Sperm quality and function deteriorate in old males of the naturally short-lived turquoise killifish

Most of the variation in male reproductive aging observed in nature may be explained by different life-history strategies, which reflect trade-offs between growth, reproduction and survival. The disposable soma theory predicts that the energy allocated in reproductive functions is traded-off with soma maintenance. In particular, males are expected to maximise the investment in reproduction at the expense of soma maintenance, due to the strong selection for reproduction. Therefore, according to the disposable soma theory, species showing rapid somatic aging and high actuarial senescence are expected to have evolved high allocation in reproduction, with limited if any reproductive aging. We tested this prediction in turquoise killifish (Nothobranchius furzeri), a vertebrate of extremely short natural lifespan. We analysed age-related changes in sperm traits, the proportion of fertilized eggs, as well as embryo survival. Surprisingly, we found that, similar to their extensive actuarial senescence, turquoise killifish males also undergo spontaneous reproductive aging, consisting in the age-dependent decline of sperm quality, decreased proportion of fertilized eggs and lower embryo survival. Our results suggest that male reproductive aging may result from processes that are not necessarily dependent on the adaptive resource allocation between reproduction and somatic maintenance. ### Competing Interest Statement The authors have declared no competing interest.