PURPOSE: Previous research has shown both squirrel monkeys and humans can identify individual faces of their own species more easily than those of other species. But it is unclear whether both species utilize the same cues to identify individual faces. This is the question we explore in this research.

METHOD: Subjects were two squirrel monkeys and two humans. The stimuli were faces of squirrel monkeys and humans, which were unfamiliar to subjects. In the training phase, subjects were trained to discriminate between two squirrel monkey faces or between two human faces so as to achieve a performance of 80% correct. In the test phase, we introduced 4 types of probe stimuli that showed only some facial features of the training stimuli (1:configuration of eyes, 2:mouth and nose, 3:only one eye, 4: outer facial boundary).

RESULTS AND DISCUSSION: Both species can identify individual faces of their own species better than those of other species. The configuration of eyes had a significant effect (binominal test < .05) on the identification performance of both species. Furthermore, monkeys unlike humans could use outside features of monkey faces. These results suggest that squirrel monkeys may have two strategies for facial processing. One strategy is similar to that of humans (processing eye configurations), while the other is specific to squirrel monkeys (the use of external features that are discriminative for squirrel monkey faces). This divergence of facial processing may due to the differential of their processing demands environments, specifically the con-specifics.