The study of non-human tool use and manufacture has a long history and remains a vibrant area of inquiry. Shumaker et al. (2011) categorize the diversity of tool behavior with modes. For example, “Cut” refers to movement of an object with a sharp edge to incise another object or surface. Modes are operationally descriptive and do not necessarily describe function. While there are many species of animals that use tools, Shumaker et al. (2011) note that there are no reliable trends that predict which types of animals will be tool users. But, unquestionably, the most frequent, elaborate, and complex forms of tool use and manufacture occur within the Primate Order, particularly among the great apes. Of those, chimpanzees (Pan troglodytes) and orangutans (Pongo spp.) are the most diverse tool users.

The cognitive abilities of orangutans are under-studied, certainly compared with chimpanzees. It is therefore most appropriate that Motes-Rodrigo et al. (2022) have chosen orangutans as their model to better understand the evolutionary origins of sharp stone tool use. Studies of the mental processes involved in the acquisition and mastery of these behaviors by orangutans offer rich opportunities and have the potential to reveal how our human ancestors acquired these same skills and abilities.

Orangutans have been documented to use stones in several different modes of tool use, specifically Drop, Throw, Pound/Hammer, and Cut (Shumaker et al., 2011). These behaviors occurred across contexts, including the wild, rehabilitation centers, and zoos. Some of these were observed to be spontaneous while others were elicited in experimental settings. There are two separate cases of orangutans using tools symbolically to request that a human cut something. Shumaker et al. (2011) introduced the mode “Symbolize” to describe a situation where a tool represents another object. In the first case, the ape drew a stick back and forth across a lock of her hair in a cutting motion after observing a familiar human use a knife to cut a stick. The orangutan then allowed the person to cut off the lock of her hair. In the second, the orangutan attempted to open a coconut. After failing to crack the shell she handed the coconut to the person, who had a machete, and repeatedly struck the coconut with a stick. The person used the machete to cut open the coconut for the orangutans. These documented findings suggest that orangutans view stones as potential tools, use them in a variety of modes, can modify stones to make sharp cutting tools, and understand that they can collaborate with trusted social partners on tool using tasks.

Which mental processes are involved in understanding and performing these tasks? Wright (1972) reports that his orangutan subjects used imitative learning to make and use sharp stone tools. In what is largely a replication of Wright’s methodology, Motes-Rodrigo et al. (2022) explore this topic through three central questions. Will an orangutan use a sharp stone tool without the benefit of a skilled demonstrator? Does increasing the perceived value of a sharp stone tool incentivize the ape to manufacture them? Is the presence of a skilled human demonstrator sufficient to elicit the manufacture and use of sharp stone tools for cutting by an orangutan?

For the first question, the experimenters worked with two apes, each offered a different apparatus that contained a conspicuous and desirable food reward inside. To obtain the food, one device required cutting a rope to release a door and the other required cutting a silicone membrane. A hammer and core stone were present as raw material for making a sharp tool, and a human-made sharp-edged stone tool was subsequently added. Neither orangutan attempted to make their own stone tool. However, one of the individuals carried a stick into the testing area, used it to penetrate the membrane, and obtained the reward. In a later trial, the same ape used the tip of the human-made stone tool to penetrate the membrane. The authors refer to this as use of a “cutting tool,” but this does not conform to Shumaker et al.’s (2011) definition of the mode Cut. Rather, the ape used both the stick and stone tool in the same mode to Penetrate the membrane and obtain food. For the ape, readily available tools (the stick and the stone) were able to be used in a manner that was clearly familiar. The use or manufacture of a sharp stone cutting tool was simply not required. From the orangutan’s perspective, the task was solved successfully.

In the planned experimental setting, what mental processes would have been required for the orangutan to spontaneously attempt to manufacture a sharp stone tool, something he had never seen or used before? The same can be asked about employing the human-made stone tool for cutting, a totally novel behavior. The only possibility for either is insight learning, sometimes referred to as mental trial and error, that would have resulted in a behavioral innovation. This form of learning, characterized by an abrupt “aha” solution to a novel problem, has been shown to be exceedingly rare in animals, including primates, and by definition occurs unexpectedly (Shettleworth, 2012). Thus, the design of this phase of the study was likely to have been biased against “success” as defined by the experimenters.

The second question was answered definitively. When the orangutans were provided with many human-made stone flakes and then asked to trade them for grapes, they quickly succeeded. Trading by zoo-living great apes with human caretakers is a nearly universal behavior that is cultivated and encouraged at early ages for management purposes. In this scenario, the experimenters presented the stones as tokens to be traded. During the testing trials, one of the apes unfastened the fixed core and used it to Hammer the surfaces of the enclosure. In the process, some pieces were detached from the core but the ape did not attempt to trade them for grapes, ignoring them instead. There is no evidence that the hammering was an attempt to create flakes to be used as tokens. From a methodological standpoint, it is worth considering the potential pitfalls of training with the use of flakes as tokens prior to their use as cutting tools. In particular, prior studies with great apes have revealed a cognitive bias known as “functional fixedness,” whereby items that serve a known purpose are not easily utilized for novel and unrelated purposes (Shettleworth, 2012).

An answer to the third question was confounded by the fact that three orangutan subjects were able to defeat the design of the apparatus and rip the membrane with their hands. No tools were needed, so there was no reason for the apes to manufacture or use them. Despite previous studies that have shown that the presence of a familiar human demonstrator positively influenced social learning of the task, that did not occur in this experiment.

Motes-Rodrigo et al. (2022) conclude that “spontaneous lithic percussion” and “directed lithic percussion” are well within the range of abilities for orangutans. There is ample evidence in many contexts to support this assertion. Orangutans routinely pound rocks or other objects on surfaces and features in their environment. The subjects did not produce sharp stone tools to use for cutting. The authors conclude that “enculturation” might be necessary for great apes to perform this behavior and describe their subjects as “unenculturated.” This explanation is problematic in terms of how enculturation is framed, and the way in which it is applied to the subjects (Furlong et al., 2008). Described simply, enculturated apes are well socialized with humans and human artifacts. Enculturation occurs across a wide continuum, but no zoo-living great ape can be described as “unenculturated.” They spend their lives observing and interacting with humans and human artifacts. A mesh divider is not a barrier to enculturation. In that situation, great apes and their human caretakers function in one extended social setting, despite perceptions to the contrary. Enculturation may certainly encourage and reveal specific cognitive skills in great apes. But contrary to what the authors state, it does not create cognitive abilities. That distinction is crucial.

The orangutans in this study did not fail, they used alternative solutions to the ones planned by the experimenters. This speaks to the importance of experimental design for great apes that considers their perspective and allows them to either succeed or fail within a methodology that authentically reveals their mental skills, abilities, and complexity.