Reproductive melanization may protect sperm from harmful solar radiation

Animal pigmentation is incredibly diverse, serving a variety of functions. However, the function of the pigmentation that surrounds the testes of some vertebrates is unknown. Why are the tissues surrounding the testes (scrotum, tunica vaginalis and/or tunica albuginea) melanized in some species but not others? We examined this question in bats, where there is extensive species-specific variation in the presence of darkly pigmented (melanized) tissues surrounding the male gonads, as well as diversity in their ecologies, mating systems, and physiologies. Using data from 136 species of bats, we found that melanin surrounding the testes and epididymis is associated with more exposed roosting sites (presumably with greater sun exposure- and UV radiation), and with the occurrence of long-term male sperm storage. These findings suggest that scrotal melanin may protect mature sperm from UV damage, and from oxidative damage in species with male sperm storage. We found no evidence of an association between group size or mating system with reproductive melanin, or that phylogeny explains the distribution of pigmentation. Although our study suggests that scrotal melanin may protect sperm, the mechanism remains unknown. We outline several avenues for future work on reproductive pigmentation aimed at investigating additional roles of reproductive melanization in male bats.