TELEVISION / Out of their trees: Birds don't do it. Bees don't do it. But educated apes can do it. Thomas Sutcliffe watches Horizon

About half way through 'Chimp Talk', Jenny Jones' documentary about ape communication for Horizon (BBC2), you saw an encounter between an experimenter and the chimp she was attempting to teach beginner's maths. A polished, leathery hand reached out between the bars and gently brushed the researcher's eyelashes. It was like a budget remake of the scene in ET when the alien says his farewell to the small child, literally signing off with a mute display of love. Something of the same poignancy, of sophisticated 'human' emotions forced to make do with clumsy expression, pervaded Jenny Jones' engrossing documentary about research into the linguistic ability of apes.

This was a return visit for Horizon, which came with a towel to mop up the cold water it had poured over previous theories of primate language ability. In the earlier documentary the latest state of knowledge (or at least the latest change of fortune in the battle between opposing schools) held that chimps couldn't actually make sentences - that in most cases they were simply imitating the unconscious prompts of those teaching them. By a nice irony, those who believe that chimps can use language are now arguing that the ability to imitate, far from being merely dumb, is a difficult skill inseparable from the development of language, whether in a human child or a young chimp. Certainly the film here, of chimps imitating a researcher making stone tools and then applying the 'new technology' for their own ends, was pretty convincing. A chimp called Kanzi is clearly capable of recognising spoken words and passes comprehension tests at about the same level as a two-and-a-half-year-old child. Another chimp was shown dealing with numerals and fractions.

There are obvious moral implications to the discovery that one of the stoutest fences we have erected between ourselves and other animals now seems a little shaky. If apes can use language then anthropomorphism is not just a sentimental example of woolly thinking but a proper recognition of kinship. Horizon's twee ending (they thanked the chimps for their help in filming) gestured towards this and its false note of equality illuminated the real moral issue - only we have the power to make decisions about how apes will live.