Carotenoid scarcity, synthetic pteridine pigments and the evolution of sexual coloration in guppies (Poecilia reticulata)

Carotenoid-based sexual coloration is the classic example of an honest sign al of mate quality Animals cannot synthesize carotenoid pigments and ultima tely depend on dietary sources. Thus, in carotenoid-poor environments, caro tenoid coloration may be a direct indicator of foraging ability and an indi rect indicator of health and vigour. Carotenoid coloration may also be affe cted, more directly, by parasites in some species. Carotenoids are not, how ever:, the only conspicuous pigments available to animals. Pteridine pigmen ts, with similar spectral properties, are displayed in the exoskeletons and wings of insects, the irides of birds and the skins of fishes, lizards and amphibians. Unlike carotenoids, pteridines are synthesized de novo by anim als. We report that the orange spots that male guppies (Poecilia reticulata ) display to females contain red pteridine pigments (drosopterins) in addit ion to carotenoids. We also examined the relationship between drosopterin p roduction by males and carotenoid availability in the field. The results co ntrasted sharply with the hypothesis that males use drosopterins to compens ate for carotenoid scarcity: males used more, not less, drosopterins in str eams with higher carotenoid availability The positive association between d rosopterin use and carotenoid availability could reflect the costs of droso pterin synthesis or it could be a consequence of females preferring a parti cular pigment ratio or hue. Male guppies appear to use drosopterin pigments in a manner that dilutes, but does not eliminate, the indicator value of c arotenoid coloration.