Sylvain Losdat, Alfonso Rojas Mora, Caroline Bellut, Remi Charge, Valentina Falchi, Gaetan Glauser, Armelle Vallat, and Fabrice Helfenstein

Sperm performance is an important component of male reproductive success. However, sperm production is costly and males need to optimize their investment in sperm quality vs. the somatic traits involved in mating success, e.g. their social status. Since oxidative stress affects both sperm performance and somatic functions, it has been hypothesized to mediate such trade-off. According to the oxidation-based soma/germline trade-off hypothesis dominant males should favour the antioxidant protection of their somatic tissues, and subordinate males should favour the antioxidant protection of their sperm. We tested this hypothesis by experimentally infecting wild-caught house sparrows Passer domesticus with Coccidia Isopora sp., an internal parasite known to deplete antioxidant resources. We predicted that (i) increased parasite load affects sperm oxidative status and sperm performance and that (ii) males with experimentally high parasite loads adjust the antioxidant protection of their soma vs. their sperm according to their social status. Despite a 5400% increase in parasite load, sperm performance and somatic and spermatic oxidative status remained unaffected, irrespective of male social status. Nevertheless, males increased their sperm performance over time, a pattern mirrored by an increase in the antioxidant protection of their sperm. Moreover, males at the lower end of the hierarchy always produced sperm with lower velocity, suggesting that they were constrained and privileged their soma over their germline. To conclude, high parasite loads do not necessarily affect sperm performance and oxidative status. In contrast, the social hierarchy and the relative investment in soma vs. sperm antioxidant protection are determinants of sperm performance.

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