Many years ago while on a visit to London Zoo I experienced first hand the wily intelligence of chimpanzees in the days when they were kept behind wire mesh. The captive troupe had rehearsed a kind of primate nonchalance that would attract a curious crowd of onlookers gathered around their caged compound. Then, with little warning, they would start to fling dung at their human audience, jumping up and down with apparent glee at the sight of the fleeing crowd.
As dirty protests go, it was relatively unsophisticated. Some years later, primatologist Mathias Osvath of Lund University in Sweden documented a rather more complex strand of protest in a chimp called Santino who lives in Furuvik Zoo. Santino showed that it was possible for chimps to plan for the future. He did this by methodically building up a cache of stones in the early morning, hours before opening time. When the first zoo visitors appeared, he began to enthusiastically hurl his missiles at the gawping humans.
Dr Osvath concluded that Santino's actions showed that chimps have a rather well developed form of intelligence, one that could envisage "life-like mental simulations of potential events". By anticipating opening time, and preparing for it with his cache of rocks, Santino and the dung-chucking chimps at London Zoo were able to construct mental pictures of the future using an element of rudimentary consciousness known as forward planning. "They most probably have an 'inner world' like we have when reviewing past episodes of our lives or thinking of days to come," Dr Osvath said at the time.
The degree to which chimps think and behave like humans has been the subject of endless speculation, and many scientific studies. When we gaze into the face of the chimpanzee, Pan troglodytes, our closest living relative with whom we share more than 98 per cent of our DNA, we are also looking into the eyes of another highly sentient being who might in many ways pass for one of us.
Indeed, chimps are capable of an array of behaviours that were once considered the sole preserve of humans and some scientists have seriously suggested that chimps, which belong to the genus Pan, should in fact be classified as Homo – the human family. They make simple tools, they are fascinated by fire and rain and have even been known to appreciate a sunset. They mourn their dead, they make war on members of the same species and, rather chillingly, they are said to be the only animal other than humans who deliberately plan the murder of rivals.
They are also capable of endearing acts of love and tenderness. They actively seek one another's affections and reassurance, they can laugh and be tickled, they conspire with one another and form alliances. They are able to recognise themselves in a mirror, are capable of "tactical deception" to fool a companion or competitor, and they even show signs of engaging in the kind of rudimentary fictional play behaviour seen in very young children.
Chimpanzees are considered to be the most intelligent of all the "non-human" primates, which include the other apes such as the gorilla and orang-utan. But their intelligence is both fascinating and frightening, in part because there is something irrevocably inhuman about the adult chimp's bestial strength and unpredictability.
Juvenile chimps can be very sweet but they grow into formidable beefcakes with a body strength several times greater than the strongest man. Yet they have the temperament of a two-year-old, as Charla Nash, a 55-year-old woman from Connecticut, can testify. In 2009, she had her face ripped off by a 200lb pet chimpanzee called Travis who up to that moment had been the adored companion of Nash's best friend.
The film, Rise of the Planet of the Apes, released next week, plays on this fear. In it, the innate strength and agility of chimps is powerfully combined with a kind of artificial, supercharged intelligence accidentally conferred to the primates by an experiment gone wrong. Scientists searching for a cure for Alzheimer's infect a chimp called Caesar with a potential new treatment, which involves the growth of new brain cells that coincidentally impart a human-like intelligence.
The film, unlike the 1968 original starring Charlton Heston and Roddy McDowall, recognises the scientific impossibility of chimps ever being able to speak – they just don't have the vocal cords. Instead, the lead character Caesar learns to communicate with his human captors with sign language, having mastered some 300 individual signs.
This is not such a fanciful concept. Experiments where chimps have been trained to learn a sign language are well-established. Washoe, a chimp who in the 1960s was captured in the wild as a baby in West Africa, was the first primate to learn American Sign Language. He was raised as a human child in Washoe County, Nevada, by a pair of husband-and-wife psychologists who taught him several hundred signs. Apparently, when Washoe first saw a swan he made the signs for "water" and "bird", which led one Harvard psychologist to say that it was like "getting an SOS from outer space".
Another baby chimp raised as a human child as part of an experiment in communication was Nim Chimpsky, the subject of a forthcoming documentary by Man on Wire director James Marsh called Project Nim. Nim Chimpsky (a play on linguist Noam Chomsky, who thought humans were the only animals capable of speech) learnt a form of sign language that enabled him to communicate with his human companions.
There are many other examples of chimps in scientific captivity who have excelled in communication skills. A chimp called Kanzi, for instance, could accurately locate the correct printed symbols on a lexigram – a computer screen full of symbols – for a given spoken word or phrase, suggesting the primate identification of spoken sounds. While a chimp called Panbanisha had been trained to understand simple English sentences, and a female called Ai was taught to count to 10 and remember five-figure numbers, just two short of the seven-digit telephone numbers most people can just about recall.
More elaborate experiments involving the use of mirrors have demonstrated that chimps are better than any other intelligent animal at recognising their own reflections. This is considered important because being self-aware provides a capacity to reflect on internal mental states – as René Descartes pointed out: "I think, therefore I am."
Scientists are particularly intrigued by the notion that chimps may be able to envisage what other chimps may be thinking about. The idea is called "theory of mind" and human children, other than autistics, develop the concept pretty early in life. Many experiments have been devised to see if chimps too have a theory of mind.
One of the simplest tests involved putting food in one of a number of boxes and asking the chimp to choose between one of two human assistants to open the box and provide them with the tasty morsel. The catch is that only one of the two assistants was in the room when the box was baited, the other having conspicuously left it at that moment.
Both assistants then point to different boxes requiring the chimps to make a choice. When faced with the dilemma of which assistant to choose from, most chimps choose the assistant who was in the room at the time the box was baited. In other words these chimps understand the idea of "false belief" in someone else's mind, and in so doing show an element of theory of mind.
However, I say only an element because there are slightly more complex versions of this well-known psychology experiment, known as the "Sally-Anne" test, which six-year-old children can easily solve, but which prove very difficult for chimps. In other words, chimps are far from having the sort of intelligence we equate with a normal human child.
This is hardly surprising given that the human brain is about three times larger in comparison to body size compared to the chimp brain. We last shared a common ancestor with each other between 5 and 7 million years ago. Some 350,000 generations separate our evolutionary history, a long period of genetic partition that explains why we are intellectually so different.
And yet, studies of wild chimps living in their natural forest habitats of West Africa show a surprising similarity of behaviours. They routinely make simple tools, for example stripping thin branches of their leaves to fish for termites or using leaves as sponges for drinking. They also pass on their technological know-how to the next generation, which has led to the emergence of peculiar cultural differences between different bands of chimps, similar to the cultural differences seen between human societies.
In the wild, the common chimp lives in a society dominated by an "alpha male", but he can only rule if he is able to enrol the help of a band of "beta males" using skilful, almost Machiavellian guile. Indeed, primatologists observing chimps in the wild and in captivity have documented many instances of "tactical deception", when one chimp tries to deceive another, whether it is pretending not to notice a banana for fear that others may see you with it, or trying to covertly mate behind the back of the alpha male.
It is this kind of Machiavellian intelligence that we humans are so good at. We have our good side, but we also lie and cheat in order to gain advantage over others. Studies of chimps suggest that this type of behaviour may go back to our shared common ancestor. When we watch chimps and how they treat one another, we are seeing how our own direct ancestors may have behaved many millions of years ago. In other words, there is a little bit of troglodyte in all of us. That's what we find so unnerving when we look into the eyes of our closest living relative.
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