The adaptive functions of routine allomaternal behavior in free-ranging Taihangshan macaques

Allomaternal behavior, such as allonursing, provisioning, transporting, cleaning, and protection by group members other than the infant's mother, is widely observed in nonhuman primates. Reduced food availability, high predation risk, infanticide, and high maternal reproductive costs are proposed as selective factors promoting allomaternal care. In this study, we examined patterns of allomaternal behavior in a multimale-multifemale group of Taihangshan macaques (Macaca mulatta tcheliensis) inhabiting a cold (winter temperatures can reach -20 °C), high-latitude (34°54'-35°42'N) environment in northern China. We found that 42.45% (45/106) Taihangshan macaques engaged in allomaternal behavior, including 59.52% (25/42) adult females, 66.67% (6/9) adult males, 75% (9/12) subadult females, and 20.83% (5/24) juveniles. Nulliparous female caregivers exhibited significantly higher frequency of allomaternal care than multiparous females, and caregiving behavior was associated with higher infant survival among inexperienced females, supporting the predictions of the "learning to mother hypothesis." Furthermore, the proportion of allomaternal care initiated by relatives was significantly higher than that initiated by nonrelatives, emphasizing the role of kinship in the evolution of allomaternal behavior in this species. Additionally, adult males participated in infant caretaking and provided more care to their genetic offspring compared with unrelated infants, which is consistent with the paternal investment hypothesis. Therefore, allomaternal caregiving appears to benefit mothers by reducing their infant care costs, while also providing inclusive fitness benefits and caregiving experience for caregivers.