Island volcanism predicts pheomelanin-based plumage colouration in a cosmopolitan raptor

Aim: Pheomelanin is a pigment responsible for yellowish-to-reddish colours of vertebrate teguments. Its biosynthesis is favoured under high concentration of intracellular thiols, which, in turn, can depend on the environmental exposure to sulphur. Thus, pheomelanin production should be more intense and frequent in environments characterized by high level of sulphur, such as volcanic regions. In this study, we aimed at addressing this hypothesis by investigating variation in plumage colour of insular populations of the cosmopolitan barn owl (Tyto alba species complex) according to the presence of soils of volcanic origin (i.e. andosols) and recent volcanic activity. Location: World. Taxon: Barn owl species complex. Methods: We measured plumage colouration of more than 2000 museum specimens from 50 islands and archipelagos worldwide. We then compared plumage colouration between populations living on volcanic (i.e. where andosols and/or recent volcanic activity are present) and non-volcanic islands/archipelagos. Results: Consistently with the prediction, plumage colouration is significantly darker (i.e. pheomelanic) on islands/archipelagos where andosols and/or recent volcanic activity are present than absent, although this environmental factor explains a small fraction of plumage colour variability across islands (<10%). Similar results were obtained when specimens' sex and climatic predictors were included in the analyses. Main Conclusions: Because excessive intracellular levels of thiols can be toxic, pheomelanin synthesis may function as a mechanism keeping these compounds below the toxicity threshold and limiting their detrimental effects on physiology. Darker plumage may also be favoured because it promotes background matching against the dark environment typical of volcanic islands (dense vegetation cover and/or dark soil). Our results add to the little evidence that the environmental exposure of compounds that are involved in melanin biosynthesis can affect animal pigmentation and suggest that soil composition may be a factor that affects melanogenesis, possibly contributing to generate spatial variation in pheomelanin-based traits in animals.