WASHINGTON -- A band of chimpanzees in West Africa routinely swing crude stone hammers to crack open nuts, a sophisticated use of tools the apes have been teaching to each new generation for more than a century.
Using carefully selected stones weighing up to 33 pounds, the chimps pound the tough shell of the panda nut to extract a high-energy kernel that is an important part of the animal's diet, researchers report Friday in the journal Science.
"It is a very skillful behavior that takes up to seven years for them to learn," said Melissa Panger, a George Washington University researcher and co-author of the study. "It looks easy, but if you sit down and try it is a very difficult task."
The panda nuts fall to the ground inside an outer husk. Inside the husk is a golfball-sized nut covered by a shell that can require up to a ton of pressure to break open. Yet, if the animals pound too hard, the nut shatters and is inedible, Panger said.
"What is remarkable is that they are controlling the force precisely," she added. Inside the shell are three nutritious kernels. During nut-smashing season, some chimps spend two or three hours a day opening as many as 100 panda nuts.
The nuts can provide up to 3,000 calories a day, researchers said.
The chimps establish nut-cracking stations, usually centered on a battered root of a hardwood tree that they use as an anvil.
Panger said the chimps leave the hammer stones beside the anvils and some of the tools have apparently been used for generations. Some animals have been seen carrying pounding stones from one anvil to another, just as a repairman might carry tools from place to place, she said.
Nut cracking demonstrates a degree of sophisticated learning because it required the animals to select hammer stones at a distant rock outcropping and then carry them to the anvil. Panger said selection of the stones requires some thought by the chimps: The crude hammers have to be flat on one side, heavy enough to smash the nuts and have a place to grasp.
"They have to use it without crushing their fingers," said Panger. "Some of these hammers have been used so many times that they have deep pits, suggesting that they have been used for many generations, over and over again."
Mothers teach their children to bang on nuts, and some young chimps have been seen hitting nuts with smaller stones, imitating their parent.
The researchers said the nut-smashing technique is known to only some bands of West African chimpanzees. It has not been seen among chimps in central Africa, although the apes there have nuts and stones available to them.
This suggests that nut smashing is a cultural, learned behavior that has not spread widely among the apes.
"There has to be knowledge of the size and hardness of the rock," said Julio Mercader, another George Washington University researcher and the first author of the study. "And that knowledge is transmitted from chimp to chimp and from generation to generation."