Darwin's theory of sexual selection is widely regarded as explaining how the peacock's magnificent tail evolved. But the theory may not be correct for that very "poster" example, the peacock. As The Design of Life authors explain, sexual selection is Darwin’s explanation for how animals acquire traits that do not help them survive (have no direct adaptive value), and may even hinder their survival:

Consider a stag whose antlers are so large that they are more deadweight than defense. Or a peacock whose large colored tail makes it easy prey. How do such structures evolve? According to Darwin, they evolve because they help to attract mates—they are a form of sexual display. (The Design of Life GN6)

Darwin championed this explanation because, as he wrote to American botanist Asa Gray (April 3, 1860), "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" Why did it make him sick?

Darwin needed to account for  animal traits that do not promote survival, and may even hinder it. Essentially, he had to incorporate an explanation for traits that hinder survival into a theory that attempts to explain traits that promote survival.

His explanation, the theory of sexual selection, has been widely accepted, and is now part of the doctrine of undirected evolution. For example the PBS Evolution Library says:

Peahens often choose males for the quality of their trains - the quantity, size, and distribution of the colorful eyespots. Experiments show that offspring of males with more eyespots are bigger at birth and better at surviving in the wild than offspring of birds with fewer eyespots.

and Berkeley's Evolution 101 says,

Sexual selection is a “special case” of natural selection. Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate. Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances, and some species deliver persuasive gifts.

His weakness is his strength?

But remember, the peacock must carry his tail whether he gets a hen or not. The tail, produced by the absence of estrogen, is generally agreed to be a deadweight for the cock bird. Mark Ridley observes in the Third Edition of Evolution (Blackwell, 2003):

The peacock's tail almost certainly reduces the male's survival: the tail reduces maneuverability, powers of flight, and makes the bird more conspicuous; its growth must also impose an energetic cost.

In that case, we might expect the generously tailed cock birds to be picked off by predators, leaving their less feathery brethren to mate. Yet that does not happen, not even in nature, where wild peacocks are as well endowed with tails as tame ones.

Evolutionary psychologist Geoffrey Miller of University College, London, thinks he has an answer:

The peacock's tail is not just an arbitrary outcome of sexual selection. It's there because it's costly, which means only those fit, healthy, strong peacocks can afford to carry around those tails.

This hypothesis, called Zahavi's handicap or the "handicap principle," states:

An individual with a well developed sexually selected character [such as a peacock's flashy tail] is an individual which has survived a test. A female which could discriminate between a male possessing a sexually selected character, from one without it, can discriminate between a male which has passed a test and one which has not been tested. Females which selected males with the most developed characters can be sure that they have selected from among the best genotypes of the male population. (Amotz Zahavi (1975)

So, according to this thesis, the hen bird realizes that the tail is a handicap for the cock bird, but, to the extent that he bears it in a cocksure manner, she also realizes that he must be a healthy mate.

Not only that but, according to Matt Ridley, the tail prevents "low quality" males from representing themselves as strong, because they do not have such a tail to drag about:

The handicap acts as an indicator of genetic quality and has to be costly to guarantee that signalling is honest: otherwise low quality males could equally well advertise and females would be unable to distinguish between them.

So, during the hyperactive mating season, not only does the hen bird make a decision about the relative fitness of the cock birds, but she accounts for their handicaps and determines that they are not a false signal. Or if not the hen bird herself, then what is doing the calculating? Her selfish genes? Do we have any evidence that such selfish genes exist, other than as a hypothetical support to an otherwise shaky theory?

Or, alternatively, the researchers may envision a complex but blind mating algorithm which simply produces this outcome. But that does not explain why an agile, tailless peacock does not simply mount more peahens. Against such a move, a mere algorithm is powerless.

Zahavi's handicap sounds like an argument put forward to protect the original theory of sexual selection from falsification, rather than an argument that addresses the nature of the birds themselves. Indeed, one problem with arguments like Zahavi’s handicap is that they render the theory of sexual selection unfalsifiable. If a peacock whose tail has fallen out due to a hereditary disorder proves agile enough to mount most of the hen birds and pass on his "tailless" trait, the theory of sexual selection can easily be altered to accommodate his success.

But in that case, what about the "costly handicap"? What becomes of all the elaborate calculations that the hen birds are supposed to be doing for the good of the gene pool? If the theory of sexual selection means to say only that birds somehow choose mates and eggs are laid, no one will dispute it. But if the theory is intended to shed light on peacocks' appearance and behaviour, it must say something explicit enough to be falsifiable by a pattern of events that does not conform to it.

Of course, all these proposed explanations depend on the assumption that the female bird (the peahen) actually makes a decision based on the size and beauty of her prospective mate's tail. But does she?

Well, does she or doesn't she?

Recent research suggests that she doesn't. Jennifer Viegas, of Discovery Channel News, reports on the work of Mariko Takahashi of the University of Tokyo and her colleagues on this very question (March 26, 2008). Their findings contradict earlier reports that the peahens are impressed by brilliant tails.

