Tool Use by Capuchin Monkeys

Capuchin monkeys are medium-size primates native to the forests of Central America and northern South America. Although these monkeys are impressive users of tools in captive settings, we have little evidence of their systematic use of tools when in the wild. Researchers have commented on this puzzling contrast by noting that capuchins have not been studied in the wild as extensively as other primates such as apes and that their more arboreal (tree-based) lifestyle limits their opportunities to use tools compared with apes. In the trees, their hands are more often needed for support; moreover, loose objects that could be used as tools are less available and less easily set aside and retrieved, and stable, strong and appropriately shaped supporting surfaces are less available in the trees than on the ground. Imagine pounding a round nut on a log or stone that rests solidly on the ground. Then imagine the same activity while sitting in a tree and pounding the nut on a sloping tree branch. Finally, activities carried out high in the forest canopy are more difficult for terrestrial humans to see than activities occurring on the ground.

All of these are plausible explanations for the rarity of observations of tool use in wild capuchins. However, although arboreality may limit opportunities for capuchins to use tools or for us to observe such activity, we know that chimpanzees and orangutans do sometimes use tools in trees. Thus arboreality alone does not preclude tool use. Instead, we must consider what aspects of capuchins' behavior and ecology might support the discovery of how to use an object as a tool in the wild. This consideration might suggest other ways we can look for tool use in wild capuchins and help us to understand why we observe it more often in captive monkeys.

One can turn the question around and ask why we see tool use at all in capuchin monkeys. Like other primates, capuchins possess the necessary sensory and anatomical characteristics for using objects as tools. They have a well-articulated hand with anatomical adaptations that favor the fine manipulation or precise positioning of objects, and they have sufficiently long limbs, postural control, and strength to generate considerable forces (when pounding for example). This, however, does not distinguish them from most other monkeys, especially those in Africa and Asia, although all other monkeys use tools less often than capuchins.

Capuchins possess two behavioral characteristics that are less widely shared with other primates and that are particularly relevant to using objects as tools. First, although using a tool is an individual endeavor, it is acquired more readily in socially supportive contexts where experts tolerate novices nearby, and capuchins are relatively tolerant of one another, particularly adults of youngsters. Second, and fundamental for the discovery of tool use, capuchins generate a great variety of explorative and manipulative behaviors that involve acting with objects and on surfaces. Capuchins reliably spontaneously combine objects with surfaces and with each other by pounding and rubbing; they also insert their hands and objects in holes and crevices. When captive capuchins encounter objects they consider benign, whether novel or familiar, they quickly approach, explore and manipulate them with enthusiastic interest. Their interest towards objects persists over time, even towards familiar objects. Although wild capuchins initially often avoid novel objects, they explore and manipulate familiar objects and substrates (layers of soil that a plant or animal uses for support) persistently, and routinely engage in many actions. This can allow them to discover the consequences of actions combining objects and surfaces.

All of these behavioral characteristics make it likely that a capuchin monkey upon encountering an interesting set of objects or an interesting surface with loose objects available, and with the motivation, time and security to investigate will produce actions with objects on surfaces. Tool use relies upon perception/action routines (e.g., pounding, inserting) that are applied to virtually any set of objects and surfaces they encounter. As a routine behavior, the monkey may occasionally combine one object or surface with another object, and so discover that using an object helps it achieve some goal. This scenario is sufficient to support the frequent discovery of tool use by captive capuchins, but one can see that it might not occur as often in natural settings.