Battle of the Nutcrackers

The other day while taking a walk around a lake near my house, I stopped to watch a small squirrel run around with a very large nut in his mouth. Naturally this made me wonder which animal out there would win the title of champion nutcracker. Maybe the winner would be the squirrel or actual nutcrackers (the Spotted or Clark’s variety). Surely these two groups would be tough competitors. But maybe their real talent lies in finding all the nuts they store and not necessarily being clever about how they open the nut. I was curious to see if there were any other species out there that could give these two a run for their money.

One species, the American crow (Corvus brachyrhychos), is in the same family as the nutcrackers (Corvidae). The American crow does things the way a lot of birds do. They drop their walnuts repeatedly onto the ground. Many species of gull use this technique to open clams and mussels. While hardly imaginative, it is successful and does require some skill. The birds have to assess how hard the shell is and this will determine the height selected for dropping the walnuts. Having to repeatedly pick up, fly up, and drop your nut again and again could be very energetically costly. At some point it’s just not going to be worth it. So the height is chosen very carefully, as is the substrate. Unlike gulls, crows learn that it is easier to crack a nut on asphalt than on soil. Of course things can get pretty competitive out there and individuals will sometimes have to choose less than optimal heights to avoid having their nuts stolen!

In the next corner we have chimpanzees. Not all chimpanzee populations crack nuts. Actually nut-cracking behavior is isolated to a small region in the West coast of Africa. Ecological factors have not been able to explain why only a small area of the Ivory coast have chimpanzees that engage in nut-cracking. Some researchers think that it is a cultural phenomenon that is passed on to individuals within a population. They eat several types of nuts, including one the people of the area enjoy, coming from an evergreen tree called Coula edulis, commonly called the African walnut. Unlike the American crows mentioned above, the chimpanzees of the Tai forest use hammers (a log or stone) and anvils (horizontal roots or rocks) to get the job done. The tools they select are correlated to the hardness of the shell. Naturally stones would be the hardest tool and they are a prized possession, especially since they are a rare find in the forest. Individuals must learn how to crack open nuts and this process take a very, very long time. Try about 13 years of practice. That’s like practicing gymnastics to make it to the Olympics. What is so hard about learning to crack a nut? First of all, an individual has to find some nuts, find a hammer appropriate for the type of nut he or she wishes to crack, and then find an anvil that will make this whole endeavor successful. Once a good anvil has been located, individuals will frequently carry the hammer and nuts to this same location. Why go through all the trouble? Because nuts are a great source of nutrients and fat, making them an energetically profitable food item, assuming of course that you can open them! On a given day, chimps will pound open about 270 nuts over a two hour period. Females rock the house when it comes to detail oriented pounding of a particular type of nut, the Panda nut. Overall females outperform males in this task, which is a good thing since moms teach their infants how to crack nuts. For the first couple of years kids are allowed to share the nut with mom. While mom is pounding away, the youngsters are often playing with tools of their own, semi-practicing. Around 4-5 years old mom stops sharing nuts but will “forget” her good tools. By sharing her tools, her offspring starts practicing with the right kind of tools for the job and some moms have even been observed correcting the techniques of offspring, actively teaching them how to crack nuts.

I bet you are thinking, “Why go on? Clearly the chimpanzee is the winner, right?”. Not so fast. In the third corner we have another primate species, the bearded capuchin monkey (Cebus libidinosus). They also use hammers to smash nuts and select these hammers carefully. One study was aimed at tricking the capuchins by providing fake rocks and other strategies designed to fool the capuchins into thinking there was a quality hammer available. There might be a reason why capuchins are often used as assistance animals to the disabled because they almost always chose the functional, or real, tool. What defined the proper tool for the job? The weight of the stone. They also frequently use an anvil like a rock or a log to crack their nuts, similar to the chimpanzees. Check out this link to a video to watch describing how a capuchin goes about this.

So far we have the American crow which drops nuts repeatedly to the ground and two species of primate that engage in complex behaviors requiring finding the right tools and learning how to crack nuts. The final contender is the Carrion crow in Japan. Like their American cousins, in some places they drop their nuts onto the pavement. However, some populations have used traffic lights to their advantage, allowing them to access a food source they normally cannot eat. Green light= drop your nut so it gets run over by a car. Red light= Walk in the crosswalk and pick up the pieces of your conveniently cracked nut. If you remain skeptical, take a look at the video narrated by David Attenborough showing the crows in action.

Among the four contenders I vote for the Japanese crow, mainly because they get the greatest payoff for spending the least amount of energy.

References:
Boesch-Achermann, H. and Boesch, C. Tool use n wild chimpanzees: New light from dark forests. Current Directions in Psychological Science.

Boesch, C. Marchesi, P., Marchesi, N., Fruth, B., Joulian, F. 1994. Is nut-cracking in wild chimpanzees a cultural behavior? Journal of Human Evolution, 26:325-338.

Brosnan, S. 2009. Animal behavior: The right tool for the job. Current Biology, 19:124-125
Cristol, D.A. and Switzer, P.V. 1999. Avian prey-dropping behavior. II. American crows and walnuts. Behavioral Ecology and Sociobiology, 10: 220-226.

Visalberghi, E., Addessi, E., Truppa, V., Spagnoletti, N., Ottoni, E., Izar, P., and
Fragaszy, D. (2009). Selection of effective stone tools by wild bearded capuchin monkeys. Curr. Biol. 19, 213–217.