Gestation length both shapes and is shaped by other life history traits in terrestrial eutherian mammals.

The length of gestation in eutherian mammals, which is key to their reproductive success, is closely connected to other life history traits, body mass, and brain mass, but the direction of relationships between these variables is unclear. Here, we used an integrative analytical framework to evaluate the evolutionary relationships between gestation length and 8 other traits on a dataset of 3,258 eutherian mammals and to infer potential causal influences. Using a random forest machine learning algorithm as well as phylogenetic comparative methods, we identify litter size as the primary predictor of gestation length, while additional traits, such as brain mass and weaning age, are significant predictors in specific mammalian orders. Using a structural equation modeling approach known as phylogenetic path analysis, we find that gestation length directly negatively influences litter size and is directly positively influenced by brain mass across eutherian orders. In contrast, the commonly observed relationship between gestation length and body size can be better understood as indirect effects through other traits. Consistent with these inferred relationships, examination of trait-trait coevolution reinforces that gestation length is strongly negatively associated with litter size, strongly positively associated with brain mass, and only moderately correlated with body mass. These findings suggest that gestation length both shapes and is shaped by other key traits, across eutherian mammals as well as sometimes in an order-dependent manner.