Scientists say red coloration—historically seen as costly in vertebrates—might represent some physiological benefit after all.
Pheomelanin, that is responsible for red hair and freckles in humans and orange and chestnut coloration in other animals, is known to increase the damage to skin cells and melanoma risk when present in large amounts.
In addition, its creation involves the consumption of glutathione, a beneficial antioxidant.
In an attempt to unearth the factors favouring the evolution of pheomelanin in spite of its costs, Ismael Galvan and Anders P. Moller of the University of Paris-Sud examined the survival from one breeding season to the next of a wild European population of barn swallows, as well as the annual survival rates of 58 species of American birds.
A recent hypothesis claims that the consumption of a component of glutathione called cysteine, which occurs when pheomelanin is produced can be beneficial under conditions of low stress.
Cysteine, which is mainly acquired through diet, can be toxic at high levels, so the production of pheomelanin may help to sequester excess quantities of this amino acid.
Galvan and Moller measured birds’ blood levels of uric acid and analyzed the coloration of their chestnut throat feathers. When they compared birds that had similar uric acid levels, they found that both the European barn swallows and the American birds with larger amounts of pheomelanin in their feathers survived better.
This study is the first to propose that the costs/benefits of pheomelanin may depend on prevailing environmental conditions, and its results suggest that the production of this pigment may even be beneficial in some circumstances.
The study has been published in the journal Physiological and Biochemical Zoology.