Teaching sign language to apes gives an insight into what they are really thinking - that they are human. Sanjida O'Connell reports
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When Penny went into the kitchen, she found a smashed cup lying on the floor. "Did you do this?" she asked Katharine angrily. Katharine said no, Mike had done it. Penny pressed her. "Are you sure it was Mike?" she asked. Katharine became uneasy and said Julie had done it. "You're lying," said Penny crossly, and Katharine admitted that she'd broken the cup.

Katharine's real name is Koko, and the surprise is that she and Mike are gorillas. There are now more than ten great apes (chimpanzees, orang- utans, gorillas and pygmy chimpanzees) who can use sign language to converse both with their human trainers and with each other. Some of the animals use American sign language for the deaf; others use keyboards with Yerkish, a language of invented symbols. When you press a key, the word lights up at the top.

These apes have not merely been trained to point to the top left-hand corner of the keyboard in exchange for a sweet; some of them fully understand the meaning of the symbol they are using. Wild apes have no need of language and have not developed it, but these ones now have a sharper tool for communicating with one another. Human companionship has also made them more similar to people than their wild counterparts. New research has shown, however, that while some apes are as skilled as two-year-old children in their use of language, they use words to communicate in a far more egotistical and unthinking way than a young child.

The best pupil so far seems to be Kanzi, a pygmy chimpanzee (or bonobo, as they are sometimes called). Kanzi has been trained by Professor Sue Savage-Rumbaugh at the Department of Biology and Language Research Centre at Georgia State University, Atlanta. He knows more words than a two-year- old child, and can understand spoken English. Kanzi and the other apes "speak" in rudimentary sentences with a rudimentary grammar.

Like people learning a foreign language, their comprehension is often far more extensive than their vocabulary. Despite their comparatively small number of words, they will use them in a creative fashion when they haven't been taught the symbol for novel events: Sherman, a chimpanzee, said there was a "scare" happening outside when he saw a chimp being carried away in a transport cage; Washoe described a water melon as a "candy fruit drink"; Panbanisha said she wanted to watch "ice TV" when it was snowing outside, and that a lady who came to visit her had hair that looked like a mushroom. According to Professor Savage-Rumbaugh, she was right.

Giving apes language has allowed people to pry into the mind of an ape far more than was thought possible. Once, when Professor Savage-Rumbaugh was driving through the woods with Kanzi's sister Panbanisha, she noticed that the bonobo appeared to be very quiet and pensive. "I was moved to ask her what she was thinking - a question I generally avoid, since I have no means of validating the answer or even knowing whether an ape understands the question. However, at this moment Panbanisha looked literally lost in thought - so I dared. She seemed to reflect upon the question for a few seconds, then answered: 'Kanzi'. I was very surprised, as she hardly ever uses Kanzi's name."

All great apes use sounds and gestures to communicate with each other, but language seems to have given them an alternative mode of communicating. Remote-controlled video cameras at Professor Roger Fouts's Primate Research Centre at Ellensburg, Washington State, have picked up the chimps signing to each other when no one else is present.

Usually they ask one another for hugs, food or games. Instead of lifting an arm and shuffling around to indicate which part of their body they want groomed, they can tell the other chimp exactly which bit to scratch. Instead of making the gesture that means play, they can specify whether they want to be chased or tickled.

The signing apes can use words to communicate and understand that a particular sign refers to a specific object or action. But research by Esteban Rivas at the University of Nijmegen in the Netherlands has shown that chimpanzees are not as adept as children when communicating their own and other apes' internal mental states - a skill that has come to be known as "Theory of Mind".

Young children will use words to describe their emotions and the emotions of others, as well as other peoples' desires for food or a toy, and what they think, know, remember, or dream. When Rivas analysed the signs used by the chimpanzees, he discovered that many of them used some of the same words as the children but in a less sophisticated way.

For example, the chimps would sign "cry" if a person did not give them a sweet they had requested, "thirsty" when they wanted a drink, or "hug" when someone had scolded them and they wished to be reassured. Words like "cry", which refer to internal states of mind, were all used as requests or to influence the behaviour of their human caretaker. Very few of the apes referred to how they themselves felt, or signed about the mental states of companions.

