Chimps teach offspring to use rock hammer

WASHINGTON — Some bands of chimpanzees in West Africa routinely swing crude stone hammers to crack open nuts, a sophisticated use of tools that has been taught 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 reported in today's issue of 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 report. "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 golf-ball-size 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 said.

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 that some of the tools apparently have been used for generations. Age dating of deposits at one anvil site showed the apes had used it as a nut-cracking station for at least 110 years, Panger said, and some animals have been seen carrying pounding stones from one anvil to another.

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 thought: The hammers must be flat on one side, heavy enough to smash nuts and have a place to grasp.

"They have to use it without crushing their fingers," Panger said. "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.

The researchers said the nut-smashing technique is seen in only some bands of West African chimpanzees.

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."

Robson Bonnichsen, a stone-tool expert at Oregon State University in Corvallis, said the research by Panger, Mercader and their co-author, Christophe Boesch of Max Planck Institute in Germany, gives important new insights into tool use by primates.

"This is really interesting," he said. "I applaud their work."