Study of non-linearities in humpback whale song units

Abstract

Unique in mammals, the vocal generator of mysticete species comprises membranes covering the two arytenoid cartilages that vibrate as the airflow passes through the trachea from the lungs to the laryngeal sac. By adjusting the airflow as well as the spacing and orientation of the two cartilages, mysticetes control the vibrations and vary acoustic qualities of the produced sounds, including the duration, amplitude, and frequency modulation of vocalizations. Humpback whales control sound production in this way to construct a complex vocal repertoire, including vocalizations with or without harmonics as well as pulsed sounds. Some vocalizations within humpback whale songs, called units, exhibit non-linearities such as frequency jumps and chaos. Here, we further describe non-linear features of units, including two additional non-linearities: subharmonics and biphonation. Subharmonics within units are probably due to higher air flow rates and to the acoustic modes of internal resonators. Biphonic vocalizations are likely generated either by an asymmetric opening of the arytenoid cartilages or by the passage of the air flow at two separate positions along the membranes. Our analyses revealed acoustic non-linearities in vocalizations emitted by six different singers during multiple breeding seasons and from populations in different oceans, suggesting that singing humpback whales often produce units with non-linear features.