Vocal registers expand signal diversity in vertebrate vocal communication

Transitions and tricks: nonlinear phenomena in the avian voice

Birds evolved a novel vocal organ, the syrinx, that exhibits a high anatomical diversity. In the few species investigated, the syrinx can contain up to three pairs of functional syringeal vocal folds, acting as independent sound sources, and eight pairs of muscles. This rich variety in vocal structures and motor control results in a wide range of nonlinear phenomena (NLPs) and interactions that are distinct to avian vocal physiology, with many fascinating mechanisms yet to be discovered. Here, we review the occurrence of classical signatures of nonlinear dynamics, such as NLPs, including frequency jumps and transitions to chaos in birds. However, birds employ several additional unique tricks and transitions of inherent nonlinear dynamical nature that further enrich their vocal dynamics and are relevant for understanding the motor control of their vocalizations. Particularly, saddle-node in limit cycle (SNILC) bifurcations can switch sounds from tonal to harmonically rich and change the physiological control of fundamental frequency. In mammalian phonation, these bifurcations are mostly explored in the context of register transitions but could be equally relevant to altering vocal fold dynamical behaviour. Due to their diverse anatomy compared to mammals, birds provide unique opportunities to explore rich nonlinear dynamics in vocal production.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Exploring nonlinear phenomena in animal vocalizations through oscillator theory

Animal vocalizations comprise a rich array of complex sounds that exhibit nonlinear phenomena (NLP), which have fascinated researchers for decades. From the melodic songs of birds to the clicks and whistles of dolphins, many species have been found to produce nonlinear vocalizations, offering a valuable perspective on the mechanisms underlying sound production and potential adaptive functions. By leveraging on the principles of oscillator theory and nonlinear dynamics, animal vocalizations, which are based on coupled oscillators, can be described and conveniently classified. We review the basic ingredients for self-sustained oscillations and how different NLP can emerge. We discuss important terms in the context of oscillator theory: attractor types, phase space, bifurcations and Arnold tongue diagrams. Through a comparative analysis of observed NLP and bifurcation diagrams, our study reviews how the tools of nonlinear dynamics can provide insights into the intricate complexity of animal vocalizations, as well as into the evolutionary pressures and adaptive strategies that have shaped the diverse communication systems of the animal kingdom.This article is part of the theme issue, 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions

Nonlinear phenomena (NLP) are acoustic irregularities that are widespread in animal and human vocal repertoires, as well as in music. These phenomena have recently attracted considerable interest but, surprisingly, have never been the subject of a comprehensive review. NLP result from irregular sound production, contribute to perceptual harshness, and have long been considered nonadaptive vocal features or by-products of sound production characterizing pathological voices. This view is beginning to change: NLP are increasingly documented in nonverbal vocalizations of healthy humans, and an impressive variety of acoustic irregularities are found in the vocalizations of nonhuman vertebrates. Indeed, evidence is accumulating that NLP have evolved to serve specific functions such as attracting listeners' attention, signalling high arousal, or communicating aggression, size, dominance, distress and/or pain. This special issue presents a selection of theoretical and empirical studies showcasing novel concepts and analysis tools to address the following key questions: How are NLP in vertebrate vocalizations defined and classified? What are their biomechanical origins? What are their communicative functions? How and why did they evolve? We also discuss the broader significance and societal implications of research on NLP for non-invasively monitoring and improving human and animal welfare.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

How to analyse and manipulate nonlinear phenomena in voice recordings

We address two research applications in this methodological review: starting from an audio recording, the goal may be to characterize nonlinear phenomena (NLP) at the level of voice production or to test their perceptual effects on listeners. A crucial prerequisite for this work is the ability to detect NLP in acoustic signals, which can then be correlated with biologically relevant information about the caller and with listeners' reaction. NLP are often annotated manually, but this is labour-intensive and not very reliable, although we describe potentially helpful advanced visualization aids such as reassigned spectrograms and phasegrams. Objective acoustic features can also be useful, including general descriptives (harmonics-to-noise ratio, cepstral peak prominence, vocal roughness), statistics derived from nonlinear dynamics (correlation dimension) and NLP-specific measures (depth of modulation and subharmonics). On the perception side, playback studies can greatly benefit from tools for directly manipulating NLP in recordings. Adding frequency jumps, amplitude modulation and subharmonics is relatively straightforward. Creating biphonation, imitating chaos or removing NLP from a recording are more challenging, but feasible with parametric voice synthesis. We describe the most promising algorithms for analysing and manipulating NLP and provide detailed examples with audio files and R code in supplementary material.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Nonlinear phenomena in mammalian vocal communication: an introduction and scoping review

