Descent of the hyoid in chimpanzees: evolution of face flattening and speech

The human supralaryngeal vocal tract develops to form a unique two-tube configuration with equally long horizontal and vertical cavities. This anatomy contributes greatly to the morphological foundations of human speech. It is believed to depend on the reduced growth of the palate and on the developmental descent of the larynx relative to the palate. Anatomically, the descent of the larynx is accomplished through both the descent of the laryngeal skeleton relative to the hyoid and the descent of the hyoid relative to the palate. We have studied the development of three living chimpanzees using magnetic resonance imaging. Our previous study showed that, as in humans, chimpanzees show rapid laryngeal descent, with changes in the relative proportion of the vocal tract during early infancy. However, this is not accompanied by the descent of the hyoid relative to the palate, although it is achieved with the descent of the laryngeal skeleton relative to the hyoid. Here, we show that subsequently the chimpanzee hyoid also descends to maintain the rapid descent of the larynx, similarly to humans. We argue that the descent of the larynx probably evolved in a common ancestor of extant hominoids, originally to confer an advantage via a function unrelated to speech. Thus, the descent of the larynx per se is not unique to humans, and facial flattening was probably the major factor that paved the way for speech in the human lineage.     

Comparative morphology of the hyo-laryngeal complex in anthropoids: two steps in the evolution of the descent of the larynx

The descent of the larynx is a key phenomenon not only in postnatal development, but also in the evolution of human speech. The positional change of the larynx is affected by the descent of the hyoid bone in relation to the mandible and cranial base, and that of the laryngeal framework in relation to the hyoid bone. The phylogeny of the spatial configuration of the hyo-laryngeal complex is one of the most important sources of information for elucidating the evolution of laryngeal descent. In the present study, the anatomy of the complex was examined in various species of anthropoids to compare the configuration, the shape of the basihyal and thyroid cartilage, and the length of the lateral thyrohyoid ligaments. Non-human hominoids share most features with humans, while cercopithecoids and ceboids have anatomical features that sharply contrast to humans, except for the form of the thyroid cartilage in ceboids. The laryngeal framework in hominoids is well separated from and assured of mobility independent of the hyoid. In cercopithecoids and ceboids, it is, by contrast, locked into and tied tightly with the hyoid so that the hyo-laryngeal complex acts as a functional unit. This spatial configuration is considered to be significantly related to the mechanism that prevents aspiration, including epiglottic movement and vestibular closure. Non-human hominoids are inferred to share the mechanism with human adults, not with cercopithecoids and ceboids, although their larynx is located as high as the latter. Consequently, it is hypothesized that the descent of the larynx evolved in two steps. The first step would have been descent of the thyroid in relation to the hyoid for the evolution of the mechanism preventing aspiration, which occurred in the common ancestor of hominoids. The second step, descent of the hyoid within the neck, occurred during hominid evolution for human speech. Electronic Publication     

The postnatal ontogeny of the sexually dimorphic vocal apparatus in goitred gazelles (Gazella subgutturosa)

This study quantitatively documents the progressive development of sexual dimorphism of the vocal organs along the ontogeny of the goitred gazelle (Gazella subgutturosa). The major, male‐specific secondary sexual features, of vocal anatomy in goitred gazelle are an enlarged larynx and a marked laryngeal descent. These features appear to have evolved by sexual selection and may serve as a model for similar events in male humans. Sexual dimorphism of larynx size and larynx position in adult goitred gazelles is more pronounced than in humans, whereas the vocal anatomy of neonate goitred gazelles does not differ between sexes. This study examines the vocal anatomy of 19 (11 male, 8 female) goitred gazelle specimens across three age‐classes, that is, neonates, subadults and mature adults. The postnatal ontogenetic development of the vocal organs up to their respective end states takes considerably longer in males than in females. Both sexes share the same features of vocal morphology but differences emerge in the course of ontogeny, ultimately resulting in the pronounced sexual dimorphism of the vocal apparatus in adults. The main differences comprise larynx size, vocal fold length, vocal tract length, and mobility of the larynx. The resilience of the thyrohyoid ligament and the pharynx, including the soft palate, and the length changes during contraction and relaxation of the extrinsic laryngeal muscles play a decisive role in the mobility of the larynx in both sexes but to substantially different degrees in adult females and males. Goitred gazelles are born with an undescended larynx and, therefore, larynx descent has to develop in the course of ontogeny. This might result from a trade‐off between natural selection and sexual selection requiring a temporal separation of different laryngeal functions at birth and shortly after from those later in life. J. Morphol. 277:826–844, 2016. © 2016 Wiley Periodicals, Inc.     

