Preliminary study on mother-approved infant handling behavior of black-and-white snub-nosed monkeys (Rhinopithecus bieti)

Mother-Approved Infant Handling (MAIH), a behaviour characterized by non-maternal individuals carrying a mother's offspring with her consent following a process of behavioural negotiation, is a well-documented phenomenon in colobine species. This study systematically documented MAIH within a semi-provisioned breeding band of black-and-white snub-nosed monkeys (Rhinopithecus bieti) in Xiangguqing, Yunnan, P.R. China. The research aimed to test the learning-to-mother hypothesis, which posits that inexperienced females engage in infant handling to acquire maternal skills. Over the study periods in 2019 and 2022, we observed MAIH events across multiple one-male units (OMUs). Our findings revealed that MAIH was predominantly initiated by juvenile and adult females, particularly within the first two months after an infant's birth. A generalized linear mixed model analysis revealed that both handler age and the number of offspring significantly influenced the occurrence of MAIH. These results support and enrich the learning-to-mother hypothesis, suggesting that MAIH provides an opportunity for inexperienced females to gain caregiving skills. While older females exhibited higher MAIH occurrences, the negative quadratic effect indicates that it declines after peaking at a certain age. This study advances our understanding of the adaptive significance of allomaternal care in primate societies and offers insights into the evolutionary origins of cooperative breeding in humans.