Evidence of human-like, holistic face processing in spider monkeys (Ateles geoffroyi)

Human subjects build mental representations of facial identity that are "holistic." This has been clearly demonstrated with the composite effect where the representation of a whole face interferes with the recognition of features. Very few studies have sought evidence of holistic representations being built by nonhumans. This study tested captive, black-handed spider monkeys (N = 2) on a standard composite task, comparable to those run previously on human subjects. In Experiment 1, the monkeys were tested with the faces of conspecifics (Ateles geoffroyi), humans (Homo sapiens), and domestic sheep (Ovis aries) together with stick objects. The results of Experiment 1 revealed that both conspecific and human faces were processed holistically. The subsequent test (Experiment 2) confirmed that, for both subjects, the face composite effect was contingent on upright orientation. A direct comparison with available human data demonstrates a level of similarity, strongly suggestive of cognitive homology.