Peafowl is a common name for two bird species of the genus Pavo and one species of the closely related genus Afropavo within the tribe Pavonini of the family Phasianidae (the pheasants and their allies). Male peafowl are referred to as peacocks, and female peafowl are referred to as peahens. Despite this, peacock is usually used to refer to both sexes, in common language.
The two Asiatic species are the blue or Indian peafowl originally from the Indian subcontinent, and the green peafowl from Southeast Asia. The third peafowl species, the Congo peafowl, is native only to the Congo Basin. Male peafowl are known for their piercing calls and their extravagant plumage. The latter is especially prominent in the Asiatic species, which have an eye-spotted "tail" or "train" of covert feathers, which they display as part of a courtship ritual.
The functions of the elaborate iridescent coloration and large "train" of peacocks have been the subject of extensive scientific debate. Charles Darwin suggested that they served to attract females, and the showy features of the males had evolved by sexual selection. More recently, Amotz Zahavi proposed in his handicap principle that these features acted as honest signals of the males' fitness, since less-fit males would be disadvantaged by the difficulty of surviving with such large and conspicuous structures.
Description
The Indian peacock (Pavo cristatus) has iridescent blue and green plumage, mostly metal-like blue and green. In both species, females are a little smaller than males in terms of weight and wingspan, but males are significantly longer due to the "tail", also known as a "train". The peacock train consists not of tail quill feathers but highly elongated upper tail coverts. These feathers are marked with eyespots, best seen when a peacock fans his tail. All species have a crest atop the head. The Indian peahen has a mixture of dull grey, brown, and green in her plumage. The female also displays her plumage to ward off female competition or signal danger to her young.
Male green peafowls (Pavo muticus) have green and bronze or gold plumage, and black wings with a sheen of blue. Unlike Indian peafowl, the green peahen is similar to the male, but has shorter upper tail coverts, a more coppery neck, and overall less iridescence. Both males and females have spurs.
The Congo peacock (Afropavo congensis) male does not display his covert feathers, but uses his actual tail feathers during courtship displays. These feathers are much shorter than those of the Indian and green species, and the ocelli are much less pronounced. Females of the Indian and African species are dull grey and/or brown.
Chicks of both sexes in all the species are cryptically colored. They vary between yellow and tawny, usually with patches of darker brown or light tan and "dirty white" ivory.
Mature peahens have been recorded as suddenly growing typically male peacock plumage and making male calls. Research has suggested that changes in mature birds are due to a lack of estrogen from old or damaged ovaries, and that male plumage and calls are the default unless hormonally suppressed.
Iridescence and structural coloration
As with many birds, vibrant iridescent plumage colors are not primarily pigments, but structural coloration. Optical interference of Bragg reflections, from regular, periodic nanostructures of the barbules (fiber-like components) of the feathers, produce the peacock's colors. Slight changes to the spacing of the barbules result in different colors. Brown feathers are a mixture of red and blue: one color is created by the periodic structure and the other is created by a Fabry–Pérot interference peak from reflections from the outer and inner boundaries. Color derived from physical structure rather than pigment can vary with viewing angle, causing iridescence.
Courtship
Most commonly, during a courtship display, the visiting peahen will stop directly in front of the peacock, thus providing her with the ability to assess the male at 90° to the surface of the feather. Then, the male will turn and display his feathers about 45° to the right of the sun's azimuth which allows the sunlight to accentuate the iridescence of his train. If the female chooses to interact with the male, he will then turn to face her and shiver his train so as to begin the mating process.
Evolutionary history Peafowl are a clade of the pavonini tribe alongside the argus pheasant. The earliest named record of the tribe is Eopavo hezhengensis from the Late Miocene of Linxia Basin, Gansu, Northwestern China. It lived in a high-elevation savanna habitat, unlike the humid forests preferred by its extant relatives. Eopavo, along with older unnamed peafowl fossils from the Himalayan area (), indicates that the peafowl dispersal likely occurred from Asia (the Tibetan Plateau) to other continents between the Late Miocene and the Pliocene.
