, played with her, and signed to her as they would a pre-school child. The Gardners recorded many controlled experiments on film, providing evidence that Washoe knew the meaning of many signs.

What abilities did Washoe display?

Washoe cussed with the sign "dirty," which was also her name for excrement. She learned to ask for favorite toys such as a doll.

Washoe asked to go outside when she wanted to. She asked for tickles and hugs, and she labeled environmental objects.

Once Washoe made up a new com­pound word. Witnessing a duck for the first time, she signed water and bird in quick succession. "Water bird" became a famous example of Washoe's creativity in language.

At the climax of their research, the Gardners participated in a film titled The First Signs of Washoe. It triumphantly heralded the demise of the "language is unique to humans" theory.

The film opened with quotations from famous linguists like Noam Chomsky saying (in effect) "only humans talk." Then the movie showed Washoe labeling various objects and situations with correct signs. It was a powerful and effective demonstration.

However, another scientist working with chimp language–Herbert Terrace–did not see talking in the Washoe film. Analyzing the sequences of human/chimp signing in slow motion, Terrace found that Washoe was often imitating her human trainers.

Also, the chimp emitted many more errors than correct signs. The humans simply ignored the errors and respond­ed to the correct sequences, much as human parents of a one year old ignore babble while responding to anything that sounds like a word.

What abilities did Nim show and not show? What did Terrace conclude?

Terrace and a group of assistants decided to repeat the chimp sign language experiment with better controls. They raised a chimp named Nim Chimpsky. The name is a reference to linguist Noam Chomsky.

Nim was taught sign language using intensive training techniques similar to those employed by the Gardners with Washoe. Like Washoe, Nim succeeded in learning to use sign language for labeling objects and actions.

However, also like Washoe, Nim never mastered the grammar of sentence construction. If Nim wanted grapes, he might emit a sequence like, "Nim eat grape grape grape eat eat Nim eat." Terrace (1985) concluded that chimps can learn labeling but they cannot learn to construct sentences using grammar as well as a human two year old.

What did Terrace see in the Washoe film?

Researchers at the Language Research Center, associated with the Yerkes Primate Research Center in Atlanta, refused to give up. Sue Savage-Rumbaugh carried out a long and careful study of language ability in the bonobo.

The star student was Kanji. Kanji learned many signs and also showed a definite ability to generate two-word sentences in which order conveyed information, a basic requirement of grammatical speech.

For example, "tickle Kanji" meant something different from "Kanji tickle." This proved a chimp could use a primitive form of grammar that researchers called a protogrammar (Gibbons, 1991).

What abilities did Kanji develop, in the research by Sue Savage-Rumbaugh?

Apparently chimps can learn to use labels and a simple form of grammar. However, the significance of this research depends on the point you want to make.

You could say, "Look, apes can use labels and even simple grammar; language is not unique to humans." Or you could say, "Look, after years of intense training, a chimp cannot use language as well as a two-year-old human child. Language is unique to humans." Both arguments can be defended.

What two contrasting conclusions could you draw?

Koko the gorilla, the longtime tutee of trainer Penny Patterson of the Gorilla Institute, developed many impressive abilities. Koko appeared to understand much spoken speech.

Koko had the distinction of being the first non-human primate to engage in an internet chat, in 1998. However, the results were not very coherent. For example, here is one typical sequence:

Q: Does she have hair? Or is it like fur?

Koko: Fine.

Patterson: She has fine hair.

Q: Koko, do you feel love from the humans who have raised you?

Koko: Lips, apple give me.

Patterson: People give her her favorite foods.

A skeptical site concluded:

"It certainly seems like Patterson is coming up with excuses for various signs."

Boysen's Chimps

Psychologist Sarah Boysen of Ohio State University explored the ability of chimps to form concepts such as "more than" and "less than." She found that two of her chimp subjects, Sarah and Sheba, were both capable of learning such discriminations easily.

Boysen then tried a variation of the experiments, using gumdrops as stimuli. Gumdrops were some of the chimps' favorite treats.

The chimpanzee was presented with two plates of gumdrops. One had more on it, the other had less.

For example, one might have five gumdrops, the other three. While the other chimp watched, the chimp being tested was asked to point to one of the plates. The plate to which it pointed was given to the other chimpanzee.

In this situation, the chimp doing the pointing should have learned to point to the plate with fewer gumdrops on it. That would satisfy its selfish motivation to get more gumdrops for itself.

Instead, something odd happened. The chimps insisted on pointing to the plate that had more gumdrops, even though this meant that more gumdrops went to the other chimpanzee.

Boysen says they seemed to know they were making a mistake when they pointed to the plate with more gumdrops. They just could not stop themselves.

Often they expressed frustration by throwing a mini-tantrum immediately after pointing at it, before Boysen removed the plate and gave it to the other chimp. (A student said this was the chimpanzee equivalent of Homer Simpson's "D'oh!")

What experiment did Boysen do with gumdrops?

