Apes aren’t the only primates to use tools

A chimpanzee cracking open nuts placed on the ground with a large stone. Notice that a young chimpanzee is also present, learning this behavior. This is a sort of Type 1 tool use where a hammer (the rock) is used on another object.

“Tool use” was once considered one of the primary factors that made Homo sapiens distinct from all other animals, but Jane Goodall’s studies of Chimpanzees at Gombe and subsequent research among living apes has shown that the tool use of humans is differentiation of grade and complexity more than anything else. Tool use has now been extended to many other groups of animals, even outside the Class Mammalia, but it still is surprising to see some animals make use of objects in their environment in inventive ways. Indeed, while the idea that humans are distinguished by the possession of tool use is dead in scientific circles, it still is alive in the public mind (I recently had a friend tell me that we were “Man the Tool-User”), and genuinely impressive utilization of tools by other animals is often related to just be a sort of “trick” or purely instinctual mechanism (I’ll save the issue of animal cognition for another day).

Of the animals that use tools, however, among the most impressive are the Capuchin monkeys (Cebus sp.). Capuchins are platyrrhine primates (New World Monkeys) that inhabit the forests of South and Central America from about Honduras to Brazil. They’re generally familiar to everyone, the proverbial “organ grinder’s monkey,” a common household pet (until recently), and a regular in film roles that required a primate (i.e. Marcel from the show Friends). Indeed, Capuchins are easy to train and highly intelligent, but despite their close proximity to people they’ve generally been overlooked as “just monkeys” for a very long time. Recent research, however, has shown that they can tell us much more about the development of intelligence and human evolution than previously thought.

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Capuchin monkeys cracking open nuts. Note the similarities between this footage and the film shown above.

The fact that chimpanzees have the highest brain-to-body size ratio out of all the African apes is well-known, but few people know that Capuchins exhibit brains of similar proportions. Such a fact is readily apparent (or at least easily researched) but Capuchins have generally been ignored because while they are primates they are not as closely related to humans as Chimpanzees, Bonobos, Gorillas, or Orangutans, but fortunately this has changed. While they can be difficult to study in the wild despite their inquisitiveness/ease of acclimation to human presence, Capuchins are primarily arboreal quadrupeds, able to run through the trees as fast or faster than researchers can make it over the forest floor. This may result in some behaviors being missed, and oftentimes studies are carried out in the dry season when foliage is a bit more sparse and allows for a better view of the monkeys. Why are such considerations important? Because the tool-use in Capuchins I’m about to discuss is more often seen in captivity than in the wild, and it’s important to consider what you may not be seeing when dealing with animals in their natural habitat.

Much of the work on Capuchin intelligence has been carried out in labs by researchers primarily interested in psychology, and as my professor once remarked when considering some of the studies, the background of the researcher can be significant as to what it studied, how it is studied, and how the results are interpreted. Be that as it may, studies in captivity involving Capuchins have shown that they can use tools and that they use tools in a variety of ways depending on the circumstances. Anyone who has used a hammer or other tool to make something recognizes that the way you grip an object has a lot to do with how effective it is going to be; it probably wouldn’t be very effective (or safe) to grip a hammer with the fingertips of both hands and try to use it to bang in a nail. Likewise, when Capuchin monkeys are given a stone and expected to throw the stone into a tub of peanut butter (as in one experiment) they need to choose a grip to accomplish the task, and while there are a number of different variations of grips they usually fall under the category of power grips or precision grips. The names belie what they are used for, and in the throwing experiment I had just mentioned the most popular grip used was called the “jaw chuck,” where an object is held in the palm of the hand with all the fingers holding it in place. The jaw chuck grip was not the most effective in this experiment, however, one monkey having better success during its attempts using a precision grip (the “cup grip,” where an object is held in a cupped hand with the fingers providing support) even though it did not catch on with the other individuals.

