A female chimp called Ai has learned how to use the Arabic numerals, 1 to 9, to represent numbers and her latest feat is to be able to memorise the order of five numbers.

"Ai's performance shows that chimpanzees can remember the sequence of at least five numbers, the same as, or even more than, pre-school children," said the Japanese researcher who led the research, Tetsuro Matsuzawa, in the journal Nature.

Professor Andrew Whiten, a primate expert from the University of St Andrews, told BBC News Online: "In one sense, as a chimpanzee researcher, I'm not that surprised. Every year we find something else that chimpanzees can do which brings them one step closer to us.

"But if you compare the performance of this chimp with what an average human can do, the chimpanzee is doing remarkably well."

Magic number

Professor Whiten explained: "We can remember a seven-digit number at first sight, our brain's 'magic number'. But here's a chimp remembering five - that's awfully close. And although their brain is large for a primate, it is only half the size of ours."

Ai is kept at the Primate Research Centre at Kyoto University. She was shown five numbers on a computer screen. Her task was then to touch each one in the correct order, but on touching the first number, the rest were covered.

Therefore to perform the task successfully, she had to memorise all the numbers. And she did.

Ai got the fourth number correct 90% of the time, compared with the predicted 13% if she was guessing randomly. She got the fifth number correct 65% of the time, compared to a 6% random chance.

Pauses observed by the team showed that "Ai inspected the numbers and their locations and planned her actions before making her first choice."

Ai, 23, is no ordinary chimp. She has been learning linguistic and other skills at the Primate Research Centre since 1978. She is also a keen painter and has her own web site and e-mail address.

Professor Whiten believes that much remains to be discovered about the abilities of chimps: "We have only studied chimpanzees for a few decades, despite sharing the planet with them for hundreds of thousands of years - we're only just getting to know them."

Chimp treatment

And he believes this type of research should inform the way in which humans treat chimpanzees: "As researchers, our job is to show what chimpanzees can do and the ways in which they are similar to you.

"It's then up to the people who want to use them in medical experiments, or even eat them as they do in some parts of Africa, to decide how comfortable they feel with what they do."

He added that when a colleague showed a video of chimpanzee performances to an African village where chimps were eaten, one of the villagers said "I can't eat him anymore, he is too close to me".