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Abstract The anatomy and function of the facial muscles of Macaca cyclopis, M. fuscata, M. irus, M. mulatta , and M. nemestrina have been correlated and analyzed by means of electromyograms made by a 12‐channel electromyograph. We made records only of those areas where there is one layer of facial muscles. Morphologically the muscular differences among the species are insignificant for the recording of EMGs. Functionally, the facial muscles moving the eyebrows and ears differ under certain conditions. There is no muscle activity during sleep: during dozing, only a slight lowering of the eyebrows was noted and sometimes an extremely low level of watching behavior. The ear and eyebrow movements during the watching responses differ among the species. The basis for threatening behavior is watching: but most species raise the ears and eyebrows more extremely in threat, and other muscle activities differ among the species. In all species, the facial muscles are contracted in fear to a greater or lesser degree. Some muscular responses, too brief to be detected by observers but recorded by the EMG, can thus be analyzed and compared with the expressions of observed behavior.
Rolf W. Seiler (Thu,) studied this question.