Researchers report the discovery of genome-wide variations in gene expression between mammalian females and males and offer new insights into the molecular origins and evolution of sexual dimorphism in mammal species, according to a new study. The findings could help explain the wide range of sex-specific differences in human health and disease. Female and male mammals often exhibit a variety of differences in biological processes and phenotypic traits. For example, in most mammal species males are larger than females, and because sex differences appear common across many species, animal models are often used to investigate sex-biased traits and diseases in humans. However, the effects of sex on gene expression, particularly in autosomal genes, aren't well known. To investigate how sex affects the genome, Sahin Naqvi and colleagues performed a genome-wide, multi-tissue and comparative survey of sex-biased gene expression across five mammalian species. Naqvi et al. collected RNA sequencing data from male and female macaques, mice, rats and dogs for 12 different tissues which represented each germ layer as well as most major organ systems. Non-human data was compared to corresponding human RNA-seq data from the Genotype Tissue Expression Consortium (GTEx), which catalogs gene expression across all major tissues in the human body. The comparative analyses revealed hundreds of conserved sex-biased gene expressions in each tissue, which contribute to differences in traits between the sexes. For example, nearly 12% of the sex difference observed in average human height can be accounted for through conserved sex biases in gene expression. The results also show, however, that most sex bias in gene expression is an evolutionarily recent adaptation and thus is not shared between all mammalian lineages - findings which warrant careful attention in the use of non-human models of sex differences.