Bluebirds, peacocks--it's mainly birds that show off blue colors. Only a few mammals sport blue skin. Now an evolutionary biologist and a mathematician have discovered how the creatures do it: Protein fibers are lined up to reflect light in a very structured way, generating blue much the same way that an oil slick takes on an iridescent sheen.
Pigments turn mammalian skin brown, yellow, or red, but there's no blue pigment. For years, biologists have assumed that skin color is affected by something like the physics that explains why the sky is blue. Blue sky arises because of the way light scatters off dust and cloud particles, which tend to reflect back toward Earth only the smaller wavelengths--the blues and purples. But when Richard Prum, an evolutionary biologist at Yale University in New Haven, Connecticut, and mathematician Rodolfo Torres of the University of Kansas, Lawrence, teamed up to study this phenomenon in mammals, they found that the skin in these animals reflects light in a much more orderly way.
Prum and Torres had first come across this alternative for making colors when they checked out the skin of shiny bright blue and green birds (Science, 24 January 2003, p. 504). For this new research, they examined the submicroscopic properties of mammalian blues, using an electron microscope and other tools to look at skin from the scrotums of a vervet monkey and two opossum species, as well as a mandrill's face and rump.
Fibers made of the protein collagen were all lined up parallel to one another and to the skin's surface. Usually, the fibers go every which way. Fibers help give skin its integrity but, in this case, they seemed to play a role in hue as well. With the help of a mathematical technique called Fourier transform analysis, the researchers determined the optical properties of this fiber array. The parallel arrangement, as well as the uniform diameter and distance between fibers, made possible the transmission of blue light, they report in the 15 June Journal of Experimental Biology.
The color appears because fibers' orderliness reflects a single wavelength, while the rest cancel out one another. In these mammals, this skin setup bounces back blue. If, for example, the fibers are spread apart, the blue pales.
"Until now, most people have had the wrong idea" about the blues, says Sönke Johnsen, a biophysicist at Duke University in Durham, New Hampshire. But Prum and Torres "have applied some good physics to this biological problem," he notes, and that should set the world straight.