From spring 1995 through spring 2001, the researchers observed peafowl (the correct name for the species) mating at Izu Cactus Park on the Izu Peninsula, about 100 kilometres southwest of Tokyo. They found that the peahens do not pay much attention to the peacocks' feathered finery. As Viegas reports,

The determination throws a wrench in the long-held belief that male peacock feathers evolved in response to female mate choice. It could also indicate that certain other elaborate features in galliformes, a group that includes turkeys, chickens, grouse, quails and pheasants, as well as peacocks, are not necessarily linked to fitness and mating success. [ ... ] Across the board, the researchers were unable to link the elaborateness of a peacock's train with his mating success. In fact, Takahashi and her team found little train variance among males in the population they studied. They also couldn't detect any link between a particular male's fitness and his train.

The new research suggests that the peacock's shrill mating scream attracts more attention from peahens than his fanned tail display. However, the tail may attract attention during the peacock's "shivering" display. According to Takahashi and colleagues:

Shivering is a display in which a peacock shows and shakes his train directly towards a visiting female at close range, producing a rustling noise (e.g. Ridley et al. 1984).During a female visit, males sometimes performed more than 20 shivering bouts. Each bout lasted 1 to >400 s and consisted of quick changes in the intensity of the noise produced; these changes were termed ‘shivers’ and were generated approximately twice per second.

British physicist David Tyler quotes Takahaski's dissent from the traditional sexual selection explanations:

To date, the peacock's train has been proposed not only as a target of current female choice (e.g. Petrie et al. 1991), but also as an indicator of good genes (Petrie 1994). However, there may be at least four problems with these hypotheses. First, male train morphology seems not to be the universal cue of choice because there is evidence both for and against the effect of male train morphology on male mating success. [. . .] Second, the ways in which females assess male trains (unless females have the ability to count eyespots per se) have been questioned repeatedly but have not been fully investigated. Third, there is no consensus on which traits characterize males with the most elaborate trains. [. . .] Fourth, to our knowledge, mate choice based on a male plumage ornament that is under oestrogen control is very rare.

Takahashi suggests that at one time the tail impressed peahens but is now "obsolete":

We propose that the peacock's train is an obsolete signal for which female preference has already been lost or weakened, but which has none the less been maintained up to the present because it is required as a threshold cue to achieve stimulatory levels in females before mating and/or it is maintained as an unreliable cue [. . .].

Tyler observes,

The alleged amazing powers of natural selection are much diminished as a result of these findings. The argument that it is "powerful enough" to maintain the feather display against the negative effects of attracting predators must be dropped. Furthermore, it appears not powerful enough to remove the display when it becomes an "obsolete signal". Darwinists need to think very hard about the way they do science. This is a clear example of how a Darwinian hypothesis has become accepted as scientific fact, yet now has been disproved by some rigorous empirical research. This is a falsified prediction. This means that numerous textbooks and web sites need to be revised. More importantly, Darwinists should cease giving the impression that they have the keys to understand the natural world. So much of this 'understanding' is like peacock feathers - lots of show and no substance. Richard Dawkins extols Darwinism as a beautiful theory, but whenever we look closely, it fails to account for the observed data.

The study, its abstract, and other resources

Title: Peahens do not prefer peacocks with more elaborate trains Mariko Takahashi, Hiroyuki Arita, Mariko Hiraiwa-Hasegawa and Toshikazu Hasegawa Animal Behaviour, 75(4), April 2008, 1209-1219 | doi:10.1016/j.anbehav.2007.10.004

Abstract: The elaborate train of male Indian peafowl, Pavo cristatus, is thought to have evolved in response to female mate choice and may be an indicator of good genes. The aim of this study was to investigate the role of the male train in mate choice using male- and female-centred observations in a feral population of Indian peafowl in Japan over 7 years. We found no evidence that peahens expressed any preference for peacocks with more elaborate trains (i.e. trains having more ocelli, a more symmetrical arrangement or a greater length), similar to other studies of galliforms showing that females disregard male plumage. Combined with previous results, our findings indicate that the peacock's train (1) is not the universal target of female choice, (2) shows small variance among males across populations and (3) based on current physiological knowledge, does not appear to reliably reflect the male condition. We also found that some behavioural characteristics of peacocks during displays were largely affected by female behaviours and were spuriously correlated with male mating success. Although the male train and its direct display towards females seem necessary for successful reproduction, we conclude that peahens in this population are likely to exercise active choice based on cues other than the peacock's train.

Paper: A version of the  paper by Takahashi et al on this subject, online.

Other resources:

 General Peafowl Information

How the peacock's colours are produced. From New Scientist: "a lattice of melanin rods and keratin on the outer layer of each barbule forms a two-dimensional photonic crystal structure. The number and spacing of the rods determines the colour of the barbule and its intensity (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2133313100)."

Domestication of the peacock

And finally, just for fun: Lonely British peacock romances a petrol (gas) pump (Times, June 17, 2006) "Ornithologists believe that Mr P is confused by the clicking sounds of the pumps, which resemble the cries of a broody peahen. ... His two brothers are also showing signs of confusion when it comes to finding a mate. One appears to have a crush on the family cat, and the other has been seen attempting to mate with a garden light."