How monkeys see others: Discrimination and recognition of monkeys' shape.

The two experiments described in this study address the question of the perceptual basis of species discrimination and body recognition in monkeys. Longtailed macaques (Macaca fascicularis) were trained to discriminate line drawings of different monkey bodies. The procedure consisted of a simultaneous discrimination between four images under continuous reinforcement. Social communication between the test animal and other group members during test sessions was almost unrestricted. In the first experiment all monkeys learned, within at least 7 sessions, to discriminate one monkey from other monkeys. Discrimination was invariant against transformations of size and rotation of the stimuli. A preference test for particular features resulted in a graded estimation of particular body features. Generalisation to different views of facial stimuli was demonstrated. In the second experiment the monkeys had to relearn a new association which involved a differentiation of the previously shown stimuli. After reaching the learning criterion it was shown that the same features as in the previous experiment were evaluated differently. The experiments generally support the view that perceptual mechanisms of the signal receiver are crucial for individual recognition. Results are discussed in contrast to a 'theory of mind' approach in primate cognition.