The male peafowl, known as the peacock, is famous for its elaborate display, a fan of iridescent feathers raised and shivered in a behavior called “train-rattling.” This structure is not the bird’s tail, but a collection of elongated upper tail coverts, called the train, which can reach over five feet in length. The presentation and accompanying pulsating mechanical sound represent a striking example of evolutionary adaptation. This behavior serves multiple purposes beyond simply looking beautiful.
The Central Role of Sexual Selection
The primary reason a peacock spreads its feathers is to secure a mate, a process driven by sexual selection. The peahen is discerning, using the male’s display as an honest signal of his health and genetic quality. This preference is explained by the “handicap principle,” suggesting that only the fittest males can survive while carrying such a large, costly, and cumbersome ornament.
The quality of the display is judged by the numerous eyespots, or ocelli, that adorn the train feathers. Research indicates that the iridescence and color saturation of the blue-green portions of these eyespots strongly correlate with a male’s mating success. The peahen’s choice is based on the visual impact and brilliance of the display itself.
The male strategically manipulates his display angle to the sun, often positioning himself at about a 45-degree angle, to maximize light reflection off the iridescent microstructures. This enhances the shimmer and proves the male possesses the motor control necessary for courtship. Choosing a male who can afford the metabolic cost of the structure selects for genes indicating robustness and parasite resistance.
Communication Beyond Mating
While attracting a female is the main function, the fanned train also serves as a tool for communication. It is employed in male-to-male competition, where the size and quality of the erect train can intimidate rivals and establish dominance over a territory. A larger, more vibrant display signals superiority, often resolving disputes without fighting.
The train is also used in defense to startle or confuse predators. When confronted, the peacock may suddenly unfurl or shake his train, presenting the numerous, large eyespots in a phenomenon similar to the “many eyes” effect. This sudden presentation can momentarily confuse a threat, allowing the peacock to flee.
The peacock’s colors may offer stealth against some threats. Since most large mammalian predators are dichromats (having two color receptors), the train’s colors may appear less conspicuous against green foliage. This allows the male to maintain a brilliant display for courtship while potentially avoiding detection.
The Anatomy and Mechanics of the Display
The physical mechanism relies on the structure of the train feathers. The long, ornate train feathers are supported and pushed upward by the shorter, stiff tail feathers, called rectrices, which are hidden beneath the train. The peacock performs “train-rattling” by vibrating these feathers rapidly.
This vibration is achieved by stridulating, or brushing, the supporting tail feathers against the train feathers at an average frequency of about 25.6 Hertz. The movement is efficient because the feathers are vibrated near their natural resonant frequency, maximizing the visual amplitude while minimizing the bird’s energy expenditure. This rapid shaking generates the low-frequency mechanical sound that accompanies the display.
The visual effect is enhanced by the specialized microstructure of the feathers, which allows the brightly colored eyespots to appear almost motionless against the background of the surrounding feather barbs. Dense microhooks in the eyespots prevent them from wobbling, creating stationary spots floating on a rapidly oscillating surface. This anatomy ensures the eyespots, the main focus of the peahen’s assessment, remain clearly visible during rattling.