Lighter plumage colour and trade-offs in heat absorption
mechanisms
While the advantage of lighter plumage coloration in snow-covered
environments might be expected under the background matching hypothesis,
it is undeniable that less melanin pigmentation reduces the integument
capacity to transform UV radiation into heat (Margalida, Negro, &
Galván, 2008). Tawny owl genome analyses show that the architecture
surrounding the candidate loci FTCD contains both a collagen
(Col6a2 ) and a β-keratin gene (BISK1 ), both playing a key
role in integument micro-structure (ref). Of particular relevance is the
identification of BISK1 in the tawny owl genome. This novel copy
of a β-keratin gene has only been recently identified in the chicken,
and because its expression is confined to the barbule cells of that
species’ contour feathers, it is suggested to be relevant for the
formation of micro-barbules in pennaceous feathers – important
insulation structures (Kowata et al., 2014). We hypothesize that the
conservation of the Col6a2 -FTCD-B1SK1 region in tawny owls
and the identification of a candidate loci strongly associated with
plumage colour also indicates the molecular basis of a trade-off,
logically linked to the melanistic grey phenotype. While empirical
evidence exists for morphological differences between morphs,
specifically that grey tawny owls have a higher number and density of
barbules than brown morphs (Koskenpato et al., 2016), it is largely
unknown whether colour morphs differ in net insulative properties. If
trade-off theory would hold true, one would expect grey (via higher
barbule number) and brown morphs (via darker coloration/higher melanin
deposition) to converge in their capacity to retain heat.