Singing more, singing harsher: occurrence of nonlinear phenomena in a primate’ song

Nonlinear phenomena (NLP) in animal vocalizations arise from irregularities in the oscillation of the vocal folds. Various hypotheses have been put forward to explain the occurrence of NLP, from adaptive to physiological ones. Non-human primates often display NLP in their vocalizations, yet the meaning of these features is still unclear. We present here the first study of NLP occurrence in the song of a singing primate: the indri (Indri indri). We found that the more an individual sings, the higher the likelihood of emitting NLP. Our results show that NLP occurrence in indri depends on phonation, i.e., the cumulative duration of all the units emitted by an individual. Furthermore, NLP occurrence depends on the timing of the signal and increases as the singing progresses. Our findings suggest that indris may be subjected to physiological constraints during the singing process which may limit the production of harmonic sounds because of the fatigue needed to perform loud and long calls. However, indris may still benefit from emitting NLP by strengthening the loudness of their signals for better advertising their presence to the neighboring conspecific groups.