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Rationale Senescence can occur not only at the level of the individual (diploid cells), but also at the level of gametes (haploid cells). In males, one way in which this haploid level senescence can occur is via pre-meiotic sperm senescence (via ageing of the germline due to advancing age of the male). Although evidence from multiple studies suggests that sperm and ejaculates of males deteriorate as males age, this evidence is not widespread across all studied taxa, and the extent to which advancing male age leads to pre-meiotic sperm senescence remains unclear. We think it is essential to consolidate the data on this topic, especially because the literature comes from different organisms and a vast variety of fields, e.g. medicine, theriogenology, and evolutionary ecology. Such diversity is likely to create methodological, taxonomic, and ecological differences, the potential effects of which are not well understood or accounted for in studies of male reproductive ageing. Broad Aims Our study aims to test, using a systematic review and meta-analytic approach, whether advancing male age leads to deterioration in sperm and whole-ejaculate traits (WET) in non-human animals (henceforth called animals). Broad methodology We will use a multiple-species, phylogenetically controlled meta-analytic approach, where we utilize study outcomes, converted into appropriate effect sizes, to test how adult male ageing affects sperm and WET. We will further test, by adding moderators to our meta-analytic model, how methodological, ecological, and biological differences between studies affects the effects of male ageing on sperm and WET. Importance Our study will help understand the overall effect of male age on sperm and WET across taxa, as well as increase our understanding of the proximate mechanisms that may lead to senescence in male reproduction, especially male infertility. This has implications for understanding how male age affects post-copulatory sexual selection and male mating decisions via changes in sperm and WET Our study has translatable uses in improving health and fertility of animals (and possibly humans) in the fields of medicine, agriculture, aquaculture, and evolutionary ecology. Our study may also help us investigate possible effects of advancing male age on epigenetic changes in sperm, via changes in sperm DNA integrity, oxidative stress, and telomeres. |
