Nut-cracking has been observed in wild chimpanzees in Guinea, Ivory Coast, Liberia, and Sierra Leone. With only one exception, chimpanzee groups in Central and East Africa have never been seen cracking nuts, despite the fact that all the necessary ingredients – nuts, stones, sticks, and roots – are all found in abundance in those parts of the continent. Nut-cracking, it seems, requires more than just the available tools and basic know-how. Even within nut-cracking groups, some individuals will go their entire lives without ever successfully cracking open a nut.

The mystery behind the apparent complication involved in learning to crack nuts prompted one research group, led by Cornelia Schrauf of the University of Vienna, to try to figure out how chimpanzees select their nut-cracking tools.

Successful nut-cracking involves selecting an appropriate surface to serve as the anvil. It should be hard and relatively flat. It should also be able to keep the nut from rolling away. Then, the chimp needs to select the appropriate hammer. It needs to be the right shape and size. It needs to be light enough to easily manipulate, but heavy enough to smash the nut in as few strikes as possible. In the Tai Forest, chimps select hammers made of stone for the hard panda nuts, but opt instead for wooden hammers when it comes to the softer coula nuts. If they’ve only got rocks around, they’ll use heavier, harder stones for panda nuts and lighter ones for coula nuts.

Researchers weren’t sure, however, if chimpanzees were actually using weight as the primary consideration for hammer selection, since weight correlates with other features of hammers, such as size, material, shape, and density.