Résumé
Humpback whales are known for their longer migratory routes and for the songs that males emit mainly during the breeding seasons. Over 60 years, researchers described the acoustic characteristics of these vocalizations emitted in a precise order making several repeated phrases. Our previous study published in 2016 showed that some vocalizations exhibited acoustic nonlinearities. The frequency jump is abrupt change in the fundamental frequency during the temporal duration of the vocalization. There may be multiple jumps within the same vocalization. The chaotic mode is the presence of noise during the vocalization, as if the cetacean suddenly lost the vibrational control of its vocal generator, then very quickly adjusted it to return to the original fundamental frequency. In this present study, we confirmed the presence of these 2 nonlinearities in the songs of humpback whales, and we found 2 new ones. Some vocalizations exhibit two frequencies that evolve differently over time, representing the first description of biphony in humpback whales. Finally, we also detected subharmonics. We propose to explain these 4 acoustic non-linearities by the mechanical description of their vocal generator, in particular by the vibrations of the membranes covering the arytenoid cartilages which hold the laryngeal sac at the level of the trachea. The individual can adjust the spacing of these cartilages as well as the intensity and direction of the air flow to select a part of these membranes. Because these non- linearities were found in 3 different songs from 3 different oceans at 3 different years, our assumption is that these acoustic features are finally non rare, and does not depend on a specific anatomical abnormality of the vocal generator of a particular singer. This study can be seen as a contribution to the identification of an individual acoustic signature, which has never been shown in mysticetes.