Who's a pretty boy, then?: Investigating the peacock's tail

Darwin made a start, but scientists are still trying to explain why the peacock has such a splendid tail, writes Sanjida O'Connell

How did the peacock get his tail? It sounds like a Just So story, but it's a question that has tormented zoologists for more than a century. It was Charles Darwin who first noted that it is the choosy peahen who plays a crucial role in the evolution of this extravagant sexual display. "We may conclude that ... those males which are best able by their various charms to please or excite the female, are under ordinary circumstances accepted. If this be admitted, there is not much difficulty in understanding how male birds have gradually acquired their ornamental characters." He continued: "In all ordinary cases, the male is so eager that he will accept any female, and does not, as far as we can judge, prefer one to the other."

At the time, Darwin's theory on female choice in animals, and birds in particular, was revolutionary and he spent pages justifying a bird's appreciation of beauty and the quality of "love" that must be felt between a pair bonding for life. More recently, biologists have delved into the reasons why birds have large, showy ornaments, like the peacock's train, which, as Darwin noticed, hamper the ability to escape from predators. A study published earlier this year in the journal Behavioural Ecology shows that a male's plumage is a direct indicator of the strength of his immune system; a signal to females of his internal workings.

In the early Eighties, a biologist called Malte Andersson carried out some seminal research in the area of female choice. Andersson studied long-tailed widow birds. The males have tails one and a half metres long and court by jumping in the air. In the prairies where they live, they can be seen for a kilometre and a half. He cut some males' tails short, down to 14cm, and elongated other males' tails. He was able to show, for the first time, that females do choose, and what they want are males with long tails, preferably artificially enhanced. A decade later, Dr Marion Petrie, from Newcastle University, observed a group of male peacocks, and then clipped the eye spots out of half of the males' tails. She did not reduce the tail length, only the number of eye spots. She discovered that females preferred males with the most eye spots.

Typically, Darwin had predicted it all before. He had written: "Does the male parade his charms with so much pomp and rivalry for no purpose? Are we not justified in believing that the female exerts a choice, and that she receives the addresses of the male who pleases her most? It is not probable that she consciously deliberates; but she is ... attracted by the most beautiful, or melodious, or gallant males. Nor need it be supposed that the female studies each stripe or spot of colour; that the peahen, for instance, admires each detail in the gorgeous train of the peacock – she is probably struck only by the general effect."

But why should females choose the most gorgeous males? One hypothesis put forward was the "Sexy Son Theory". The sons of a female who mated with a good-looking male would be attractive to a new generation of females and thus continue their genetic line. Another theory was suggested by an Israeli zoologist, Amotz Zahavi, who thought that cumbersome ornamentation was a way of showing females how fit the males were, a "Look at me, I'm so strong I can escape from predators in spite of my tail" kind of a theory.

But what exactly were the males demonstrating, what was this "fitness"? Good genes seemed a slightly weak explanation. Biologists William Hamilton and Marion Zuk then came up with a new suggestion, that the males were signalling to the females that they were if not parasite-free, then parasite "light", since a glossy plumage can only be maintained by a male who was not heavily infested. The trouble was, there was little evidence for the theory.

Anders Moller, from the Laboratoire d'Ecologie at the French national research centre in Paris, believes it is because people have been looking at the wrong sort of parasites, perhaps ones that are relatively benign to the birds, if not to us. "If you look at our own species, we are attacked by hundreds of different species of parasites," says Moller. "So if you wanted to study our parasite burden, you'd have to identify all the parasites, from tapeworms to head lice, see how abundant they are and how they affect us. It would be practically impossible, so we decided to focus on the immune system." He teamed up with Petrie, and the two of them went back to peacocks. The researchers took blood samples from the birds and recorded the numbers of B- and T-cells, the white blood cells produced to defend the body against pathogens, as well as measuring the peacocks' tails and the number of eye spots they had.

Prior to this study, Moller had been interested in why males should have multiple ornaments. For instance, male peacocks not only have a long tail, they are brightly coloured, have eye spots, a crest on their head, spurs on their feet and a mating call. Moller's theory was that these ornaments signal different aspects of the male's quality to a female. Indeed, the results from the study seem to support his hypothesis. He and Petrie discovered that the condition and length of the peacock's tail was related to the production of B-cells, and the size of the eye spots to T-cell production.

"Our main finding is that females are looking at different aspects of a male's immune competence," says Moller. Males, in effect, are walking billboards advertising their health and status. And these things matter. Previous research has shown that in chickens and quail, at least, the immune system is under genetic control so offspring will inherit their parents' ability to fight parasites. It thus pays for females to be choosy since their chicks, in turn, will survive better and mate with other, equally picky females.

However, some traits may not be indicators of general health. For instance, the male jungle fowl shows a deterioration in its comb when it is infested with a gut parasite, but the parasite has no effect on its plumage. Moller has instead proposed an unreliable signal hypothesis. Some ornaments are unreliable signals for females, but they are maintained because they don't cost the bird much to produce them and there is a female preference for them. He predicts that multiple sexual ornaments should be seen in species that have intense sexual selection, such as those that are polygamous. In species with only one ornament, there should be more evidence to show that this ornament is dependent on the condition of their immune system. But it is just possible that Moller's original theory of males as billboards explains these variations, too. He says: "Another possibility would be that we have not measured all aspects of immune defence."

Although there are still many unanswered questions, we are now closer to finding out how the peacock got his tail.