Monkeys pick best way to crack nut
Wild capuchin monkeys are very selective about the best way to crack a nut, according to scientists.
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.
Qing Liu, who led the study, said it was the first demonstration of the animals' ability to "measure" how appropriate a particular anvil was for a specific job.
Previously, researchers had found that the monkeys selected just the right stone for nut-cracking.
But it seems they are also good at choosing the most effective anvil surface, said Ms Liu. "Even when this is not something visible."
"The effectiveness is equivalent to the number of strikes needed to crack a nut in that pit - the lower the better."
"The detection of such a complex property has not been demonstrated in animal tool use before, as far as I know."
Because capuchin monkeys are social animals, they are also likely to watch and learn from other monkeys attempting the same task.
But if the pit they start with is ineffective, the animals usually "self-correct", evaluating their success and making an independent choice.