Environmental gradients and pigment concentrations
Total carotenoid concentration was significantly associated with environmental gradients. Individuals living in arid environments with low vegetations productivity (and thus potential carotenoid limitation; environmental PC1) had a lower concentration of total carotenoids (Figure 2a, Table 1, r2= 0.16). This association was driven by dietary yellow carotenoids (Table S2) because they comprise the great bulk of total carotenoids; there was no association between red ketocarotenoids and environmental PC1 (Table S2). Individuals in less productive environments also had a higher concentration of total pteridines (Figure 2b, Table 1, r2= 0.14), and therefore there was a significantly lower ratio of carotenoids to pteridines in more productive environments (Figure 2c, Table 1, r2= 0.17).
To further explore the link between environmental PC1 and pigment concentrations, we examined the association between specific environmental variables and total carotenoids, total pteridines, or their ratio (Table S3). The strongest drivers of pigment concentration were aridity and radiation of the warmest quarter. Species with low carotenoids, high pteridines and low ratio of carotenoids to pteridines were found in arid environments with high summer solar radiation.
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).