In a different experiment, where a tub of peanut butter was covered by an acetate barrier and stones with one sharp edge were placed in the cage, the jaw chuck was even more popular than in the throwing trials, even though similar “power grips” were used as well. Indeed, while the monkeys did use (experiment?) with a number of precision and power grips, the jaw chuck was the most popular overall. Another set of tests, however, showing that monkeys might not use tools at all if they don’t have to. When a tub of peanut butter was covered in 5cm of soil and the monkeys were provided with sticks, the monkeys simply dug with their hands (like baboons do) if the soil was loose. If the soil was hard, however, some of them used sticks, even modifying the sticks by removing leaves and biting off little bits, to reach their prize. This is significant because some people like the !Kung San of the Kalahari use sticks to dig for roots and tubers today and the ability to dig for food underground is considered to be a very important factor in human evolutionary history.

As seen in the video above, however, digging in the dirt isn’t the only thing Capuchins do. They also crack open nuts in a way very reminiscent of Chimpanzees, although not all Chimpanzees exhibit this behavior. Some, like the ones in the Tai Forest do use tools to open nuts (as do other populations), but some populations don’t use tools and some don’t use them in the same way. As I mentioned in my post about Mt. Assirik chimpanzees, the chimpanzees there use the large Baobab tree limbs and trunks as anvils to crack open the fruit of that tree, using a level of tool use lower than that of other populations that put a nut on an anvil and then use a hammer (the Mt. Assirik chimps are just using the tree as an anvil). Again, as described in my earlier post about the Mt. Assirik chimpanzees, tool use can evolve given the proper ecological opportunities and cognitive steps, going from simply using a hammer or anvil on an object to using two tools (hammer and anvil) to open an object to the production of more complex and specialized tools under the proper conditions. In the case of the Capuchins, the monkeys have been known to bang stones together (holding one in each hand), use stones to crack nuts, throw stones against the ground, and hit stones with other stones making a “bipolar” object (it flakes on two sides if held on a stone anvil). Unfortunately I don’t know what becomes of these objects as it seems that Capuchins do not keep or further modify tools they make when they are finished using them, but it could represent the beginnings of tool manufacture, the behaviors requiring the cognitive leap to move ahead.

The cognitive abilities of Capuchins is one of the ways that they differ from Chimpanzees, in fact. While Chimpanzees often recognize themselves if presented a mirror, Capuchins do not (although some have used mirrors to look around objects for hidden food). Capuchins also fail some cognitive tests passed by Chimpanzees, and it seems that while both primates exhibit some similar behaviors the convergence is even more striking because Capuchins are different in terms of their intelligence. Still, the fact that Capuchins can use tools and show convergences with chimpanzees shows us that certain “milestones” that were once considered hallmarks of human evolution can show up multiple times in multiple lineages, recalling the “branching bush” of evolution rather than the orthogenic line.

There are problems with the lab studies, however, and more study needs to be undertaken of wild populations to determine how tools are being used (or even made) in natural groups rather than animals in cages. The behavior of the captive animals will only make sense in terms of evolution when compared to that of wild groups, and it would be a mistake to assume that everything Capuchins do in captivity they must also be doing in the wild. Perhaps they are and we haven’t seen it yet, but perhaps it’s a matter of ecology. If a Capuchin lives in an area with lots of soft fruit and food that does not require tools, they’re not likely to turn to tools to solve some of the problems presented by their environment. If the environment is harsher, however, and the monkeys are not naturally well-equipped to crack open nuts or get the most flesh off bone possible (because Capuchins do eat meat when they can get it), tool use is much more likely to emerge if the cognitive connections can be properly made. Some are more proficient than others, and it make take a while for certain behaviors to become established, but the tool use of Capuchins teaches us some important lessons about evolution and how it is never finished shaping life in the most surprising ways.

References;

Visalberghi, E., and McGrew, W.C. “Cebus meets Pan.” International Journal of Primatology, Vol. 18, No. 5, 1997

Westergaard, G.C. “What Capuchin Monkeys Can Tell Us About the Origins of Hominid Material Culture.” National Institute of Child Health and Human Development,1998.

Westergaard, G.S., and Suomi, S.J. “Capuchin Monkey (Cebus apella) Grips for the Use of Stone Tools.” American Journal of Physical Anthropology, 103: 131-135 (1997)

Westergaard, G.C., and Suomi, S.J. “The Production and Use of Digging Tools by Monkeys: A Nonhuman Primate Model of a Hominid Subsistence Activity.” Journal of Anthropological Research, Vol. 51, No. 1 (Spring, 1995)