A study of vocal nonlinearities in humpback whale songs: from production mechanisms to acoustic analysis.

Dorian Cazau,Jeffrey T Laitman,Olivier Adam,Thierry Aubin,Joy S Reidenberg

doi:10.1038/srep31660

Abstract

Although mammalian vocalizations are predominantly harmonically structured, they can exhibit an acoustic complexity with nonlinear vocal sounds, including deterministic chaos and frequency jumps. Such sounds are normative events in mammalian vocalizations, and can be directly traceable to the nonlinear nature of vocal-fold dynamics underlying typical mammalian sound production. In this study, we give qualitative descriptions and quantitative analyses of nonlinearities in the song repertoire of humpback whales from the Ste Marie channel (Madagascar) to provide more insight into the potential communication functions and underlying production mechanisms of these features. A low-dimensional biomechanical modeling of the whale’s U-fold (vocal folds homolog) is used to relate specific vocal mechanisms to nonlinear vocal features. Recordings of living humpback whales were searched for occurrences of vocal nonlinearities (instabilities). Temporal distributions of nonlinearities were assessed within sound units, and between different songs. The anatomical production sources of vocal nonlinearities and the communication context of their occurrences in recordings are discussed. Our results show that vocal nonlinearities may be a communication strategy that conveys information about the whale’s body size and physical fitness, and thus may be an important component of humpback whale songs.

Highlights

Tyson et al.[6] reported the occurrence of subharmonics, deterministic chaos, biphonation, and frequency jumps in North Atlantic right whales (Eubalaena glacialis)
While the presence of nonlinear phenomena in mysticeti vocal sounds has been observed and quantified, the question of their recognition as essential vocal features of humpback whale songs remains. This tendency to disregard nonlinear features may be explained by their transitory characteristics and intermittent occurrences within various sound units, which make them unreliable cues for the definition of characteristic sound units
Reidenberg and Laitman[9] discovered the presence of a vocal fold homolog, overturning the long-held and widespread misconception that mysticetes lack vocal folds[10]. This misconception may have colored the classification of sound units (e.g., Au et al.11), as no distinctions have ever been made between noise-like and chaotic vocalizations, their production mechanisms are completely distinct with respectively no vibrations and irregular vibrations of vocal folds

Summary

Introduction

Tyson et al.[6] reported the occurrence of subharmonics, deterministic chaos, biphonation, and frequency jumps in North Atlantic right whales (Eubalaena glacialis). Previous studies on humpback whale vocal production[17,18] focused on the supraglottal part (i.e. from the vocal fold to the nose) of the respiratory tract, using a non-interactive linear source-filter model to investigate occurrences of formant-like acoustic resonances. The interactive nonlinear source-filter model[23] is used to determine potential mechanisms that generate chaos and frequency jumps This model includes a two-mass mechanical system to simulate the physics of the paired vocal folds (the two “arms” of the U-fold), that represents a good compromise between the most basic mechanism of self-oscillating flow-induced tissue oscillation (i.e., the phase difference between upper and lower parts of vocal folds), and the more extreme mechanism involving regulation by muscle activity affecting exact flow rates and vocal fold oscillation characteristics. The simulated vocalizations of diagram C have been obtained with an upward fundamental frequency modulation, and those of diagram D have been obtained with a laryngeal sac extension

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Conclusion