First-time mother Kianga gave birth to the male ape at Twycross Zoo in Leicestershire on 9 February.
Keepers were concerned about its health to begin with as Kianga was not holding it properly to allow suckling.
But Cheka, a more experienced mother, noticed this and showed her how to feed the baby properly.
The newborn, which does not yet have a name, is said to be developing well.
Dr Charlotte Macdonald, director of life sciences at the zoo, said: "We were astonished to find out about the wonderful intervention by Cheka, as it is unique to observe such an important social teaching behaviour."
Cheka has reared several babies, with youngest, Ndeko, born at the zoo in August.
"It is very exciting to welcome a second baby to our bonobo group in such a short space of time," said Dr Macdonald.
Both births were part of a carefully planned European Endangered Species Programme, which ensures healthy genetic lines.
Dr Macdonald said managing the breeding of bonobos has to be carefully done because of their "promiscuous behaviour".
"Bonobos use sex as a communication tool, so it is not always used for reproduction, but also to establish social hierarchies, or to avoid potentially aggressive situations," she said.
A UK zoo has announced the birth of a bonobo ape - one of only 11 births of the endangered animal in zoos across the world in the last year.