Age- and sex-related variations in clear calls of Papio ursinus

Changes in vocalizations during ontogeny can in principle be related to three factors: 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 behavior of nonhuman primates. Nonetheless, subjects of different age and sex often exhibit prominent variation in the structure of their vocalizations. We investigated ontogenetic changes and the emergence of sex-related differences in the acoustic structure of Chacma baboon (Papio ursinus) clear calls. We recorded the vocalizations emitted by individuals separated from the rest of the group or from particular individuals, in a group of baboons in the Moremi Wildlife Reserve, Botswana. We analyzed calls from 58 baboons 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 modulation, varied with age and sex of the caller. We discuss how body size may explain the variations. Some of the variables exhibited a different profile of variation with age between the sexes, with significant differences becoming apparent around puberty. The emergence of these sexual differences may be explained by the onset of sexual dimorphism in body size and mass. To which degree the hormonal status contributes to variation in the calling remains to be investigated.