Which New World monkey uses rocks to crack open nuts?

The New World monkey that uses rocks to crack open nuts is the capuchin monkey. In the wild, capuchins place a hard nut or fruit on a large stone anvil and strike it with another stone used as a hammer.

This is not a single trick performed by one trained animal. Several capuchin populations use stone tools as part of their natural foraging, especially robust capuchins in Brazil. The monkeys select stones of different sizes and weights, transport them to useful anvils, and adjust their technique to the food they are opening.

The skill develops gradually and is influenced by practice and social learning. Some wild capuchins also use stones to dig for tubers, sticks to probe crevices, and tools to extract other foods. Archaeological evidence from Brazil suggests that some capuchin stone-tool traditions have continued for at least 3,000 years.

Other New World monkeys may use objects occasionally, but capuchins are the canonical answer because their wild hammer-and-anvil behavior is frequent, complex, and well documented.