Developmental changes in the shape of the supralaryngeal vocal tract in chimpanzees

The hyoid bone and larynx in human neonates are positioned as high as in other mammals. However, during postnatal life, they descend relative to the hard palate more rapidly compared with the horizontal growth of the oral cavity. This process is completed through the descent of the laryngeal skeleton relative to the hyoid, and through the descent of the hyoid relative to the cranial base. Thus, the human supralaryngeal vocal tract (SVT) develops to form a two-tube configuration with equally long horizontal and vertical parts. Longitudinal studies on living chimpanzee infants show that the descent of the larynx is more rapid than the horizontal growth of the oral cavity. This is primarily attributed to the descent of their larynges relative to the hyoid bone, but this is not accompanied by the descent of the hyoid. The present study, using embalmed specimens of chimpanzees, also shows that the horizontal and vertical parts of the SVT grow in chimpanzees similarly to humans during infancy. However, in chimpanzees, the horizontal part of the SVT grows greatly, whereas the vertical part of the SVT grows only slightly during the juvenile period. As a result, the chimpanzee larynx does not descend rapidly relative to the oral elongation during that period. Such differences may be related to the structural and morphological development of the facial skeleton and mandible, which affects prognathism and hyoid descent. These results support the hypothesis that the descent of the larynx evolved in at least two steps during hominoid evolution.     

Development of the supralaryngeal vocal tract in Japanese macaques: implications for the evolution of the descent of the larynx

The configuration of the supralaryngeal vocal tract depends on the nonuniform growth of the oral and pharyngeal portion. The human pharynx develops to form a unique configuration, with the epiglottis losing contact with the velum. This configuration develops from the great descent of the larynx relative to the palate, which is accomplished through both the descent of the laryngeal skeleton relative to the hyoid and the descent of the hyoid relative to the palate. Chimpanzees show both processes of laryngeal descent, as in humans, but the evolutionary path before the divergence of the human and chimpanzee lineages is unclear. The development of laryngeal descent in six living Japanese macaque monkeys, Macaca fuscata, was examined monthly during the first three years of life using magnetic resonance imaging, to delineate the present or absence of these two processes and their contributions to the development of the pharyngeal topology. The macaque shows descent of the hyoid relative to the palate, but lacks the descent of the laryngeal skeleton relative to the hyoid and that of the EG from the VL. We argue that the former descent is simply a morphological consequence of mandibular growth and that the latter pair of descents arose in a common ancestor of extant hominoids. Thus, the evolutionary path of the great descent of the larynx is likely to be explained by a model comprising multiple and mosaic evolutionary pathways, wherein these developmental phenomena may have contributed secondarily to the faculty of speech in the human lineage.     

In reference to phonation larynx fixation: computer graphic record

The vocal apparatus serves phonation. It represents a biocybernetic self-regulating system, disposing of a feedback network of the central nervous system. The larynx is a self-induced vibrating system. The larynx, functioning as the phonation apparatus of the vocal apparatus, is a source of human voice. In every individual its frequency range corresponds to about eight semitones in speech and about two octaves of the so-called chest register in singing, denoted also as a thoracic or modal voice. This is followed by one more octave of the so-called cranial register or falsetto voice. We were interested in changes of the larynx positions at intonation in the fundamental singing registers, both modal and falsetto, in professional male singers. At our disposal were 11 professional male singers. We investigated changes in the position of the laryngeal structures simultaneously with the aid of an X-ray apparatus, the acoustic and mechanical signals registered by means of the B & K 4369 acceleration recorder. It has been found that at phonation with the modal voice a change in the position of the laryngeal structures takes place in two different ways, whereas the larynx movements at falsetto remain the same. It has been suggested that a complex fixation apparatus participates in the phonation larynx movements. Of the same complex character are also the problems connected with the examination of the entire vocal apparatus. For the purpose of compiling the present pieces of knowledge in the field of human voice studies, we have made the most advantageous use of the presently most complex system Authorware for the production of some interactive multimedial programmes on personal computers.     

