CHAOS IN THE CANOPY- NON-LINEAR PHENOMENA OCCURRING IN THE SONG OF THE INDRI (INDRI INDRI)

Non-linear phenomena (NLP) in animal vocalizations arise from irregularities in the oscillation of the vocal folds, which are considered a highly non-linear system. Physical constraints may play a role in the emission of NLP, although different explanations for the adaptive significance of these features have been provided. Previous studies argued that the occurrence of NLP may reflect the emotional state of the emitter, e.g., indicating stress, and be associated with the context of emission, e.g., alarm calls in the presence of predators, or individual characteristics as age, sex, and reproductive status. The indri (Indri indri), the largest living lemur inhabiting the dense rainforests of Eastern Madagascar, exhibits a unique vocal repertoire that shows different kinds of NLP, i.e., deterministic chaos, subharmonics, and frequency jumps. The most distinctive vocalization of this species is the loud song which indri groups daily emit to advertise their presence to conspecifics at long distance. The song starts with harsh sounds and follows as harmonic frequency modulated notes. We investigated the occurrence of NLP within the harmonic part of the indri song at the individual level. Specifically, we counted the total occurrence of NLP and we tested whether nonlinearities may be associated with individual characteristics of the emitter, i.e., sex and age, song duration, and seasonality. We found that the longer the song duration, the higher the number of NLP. Sex, age, and season of emission were not correlated with the emission of NLP. Our results suggest that NLP might result from the vocal fatigue needed for sustaining loud calls over time rather than be associated with individual characteristics and do not support adaptive explanations. This is the first attempt to investigate NLP within the vocal repertoire of indris and to provide potential explanations for such features arising within the harmonic part of their song.