A test of postural development was devised and applied to primate infants consisting of 5 species of macaques, chimpanzees, orangutans, and humans. The test consisted of 11 sorts of techniques, each of which induced certain postural reactions. Longitudinal observation showed that these induced postural reactions had a common develapmental process across the primate species. Such reactions developed in four stages as follows. First stage: the flection of forelimbs and hindlimbs. Any reaction of the limbs was not induced. Second stage: the extension of forelimbs but the flection of hindlimbs. The elbow extended with the pronation of the forearm. Third stage: the extension of forelimbs and hindlimbs. The knee extended with the pronation of the foot. Fourth stage: stepping of the hindlimbs. Postural reactions developed through these four stages in the first 2 to 3 months in the macaque infants, the first 11 to 12 months in the apes and humans. Although sponmneous postures and locomotions in infants are different among the primate species, induced postural reactions have common characteristics which provide a useful scale for the cross-species comparison of postnatal behavioral development. The development of object manipulation was compared among species in reference to the developmental stages of postural reactions. Approaching behavior appeared in the second stage, while increased dexterity and a variety of manipulations was marked in the third stage in all of the species. However, operational manipulation in human infants always exceeded that of the other primates.

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The present study shows that chemical signals are used as sociosexual communication among prosimians, New World as well as Old World monkeys. The abundance of scent glands and of behaviors concerned with their use exhibits that olfactory communication is of great importance in prosimians and callitrichids. Although cebids share a reduction in the complexity of the gland structure with Old World monkeys, the relative scarcity of specialized scent-marking behaviors is complemented by variable types of urine-concerned behaviors. It suggests that olfactory communication might play a more powerful role in social interaction in general throughout the order primates than is commonly supposed.

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