RESULTS
Skin tissues from all 27 species contained both carotenoid and pteridine pigments (Figure 1). Carotenoids with the highest concentrations among our samples were lutein/zeaxanthin (yellow) and β-carotene (orange), while the pteridines with the highest concentrations were isoxanthopterin and pterin-6-carboxilic acid (colourless) but there were also substantial concentrations of yellow xanthopterin and red drosopterin (Figure S1).
Total carotenoid concentration was significantly associated with environmental gradients. Individuals living in environments with higher productivity (and thus higher carotenoid availability; environmental PC1) had a higher concentration of total carotenoids (Figure 2, Table 1, r2=0.16). Furthermore, individuals in more productive environments had a lower concentration of total pteridines (Figure 2, Table 1, r2=0.14), and therefore there was a significantly higher ratio of carotenoids to pteridines in more productive environments (Figure 2, Table 1, r2=0.17). There was no significant association between sexual selection indices and pigment concentration in the whole dataset analysis (Table 1) or at the species-level (Table S1), although there was a trend for higher total carotenoid concentration in species with higher sexual size dimorphism (Figure 2D).
To test for direct compensation (carotenoid mimicry), we examined the relationships between carotenoids and pteridines of a similar hue. There was no association between either red ketocarotenoids and drosopterin, or yellow-orange dietary carotenoids and xanthopterin (Figure 3). Interestingly, there was a significant positive correlation between the concentration of dietary carotenoids and ketocarotenoids, and between colourless pteridines and xanthopterin, and a negative correlation between xanthopterin and ketocarotenoids (Figure 3).
Variation in skin colours was associated with the concentration of pteridines but not carotenoids (Figure 4). Specifically, tissues with higher concentrations of drosopterin had redder hues (lower hue values), and tissues with higher xanthopterin, colourless pteridines and total pteridines had more saturated colours (Table 2). Tissues with higher concentrations of colourless pteridines also had lower luminance (darker). Yellow-red (including browns, N=150) tissues had higher concentrations of drosopterin (Figure 4C), colourless pteridines, and ketocarotenoids compared to black/grey/white tissues (N=36; 186 tissue samples in total), whereas dietary carotenoid and xanthopterin concentrations were similar in all skin colours (Table S2, Figure S2). Additionally, the luminance of skin colours was associated with habitat productivity (PC1 95% CIs 2.56 – 10.18), with darker colours in more productive environments (Figure S3), but environmental PCs did not predict hue or saturation (Table S3).