Genetic sex determination, sex chromosome size and sex-specific lifespan across tetrapods

Sex differences in lifespan are ubiquitous across the tree of life and exhibit broad taxonomic patterns that remain a puzzle, such as males living longer than females in birds and vice versa in mammals. The prevailing “unguarded-X” hypothesis explains sex differences in lifespan by differential expression of recessive mutations on the X or Z chromosome of the heterogametic sex but has only received indirect support to date. An alternative hypothesis is that the accumulation of deleterious mutations and repetitive elements on the Y or W chromosome might lower the survival of the heterogametic sex (“toxic Y” hypothesis). Here, we use a new database to report lower survival of the heterogametic relative to the homogametic sex across 136 species of birds, mammals, reptiles and amphibians, as expected if sex chromosomes shape sex-specific lifespans, and consistent with previous findings. We also found that the relative sizes of both the X and the Y chromosomes in mammals (but not the Z or th..., ,