The peacock loses its tale

Darwin's ideas on sex discredited

Posted Mar 29, 2011

Yes, I know how to spell! The peacock has lost its narrative, not its colorful plumage.

The tale so far
Up to now, the story told by biologists was that the peacock uses its colorful tail to charm the females. For their part, females run around in search of the one with the gaudiest plumage. In doing so, they select the best genes for their offspring who are consequently more likely to survive and reproduce.

In recent years, almost all of the key claims of the peacock story have come under withering scrutiny. Thus, a study of feral Indian peafowl in Japan (1) found that:

Females do not prefer to mate with gaudy males.
Males differ little in plumage quality anyway.
Plumage quality is not related to male health or vigor.

This looks like a fairly grisly hatchet job on Darwin's thesis of females being charmed by male "ornaments" as a sign of biological quality. Yet, there is more. It turns out that the ornaments might not even be distinctively male.

Authors of the study suggest that both sexes of Indian peafowl were brightly colored to begin with but that the females lost their gaudy feathers in order to protect them from predation. This is an occupational hazard amongst female birds particularly if they incubate the eggs without any male assistance.

Moreover, there is plenty of evidence from other species, from swallowtail butterflies to dragon lizards (240 different species) that ornaments usually evolve in both sexes at the same time. Females are more likely to lose the ornaments thereby producing the pattern observed by Darwin.

Why are female lizards ornamented to begin with? Most likely the ornaments evolved for purposes of communication, being used most often in the context of territorial defense. Amongst lizards, at least, bright coloration may have little or nothing to do with mate selection.

Returning to birds, peafowl are not alone in their failure to conform to Darwin's ideas. Some males come in several different forms, such as the European ruff that either has a black ruff, a white ruff, or no ruff at all (like the females). Even more damaging to Darwin's ideas on the nature of sexual competition, males with white ornaments cooperate with those having black ornaments and females prefer to mate with such a pair rather than with a single black-collared male.

Then there are sunangel humming birds from the Venezuelan Andes, that boast a broad band of distinctively colored feathers around the throat and chest known as a gorget. Some females have ornaments just like the males and some males have the same colors as females. Of 42 species studied in museum specimens, 18 had either masculine females, or feminine males, or both (2).

Then there is widespread homosexuality in nature, of which I wrote previously. The most plausible explanation for this phenomenon is that it cements social bonds permitting a high level of cooperation that would otherwise be impossible. Advocates of sexual selection theory see it as a trick, a way of depleting the sperm of rival males.

Although sexual selection enthusiasts have not gone down without a fight, I think it is fair to say that the peacock has lost its tale. There is no doubt that the colorful plumage is used to excite females during courtship but one tail is as good as another, it seems.

Those of us who have been charmed ourselves by Darwin's story for how the peacock got its tail, will be sorry to see it go. It has that rare marriage between science and esthetics that is so compelling. It is beautiful, therefore it must be true. Not so alas.

The peacock's tale appeals to us as much for what it has to say about humans as about peafowl. In particular, it depicts males and females as being very different, having different agendas and therefore being in continual conflict. Perhaps we are not so different after all. Perhaps it is in our best interest to cooperate with one another, particularly if we are raising children together.

1. Takahashi, M. et al. (2008). Peahens do not prefer males with more elaborate trains. Animal Behaviour, 75, 1209-1219.
2. Roughgarden, J. (2009). The genial gene: Deconstructing Darwinian selfishness. Berkeley, CA: University of California Press.