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 color; that the peahen, for instance, admires each detail in the gorgeous train of the peacockshe 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 had mated with a good-looking male would be attractive to a new generation of females and thus continue the genetic line.
Another theory was suggested by Israeli zoologist Amotz Zahavi, who thought that cumbersome ornamentation was a way of showing females how fit the males werea "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 signaling to the females that they were, if not parasite-free, then parasite "lite," since a glossy plumage can only be maintained by a male who was not heavily infested.
The trouble was, there was little evidence for this theory.
Anders Moller of the Laboratoire d'Ecologie at the French national research center 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," said 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."
Moller 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, but they are brightly colored and 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," said 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.
Thus, it pays for females to be choosy because their chicks, in turn, will survive better and mate with other, equally picky females.
Some traits, however, may not be indicators of general health.
For instance, the comb of a male jungle fowl deteriorates when the bird 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 possible that Moller's original theory of males as billboards explains these variations, too. "Another possibility would be that we have not measured all aspects of immune defense," he said.
Although there are still many unanswered questions, scientists are moving closer to finding out how the peacock got his tail.