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).