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Every Summer, the peacocks that roam free within Whipsnade Park expose their magnificent trains to the critical gaze of the peahens. They re-enact the mystery that tormented Charles Darwin: how in this competitive world could birds have evolved such an obvious extravagance? Marion petrie and her colleagues are now trying to settle the matter. Darwin advanced the idea of evolution through natural section, i.e., the 'fittest' are likely to survive. The peacock's tail, by contrast, was at best a waste of space and a severe encumbrance that he felt obliged to refer as 'beauty for beauty's sake'. Russel Wallace was even more Darwinian. If peahens chose peacocks with the showiest trains, then it must be that they knew what they were about. The cocks must have some other quality, not necessarily obvious. The train was not an end in itself, but an advertisement for some genuine contribution to survival.

Now, 100 Years later, the wrangle is still unresolved. But 200 birds at Whipsnade offer unique opportunities for study. Marion petrie's team has identified two questions. Frist, do peahens really choose the males with the showiest trains? And, secondly, do the peacocks with the showiest trains have some extra, genuinely advantageous quality? Long observation from hides, backed up by photographs, suggests that the cocks with the most eye-spots do indeed attract the most mates. Showy male birds in general are the most parasite-free and advertise their disease- free state. But Petrie and her colleagues have not been able to assess the internal parasites in the Whipsnade peacocks. This year, however, she is comparing the offspring of cocks that have in the past proved attractive to hens with the offspring of cocks that hens find unattractive. R A Fisher's suggestion in the nineteen-thirties has become known as the 'Fisher's Runway'. This occurs when a female first picks a male with a slightly better tail than the rest. The sons of that mating will inherit their father's tail and the daughters will inherit their mother's predilection for long tails. This is how the runway begins. Within each generation. the males with the longest tails will get most mates and leave most offspring; and the females' predilection for long tails will increase commensurately.