show Abstracthide AbstractReconstructing ancestral characters on a phylogeny is a difficult task. It was proposed that ancestral life-history traits can be indirectly estimated with the help of molecular data. We challenge this approach in the Cetartiodactyla by analysing transcriptome data of 41 species, of which 22 were newly sequenced. We report a significant effect of body mass on molecular substitution rates and demonstrates the potential of phylogenomic methods for ancestral trait reconstruction.