A study of wild great tits suggests that sexual ornamentation, in this case vibrant plumage, may reliably signal the quality of a male's sperm. Sperm can suffer damage by free radicals when an animal is stressed, for example when fleeing predators or even working too hard. One way of mopping up free radicals is with antioxidants. As the plumage of some birds is made from antioxidants called carotenoids, it had been proposed that the intensity of the color could indicate the bird's capability for dealing with free radicals, and therefore maintaining its sperm quality. Fabrice Helfenstein, an evolutionary ecologist at the University of Bern, Switzerland, tested this by upping the parental workload of wild male great tits, Parus major - which have yellow breast plumage - by adding two extra chicks to their nests. After five days of looking after their big brood, males with paler plumage suffered a greater reduction in sperm motility than more colorful males. Supplementing the birds' diet with carotenoids markedly improved the sperm quality of the paler males.
Flashier Great Tits Produce Stronger Sperm, Bird Study Shows