Vocal Tract Morphology Determines Species-Specific Features in Vocal Signals of Lemurs (Eulemur)
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The source-filter theory describes vocal production as a two-stage process involving the generation of a sound source, with its own spectral structure, which is then filtered by the resonant properties of the vocal tract. This theory has been successfully applied to the study of animal vocal signals since the 1990s. As an extension, models reproducing vocal tract resonance can be used to reproduce formant patterns and to understand the role of vocal tract filtering in nonhuman vocalizations. We studied three congeneric lemur species —Eulemur fulvus, E. macaco, E. rubriventer— using morphological measurements to build computational models of the vocal tract to estimate formants, and acoustic analysis to measure formants from natural calls. We focused on call types emitted through the nose, without apparent articulation. On the basis of anatomical measurements, we modeled the vocal tract of each species as a series of concatenated tubes, with a cross-sectional area that changed along the tract to approximate the morphology of the larynx, the nasopharyngeal cavity, the nasal chambers, and the nostrils. For each species, we calculated the resonance frequencies in 2500 randomly generated vocal tracts, in which we simulated intraspecific length and size variation. Formant location and spacing showed significant species-specific differences determined by the length of the vocal tract. We then measured formants of a set of nasal vocalizations (“grunts”) recorded from captive lemurs of the same species. We found species-specific differences in the natural calls. This is the first evidence that morphology of the vocal tract is relevant in generating filter-related acoustic cues that potentially provide receivers with information about the species of the emitter.