Male Reproductive Senescence in Mammals Is Pervasive and Aligned With the Slow-Fast Continuum

Reproductive senescence, the decline in any component of offspring recruitment with increasing age, has been well documented in mammalian females. Male reproductive senescence, however, is much less understood, partly due to the past complexities of getting reliable paternity assignment in the wild. Through a standardised literature search, we compiled age-specific reproductive data on both mating and reproductive success on 57 species encompassing 73 populations. We then explored the occurrence of male reproductive senescence and sought to identify key ecological and evolutionary drivers modulating its intensity. We found evidence of reproductive senescence in 59.6% of the studied species, with a detection probability increasing with larger sample sizes and longer age ranges relative to lifespan (i.e., relative maximum age measured as the ratio between the oldest age sampled and the species maximum longevity). As expected, both the onset and the rate of male reproductive senescence were aligned with the species position along the slow-fast continuum, with fast-living species exhibiting earlier and stronger reproductive senescence than slow-living species. Moreover, the rate of reproductive senescence also increased with the level of sexual size dimorphism, highlighting that males from species displaying a high level of sexual selection suffer from a steeper rate of reproductive senescence.