Attracting a desirable mate is all about beauty in the eye of the beholder!Animals have developed spectacular manes, lustrous scales or brilliant plumage to catch the attention of a passing suitor. Some birds have added vividly coloured skin patches to their repertoire of attractive decorations. However,only a few of these bright skin shades are produced by pigments; many of the colours towards the blue end of the spectrum are generated by the optical properties of the skin itself. Richard Prum explains that it was accepted that blue colours in bird skins were generated by Rayleigh scattering, in the same way as the sky's colour; randomly arranged particles in the atmosphere only scatter short wavelength light to give the sky its familiar tone. But when Prum began looking at the microscopic collagen structures in coloured bird skins, he realised that the skin's colour wasn't produced by randomly scattered light, it was generated by a few wavelengths of light scattered from arrays of collagen fibbers in the skin to produce the blue-green effect(p. 2409); the same as oil-film colours.

Prum explains that he first stumbled across this strange class of structural colours because he was intrigued by the ornate decorative structures that birds use to attract a mate. Knowing that these skin colours weren't iridescent, because the colour didn't appear to shimmer, everyone had assumed that the effect was generated by Rayleigh scattering from randomly arranged structures in the skin. But when Prum used transmission electron microscopy to look at the blue skin from a Madagascan velvet aity, he realised that this couldn't be true. The collagen structures were arranged regularly,almost like a crystal, so that light reflected off each layer to produce the blue and green tones. Was the velvet aity unique, or had other birds come up with the same solution to the structural colour problem?

Prum sent bottles of wattle [preserving] juice' out far and wide, asking friends and colleagues to send back samples of skin, gathering vivid samples from more then 30 bird species across the planet. Time and again, the skin's collagen structures were far too ordered to produce the colours by Rayleigh scattering. But were they ordered enough to generate colours in the same way that thin layers of oil only reflect a few wavelengths of light to make rich colours? Prum teamed up with Rodolfo Torres, a mathematician, to see if Torres could derive information from the layered collagen structures, and use it to predict the vivid spectrum of colours that the skin produced.

Amazingly, Torres' mathematical analysis of the skins' structures produced predictions that were remarkably similar to the measured spectra. The skins'hues weren't produced by Rayleigh scattering from individual collagen fibers,but constructive interference among blue waves scattered by multiple collagen fibres. And Prum thinks that this effective way of generating skin colours could have evolved more than 50 times over the course of evolution.

Prum adds No one had tested to see whether Rayleigh applied'. He explains that every one assumed that the colours were produced by Rayleigh scattering because they weren't iridescent. `The take home is that biologists were lazy physicists, they had accepted a lousy criterion of accepting [that the colours were generated] by Rayleigh scatterers' concludes Prum.

Prum, R. O. and Torres, R. (
2003
). Structural colouration of avian skin: convergent evolution of coherently scattering dermal collagen arrays.
J. Exp. Biol.
206
,
2409
-2429.