As the days get colder, and you long for the early days when humans were covered in fur like other mammals, consider a new theory which blames the loss of our hairy covering on our sexual urges. The theory challenges the orthodox view that we lost our hair in order to cope with the heat of tropical climates.

Charles Goodhart, fellow of Caius College, Cambridge, thinks that our nakedness is the result of a long line of males preferring less hairy females. This led to hairless genes being propagated - and thus to runaway sexual selection. If heat had been the cause of fur loss, he says, many more primates would also have lost their hair.

Dr Goodhart, who has persuaded The Biologist to publish his theory next April, says that males have also driven women to develop long hair on their heads. "A man's uncut hair reaches scarcely down to his waist," he told the Linnaean Society recently. "A woman's hair is so long as to be a positive encumbrance to life under primitive conditions and . . . its excessive length must have been evolved by men preferring their women to have long silky hair."

Dr Goodhart says his theory was omitted from the proceedings of an International Primatological Society congress on the grounds that it was too speculative. He also says that males have retained beards and moustaches to use in threat displays to other males.

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Females lost their facial hair because they do not take part in such displays. Pubic and armpit hair has remained because it traps scents which are useful in sexual attraction.

Objectors to his theory include Chris Stringer, of the Human Origins Group at the Natural History Museum, who maintains that humans lost their hair in order to control body temperatures better.

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But Robert Kruszynski, also of the Human Origins Group, says that the two theories are compatible. "The ability to sweat was the probable cause of hair loss in males and females," he said. "But when it gets to the differences between males and females we need an additional theory."