Gestation length drives brain size and litter size variation in eutherian mammals

The length of gestation in eutherian mammals, which is key to their reproductive success, is closely connected to other life history traits and with body mass and brain mass, but causal relationships between these variables are unclear. Here, we used an integrative analytical framework to evaluate the evolutionary relationships between gestation length and eight other traits on a dataset of 3,258 eutherian mammals and infer causality. We identify variation in generation length and litter size as the primary predictors of eutherian gestation length variation, whereas additional traits, such as brain mass, significantly predict gestation length only in specific mammalian orders. Using a structural equation modeling approach known as phylogenetic path analysis to infer causality, we find that gestation length variation positively influences brain mass variation and negatively influences litter size variation. Furthermore, body mass causally influences gestation length variation only in certain orders. Consistent with these causal inferences, examination of trait-trait coevolution reinforces that gestation length is strongly positively associated with brain mass, strongly negatively associated with litter size, and only moderately correlated with body mass. These findings reveal how gestation length directly and indirectly influences, and is influenced by, other key eutherian traits. Our study establishes a robust framework for identifying causal relationships within suites of correlated and co-evolving traits.