This chapter presents the results of a qualitative study concerning those displays of the catarrhine monkeys and apes which are mainly characterized by the participation of the muscles and the structures of the facial region. It closely follows van Hooff. It gives a survey of the conditions which during evolution have favoured the development of a facial musculature which eventually could become the 'organ' of facial expressions. They consist of a combination of postures or movements of such facial elements as the jaws, the hps, the tongue, the ears, the eyebrows and the upper-head skin, the eyes and the eyelids. Apart from these, there are the postures and the movements of the body or parts of the body and the vocalizations. It is possible to distinguish in the catarrhine monkeys and the apes a number of compound expressions. Some of these can be observed frequently in most catarrhine monkeys and apes.
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