Superiority of conspecific faces and reduced inversion effect in face perception by a chimpanzee (Pan troglodytes).
Masaki Tomonaga, Shoji Itakura, Tetsuro Matsuzawa
Faces represent special stimuli for certain nonhuman primates as well as for humans. Various facial expressions have been devel oped for visual communication in a social context [l]. Recent laboratory research also indicates that monkeys and apes can visually recognize both individuals and species [2 5]. A limited number of experimental studies of face perception or recognition have also been conducted with primates [5 9]. For example, Matsuzawa  trained a female language trained chimpanzee, Ai, to name chimpanzee and human individuals with single upper case letters of the alphabet, and found that Ai showed superiority in naming conspecific in dividuals.
In contrast to nonhuman primates, percep tion or recognition of faces in humans has been extensively studied by psychologists con cemed with developmental, cognitive, social and neural aspects [10, 11]. These studies revealed certain phenomena specific to face perception or recognition. One is the inver sion effect: humans show more difficulty in perceiving and recognizing faces when pre･ sented upside down than when normally oriented . Another interesting finding is the so called 'other-race' effect: faces of an individual's own race are more easily recog nized than faces of another race .
The aim of the present experiment was to test the chimpanzee ʼs ability to recognize faces of another species (human) and rotated faces in comparison to the performance of human subjects.