The evolution of gestation length in eutherian mammals

Eutherian mammals exhibit striking variation in their gestation lengths, which has traditionally been linked to and allometrically scales with variation in other life history traits, including body mass and lifespan. To understand how the phenotypic landscape of gestation length variation, including its associations with body mass and lifespan variation, changed over mammalian evolution, we conducted phylogeny-informed analyses of 845 representative extant species. We found that gestation length substantially differed in both whether and how strongly it was associated with body mass and lifespan across mammalian clades. For example, gestation length was not associated with lifespan or body mass in Chiroptera and Cetacea but was strongly associated only with body mass in Carnivora. We also identified 52 evolutionary shifts in gestation length variation across the mammal phylogeny and 14 shifts when we jointly considered variation of all three traits; six shifts were shared. Notably, two of these shifts occurred at the roots of Cetacea and Pinnipedia, respectively, coinciding with the transition of these clades to the marine environment. These results suggest that evolutionary constraints on gestation length have varied substantially across mammalian phylogeny and that the genetic architecture of gestation length may differ between mammal clades.

Methods
We retrieved data on gestation length, lifespan, and body mass for 845 representative extant species of eutherian mammals. This information was sourced from four databases: PanTHERIA, AnAge, EltonTraits, and MOM-Mammals.