STRUCTURAL COLOUR: STAR SPECIES NUMBER 6
Special Power: The Power to Make Charles Darwin Sick!
About 150 years ago, and “almost a lifetime” either side, Charles Darwin was beleaguered by the problem of the peacock’s tail. Just the sight of a feather, he wrote in April 1860, “makes me sick!” The plumage of the male bird represented a hole in his theory of evolution. According to Victorian thinking, beauty was divine creation: God had designed the peacock for his own and humankind’s delight. In On The Origin of Species published the previous year, Darwin had challenged the dominant theory of creationism, arguing that man had been made not in God’s image but as a result of evolution, with new species formed over generations in response to their environment.
But beauty, and a supposed aesthetic sense in animals (“We must suppose [that peahens] admire [the] peacock’s tail, as much as we do,” he wrote), took Darwin the best part of his life to justify – not least because the theory he eventually landed upon went against the grain of his entire worldview. Sexual selection was of strategic importance to Darwin, says Evelleen Richards, an honorary professor in history and philosophy of science at the University of Sydney: it was a naturalistic account for aesthetic differences between male and female animals of the same species, shoring up his defence of natural selection.
“No one had come up with this theory in quite the same way as Darwin, and yet it was built into his thinking on natural selection: sexual selection explains what natural selection cannot,” she says. Natural selection was the “struggle for existence”, sexual selection was the “struggle for mates”. It attributed the development of plumage, courtship dances, song and other so-called “secondary sexual characteristics” to females’ choices of mates, creating a positive feedback mechanism over generations.
“A girl sees a handsome man and without observing whether his nose or his whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance and says she will marry him,” he wrote in 1868. “So I suppose with the peahen; and the tail has been increased in length merely by on the whole presenting a more gorgeous appearance.”
Richards argues that, more than natural selection, Darwin’s theory of sexual selection was uniquely his own and, perhaps as a result, often misunderstood. His theorising drew upon a wide range of influences, many of them deeply personal, including his grandfather Erasmus’s radical writings on evolution and his own relationship with his wife.
The ‘peafowl’ include three species of birds in the genera Pavo and Afropavo of the Phasianidae family, the pheasants and their allies. The two Asiatic species are the blue or Indian peafowl originally of the Indian subcontinent, and the green peafowl of Southeast Asia. The term peacock is properly reserved for the male; the female is known as a peahen, and the immature offspring are sometimes called peachicks.
Peacocks 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 colouration and large “train” of peacocks have been the subject of extensive scientific debate. As stated above, Charles Darwin suggested they served to attract females, and the showy features of the males had evolved by sexual selection. More recently, Amotz Zahavi has proposed in his handicap theory 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.
As with many birds, the peacock’s vibrant iridescent plumage colours are not primarily pigments, but structural colouration. Optical interference Bragg reflections, based on regular, periodic nanostructures of the barbules (fiber-like components) of the feathers, produce the peacock’s colours. Slight changes to the spacing of these barbules result in different colours. Brown feathers are a mixture of red and blue: one colour 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.
Such structural colouration causes the iridescence of the peacock’s hues. Interference effects depend on light angle rather than actual pigments.