Mammals are characterized by lengthy maternal care, and this has been linked to manifold attributes of this successful and charismatic group that includes our own species. However, living mammals display remarkable diversity in reproduction, from egg-laying monotremes, to pouch-bearing marsupials, to long-gestating placentals. The obvious differences in species numbers, geographic breadth, ecological and anatomical diversity across these three groups begs the question of how much their divergent reproductive strategies are responsible for their success, or lack thereof.