A juvenile chimpanzee in the Ivory Coast's Tai Forest watches as her mother carefully places a soft coula nut onto a hard, flat rock. In her other hand, mom has a chunk of hard wood. Mom smashes the nut with her makeshift hammer, once, twice, three times. Having broken the outer shell, she plucks out the edible bits, reaching for another nut and starting the process again. The juvenile wants a snack too. Even though she's learned the basic process by watching the older chimpanzees in her troop, she'll be at least six years old before she's proficient enough at this sophisticated form of tool use to crack nuts open herself. Not that she'll stop trying, in the meantime.

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.

Schrauf and her colleagues assessed the means by which a group of captive chimpanzees (Pan troglodytes) chose their hammers in three different experiments. In the first one, they provided the nuts and the anvil along with three different hammers of equal size and shape. Despite their equivalent dimensions, the cubic hammers weighed 300, 600, or 1200 grams. The second experiment was the same, except the hammers were spheres instead of cubes. The third experiment used spherical hammers as well, but the researchers used a wider weight discrepancy among them: 200, 800, and 1400 grams.

In general, the chimps preferred the weightier hammers across all three experiments. Since all the hammers were visually identical, it was certain that they were choosing the heavier hammers only because of the weight. Further, the heavier the hammer, the fewer strikes were required to crack the nuts and the less time was needed to do so. Therefore, the researchers had effectively established that the chimpanzees at least implicitly understood the relationship between a hammer's weight and its efficiency.

The results become more intriguing when each chimpanzee's individual responses are considered. Loi, the alpha male of the group, was the most proficient nut cracker, outperforming the others from the beginning of the first experiment. He showed a clear preference for the heavier hammers, switching from a lighter to a heavier hammer more often than from heavy to light. Most of the other chimps showed little discrimination among the hammers at the beginning of the first experiment, but after the first few trials of the second experiment, they began to opt for the heavier tools. By the end of the final experiment, most chimpanzees' performance matched Loi's. In fact, by the third experiment, the chimps were more likely to switch lighter hammers out for heavier ones before even trying to use them.

On the other hand, Natsuki, who was the only infant in the experiment, never improved her performance, despite an equivalent amount of experience as the other chimps. Even in the third experiment, where the heaviest hammer was seven times heavier than the lightest hammer, she still showed no preferences, selecting each of the three hammers the same number of times. She also never switched tools after being unable to crack open a nut. This, according to the researchers, is consistent with observations from the wild.

Together, these patterns demonstrate that experience is key to gaining expertise in nut-cracking, but experience alone isn't enough. "Maturational aspects also play an important role in determining tool selectivity and proficiency," the researchers say. In other words, young chimpanzees need to do some growing up before they'll be ready to successfully tackle adult-sized tasks.

Is there a lesson to be learned here for human primates?

Schrauf C, Call J, Fuwa K, Hirata S, (2012) Do Chimpanzees Use Weight to Select Hammer Tools? PLoS ONE 7(7): e41044. doi:10.1371/journal.pone.0041044

For more on tool use in chimpanzees and other species:

How Do You Figure Out How Chimps Learn? Peanuts.

Chimpanzees Help, But Only When Asked

Dingoes Ate My Nametag: Tool Use in a Dingo

Dogs, But Not Wolves, Use Humans As Tools

Images: Header photo from van Schaik (2012) Current Biology; additional photos from Schrauf et al. (2012) PLoS ONE.