Domestic cat larynges can produce purring frequencies without neural input

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'.

Vocal registers expand signal diversity in vertebrate vocal communication

Among air-breathing tetrapods, the most common sound production mechanism is flow-induced self-sustained tissue oscillation, aka voiced sound production, driven by inherently nonlinear physical processes. Some signature features like deterministic chaos have received particular attention in bioacoustics as nonlinear phenomena (NLP). However, one type of NLP that extends frequency ranges and enriches timbres has received much less focus in comparative bioacoustics: vocal registers. Controlled by muscle activity, vocal registers constitute distinct periodic vibratory states of vocal tissues. Transitions between vocal registers often lead to abrupt fundamental frequency jumps, which are, e.g., deliberately used in human alpine yodelling, for example. Theoretical work suggests that register transitions are caused by saddle-node-in-limit-cycle bifurcations. Here, we review the biophysical underpinnings of vocal registers and what signatures they leave in vocal fold kinematics and acoustics in the best studied species: humans. Apart from human speech and song, registers have been described only in a few animal taxa, but the occurrence of signature features suggests that vocal registers could be much more common across vertebrates than currently appreciated. We suggest that registers are a fundamental trait of voice production and that they are favoured in selection because they vastly extend and diversify the acoustic signalling space. 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'.

A New, Easy-to-Learn, Fear-Free Method to Stop Purring During Cardiac Auscultation in Cats

BACKGROUND

Purring in cats can interfere with cardiac auscultation. If the produced noise is loud enough, purring makes it impossible to perform a meaningful auscultation as it is much louder than heart sounds and murmurs. Our study introduced and tested a new, simple, fear-free, cat-friendly method to stop purring during auscultation.

METHODS

The technique involves grasping the cat's larynx from ventral with one hand, while simultaneously holding the stethoscope in the other hand to perform the auscultation.

RESULTS

The incidence of purring was evaluated in 582 cats, in a veterinary teaching hospital and in a cat-friendly private practice. Fifty-one (8.8%) cats were purring during their physical examination. The tested method had a success rate of 89% in terminating purring. A comparison between investigators (a veterinary student versus an experienced veterinary cardiology specialist) showed no significant difference in the effectiveness of the method (p = 0.57). The incidence of purring was not significantly different between the teaching hospital and the cat-friendly practice (p = 1.00). Sick and older cats purred more often than healthy and younger cats.

CONCLUSIONS

This new, simple, easy-to-master method is an improvement over previously reported techniques and supports the need for stress-free, cat-friendly handling in veterinary practice.

Vocal communication: The enigmatic production of low-frequency purrs in cats

Cat purring, the unusual, pulsed vibration that epitomizes comfort, enjoys a special status in the world of vocal communication research. Indeed, it has long been flagged as a rare exception to the dominant theory of voice production in mammals. A new study presents histological and biomechanical evidence that purring can occur passively, without needing muscle vibration in the larynx controlled by an independent neural oscillator.