The Ape Ecological Niche: Posture and Hand Use in Gibbons and Macaques and the Influence of Manual Skill on Cognitive Development in Apes and Humans

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