Identification and quantification of carotenoids in supra-orbital combs of male black grouse (Lyrurus tetrix)

Coloured patches of skin are seen across the animal kingdom and serve multiple functions. For many species, carotenoid pigments are often incorporated into the skin, hair or feathers and feature prominently in colourful signals used as sexual signals. Most animals cannot synthesise carotenoids de novo and, as a result, carotenoid-based signals have long been considered indicators of individual quality. We identified and quantified the carotenoids within the eye combs of black grouse (Lyrurus tetrix) using ultrahigh performance liquid chromatography–mass spectrometry. We found that black grouses incorporate two ketocarotenoids, astaxanthin and canthaxanthin, into their eye combs in different proportions depending on total eye comb size, with individuals with smaller combs incorporating significantly more canthaxanthin than astaxanthin. Additionally, a higher ratio of astaxanthin to canthaxanthin significantly decreased brightness, but was positively associated with both the λUV peak and the λR peak. The results found here highlight the need to also consider the additional effects of tissue structure on carotenoid, and overall colouration.