Meeting Abstract
Mammals are unique in their capacity to care for young by milk production. In therian mammals (marsupials and placentals), lactation occurs at the mammae, which includes the mammary glands and nipples, or teats. Mammary number varies widely across the clade, up to 26 in tenrecs. Given the obvious link between these structures and parental care, it was proposed that mammary and offspring number coevolve, perhaps with milk production serving as a constraint on litter size. Litter size ranges from 1-32 offspring per litter, but no formal comparisons to mammary number have been conducted across mammalian orders. Previous analyses within Rodentia showed that mean offspring number per litter is approximately one half the mammary number. Here we revisit this ‘one-half rule,’ which has been broadly extrapolated as general for mammals, and for the first time consider this basic question of reproductive biology in a phylogenetic context. We surveyed primary literature regarding mammalian reproduction, including the online data repositories PanTHERIA and VertNet, to generate a data set that includes information on 2840 species belonging to 1022 genera and all 27 orders. These data were mapped onto a new time calibrated and species-level phylogeny of Mammalia based on a 31-gene supermatrix and built to incorporate uncertainty in branching relationships. Phylogenetic generalized least squares analyses confirm that mammae number and mean litter size coevolve, but suggest that their relationship is closer to one-to-one across all mammals, and that rodents might be unique in their one-half relationship.
