Apes share a distinct set of morphological and anatomical characteristics that allow us to use our arms and hands in unique ways. Apes also have relatively larger brains with similar sulcal patterning indicating there is a distinctively hominoid brain structure. These features in great apes have consistently been linked with higher cognitive skills and are considered to be the precursors leading to the exceptional developments of humans over evolutionary time – establishing the physiological basis that allows us to make and use tools to modify our environments and build our unique cultures. This study examines the general model that orthogrady and suspensory postures were the antecedents for enhanced manipulative ability in apes, which consequently set the stage for enhanced cognitive abilities in early hominoids. The primary hypothesis is that if single-handed prehensility is enhanced by suspensory orthogrady, then we may predict ape feeding style will differ significantly from that of pronograde monkeys, allowing them to access foods in unique ways. Using sympatric white-handed gibbons, Hylobates lar, representative of orthograde apes, and pig-tailed macaques, Maccaca leonina, representative of pronograde monkeys, as models, the comparative feeding styles of primates were analyzed focusing on their positional behaviour and manual skill. Results support the hypothesis that gibbons exhibit a unique feeding style associated with their orthogrady/suspensory postures in comparison with pronograde macaques. This was demonstrated by their increased access to food in the trees with more stable postures, an expanded foraging radius, and more frequent use of the terminal branches, and was evident in their complex manipulative skills with larger manual repertoires, more variability in wrist use, and more sophisticated manual techniques. Moreover, significant differences in positional behaviour and manual skill demonstrated by gibbons and macaques were evident even when feeding on the same types of foods within their shared environment. This study proposes that the combined uniquely ape traits to forage in suspensory orthograde postures with precision dexterity have allowed apes to become highly selective feeders within their environments, leading to advance manual dexterity and cognitive prowess in apes.

Gibbons (Hylobatidae), taxonomically apes, have been largely ignored in cognitive research. This is surprising given their unique phylogenetic position, being intermediate between the monkeys and great apes, and the available diversity of extant species. They are therefore, ideally placed to study the evolution of cognitive abilities in the hominoid line; they offer the opportunity to determine how the mental capacities of primates have changed through the transition from monkey to ape. This research aimed to begin to fill the void in our knowledge regarding the cognitive abilities of this family through investigations of their object manipulation and tool-use skills, relating the findings to the evolution of the hominoid brain. In a raking-in task, where the gibbons were presented with a tool that could be used to draw in an out-of-reach food item, these apes evidenced potentially insightful comprehension of object relationships when the tool and goal object were presented in direct alignment. They also proficiently used a rake to retrieve a reward while avoiding a trap that presented an impediment to goal attainment; however, in general, they required a period of learning to perform consistently. Once the necessary relationships between the tool and goal object were not physically situated in the task layout, as in true tool-use manipulation, the gibbons performed poorly. In a raking-in task where the necessary orientation for success had to be produced by the subject, no individual evidenced foresightful comprehension of the required action. There was some suggestion of learning the correct behaviour through associative processes. This finding was also supported by evidence from dipping experiments where the gibbons were provided with a transparent box containing a liquid reward and sticks that could be used as tools to access it. No individual developed dipping behaviour. The gibbons therefore, performed well on tasks when the salient relationships between tool and goal were directly perceivable. Once they became responsible for producing that relationship, performance was poor. When the necessary orientation between the tool and goal was not provided by the experimenter, the gibbons evidenced low motivation to manipulate the objects. Given the gibbons’ requirement for direct visual feedback to comprehend the causal interactions between objects, this likely hindered their learning process. Failure therefore on the true tool-use tasks may not represent a particular cognitive limitation in these apes. A consistent finding was that the hoolock gibbons (Bunopithecus) were the most attentive and effective of the four gibbon genera. This is potentially due to the more variable natural environment experienced by these apes, driving selection for greater exploratory tendencies and flexibility of behaviour. The findings from this, and other work on primate cognition, suggest that contrary to propositions put forward by proponents of modular accounts of hominid brain evolution, the cognitive architecture of non-human primates contains neural mechanisms capable of processing technical information that may not be completely encapsulated. Suggestions that no non-human possesses specialised cognitive machinery for understanding objects as tools are also challenged.

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