Sexual selection for plumage
Charles Darwin first suggested in The Descent of Man and Selection in Relation to Sex that peafowl plumage may have evolved through sexual selection:
The peacock's train and iridescent plumage are perhaps the best-known examples of traits believed to have arisen through sexual selection, though with some controversy.Male Peacock's Feather Fails to Impress Females: Study . The Indian News. 27 March 2008. Male peafowl erect their trains to form a shimmering fan in their display for females. Marion Petrie tested whether or not these displays signalled a male's genetic quality by studying a feral population of peafowl in Whipsnade Wildlife Park in southern England. The number of eyespots in the train predicted a male's mating success. She was able to manipulate this success by cutting the eyespots off some of the males' tails: females lost interest in pruned males and retained interest in untrimmed ones. Males with fewer eyespots, thus having lower mating success, suffered from greater predation. She allowed females to mate with males with differing numbers of eyespots, and reared the offspring in a communal incubator to control for differences in maternal care. Chicks fathered by more ornamented males weighed more than those fathered by less ornamented males, an attribute generally associated with better survival rate in birds. These chicks were released into the park and recaptured one year later. Those with heavily ornamented feathers were better able to avoid predators and survive in natural conditions.Zuk, Marlene. (2002). Sexual Selections: What we can and can't learn about sex from animals. University of California Press; Berkeley, CA. Thus, Petrie's work shows correlations between tail ornamentation, mating success, and increased survival ability in both the ornamented males and their offspring.
Multiple hypotheses involving female choice have been posited. One hypothesis is that females choose mates with good genes. Males with more exaggerated secondary sexual characteristics, such as bigger, brighter peacock trains, tend to have better genes in the peahen's eyes. These better genes directly benefit her offspring, as well as her fitness and reproductive success.
Runaway selection is another hypothesis. In runaway sexual selection, linked genes in males and females code for sexually dimorphic traits in males, and preference for those traits in females.Caldwell, Roy, and Jennifer Collins. "When Sexual Selection Runs Away ." Evolution 101: Runaway Selection. N.p., n.d. 24 November 2014. The close spatial association of alleles for loci involved in the train in males, and for preference for more exuberant trains in females, on the chromosome (linkage disequilibrium) causes a positive feedback loop that exaggerates both the male traits and the female preferences.
Another hypothesis is sensory bias, in which females have a preference for a trait in a non-mating context that becomes transferred to mating, such as Merle Jacobs' food-courtship hypothesis, which suggests that peahens are attracted to peacocks for the resemblance of their eye spots to blue berries.
Multiple causalities for the evolution of female choice are also possible. In contrast to Petrie's findings, a seven-year Japanese study of free-ranging peafowl concluded that female peafowl do not select mates solely on the basis of their trains. Mariko Takahashi found no evidence that peahens preferred peacocks with more elaborate trains (such as with more eyespots), a more symmetrical arrangement, or a greater length. Takahashi determined that the peacock's train was not the universal target of female mate choice, showed little variance across male populations, and did not correlate with male physiological condition. Adeline Loyau and her colleagues responded that alternative and possibly central explanations for these results had been overlooked. They concluded that female choice might indeed vary in different ecological conditions.
Handicap hypothesis
Amotz Zahavi used the excessive tail plumes of male peafowls as evidence for his "handicap principle". Since these trains are likely to be deleterious to an individual's survival (as their brilliance makes them more visible to predators and their length hinders escape from danger), Zahavi argued that only the fittest males could survive the handicap of a large train. Thus, a brilliant train serves as an honest indicator for females that these highly ornamented males are good at surviving for other reasons, so are preferable mates. This theory may be contrasted with Ronald Fisher's hypothesis that male sexual traits are the result of initially arbitrary aesthetic selection by females.
Plumage colours as attractants
A peacock's copulation success rate depends on the colours of his eyespots (ocelli) and the angle at which they are displayed. The angle at which the ocelli are displayed during courtship is more important in a peahen's choice of males than train size or number of ocelli. Peahens pay careful attention to the different parts of a peacock's train during his display. The lower train is usually evaluated during close-up courtship, while the upper train is more of a long-distance attraction signal. Actions such as train rattling and wing shaking also kept the peahens' attention.
Redundant signal hypothesis Although an intricate display catches a peahen's attention, the redundant signal hypothesis also plays a crucial role in keeping this attention on the peacock's display. The redundant signal hypothesis explains that whilst each signal that a male projects is about the same quality, the addition of multiple signals enhances the reliability of that mate. This idea also suggests that the success of multiple signalling is not only due to the repetitiveness of the signal, but also of multiple receivers of the signal. In the peacock species, males congregate a communal display during breeding season and the peahens observe. Peacocks first defend their territory through intra-sexual behaviour, defending their areas from intruders. They fight for areas within the congregation to display a strong front for the peahens. Central positions are usually taken by older, dominant males, which influences mating success. Certain morphological and behavioural traits come in to play during inter and intra-sexual selection, which include train length for territory acquisition and visual and vocal displays involved in mate choice by peahens.
Alternate selection mechanisms It has been suggested that a peacock's train, loud call, and fearless behavior have been formed by natural selection (with or without sexual selection too), and served as an aposematic display to intimidate predators and rivals. This hypothesis is designed to explain Takahashi's observations that in Japan, neither reproductive success nor physical condition correlate with the train's length, symmetry or number of eyespots.
Behaviour