Sounds and size: identification of acoustic variables that reflect body size in hamadryas baboons, Papio hamadryas

Numerous animal species produce calls to repel competitors and attract mates. Because body size often determines fighting ability and reproductive success, it is of critical importance to assess the reliability of acoustic variables that may serve as indicators of body size. In mammals, the fundamental frequency (F0) is determined by the subglottal air pressure and the length and tension of the vocal folds, while the location and spacing of the formants (‘formant dispersion’) are related to vocal tract length. Because F0 can be modulated, it has been suggested that formant dispersion but not F0 serves as a reliable predictor of body size. To test this assumption, we examined grunts recorded from hamadryas baboons. We used linear predictive coding to extract the formants, and an autocorrelation method to measure the fundamental frequency. We found that, across age classes and among adult females, the fundamental frequency was more closely related than formant dispersion to most physical measurements. Depending on the call type, and possibly the sex of the caller, the fundamental frequency can thus serve as a reliable indicator of physical characteristics of the caller.