The antioxidant function of many animal pigments: are there consistent health benefits of sexually selected colourants?

Several classes of pigments impart flamboyant colours on animals, including the red, orange and yellow carotenoids. Carotenoid-based colours have served as ideal models for studying the honesty-reinforcing mechanisms underlying sexually selected traits, because the very pigments used to become colourful also have antioxidant and immunoregulatory properties that allow individuals to signal their superior health to prospective mates. It is unclear, however, whether other chemical colourants in animals confer similar physiological benefits. Here I make the observation that the other major groups of animal pigments, including melanins, pterins, porphyrins, psittacofulvins and flavonoids, also exhibit antioxidant activity in living systems. Thus, many types of pigment-based colour ornaments in animals have the potential to honestly reveal health state via the immunomodulatory action of the pigments themselves. To evaluate this hypothesis, the extent to which these sets of pigments serve antioxidant functions in colourful animals and to which animals face a trade-off shunting pigments to physiological functions versus colour displays should be investigated.