Placental Interdigitation Is Associated with Reproductive Pace across Mammals

The placenta is a transient organ mainly derived from fetal tissue that is essential for establishing metabolic and immune interactions that support healthy pregnancies in eutherian mammals. Across mammals, placentas show high morphological diversity in both the degree of tissue invasiveness and in the complexity of tissue integration between maternal and fetal tissues. This variation is thought to reflect strong selective pressures on reproductive strategies, balancing maternal investment, offspring development, resource transfer, and reproductive pace. Previous studies have explored the relationship between placental morphology and mammalian life history focusing mainly on placental invasiveness and giving less attention to placental interdigitation. Here, we expand the scope of previous comparative work by analyzing 509 mammalian species to examine how three life history traits (gestation length, interbirth intervals, and relative litter mass) are associated with placental morphology. We also test whether placental morphology covaries with broader multivariate life history structure using phylogenetic principal component analyses (pPCA). We found that, relative to invasiveness, placental interdigitation was more strongly associated with reproductive timing. Labyrinthine placentas were associated with shorter gestation lengths than villous and trabecular placentas, and species with villous placentas were associated with the longest interbirth intervals. Placental invasiveness showed no association with life history variation. Neither invasiveness nor interdigitation was associated with relative litter mass. Our results suggest that, although diverse census timing may have limited the extent of our interpretation, placental interdigitation is associated with reproductive timing (gestation length and interbirth intervals), whereas neither interdigitation nor invasiveness is associated with relative litter mass, an approximate measure of offspring mass produced per reproductive event relative to adult body mass.