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The biomechanics behind the astonishing courtship display of the peacock


Male peacocks fan their colourful tail feathers and shake them, but somehow the iridescent eyespots on their feathers remain almost motionless, like a fixed gaze.

The biomechanics behind peacock tail-rattling display was revealed by a team of researchers from the US and Canada: resonance, sophisticated feather structures and measured muscle application are responsible for the spectacular tail-rattling effect.


As if its spectacular tail wasn't eye-catching enough, when the peacock shakes his tail feathers, a visual trick comes into play: in contrast to the rest of the moving feathers, the iridescent eyespots remain almost still, so that they seem to come to the fore and float. This effect was already explained in a research study by an international team of researchers.


Peacock's tail-rattling display


The peacock is the most flamboyant subject of the bird kingdom: the male animals unfold an astonishing splendour of feathers, which they stage in a noisy manner during courtship. Those striking male ornaments like the peacock's tail seemed to be rather difficult to explain by natural selection, as they appeared to be a disadvantage rather than an instrument for survival.


It is now known that peahens prefer the most spectacular candidates for mating. The reason behind this is the dazzling display of tail feathers, which reflects good genetic material.

Darwin's so-called sexual selection has ultimately led to the appearance of peacock tail feathers becoming more and more spectacular in the course of evolution.


What is responsible for the effect?


When a peacock displays his tail feathers, his muscles vibrate rhythmically, so that a vibration runs through his feather decoration, accompanied by a loudly audible hissing sound. When the vibration occurs, the blue-green iridescent eyespots seem to float in front of the moving background, because they move much less than the rest of the feathers.


A research team led by Roslyn Dakin, a zoologist at the University of British Columbia and Suzanne Kane, a biological physicist at Haverford College in Pennsylvania, have analyzed peacocks tail-rattling display using high-speed recordings and conducted vibration experiments with individual tail feathers of 14 peacocks.


Ultimately, it is the peacock's fine structure of the feathers that is responsible for the eyespot effect, the researchers report. It is known from the flight feathers of all birds that the fine fibers emanating from the shaft are held together laterally by small micro hook structures, thus creating a flexible surface. This is exactly the same with the iridescent eyespots on a peacock's tail - but not with the surrounding feather areas.


Feather structure, resonance and dexterity


Scanning electron microscope (SEM) revealed that the blue-green iridescent spots of the bird are made up of tightly interwoven feathers that are closed with microscopic hooks. This is why the eyespots are denser than the surrounding delicate feathers. As a result, the special decorative spots are more rigid and hardly move at all, while the remaining feather areas vibrate conspicuously.


The researchers found that peacocks stroke their tail feathers against the ornamental eyespots at or near resonance - a vibrating 25.6 hertz sweet spot. The rustle generates sound waves that are within the peahen hearing range, which could be a contributing factor in mate selection. Feather shaking at resonance maximizes the amplitude of the peacock's tail vibrations and reduces the force required to generate these movements. This results in an energy-saving effect on behaviour, the scientists say.


However, they were also able to show in this context that peacocks with particularly long tail feathers use more energy for the trembling effect. They shake their feathers at higher frequencies than males with shorter tail feathers. According to the researchers this might be another piece of evolutionary information; perhaps these peacocks are working harder to demonstrate their muscle strength and thereby improve chances of passing their genes on.


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Original research:

Dakin R, McCrossan O, Hare JF, Montgomerie R, Amador Kane S (2016) Biomechanics of the Peacock’s Display: How Feather Structure and Resonance Influence Multimodal Signaling. PLoS ONE 11(4): e0152759.