Our understanding of the evolution of human speech has been expanded by an increased knowledge of vocal anatomy and physiology in non-human primates. Comparative approaches provide evidence supporting the primate origins of many speech faculties. The descent of the larynx enables the two-tube configuration of the supralaryngeal vocal tract (SVT) in humans; however, this configuration is also found in chimpanzees and macaques. The acoustic properties of voices produced in helium gas support the view that vocalizations are usually produced through SVT resonance, with the sound source generated by vibration of the vocal folds in gibbons and marmosets, as seen in human speech. Nonhuman primates produce a wider range of vocal repertoire than previously thought, reflecting their varied manipulations of the vocal apparatus to modify SVT topology. These species often actively descend the hyoid and larynx to produce calls. This ‘active’ descent is one of the options for SVT modification in non-human primates. However, this is distinct from human speech, where a ‘static’ descended larynx moves in a restricted range during speech. Instead, humans modify SVT configuration by combinations of contraction and relaxation of the tongue muscles, to produce their vocal acoustics. The components of the vocal apparatus act under the constraint of anatomy, and various associations of anatomy and vocal actions are expected to be found in a variety of types of vocalization in non-human primates. Increasing knowledge of their anatomy and physiology promises better understanding of primate origins and of the evolutionary history of physical faculties in human speech.

The study of sound production mechanisms is a crucial, yet understudied, aspect of vocal communication research in vertebrates. In excised larynx experimentation (ELE), phonation is simulated ex vivo by forcing air through a larynx specimen mounted on a laboratory bench. The method provides unique insights into vocal production and allows inference of in vivo conditions. Here, we provide a historical overview of how this technique has been implemented, from antiquity to current state-of-the-art setups. We review the advances made by applying ELE to human voice and biophysics research. We then highlight the promising research output resulting from ELE in animal bioacoustics, a research field that has largely overlooked the use of this method until very recently, but is now increasingly relying on this tool. We continue by discussing the limitations of ELE, depending on the focus of investigation. Finally, we suggest how this approach should be implemented and can be applied to various research questions. We conclude by underlining the value that ELE contributes to the comprehension of human voice as well as mammalian and avian vocal communication within an interdisciplinary approach.

Electroglottography (EGG) is a low-cost, non-invasive method for documenting laryngeal sound production during vocalization. The EGG signal represents relative vocal fold contact area and thus delivers physiological evidence of vocal fold vibration. While the method has received much attention in human voice research over the last five decades, it has seen very little application in other mammals. Here, we give a concise overview of mammalian vocal production principles. We explain how mammalian voice production physiology and the dynamics of vocal fold vibration can be documented qualitatively and quantitatively with EGG, and we summarize and discuss key issues from research with humans. Finally, we review the limited number of studies applying EGG to non-human mammals, both in vivo and in vitro. The potential of EGG for non-invasive assessment of non-human primate vocalization is demonstrated with novel in vivo data of Cebus albifrons and Ateles chamek vocalization. These examples illustrate the great potential of EGG as a new minimally invasive tool in primate research, which can provide important insight into the ‘black box’ that is vocal production. A better understanding of vocal fold vibration across a range of taxa can provide us with a deeper understanding of several important elements of speech evolution, such as the universality of vocal production mechanisms, the independence of source and filter, the evolution of vocal control, and the relevance of non-linear phenomena.

One noteworthy, but unexplained, aspect of the evolution of human speech is the loss of laryngeal air sacs during hominin evolution. Very little is known about the adaptive significance of this curious trait, or the selection pressures that may have driven the evolution of air sacs among primates, and their later loss in Homo. Here, I review the literature on the loss of laryngeal air sacs during the evolution of speech, and argue that sexual selection may have been a key factor. Although air sacs do not fossilize, the presence or absence of air sacs appears to be correlated with the anatomy of the hyoid bone, and fossil hyoid evidence suggests that air sacs were lost in hominins between 3.3 million and 530000 years ago. Air sacs are hypothesized to have an acoustic function, and some authors have argued that hominins may have lost their air sacs because they would make speech less clear. In other primates, such as gorillas and howler monkeys, air sacs appear to play a role in acoustic size exaggeration and may be linked to reproductive competition. I explore the hypothesis that changes in social organization and mating system towards reduced male–male competition may have relaxed the selection pressure maintaining loud, low-frequency calls in hominins, making air sacs obsolete. While much of the above will remain hypothetical until more concrete data are gathered, we can speculate by saying that air sacs may not have been necessary for the type of quiet vocal interaction that typifies human communication. Perhaps more recent Homo species, with lower levels of sexual dimorphism and increased social tolerance and complexity, began to communicate in a more complex way, eventually leading to spoken language.

Animal vocalizations range from tonal sounds to irregular atonal sounds and are generated from non-linear oscillations of the vocal folds as well as from turbulent noise in the glottis. Comprehensive study on bioacoustic signals indicates the existence of a diversity of non-linear phenomena, such as limit cycles, subharmonics, biphonation, chaos, and bifurcations in animal vocalizations, which may provide keys to understanding animal communications. In this paper, we review the concept of nonlinear dynamics and its methodology as applicable to bioacoustics. Acoustical analysis of recorded sounds, simulation of a biomechanical model of the voice production system, and physical experiment of the vocal tract and vocal folds are presented to demonstrate non-linear features inherent in animal vocalizations. We focus on source–filter interaction as one of the main regulators of the non-linear property that can lead either to efficient vocalization or to voice instability in animal sounds. A combination of different approaches is suggested to be of great use for extracting the essential features of non-linear dynamics in animal vocalizations.

This study aimed to show secular changes in the cephalic index (CI) in Japan when brachycephalization was reported to have ceased, and examined possible causes of the observed changes. Head length and breadth data measured on 4034 Japanese adults, born between 1910 and 1996, were used to examine the association between birth year and CI using linear and quadratic regression analyses. Possible causes of secular changes in CI were investigated by examining the secular changes in eight body-size variables and 16 environmental indicators, obtained from the government statistics records. Results showed that a trend reversal from brachycephalization to debrachycephalization occurred among the generation born in 1960–1964, followed by trend reversals in birth length (BL), energy intake, and birth weight (BW) in that order. Previous studies showed that the reverse trend in BW was related to a reduction in fetal growth rate, which resulted from worsening fetal nutritional status, which was also attributable to decreasing maternal energy intake. Since the head measurement at birth is approximately 60% of the adult size, the main cause of debrachycephalization may be worsening fetal nutritional status. From the maternal nutritional perspective, reverse trends in CI and BL began with rapid recovery from the post-war food shortage. However, the body mass index (BMI) of 20-year-old females decreased due to the very small increase in weight despite a rapid increase in height, which was partly achieved by weight loss after the age of 17. As speculated, debrachycephalization began when the negative effects of decreasing BMI among the young adult females outweighed the positive effects of the post-war recovery of the food situation. The results of this study may help to understand the plasticity of the human body.