Dominant frequency of loud mew calls of felids (Mammalia: Carnivora) decreases during ontogenetic growth

• 1 A negative correlation between body weight and frequency characteristics of a species' vocalizations exists in mammals, due to the acoustics of vocal sound production (‘source‐filter theory’; source = larynx; filter = supralaryngeal vocal tract) and the strong positive correlation between body weight and vocal tract length.
• 2 A negative correlation is hypothesized to exist between increasing body weight and frequency characteristics of calls during ontogeny as well.
• 3 This hypothesis is tested for mean dominant frequency (maximum spectral energy peak) of intense mew calls in juveniles of five species of the Felidae: lion Panthera leo, jaguar Panthera onca, leopard Panthera pardus, tiger Panthera tigris and puma Puma concolor.
• 4 In the five felid species in which the hyoid is incompletely ossified (genera Panthera and Uncia), the larynx undergoes a considerable ontogenetic descent, resulting in a proportionally longer vocal tract in adult individuals than in all other species of the family, which have a fully ossified hyoid without a descent.
• 5 In all five species studied here, mean dominant frequency decreases as body weight increases during growth. In the four Panthera species (with laryngeal descent) dominant frequency is determined by the vocal tract (the filter), and dominant frequency is largely similar at similar weights, indicating a similar correlation between the ontogenetic increase in body weight (and vocal tract length) and the decrease in mean dominant frequency. In the puma (without laryngeal descent) dominant frequency is determined by the larynx (the source), it is considerably higher than in the Panthera species, and the course of its ontogenetic decrease differs considerably from that in Panthera.
• 6 The data do not support a uniform scaling relationship between body weight and mean dominant frequency of intense mew calls in the Felidae during ontogenetic growth.

     

Sexual Dimorphism of the Larynx of the Mongolian Gazelle (Procapra gutturosa Pallas, 1777) (Mammalia, Artiodactyla, Bovidae)

The larynges (except for the epiglottis) of two adult Mongolian gazelles, one male and one female, were dissected. This species is characterized by a pronounced sexual dimorphism of the larynx. Dimorphism with regard to the size of the entire larynx and of the thyroid cartilage is about 2:1 whereas the difference of mean body mass is about 1.3:1 between males and females. Unexpectedly, and in contrast to other bovids, the larynx of the male Mongolian gazelle has a paired lateral laryngeal ventricle. However, in contrast to horse, dog, pig and many primate species also possessing such a paired ventricle, its rostral opening in the Mongolian gazelle is situated lateral to the corniculate process of the arytenoid cartilage. The neck of the laryngeal ventricle is embraced by the bifurcated cuneiform process of the epiglottis. Despite the enlarged laryngeal cartilages, the vocal process of the male arytenoid cartilage is relatively shorter than that of the female. The male thyroarytenoid muscle is clearly separated into a rostral ventricular muscle and a caudal vocal muscle whereas the female's, as in other bovids, is almost uniform. The lateral sac of the two-chambered laryngeal ventricle in the male projects laterally between the ventricular and the vocal muscle. As in the domestic bovids and in many other artiodactyls the larynx of the male Mongolian gazelle is lacking any rostrally directed membraneous portion of the vocal fold. Instead, the thick and tough bow-like vocal fold projects caudally into the infraglottic cavity and is supported by a peculiar pan-like fibroelastic pad. This resilient element, situated medial to the bipartite thyroarytenoid muscle, might be a homologue of the vocal ligament, eventually including lateral portions of the elastic cone. A fibroelastic pad is absent in the female. The resilient floor of the laryngeal vestibulum, ventral to the fibroelastic pad, is rostrally and caudally subducted by tube-like spaces. Evolutionary enlargement of the male larynx, including the vocal folds, and of the caudal portions of the vocal tract may have shifted the fundamental and formant frequencies to a lower register. The paired lateral laryngeal ventricle might produce an amplitude increase of the vocalizations assisted by differential action of the bipartite thyroarytenoid muscle. In addition, the peculiar shape, size and tough consistency of the male vocal folds may, as in roaring felids, assist in producing high amplitude and low frequency vocalizations. Perhaps the biological role of the enlarged male larynx of Procapra gutturosa has evolved in relation to its mating system. In the rutting season, dominant males establish individual territories and maintain harems. During prolonged courtship prior to mating, these males perform an acoustic display uttering loud and guttural bellows. In addition, the bulging ventral neck region of males may serve as an optical attractant for the females. Thus, the evolution of the enlarged larynx of the male Mongolian gazelle may have been favoured by sexual selection.     

Primate vocal communication: a useful tool for understanding human speech and language evolution?

