Why Does a Blind, Fossorial Mammal Have Iridescent Fur?

Golden moles (Chrysochloridae) are members of the mammalian Order Afrosoricida, an African group that also includes then tenrecs of Madagascar. Golden moles and true moles (Talpidae) evolved their fossorial habits independently. However, both groups have soft velvety fur. Scientists working with golden moles noticed that their silky fur often appears slightly iridescent (Figure 1).

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Figure 1. A Grant’s golden mole (Eremitalpa granti) showing it’s silky fur and leathery nose patch. (Courtesy of G. Rathbun)

Iridescence is common among insect and bird species, but has not been described before in mammals. Iridescence occurs when light is scattered by nanoscale ridges that are repeated over the surface of a structure such as a bird’s feather or a beetle’s carapace. In doing so, the structure appears to change in hue as the angle of view or angle of light changes.

Recently, a group of scientists reported in the journal
Biology Letters (2012) that golden moles have iridescent fur (Figure 2).


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Hair3
Figure 2. SEM images of iridescent and non-iridescent hairs from the golden mole species Chrysochloris asiatica, showing an iridescent (b) and non-iridescent hair (c) with an optical image (insert) of the cross section of a single hair. TEM images of a cross section of an iridescent (d) and non-iridescent hair (e), showing repeated dark and light bands in the cuticle. (From Snyder et al., 2012)


The fur of golden moles is sometimes described as having a greenish to purplish sheen. How are these iridescent colors produced? According to Snyder’s team (2012) golden moles have unusual hair structure that contributes to its weak iridescence. The hairs are flattened distally with extremely thin cuticular scales that make the hair surface appear almost smooth. Inside the hair cuticule, are repeated bands of light and dark, which resemble the wing cases of some iridescent beetles (Figure 3). Reflectance measurements suggest that the iridescent colors are produced when the multiple light-dark layers produce thin-film interference. This is enhanced by the fact that the hairs are flattened and have a relatively smooth reflective surface.

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Figure 3. (a) TEM image showing the alternating light and dark bands in the hair cuticule, and (b) optical modeling curves of reflectance for the golden mole Amblysomus hottentotus (solid line) and predicted (dashed line). (From Snyder et al., 2012)

Usually, animal coloration is the result of selection for camouflage or sexual ornamentation. Why would iridescent coloration evolve in a blind mammal that spends most of its life in darkness burrowing through the sand? Snyder and colleagues suggest that iridescent fur is an epiphenomenon; a by-product of evolution acting on some other aspect of the trait. They hypothesize that the smooth, flatten hairs with multiple layers evolved to reduce friction and damage as they burrow, and that the iridescence arises as a by-product of these mechanical functions.


References

Snyder, H., Maia, R., D'Alba, L., Shultz, A., Rowe, K., Rowe, K., & Shawkey, M. (2012). Iridescent colour production in hairs of blind golden moles (Chrysochloridae) Biology Letters DOI: 10.1098/rsbl.2011.1168