Testicular melanization has evolved in birds with high mtDNA mutation rates

Melanin is mainly found in the integument of animals, but it also appears in several extracutaneous tissues. The presence of melanin in testes has been anecdotally reported in all vertebrate groups, but the causes and functions of this melanin remain unknown. Similar to other extracutaneous melanins, testicular melanin may protect male germ cells from oxidative stress. Given the high respiratory activity of spermatozoa, oxidative stress generated by mitochondrial dysfunction as a consequence of mtDNA mutations directly affects sperm viability. Thus, natural selection may favour testicular melanization in males of species with high historical mutation rates in the mitochondrial genome. Here, we tested this hypothesis using information on occurrence of testicular melanization and mutation accumulation as reflected by cytochrome b mtDNA base pair substitution rates in a large set of 134 species of birds, controlling for the confounding effects of body mass, reproductive activity and phylo...