In a variety of mammalian species, mothers and others care for and/or carry deceased newborns, and sometimes other conspecifics. The rationale for such behavior remains elusive. Based upon field observations of olive baboon (Papio anubis), African elephant (Loxodonta africana), and Thornicroft’s giraffe (Giraffa camelopardalis) responses to recently dead conspecifics, combined with reports in the literature, a hypothesis is proposed to account for this activity. Among female mammals, lifetime reproductive success is more dependent upon rearing, than production, of offspring. The successful nurturing of progeny is associated with a strong maternal–offspring bond. One of the most important chemicals involved in both lactation and mother–infant bonding is oxytocin, a tiny molecule that has a lengthy evolutionary history and is implicated in the formation of social bonds across mammals. Evolution has extended the impact of oxytocin by adopting it beyond the original mother–infant bond to the establishment of social bonds that are required among group-living animals. Hence, sociality is a consequence of the same fundamental biological mediator of mother–offspring bonding, and this intricate connection between physiology and behavior has produced a situation where sometimes animals will care for or carry dead companions. Ways to test this hypothesis, as well as a potential way to refute it, are proposed.