biostudies-literatureUnknown36(2)Sultanova ZEuropean Regional Development FundSex 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 exJournal of evolutionary biology480-494https://www.ebi.ac.uk/biostudies/studies/S-EPMC10107984biostudies-literatureGenetic sex determination, sex chromosome size and sex-specific lifespans across tetrapods.PMC10107984Downing PACarazo PSultanova ZfalseGenetic sex determination, sex chromosome size and sex-specific lifespans 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 ex2023-01-01T00:00:00Z2023 Feb2025-04-27T02:37:51.879Z2025-04-06T18:38:18.641ZS-EPMC101079843653735210.1111/jeb.14130