More than 87% of golden snub-nosed monkey infants evaluated in a five-year field study were nursed by females other than their mothers - a phenomenon called allomaternal nursing. This is the first evidence of allomaternal nursing in an Old World nonhuman primate, the study's authors say. It occurred predominantly among relatives and was usually reciprocal; around 90% of mothers nursed another female's baby if that female had previously nursed their own. The behavior - costly for non-mothers - likely arose in tolerant kin-based support networks where shared care helped the animals navigate harsh, unpredictable environments, the authors say. The findings may help understand the role of allomaternal nursing behavior in human evolution. Allomaternal nursing has been seen in a variety of mammals, from rodents to humans. The behavior is believed to enhance infant survival and reduce postnatal reproductive costs incurred by the infant's mother. These costs are particularly high for monkeys such as the golden snub-nosed monkey that live in high-elevation temperate forests with extremely cold, five-month-long winters, and strong seasonal changes in food availability. Allomaternal nursing, however, has never before been observed in Old World monkeys. In this study, the authors observed groups of golden snub-nosed monkeys in the Shennongjia National Nature Reserve located in central China. Over the course of five birth seasons, the authors discovered that 40 out of 46 infants (87%) suckled from one or more females that were not their mothers, with 22 out of 46 (48%) suckling from at least two additional females. Allomaternal nursing was seen predominantly during the first three months of an infant's life. Four of the six infants, who did not receive nursing from another female died during winter, whereas only six of the 40 infants who were allonursed died. Relatedness - usually either the grandmother or aunt of the infant - and reciprocity played important roles in allomaternal nursing, the authors say. Around 90% of mothers (25 of 28) reciprocated nursing during the current or following year if that female had nursed their own infant.