Vocal tract flexibility and variation in the vocal output in wild indris.

Acoustic output in human speech is commonly considered to result from the combination of a source of sound energy modulated by the transfer function determined by the supralaryngeal vocal tract. This process is central to the source-filter theory of voice production (Fant 1960) and applies well to the description of vocal sounds produced by other terrestrial mammals (see McComb & Reby 2009 for a review). This source-filter theory of voice production postulates that phonation consists of the combination of two independent events. First, air passes from the lungs through the glottis, where an harmonic signal ("source signal") is generated by the vibration of the vocal folds. Then the source signal passes through the supra-laryngeal vocal cavities, where vocal tract resonance properties selectively attenuate or reinforce certain harmonics of the source signal. In this second step, the vocal tract acts as a filter. The most emphasized frequencies are called 'formants'. In non-human primates, including lemurs, the resulting signal, a linear combination of the two previously described independent mechanisms, finally radiates through the mouth or nostrils into the environment (Fitch & Hauser 1995). Previous research also demonstrated that articulation of the mandibles may affect formants (Gamba & Giacoma 2006), then the acoustic structure of the vocalization is directly affected by both the anatomy of the vocal tract and the articulation of the vocal apparatus. From voice research, we know that the length of the vocal tract of a speaker affects both formant position and formant spacing (Titze 1994). In humans, singers often open their mouths as wide as possible to produce powerful high tones. This happens because the vocal tract can adopt specific shapes to project certain pitches and resonance overtones better (Joliveau et al. 2004).