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How The Peacock Got Its Tail

Indian Peacock

According to Greek mythology Zeus, the king of gods, fell in love with Io, a mortal priestess of Hera, the goddess of women, marriage, family and childbirth. Hera also happened to be Zeus’ wife and sister, known for her jealous nature, often taking revenge on Zeus’ various lovers and children.

To protect her from the wrath of his sister-wife, Zeus turned Io into a white cow, which he claimed had appeared out of nowhere. Hera saw right through this lie and persuaded Zeus to give her the cow which she asked her watchman Argus, an all-seeing giant with one hundred eyes, to guard.

Zeus formed a rescue plan and ordered his messenger Hermes to kill Argus. Hermes, disguised as a shepherd, approached the giant, playing a flute to hypnotise him. When that didn’t work, he started telling him mundane stories to bore him to death. Once Argus had fallen asleep, Hermes pulled out all of his eyes and then bashed him over the head with a stone.

The sacrifice of Argus allowed Io to wander the earth freely, although Hera continued to torment her sending a gadfly to sting her continuously until she descended into madness. To commemorate her loyal servant Argus she took all one hundred of his eyes and set them in the tail of a peacock.

A fanciful tail

If you thought that story was complex the real story of how the peacock got its tail is one that has puzzled scientists for centuries.

There are three species of peafowl; the Indian or blue peafowl, the green peafowl of South East Asia, and the Congo peafowl, native only to the Congo basin.

They are members of the pheasant family, heavy birds that live on the ground, that also includes partridges, chickens, and turkeys. Male peafowl, or peacocks, are known for their extravagant plumage, which is especially prominent in the Indian and green peafowl species.

Indian peacocks have iridescent green and blue plumage, while green peacocks are green and bronze. Both species have an ornate train decorated with eyespots. These long trains are not tail quill feathers but are formed from elongated upper tail coverts which grow over the rectrices. Their actual tails are short and grey but can only be seen from behind when the train is fanned, or during moulting season when males shed their train feathers.

Green Peacock
Green peacock

The Congo peacock is less impressive and does not display his covert feathers during courtship displays but uses his tail feathers, which are shorter and have less pronounced eyespots than those of the Indian and green peacocks.

Male and female green peafowl are quite similar in appearance, although she has shorter upper tail coverts. Indian and Congo peahens are duller than the peacocks with Indian peahens lacking the train altogether.

Before breeding season peacocks establish small territories known as leks. They are typically polygynous, which means that dominant males will mate with several females in one season.

A new train of thought

The theory of evolution by natural selection, first formulated by Charles Darwin in his book On the Origin of Species, published in 1859, is widely accepted as the best explanation for how life on earth diversified over time.

In his seminal work, that underpins all evolutionary biology, Darwin argued that changes to an organism that allow it to better adapt to its environment will help it survive longer and increase the chances of it having more offspring. Over generations, as the changes became more pronounced, new species were formed in response to their environment.

The theory is sometimes described as ‘survival of the fittest’ but this can be misleading as it’s not necessarily the strength of an organism that is the factor that improves its survival rate, but rather its ability to fit into its environment.

It was Darwin’s observations of the different species of finches on the Galapagos Islands that helped him formulate his theory. Although the finches were in many ways almost identical, they showed a wide diversity in the size and shape of their beaks.

Darwin’s finches

Species diversity

Over time, Darwin concluded, their beaks had changed depending on the food sources available on the different islands. On islands where there was an abundance of insects the finches had evolved thin, sharp beaks, while on islands where seeds were the main food supply the finches had evolved much bigger, blunter beaks which enabled them to crack open the seed cases.

The finches that were best able to keep themselves nourished would have more of a chance of reaching sexual maturity and producing offspring. Those offspring in turn would inherit some of the traits that had enabled their parents to survive, and so on down the generations.

However, another species of bird posed a problem for Darwin as it didn’t seem to fit his theory. Just a year after the publication of On the Origin of Species, he wrote in a letter to his friend Asa Gray, “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”

The problem of the peacock

Darwin was worried that the peacock’s ornamental tail could put his theory at risk. If natural selection was driven by an organism’s ability to survive by overcoming the challenges of finding food or escaping from predators, how could the peacock’s seemingly useless tail give it any advantage?

It’s heavy and cumbersome which makes flight difficult and it’s harder to hide from predators with something so showy. The evolution of the peacock’s tail at first appears to be completely at odds with the theory of natural selection by survival of the fittest.

Congo Peafowl
Congo peafowl

Darwin resolved this paradox by proposing a further mechanism – natural selection by means of sexual selection. He suggested that this happens in one of two ways; competition between males, and female choice.

Competition between males, or intrasexual selection, means males develop traits or characteristics which enable them to dominate other males and win the ability to mate with females. Female choice, or intersexual selection, means males develop traits or characteristics that are more attractive to females.

Darwin’s theory of sexual selection helps explain why there are marked differences between the males and females of some species of animals, including the peafowl. Whereas natural selection is a struggle for survival, Darwin noted that sexual selection is ‘a struggle between the males for possession of the females”.

Sexual selection often relies on a trade-off with natural selection. In the case of the peafowl, the long ornamental train’s hindrance to survival is cancelled out by it being more attractive to females which increases the chance of breeding success.

Darwin was not without his critics. Alfred Russell Wallace, the British biologist who had independently conceived of the idea of natural selection, wrote to Darwin in 1868 expressing his concerns.