Boysen herself was surprised the chimps could not maximize their gain. She said, "It was the first task in 20 years that I'd failed to teach a chimpanzee" (Fischman, 1993).

Then she tried a simple variation. She replaced the gumdrops with plastic poker chips. Two plates with gumdrops remained in the background. If a chimp pointed to the plate with fewer poker chips, it got the plate with more gumdrops on it.

Now the chimps had no trouble with the task. They pointed to the plate with fewer poker chips on it and maximized their gain

Before substituting poker chips for gumdrops, the chimpanzees seemed at the mercy of their desire for food. "The chimps understood the rule," Boysen says, "but they couldn't act on it" because of a biological imperative to get more food.

Moving into the symbolic realm, by using poker chips in place of the gumdrops, allowed the chimps to transcend their natural tendancy to grab at desirable food. Instead, they could use abstractions like "more" and "less" to maximize their gain (Fischman, 1993).

Why was Byrne enthusiastic about Boysen's findings?

Richard Byrne of the University of St. Andrews in Scotland was enthusiastic about Dr. Boysen's experiment. He said it provided a clue about what is special in human intelligence.

Suppose, Byrne suggested, the crucial advance in human cognition is the ability to use symbols. We use beliefs, language–in short, abstract thinking–to suppress selfish appetitive behavior and plan for the future.

Early humans had to store food for hard times; they had to plan for the future. Only creatures capable of deferring immediate gratification and planning for the future could engage in such strategic behavior. Humans have such a capability; chimpanzees apparently do not.

How does Colinvaux's "CPSB" concept relate to this?

Colinvaux (1991) made a similar point. He proposed that the "selectively most important aspect of intelligence is the property of overriding innate behavior, for which the term Conscious Prevention of Stimulated Behavior (CPSB) is proposed."

That sounds much like the talent that Boysen's chimps were lacking. They could not prevent themselves from responding to the simulation of a large place full of gumdrops by pointing at it...until they used symbols to substitute for actual gumdrops.

This line of thinking is consistent with a point made when discussing con­sciousness in animals. The biggest difference between cognitive processes of humans and other animals is language.

We can label something and make plans for it (e.g., "This is the seed corn for next year, so don't eat it"). This gives us a huge advantage over every other species.

Emotions in Chimpanzees

Language is not everything. Robert Yerkes preferred to emphasize that chimpanzees had human-like emotions.

Yerkes was a gentle man who loved animals. In his book Almost Human (1925) Yerkes cites many examples of human-like behavior in great apes, including chimpanzees. For example:

Chimpita...took safe refuge in a mango-tree and refused to come to his keeper. "So," says Madam Abreu, "I went to the tree and, speaking to him, pretended that I was injured in the arm and suffering.

Immediately, on seeing that I was in trouble, he jumped from the tree, and coming to me held my arm and kissed it strongly. And so we were able to catch him." (p.124)

What story, related by Yerkes makes it sound like a chimp can empathize with an injured human?

Of course, this is an anecdote. It would have impressed Romanes back in 1884. But anecdotes are not very good forms of scientific evidence. They are stories, and who is to say if they are reported accurately?

Fentress (1992) made a different case, arguing that anecdotes are a valuable source of insights into animal behavior. Faulkes (1993) strongly agreed, calling anecdotes a "gold mine" for inspiring future research.

What are some arguments in defense of reporting anecdotes about animal behavior?

Consider the following example from Terrace's book Nim. Terrace was famous for his skeptical stance about chimp language. But he was impressed by the chimp's capacity for emotion. He reported this anecdote, which could inspire controlled studies but also speaks for itself.

What anecdote about chimpanzee empathy was reported by Terrace?

Tears brought out especially tender behavior on Nim's part. I once saw him rush over to Jennie while she was crying, leap into her arms, and stare intently at her eyes. He then touched her cheeks very gingerly and gently tried to wipe away her tears. (Terrace, 1979, p.38)

Like Yerkes, Terrace was touched by the chimpanzee's display of empathy. Perhaps with regard to language and comprehensive planning, humans are unique on earth. However, in our emotional lives, we seem to have a lot in common with non-human animals.


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Colinvaux P. A. (1991). A model for the selective advantage of intelligence to breeding females. Evolutionary Theory, 10, 15-32.

Faulkes, Z. (1993) Who watches the watchmen? Review of Davis & Balfour on Human-Animal Bond. Psycoloquy: 4(40) Human Animal Bond (4). Retrieved from: .

Fentress, J. C. (1992) History of developmental neuroethology: Early contributions from ethology. Journal of Neurobiology, 23, 1355-1369.

Fischman, J. (1993). New clues surface about the making of the mind. Science, 262, 1517.

Gibbons, A. (1991). Deja Vu all over again: Chimp-language wars. Science, 251, 1561-1562.

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Sands, S. F. & Wright, A. A. (1982). Monkey and human pictorial memory scanning. Science, 216, 1333-1334.

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Woodruff, G., Premack, D., & Kennel, K. (1978). Conservation of liquid and solid quantity by a chimpanzee. Science, 202, 991-994.

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