Researchers from the University of Georgia, US, filmed the wild monkeys selecting the correct anvil and hammer for the job. The footage reveals that the animals are "selective about the materials they use in tool use", say the scientists.
The research team have reported their findings in the journal Animal Behaviour.
The monkeys use pits in logs as anvils, to lodge the nuts in place while they use large stones as hammers to bash through the shells. In the footage, the monkey can be seen "weighing up" the most appropriately sized pit.