Intense red colors in birds are often due to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signaling of carotenoid color displays. Here we investigate hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, reflectance-based redness (hue), and circulating sex steroids. A coloration function of CYP2J19 is supported by seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, upregulation was similar in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative functions of hepatic CYP2J19 or its products, such as detoxification or antioxidants, respectively. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, likely of adrenal origin and compatible with luteinizing hormone-induced and (in females) estrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signaling, CYP2J19 expression was not related to male body condition or plumage redness.

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