Changes in vocalizations during ontogeny can in principle be related to three classes of factors, namely growth, maturation, and experience, i.e. learning. While learning is a prerequisite for the proper development of speech, it hardly appears to play a role in the development of the species-typical vocal behaviour of nonhuman primates. Nonetheless, subjects of different age and sex often exhibit prominent variation in the structure of their vocalizations. Here we investigate ontogenetic changes and the emergence of sex-related differences in the acoustic structure of Chacma baboon (Papio hamadryas ursinus) clear calls. These vocalizations, uttered by individuals separated from the rest of the group or from particular individuals, were recorded in a group of baboons living in the Moremi Wildlife Reserve, Botswana. We analyzed calls from 58 animals of both sexes and all age classes. While the structure of the call appeared to be fixed from birth, call duration, the distribution of energy, and the fundamental frequency of the calls, including its modulation, varied with age and sex of the caller. We discuss how body size may explain these variations. Some of these variables exhibited a different profile of variation with age between the sexes, with significant differences emerging around puberty. The emergence of these sexual differences may be explained by the emergence of sexual dimorphism in body size and mass. To which degree the hormonal status contributes to variation in the calling remains to be investigated.