Iridescence -- a lustrous rainbow-like play of color caused by differential refraction of light waves -- has just been detected in the fur of golden moles.
Aside from the “eye shine” of nocturnal mammals, seen when a headlight or flashlight strikes their eyes, the discovery marks the first known instance of iridescence in a mammal. The findings, published in the latest Royal Society Biology Letters, reveal yet another surprise: the golden moles are completely blind, so they cannot even see their gorgeous fur.
“It is densely packed and silky, and has an almost metallic, shiny appearance with subtle hints of colors ranging between species from blue to green,” co-author Matthew Shawkey told Discovery News.
Shawkey, an associate professor in the Integrated Bioscience Program at the University of Akron, was first inspired to study golden moles after an undergraduate student of his, Holly Snyder, wrote her honors thesis about iridescence. Snyder is lead author of the paper.
For the study, the scientists pulled hairs from specimens of four golden mole species. Using high tech equipment, such as scanning electron microscopy and transmission electron microscopy, the researchers analyzed the structure of the hairs, down to their smallest elements.
The researchers determined that the hairs are indeed luminescent. They further discovered that each hair has a flattened shape with reduced cuticular scales that provide a broad and smooth surface for light reflection. The scales form multiple layers of light and dark materials of consistent thickness, very similar to those seen in iridescent beetles.
Optical modeling suggests that the multiple layers act as reflectors that produce color through interference with light. The sensitivity of this mechanism to slight changes in layer thickness and number explains color variability.
What remains a mystery is why blind animals would have such eye-catching fur.
Ancestors of the moles were sighted, so it’s possible that the iridescence is a carryover from those times. “However, the moles have diverged considerably from these ancestors so there had to be some selection pressure other than communication to keep their color intact,” Shawkey said.
Another possibility is that the fur somehow wards off the mole’s sighted predators. But Shawkey said shiny fur “would seem to make them more conspicuous,” doing just the opposite. The moles are not poisonous, so the coloration does not serve as a warning to other animals.
The researchers instead think that iridescence may be a byproduct of the fur’s composition, since the structure also streamlines the mole’s profile and creates less turbulence underground, permitting the animals to move more easily through dirt and sand.
“Many of the nanostructures producing iridescent colors have non-optical properties like enhanced rigidity (think mother of pearl) or enhanced water repellency (such as seen in Morpho butterflies),” Shawkey explained. “In the former case, the color, like in the moles, clearly has no communication function and is a byproduct.”
Iridescence has been around for at least 50 million years, since beetles from that time with the unique coloration have been unearthed. An ancient, iridescent bird feather dating to 40 million years ago has also been documented, as have early shells. Now peacocks, hummingbirds, sunbeam snakes, birds of paradise, the rainbow skink, and many fish flash their iridescence.
Daniel Osorio, a professor of neuroscience at the University of Sussex, has studied iridescence in birds.
Surprisingly, one of the most beautiful examples may belong to the common feral pigeon. The pigeon’s neck feathers shift from green to magenta, but often look drab gray to human eyes.Osorio told Discovery News, “In fact, this gray may be a remarkable and very unusual color to birds that can probably see more colors than us.”In the future, Shawkey and his team hope to study the phenomenon more, to better understand the function of iridescence in the moles and other species.