Multilayer thin-film produces recurrent evolution of iridescence in mammals

Iridescent coloration is a vibrant structural colour that is widespread in nature, but in mammals is thought to be limited. Although multiple rodent and Eulipotyphlan species have been anecdotally described as iridescent, empirical evidence outside of the Chrysochloridae (golden mole) family is lacking. As iridescence in golden moles is created through a thin-film mechanism from a compressed cuticle structure, and the structure of hair is highly conserved, we expect iridescence to be present, and produced by the same mechanism, in mammals that share similar hair properties. Here, we test this hypothesis by first collecting and analysing existing documentation of iridescence in mammals, finding written evidence spanning 25 genera across eight mammalian families. We then identified the underlying mechanisms of iridescence for 14 species from Rodentia and Afrosoricida (including one outside Chrysochloridae), and showed that iridescence in these species is created through the same system of thin, alternating layers of keratin and probably lipid-rich material within the cuticle, in similar proportions as Chrysochloridae. These data suggest that iridescence in mammals is more common than originally thought, adding another dimension to mammal coloration research.