Blue whale (Balaenoptera musculus) skin contains eumelanin and pheomelanin

Melanin is a widespread pigment of the animal tegument. Two variants of melanin exist: (1) eumelanin, a dark brown pigment, and (2) pheomelanin, a reddish-orange pigment. While eumelanin is photoprotective, pheomelanin has been linked to cellular damage and high cancer risk. Despite this negative effect, pheomelanin is present in many, but not all, species, suggesting that it could confer an evolutionary advantage. To date, it is unknown whether the cetacean epidermis contains both melanin variants. Herein, we implemented a simple technique, previously developed for bird feathers, to quantify eumelanin and pheomelanin in blue whale (Balaenoptera musculus) skin, and we explored the potential biological role of pheomelanin based on histological analysis and gene expression quantitation. Both melanin variants were observed in the epidermis, with eumelanin being 42% more abundant than pheomelanin. Blue whale skin pigmentation or mottling type was predicted by the ratio of eumelanin to pheomelanin (EPR), with darker whales showing a higher EPR. Tyrosinase transcription levels influenced the EPR, with higher transcription being associated with higher EPR. Neither transcription of tumor suppressor gene p53 nor occurrence of epidermal photo-damage were related to the concentration of melanin variants, although there appeared to be a trend in which whales with blisters and microvesicles tended to have a lower EPR. Our study expands current understanding of epidermal pigmentation and photoprotection of large cetaceans.