Nonlinear phenomena (NLP) are common elements of mammalian vocalizations. Resulting from irregular sound production, they contribute to perceived harshness and are often present in calls conveying urgency or arousal. Initially dismissed as by-products of vocal production, NLP are increasingly recognized for their adaptive potential. However, NLP have never been the subject of a comprehensive review across vertebrate taxa. Here, we introduce NLP and examine developments in NLP studies in mammals. We found 220 papers published between 1962 and 2023, with publication rates increasing with time. The studies covered a wide range of taxonomic groups but were dominated by artiodactyls, carnivores, bats, rodents and primates. Tinbergen's questions offer a framework for future investigations, highlighting that while much research has been conducted on adaptive function, our understanding is still lacking in terms of ontogeny, mechanisms and evolution. The existing literature is a testimony to the importance of NLP in animal vocalizations. With the use of novel tools for analysis and playback studies, NLP research can become more cohesive and impactful, fostering better understanding among researchers. We look forward to a new age of NLP research, which we anticipate will lead to a paradigm shift in our understanding of vocal communication in mammals.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

'Monkey yodels'-frequency jumps in New World monkey vocalizations greatly surpass human vocal register transitions

We investigated the causal basis of abrupt frequency jumps in a unique database of New World monkey vocalizations. We used a combination of acoustic and electroglottographic recordings in vivo, excised larynx investigations of vocal fold dynamics, and computational modelling. We particularly attended to the contribution of the vocal membranes: thin upward extensions of the vocal folds found in most primates but absent in humans. In three of the six investigated species, we observed two distinct modes of vocal fold vibration. The first, involving vocal fold vibration alone, produced low-frequency oscillations, and is analogous to that underlying human phonation. The second, incorporating the vocal membranes, resulted in much higher-frequency oscillation. Abrupt fundamental frequency shifts were observed in all three datasets. While these data are reminiscent of the rapid transitions in frequency observed in certain human singing styles (e.g. yodelling), the frequency jumps are considerably larger in the nonhuman primates studied. Our data suggest that peripheral modifications of vocal anatomy provide an important source of variability and complexity in the vocal repertoires of nonhuman primates. We further propose that the call repertoire is crucially related to a species' ability to vocalize with different laryngeal mechanisms, analogous to human vocal registers. This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Application of nonlinear dynamics theory to understanding normal and pathologic voices in humans

The theory of nonlinear dynamics was introduced to voice science in the 1990s and revolutionized our understanding of human voice production mechanisms. This theory elegantly explains highly complex phenomena in the human voice, such as subharmonic and rough-sounding voice, register breaks, and intermittent aphonic breaks. These phenomena occur not only in pathologic, dysphonic voices but are also explored for artistic purposes, such as contemporary singing. The theory reveals that sudden changes in vocal fold vibratory patterns and fundamental frequency can result from subtle alterations in vocal fold geometry, mechanical properties, adduction, symmetry or lung pressure. Furthermore, these changes can be influenced by interactions with supraglottal tract and subglottal tract resonances. Crucially, the eigenmodes (modes of vibration) of the vocal folds play a significant role in these phenomena. Understanding how the left and right vocal fold eigenmodes interact and entrain with each other, as well as their interplay with supraglottal tissues, glottal airflow and acoustic resonances, is essential for more sophisticated diagnosis and targeted treatment of voice disorders in the future. Additionally, this knowledge can be helpful in modern vocal pedagogy. This article reviews the concepts of nonlinear dynamics that are important for understanding normal and pathologic voice production in humans.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.