Is male reproductive senescence minimized in Mus species with high levels of sperm competition?

Abstract

Sperm competition, an evolutionary process in which the spermatozoa of two or more males compete for the fertilization of the same ovum, gives rise to several morphological and physiological adaptations. Generally, high levels of sperm competition enhance sperm function. In contrast, advanced age is known to lead to reproductive senescence, including a general decline in sperm function. Sperm competition and advanced age may thus have opposing effects on sperm function. Here, we tested the hypothesis that the increase in sperm function in species experiencing high levels of sperm competition will counteract the negative effects of advanced age. We measured a comprehensive set of reproductive traits in young and old males in three species of mice of the genus Mus, which differ greatly in their levels of sperm competition. Our prediction was that the expression of reproductive senescence would be highest in the species with low levels of sperm competition and lowest in the species with high levels of sperm competition. Surprisingly, we did not find a strong signal of reproductive senescence in any of the three Mus species. Overall, our results did not clearly support our hypothesis that high levels of sperm competition minimize the negative effects of ageing on sperm function.