Every summer, the peacocks that roam free within Wild Animal Park in Bedfordshire their expose magnificent trains to the critical and often disdainful gaze of the hens. They re-enact the mystery that tormented Charles Darwin to his dying day: how in this competitive world, where nature as Tennyson said is red in tooth and claw, could birds have evolved such an obvious extravagance? How do they get away with it?

The zoologist Marion and her colleagues at the Open University are now exploiting the quasi-wild conditions of Park to try, a century after Darwin's death, to settle the matter. Darwin argued that living creatures came to be the way they are by evolution, rather than by special creation, and that the principal mechanism of evolution was natural selection. That is, in a crowded and hence competitive world, the individuals best suited to the circumstances - the 'fittest' are the most likely to survive and have offspring. But the implication is that the fittest would generally mean toughest, swiftest, cleverest, most alert.

The peacock's tail, by contrast, was at best a waste of space and in practice a severe encumbrance and Darwin felt obliged to invoke what he felt was a separate mechanism of evolution, which he called 'sexual selection'. The driving mechanism was simply that females liked in his words "beauty for beauty's sake". But Darwin's friend and collaborator, Alfred Russel Wallace, though in many ways more romantic than Darwin, was in others even more Darwinian. Beauty for beauty's sake' he wanted nothing of. If peahens chose cocks with the showiest trains, he felt, then it must be that they knew what they were about. The cocks must have some other quality, which was not necessarily obvious to the human observer, but which the hens themselves could appreciate.