Female Chinese golden snub-nosed monkeys happily and regularly suckle infants that are not biologically theirs, a five-year study has found.
The phenomenon, known as allomaternal nursing, is known in some species of rodents, among New World monkeys, and, in a few societies, in humans, but this is the first time it has been observed in Old World primates.
The discovery is reported by a team of researchers led by Zuofu Xiang from the Central South University of Forestry and Technology in Changsha, China, and published in the journal Science Advances.
To make their findings, the researchers studied a small colony of the monkeys (Rhinopithecus roxellana) in the Shennongjia National Nature Reserve in central China.
Over the course of five birth seasons, they found that 40 out of 46 infants suckled from at least one adult female who was not its mother, with 48% of the cohort suckling from two.
Among the adult females, the practice was almost always reciprocal, with 90% of those whose infants had been suckled by another female in any given year returning the favour the following year.
Allomaternal nursing is costly for the adults involved – milk production is energy-intensive and generating enough to feed an extra mouth even more so – but seems to confer a fitness advantage on the young.
The monkeys live high in tall trees in a very harsh environment, typified by long, cold winters.
Xiang and colleagues noted that out of the six infants who received only milk from their biological parent, four died. Of the 40 who were co-suckled, only six died – a much lower rate.
All of the suckling monkeys were related to the offspring at the levels of aunt or grandmother. The researchers suggest that the practice is an expression of “kin selection” – an evolutionary tactic that prioritises the survival of genetic relatives above that of any individual.