HOW the peacock got his tail is one of natural history’s “Just So” stories that biologists like to think they have cracked. His tail is for showing off to the ladies just how fit he and his genes are. A less-than-perfect tail means no offspring. Genes for spectacular male tails are thus preserved and promoted over the generations in a process that is called sexual selection.

There is, though, a problem with this story. Peahens, though not as showy as cocks, are by no means dowdy. Their heads have fetching crests, and their necks are a beautiful, iridescent blue. Such flummery is costly to grow and likely to attract predators. If you do not have to strut your stuff to get a mate, why do you need it?

Even more confusingly, in many species both sexes are equally showy—the Gouldian finches overleaf, for example (the female is on the right). So, though no one thinks the theory is incorrect, as far as it goes, it clearly does not go far enough. To understand things better James Dale of Massey University, in Auckland, New Zealand, and his colleagues have therefore examined the plumage of both sexes of all 5,983 species of passerine bird (peafowl, not being passerines, are not among them), and compared them in exquisite detail.

First, the team had to devise a way to deal with the 11,966 types of plumage they had set out to examine. Using the “Handbook of the Birds of the World”, regarded by ornithologists as the definitive work in the field, they picked six points on a bird’s body (nape, crown, forehead, throat, upper breast and lower breast) and performed a spectral analysis of each to measure how red, green and blue it was. The average let them plot each plumage as a dot on a graph with three axes—red, green and blue.

To translate this into a “showiness” score, the researchers started from the fact that, despite the exceptions, showiness is still more a male than a female phenomenon. They therefore calculated, as a proxy for showiness, a “maleness” score for each dot, regardless of which sex it represented, by counting the sexes of its nearest 120 neighbours in the graph (ie, the nearest 1% of dots). They then ran these scores against characteristics, such as the size of a species, its habitat and its pattern of family life.

Their first observation, as they report this week in Nature, was that in species where a few males monopolise all the females, with a consequent lack of male involvement in parental care, males were more colourful than females. This is what the theory of sexual selection would predict. What it would not predict in its simple form, though, was a second finding—that females in co-operatively breeding species (those in which, for lack of other opportunities, several females collaborate to raise the young of only one of them) are more ornamented than those in which all adult females have a chance of breeding. In this case it is females who are competing for the right to reproduce, thus putting themselves in a more male-like position.

Another widespread belief Dr Dale and his colleagues confirmed is that tropical species are more colourful than those from temperate climes. But again, there was a twist—the effect was much more marked in females than in males. Something about the tropics favours colourful females. It may be that tropical birds, which face more intense competition for food and nesting sites than temperate ones do because the tropics have more species, form more stable and collaborative pair-bonds than do temperate birds. In these circumstances males also need to be choosy, and females competitive. Selection for gaudy plumage therefore works in both directions.

The final effect the researchers found was that big species are more colourful than small ones. That is true of both sexes, and probably reflects the fact that bigger birds are more difficult prey and thus have less need to hide. When released from the threat of predation, then, females tend to be gaudier. That suggests gaudiness is always good when it can be got away with (for even in a promiscuous species, pretty females are likely to be at an advantage to ugly ones)—and probably explains the decorated necks and heads of peahens, which are among the biggest of birds.

Putting these results together, then, suggests that what is happening in the arena of sexual selection is as much to do with females as with males. Just as females are half the world, so the conventional explanation of the peacock’s tail, though not wrong, is only half the story.