Esteban Rivas concluded that the apes can show behaviour which indicates a rudimentary understanding of others' states of mind, but that they are far more egotistical and desire-driven than young children. They use these signs merely to convey what they want, and they are not particularly motivated to describe how they and their companions feel.

There is no doubt, however, that teaching animals language can make them more human-like in other ways. Professor Savage-Rumbaugh says of the apes that use signing and symbols: "They clearly have a language capability of a human sort, limited though it is." She and Melinda Carpenter and Professor Michael Tomasello from Yerkes Regional Primate Centre at Emory University, Atlanta, ran some tests on apes accustomed to using sign language; they found that the chimpanzees and bonobos with the most practice at sign language and the most exposure to people reacted far more like children when they played with toys. They imitated how a human demonstrator played, and looked up at the demonstrator (wild chimps do not usually make eye contact).

Another thing seen in "signing" chimps, but rarely seen in wild ones, is food-sharing. Wild chimps will beg each other for food, usually to no avail. The chimp with the food will carry on eating until he or she is full, and only then - if there is any food left - will the beggar get some. Austin and Sherman, two of Professor Savage-Rumbaugh's chimps, have started to share their food. When given different types of food, they ask each other for portions of the other chimp's. Professor Savage- Rumbaugh makes things easier for them, by giving each two portions of the same food so they can simply hand over one to their companion chimp and keep the other for themselves.

However, sometimes one chimp will eat both portions of one type of food, even though the other chimp is asking him to share. But he will then give both portions of another kind of food to the chimp who is requesting it. Professor Savage-Rumbaugh has an explanation for this. It is as if one animal is saying to the other: "I'm sorry, I couldn't help eating all of that food, so you have all the other sort."

Professor Fouts stresses that language can affect how you view the world. Different languages give people a different outlook on life: "You're English, you speak Eng- lish. The French speak French. Americans are weird; they speak a kind of English. Language and culture are entwined." The apes that use sign language no longer act truly like apes, and they do not consider themselves to be apes. Washoe, one of the oldest signing chimps, saw some chimpanzees when she was five. When she was asked what they were, she said they were "black bugs".

Once humanised or, to use the jargony American word, "enculturated", there is a peculiar cruelty in depriving these apes of the mental stimulation of language. Booee, a chimpanzee who uses sign language, is hepatitis- C positive. There is no cure. He was infected with the virus as part of hepatitis research at the Lemsip Corporation in New York.

Professor Fouts taught Booee sign language at the Institute of Primate Studies at Oklahoma, but resigned when the Institute sold the chimp and other apes for medical research. Years later, when Professor Fouts visited Booee, the animal remembered him and signed his name. Booee is still held at Lemsip, and Jane Goodall and others are attempting to raise enough funds to buy him back.

Professor Fouts is constantly struggling for cash to care for his chimpanzees. Every year he tries to get money from the National Institutes of Health and the National Science Federation, the two main funding bodies in the States. Every year they say no. "I've committed one of the greatest sins in science by criticising my fellow scientists for bad research and inhumane behaviour, and I've been excluded."

His wife, Deborah, puts it very eloquently: "These chimps have an uncertain future. What are we going to do in 15 years' time when we have no grant, no endowment to care for the animals? Washoe is the oldest and she could live another 25 years. What are we going to do when we're not strong enough to care for her and we still have no money?"

Arguably, sign language has benefited individual apes and enriched their lives in certain ways. But it is not true to say that wild chimpanzees and other great apes do not have an equally rich or varied life in the wild. What we have done, we cannot undo; neither should we abandon animals over which, in a sense, we have guardianship. This does not mean we should continue to raise chimpanzees and other great apes to exist in this ape- human half-world, because it helps us understand them better. While we may have leanings towards being Dr Doolittle, we should be patient and accept and learn from them on their own terms. In the meantime, these apes continue to intrigue and delight us with their simian-human outlook on the world.

The film Congo, which is based on a book by Michael Crichton, starred a gorilla called Amy. She used sign language for the deaf, and wore a virtual-reality glove with a speaker attached to it which translated her hand gestures into computerised American speech. It's certainly true that Amy is vocal. "Me want green drop drink," she once said. If the apes that use sign language were allowed to drink alcohol, what better way to ask for a dry Martini? !