Capuchin monkeys rely on more than brute force when they crack a palm nut with a stone - they also apply basic rules of physics, scientists have shown.
The monkeys seem to understand the importance of ensuring the nut is in a stable position on a wooden log before striking it.
Studies reveal that they take time to knock the nut into a dip in the log, with its flattest face downwards. Only then do they whack the nut with a stone from a standing position.
They also take care to choose a stone of the right weight, thrown with the correct amount of force.
Blindfolded humans asked to carry out the same task manoeuvred the nut in just the same way, judging its position by touch.
Researchers videoed several wild bearded capuchin monkeys cracking nuts in Brazil. The scientists provided them with a suitable log "anvil" and placed a number of nuts and stones nearby.
Nuts were marked with a "stop meridian" line which when facing upwards indicated they were in the most stable position.
The US scientists, led by Dr Dorothy Fragaszy, from the University of Georgia, wrote in the online journal Public Library of Science ONE: "Monkeys typically knocked the nuts on the anvil a few times before releasing them in a pit.
"They positioned the nuts so that the stop meridian was within 30 degrees of vertical with respect to gravity more often than expected, and the nuts rarely moved after the monkeys released them."
In a parallel experiment, the researchers asked a group of blindfolded people to position palm nuts on the log as if about to crack them. Like the monkeys, they adjusted the nuts by feel to place them in the most stable position.