Environmental gradients predict the ratio of environmentally acquired
carotenoids to self-synthesised pteridine pigments
Abstract
Carotenoids are important pigments producing integument coloration;
however, their dietary availability may be limited in some environments.
Many species produce red to yellow hues using a combination of
carotenoids and self-synthesised pteridine pigments. A compelling but
untested hypothesis is that pteridines replace carotenoids in
environments where carotenoid availability is limited. Based on a
phylogenetic comparative analysis of pigment concentrations in agamid
lizards, we show that environmental gradients predict the ratio of
carotenoids to pteridines; carotenoid concentrations are lower and
pteridine concentrations higher in arid environments with low vegetation
productivity. Both carotenoid and pteridine pigments were present in all
species, but only pteridine concentrations explained colour variation
among species and there were no correlations between carotenoid and
pteridine pigments with similar hue. These results suggest that
pteridine pigments replace carotenoids in carotenoid-limited
environments, irrespective of skin hue, presumably because it is
metabolically cheaper to synthesise pteridines than to acquire and
sequester carotenoids when carotenoids are rare.