Acoustic correlates of physiological stress in a wild primate

Vocalizations potentially encode information about physiological states, yet there is little direct evidence linking vocal parameters to physiological stress in non-humans, including primates. We investigated whether male mantled howler monkey (Alouatta palliata) loud calls reflect physiological stress by analyzing the relationships between the acoustic parameters of loud calls and fecal glucocorticoid metabolite (fGCM) concentrations. Howler monkeys produce loud calls primarily in the context of intergroup competition, which has the potential to elicit physiological stress responses. We expected that elevated fGCM would be associated with loud call acoustics through changes in laryngeal tension (increasing fundamental frequency and vocal perturbations), respiratory control (affecting call duration and temporal patterning), and vocal tract configuration (modifying spectral properties). We analyzed 93 high-quality loud calls and assayed 242 fecal samples collected over a 10-year period from 23 adult males across seven groups in Los Tuxtlas (Mexico). We calculated 26 loud call acoustic measurements including spectral, temporal, and non-linear variables. Mixed-effects modeling revealed that acoustic features collectively explained 71 % of the variation in fGCM. Loud calls produced at higher fGCM were characterized by increased pitch, greater pitch instability, altered vocal tract resonances, increased voice roughness, and reduced tonal clarity. Among these features, changes in pitch showed the strongest association with fGCM. These findings establish a link between stress physiology and vocal production in howler monkeys, suggesting that internal physiological states manifest in acoustic signals that could convey information about caller condition.