Why do these monkeys nurse each other’s babies?

Lactating and nursing costs a lot of energy. There must be some benefit to spending it on someone else’s infant.

Being a mom is hard enough, but imagine mothering someone else’s kids, too. That’s the norm for golden snub-nosed monkeys, according to a new study in the journal Science Advances. Scientists found that over a five-year period, more than 87 percent of golden snub-nosed monkey infants were nursed by females other than their mothers—a phenomenon called allonursing.

While allonursing has been documented in a number of rodent and carnivore species, as well as some primates, it is not common. This is the first evidence of allonursing in an Old World monkey.

Allonursing on a regular basis has only been reported in about 40 mammal species, and researchers weren’t expecting to see this behavior when they first started observing the behavior of

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