“I am glad you have got good materials on sexual selection. It is no doubt a difficult subject. One difficulty to me is, that I do not see how the constant minute variations, which are sufficient for nat. select to work with,—could be sexually selected. We seem to require a series of bold and abrupt variations. How can we imagine than an inch in the tail of the peacock, or 1/4 inch in that of the Bird of Paradise, would be noticed and preferred by the female?”

Darwin replied:

“In regard to sexual selection. A girl sees a handsome man, & without observing whether his nose or whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance & says she will marry him. So I suppose with the pea-hen; & the tail has been increased in length merely by on the whole presenting a more gorgeous appearance. J. Jenner Weir, however, has given me some facts showing that birds apparently admire details of plumage.”

By the 1880s the ideas around sexual selection had been deemed too controversial to be studied further by most scientists, and in 1915 Wallace published a paper which stated that there was no evidence to show animals displayed any sexual preference.

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Although sexual selection goes some way to explain sexual dimorphism, it doesn’t explain the extreme sexual dimorphism exhibited in peafowls, birds-of-paradise, and some species of duck. Neither does it explain what advantage the offspring of those females who choose attractive mates over the offspring of those who don’t. In other words, why did females evolve to be choosier?

The runaway train

Ronald Fisher, the mathematical biologist, was one of the few scientists to continue working on Darwin’s sexual selection theory and in the 1930s he proposed a further mechanism which could account for the exaggerated ornaments found in some males including the peacock’s train.

Fisherian runaway selection proposes that a secondary sexual trait becomes correlated with a preference for the trait in the opposite sex. As subsequent generations mate they generate a positive feedback loop which accelerates the evolution of the trait.

For example, imagine a species of bird where males that have slightly longer tails can fly better than those with shorter tails, which means they can find food more easily and escape predators. Males with longer tails are therefore more likely to live long enough to breed, and according to the sexy son hypothesis, a female will choose to mate with a long-tailed male as she will produce sons with longer tails.

Peahen And Peacock Displaying

Over time the trait spreads throughout the population until most males have longer tails and most females have a preference for longer tails. At some point the process runs out of control, and the length of the tail begins to have a disadvantage for survival. But by then females have evolved to be so choosy that they only pick males with the exaggerated trait. It is only natural selection that prevents the trait from getting so extreme that it would lead to the species’ extinction.

Although Fisher was able to explain part of the reason why the male peacock has such a flashy tail, the puzzle was far from complete. The main problem with the theory is that large, sexy tails may hide an underlying physical weakness.

Strong, sexy signals

In the 1970s Avishag Zahavi and her husband Amotz Zahavi, evolutionary biologists from Israel, put forward a new, if somewhat controversial idea they called the handicap principle.

They proposed that females prefer males with ‘handicaps’. – characteristics that impede survival chances – because handicaps are indicators of genetic superiority.

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.

The principle relies on three chief tenets:

  • Animals use signals to communicate with each other
  • In order to be effective these signals must be honest
  • Animals displaying honest signals incur a cost in doing so

The final point means that only strong individuals are able to display seemingly unnecessary ornaments. A peacock with an elaborate train is saying to potential mates that he is able to survive in spite of the handicap he is carrying. A weaker bird would only be able to survive if his train was not as long.

During courtship displays a male peacock holds his train erect in a semi-circle high over his back. From time to time he shakes it vigorously and turns to show off its full beauty to the female. This takes an enormous amount of energy, another signal to demonstrate his physical strength.

Male peacocks also develop their train feathers at a time of year when food is scarce, so a peacock with an elaborate train is signalling that he is physically fit enough to grow the train in spite of the stress of searching for food.

Darwin himself in The Descent of Man and Selection in Relation to Sex, touched on the handicap principle, but did not explore it deeply in relation to sexual selection:

The females are most excited by, or prefer pairing with, the more ornamented males, or those which are the best songsters, or play the best antics; but it is obviously probable, as has been actually observed in some cases, that they would at the same time prefer the more vigorous and lively males.

The Zahavis published their ideas in a book but were unable to take the theory any further and mathematical models seemed to show that it was not viable.

The idea lay dormant for over a decade until Alan Grafen, a Scottish professor who applies mathematical and logical theory to evolutionary problems, devised a model showing that the principle could work in natural populations.

Darwin’s theory of sexual selection, Fisher’s runaway hypothesis, and the Zahavis’ handicap principle may be applied to any species of animal but none of them prove whether peahens do indeed prefer peacocks with the most ornate trains.

In 1987 Marion Petrie, Emerita Professor of Behavioural Ecology at Newcastle University, set out to test the idea.

Petrie and her colleagues at Whipsnade zoo counted the eyespots in the trains of a populations of feral peacocks and then observed them lekking. She found that males with more spots had more success mating, and that this was directly as a result of female choice.

Indian Peacock
Indian peacock

To test her hypothesis further Petrie removed the eyespots from some of the peacocks’ trains and found that peahens lost interest in these birds becoming more interested in those with intact trains.

She also found that the chicks fathered by peacocks with more eyespots weighed more and were able to better escape predators and survive in natural conditions.

We do not yet know how females can determine which males have the most spots. It could be that males with more spots are simply able to put on a more impressive display, due to increased physical strength.


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3 Responses

  1. Well that was an interesting read. Wish we had learnt stuff like this at school – would have been more intresting and given us a better understanding of the amazing Natural World around us. I didn’t know there were different kinds of peacocks – have only ever seen them in zoos.

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