Harsh environments promote alloparental care across human societies.

Alloparental care is central to human life history, which integrates exceptionally short interbirth intervals and large birth size with an extended period of juvenile dependency and increased longevity. Formal models, previous comparative research, and palaeoanthropological evidence suggest that humans evolved higher levels of cooperative childcare in response to increasingly harsh environments. Although this hypothesis remains difficult to test directly, the relative importance of alloparental care varies across human societies, providing an opportunity to assess how local social and ecological factors influence the expression of this behaviour. We therefore, investigated associations between alloparental infant care and socioecology across 141 non-industrialized societies. We predicted increased alloparental care in harsher environments, due to the fitness benefits of cooperation in response to shared ecological challenges. We also predicted that starvation would decrease alloparental care, due to prohibitive energetic costs. Using Bayesian phylogenetic multilevel models, we tested these predictions while accounting for potential confounds as well as for population history. Consistent with our hypotheses, we found increased alloparental infant care in regions characterized by both reduced climate predictability and relatively lower average temperatures and precipitation. We also observed reduced alloparental care under conditions of high starvation. These results provide evidence of plasticity in human alloparenting in response to ecological contexts, comparable to previously observed patterns across avian and mammalian cooperative breeders. This suggests convergent social evolutionary processes may underlie both inter- and intraspecific variation in alloparental care.