Demographic pathways linking sex-chromosome system to adult sex ratio variation across tetrapods

Sex chromosomes determine male and female phenotypes, and the resulting sex differences can have significant impacts on ecology and life history. One manifestation of this link is that ZZ/ZW sex-determination systems are associated with more male-skewed adult sex ratio (ASR, proportion of males in the adult population) than XY/XX systems across tetrapods (amphibians, reptiles, birds, and mammals). Here we investigate four demographic processes: male and female offspring production, sex differences in juvenile and adult mortalities and in timing of maturation that can contribute to ASR variation between XY/XX and ZZ/ZW systems, using phylogenetic analyses of a large dataset collected from tetrapod species in the wild. We show that sex differences in adult mortality reliably predict ASR, and it is also more male-biased in XY/XX species than in ZZ/ZW species. Sex differences in juvenile mortality or in maturation time also contribute to ASR skews, but do not differ consistently between XY/XX and ZZ/ZW systems. Phylogenetic path analyses confirm an influence of sex-determination system on ASR through sex-biased adult mortality. Thus, these results infer that sex chromosomes can impact, via demographic pathways, frequency-dependent selection emerging from the relative number of males and females. We call for follow-up studies to uncover the potentially complex web of associations between sex determination, population dynamics, and social behaviour.