New research by a Yale scientist and his collaborator sheds light on a colorful riddle of the animal kingdom -- how the bright blue skin found in certain monkeys and marsupials is produced.
Only a few mammals have bright blue coloration. The West African mandrill has a bright blue face and rump, while the vervet monkey has a brilliantly blue scrotum. The mammals use these colors to communicate social and sexual signals.
In the June 15 issue of The Journal of Experimental Biology, evolutionary biologist Richard O. Prum and mathematician Richard H. Torres present new evidence demonstrating how these blue skin colors are produced. Prum is the William Robertson Coe Professor of Ornithology in Yale's Department of Ecology and Evolutionary Biology and the curator of ornithology at the Peabody Museum of Natural History. Torres is a professor in the Department of Mathematics at the University of Kansas at Lawrence. Their research was funded in part by grants from the National Science Foundation.
For a century, biologists have known that the blue in mammal skin is a structural color -- that is, a color created by the interaction of light with tiny physical structures rather than by pigment molecules. Other examples of structural colors include the blue sky, a rainbow, an oil slick and the hues of an opal.
For years, biologists believed this blue mammal skin color was caused by Tyndall scattering, or incoherent scattering. This mechanism produces color by light scattering off of a diffuse mixture of particles. The blue of the sky is produced in this way.
Prum and Torres have found evidence that the blue colors in mammal skin are produced instead by the coherent scattering, or interference -- the mechanism that creates the colors of an oil slick or an opal. In those cases, most of the light waves scattered off of the different surfaces of the substance (the top and bottom of the oil slick, for example) cancel each other out, while those wavelengths that are equal to the difference in the distance traveled are reinforced, producing a brilliant color.
Using electron microscope images of blue skin from four different species of mammals, Prum and Torres concluded that the skin is too ordered to produce the color by incoherent Tyndall scattering.
The blue color of mammal skin, say the researchers, is made by light scattering from the highly ordered arrays of parallel collagen fibers in the skin. Collagen is a common protein in the spaghetti-shaped fibers that make up much of human skin, but the collagen fibers in blue monkey skin are much more ordered than typical collagen, explain the researchers: Each fiber is about 100 nanometers in diameter (or about 4 millionths of an inch) and runs in parallel with its neighbors, like uncooked spaghetti in a box.
In fact, in order to produce the correct color, these mammal species must grow many collagen fibers with exactly the correct diameters and distances between fibers, note the researchers. Less than one millionth of an inch difference will produce the entirely wrong color, they note, resulting in a patch of skin that will not communicate an appropriate social signal or attract any mates.
Prum first discovered this kind of structural color in the blue and green skin of various birds. "We were amazed to find out that the same exact anatomical structures have evolved convergently in the skin of mammals," Prum reports. "But interestingly, blue mammal skin has evolved in only the two groups of mammals that are known to have advanced color vision: Old World monkeys and marsupials."
The most common place for blue skin to evolve in mammals is on the scrotum, note the researchers, and at least three different primate and marsupial lineages have evolved blue scrotum coloration. In the vervet monkey, the blue scrotum features prominently in male-male interaction displays and in maintaining social status within the troop.
Prum hopes that these new details on the anatomical and physical basis of these colors will further an understanding of how social signals have evolved in some of humans' closest relatives.
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