Size-related changes and chemical basis of melanin-based body coloration in the amphisbaenian Trogonophis wiegmanni

Melanin is a fundamental pigment in animal coloration as it is involved in many different adaptive functions such as signaling or thermoregulation. Two forms of melanin are known to produce different colors (eumelanin: black; pheomelanin: yellow or brown). Here, we explored whether there were intersexual differences and/or size-related changes in melanization of the amphisbaenian Trogonophis wiegmanni, a fossorial species with a characteristic black and yellow color pattern of scales. We found a clear effect of body length, but not of sex, on the levels of melanization, which was also dependent on the body region; the proportion of melanin-dependent blackish coloration increased with body length in the dorsum, but decreased with body length in the ventral belly area. We also studied the chemical basis of this coloration using Raman spectroscopy and found that eumelanin was present in both black and yellow scales, but pheomelanin was not found in yellow scales, suggesting that this yellow coloration is caused by the reduction of eumelanin or dispersion of melanosomes and/or perhaps by other pigments.