Reproductive melanization may protect sperm from harmful solar radiation

Abstract

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.

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Acknowledgements

We thank Kate Doyle and the UMass Natural History Collection; Mariana Muñoz-Romo, John Cubit, Al Hicks and Jennifer Wen for photographs; M. Brock Fenton and Nancy Simmons for support in field endeavors, Tom Eiting, Theodore Garland, Jr., Anthony Ives, Diane Kelly, Chi-Yun Kuo, Casey Gilman, Rachael Juvinall, Jenna Okun, Christopher Delmore and Robert Timm for useful comments. We also wish to thank two anonymous reviewers for insightful comments. Material is based upon work supported by NSF Postdoctoral Research Fellowship in Biology to TJO under Grant No. (DBI-1202871).

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Correspondence to Teri J. Orr.

Electronic supplementary material

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Supplement 1

Data used in analyses. Roost exposure categorization is based on roosting type and thus both use the same citations. Pigment was assigned from Kermott and Timm or other sources (as referenced in supplementary table citations) and verified when possible by additional sources as present (1) or absent (0). In a few cases where species is in parentheses none was specified in the original source (genus level only). Finally, in one case our observations differed from those of Kermott and Timm (Saccopteryx bilineata) in which we noted reproductive melanization but Kermott and Timm did not. In this case we went with our personal observations in the lab and field. Given the diminutive size of this species as well as cryptic nature of the genitalia (males and females are difficult to discern even when males are reproductive) we believe this to present a unique case. (XLSX 30 kb)

Supplementary material 2 (DOCX 28 kb)

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Delmore, J.L., Brennan, P.L.R. & Orr, T.J. Reproductive melanization may protect sperm from harmful solar radiation. Evol Ecol 32, 127–139 (2018). https://doi.org/10.1007/s10682-018-9932-9

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Keywords

  • Melanin
  • Reproductive pigments
  • Roost
  • Scrotal pigments
  • Sperm storage
  • UV radiation