Language is a uniquely human trait, and questions of how and why it evolved have been intriguing scientists for years. Nonhuman primates (primates) are our closest living relatives, and their behavior can be used to estimate the capacities of our extinct ancestors. As humans and many primate species rely on vocalizations as their primary mode of communication, the vocal behavior of primates has been an obvious target for studies investigating the evolutionary roots of human speech and language. By studying the similarities and differences between human and primate vocalizations, comparative research has the potential to clarify the evolutionary processes that shaped human speech and language. This review examines some of the seminal and recent studies that contribute to our knowledge regarding the link between primate calls and human language and speech. We focus on three main aspects of primate vocal behavior: functional reference, call combinations, and vocal learning. Studies in these areas indicate that despite important differences, primate vocal communication exhibits some key features characterizing human language. They also indicate, however, that some critical aspects of speech, such as vocal plasticity, are not shared with our primate cousins. We conclude that comparative research on primate vocal behavior is a very promising tool for deepening our understanding of the evolution of human speech and language, but much is still to be done as many aspects of monkey and ape vocalizations remain largely unexplored.     

The anatomy, physiology, acoustics and perception of speech: essential elements in analysis of the evolution of human speech

Inferences on the evolution of human speech based on anatomical data must take into account its physiology, acoustics and perception. Human speech is generated by the supralaryngeal vocal tract (SVT) acting as an acoustic filter on noise sources generated by turbulent airflow and quasi-periodic phonation generated by the activity of the larynx. The formant frequencies, which are major determinants of phonetic quality, are the frequencies at which relative energy maxima will pass through the SVT filter. Neither the articulatory gestures of the tongue nor their acoustic consequences can be fractionated into oral and pharyngeal cavity components. Moreover, the acoustic cues that specify individual consonants and vowels are “encoded”, i.e., melded together. Formant frequency encoding makes human speech a vehicle for rapid vocal communication. Non-human primates lack the anatomy that enables modern humans to produce sounds that enhance this process, as well as the neural mechanisms necessary for the voluntary control of speech articulation. The specific claims of Duchin (1990) are discussed.     

The Evolution of Language: A Comparative Review

For many years the evolution of language has been seen as a disreputable topic, mired in fanciful “just so stories” about language origins. However, in the last decade a new synthesis of modern linguistics, cognitive neuroscience and neo-Darwinian evolutionary theory has begun to make important contributions to our understanding of the biology and evolution of language. I review some of this recent progress, focusing on the value of the comparative method, which uses data from animal species to draw inferences about language evolution. Discussing speech first, I show how data concerning a wide variety of species, from monkeys to birds, can increase our understanding of the anatomical and neural mechanisms underlying human spoken language, and how bird and whale song provide insights into the ultimate evolutionary function of language. I discuss the “descended larynx” of humans, a peculiar adaptation for speech that has received much attention in the past, which despite earlier claims is not uniquely human. Then I will turn to the neural mechanisms underlying spoken language, pointing out the difficulties animals apparently experience in perceiving hierarchical structure in sounds, and stressing the importance of vocal imitation in the evolution of a spoken language. Turning to ultimate function, I suggest that communication among kin (especially between parents and offspring) played a crucial but neglected role in driving language evolution. Finally, I briefly discuss phylogeny, discussing hypotheses that offer plausible routes to human language from a non-linguistic chimp-like ancestor. I conclude that comparative data from living animals will be key to developing a richer, more interdisciplinary understanding of our most distinctively human trait: language.     

Some aspects of dimorphism and human speech

The production of human speech involves thre supralaryngeal airway filtering the acoustic energy generated by either laryngeal or noise sources. Both the supralaryngeal vocal tract and the larynx are subject to sexual dimorphism in anatomically modernHomo sapiens, yielding acoustic signals that can convey a speaker’s sex. Formant frequencies can be lower because the adult male supralaryngeal vocal tract is often longer. The fundamental frequency of phonation also can be lower because of the enlargement of the thyroid cartilage of the larynx and concomitant increases in the length and mass of the vocal cords. Secondary, learned supralaryngeal vocal tract maneuvers can also lower or raise formant frequencies. These acoustic effects yield male-female «microphonetic» dialects that are ultimately related to the average dimorphism found in the adult population. The dimorphism anatomy of the supralaryngeal vocal tract and larynx of fossil hominoids can be inferred from their skulls.     

Mobile larynx in Mongolian gazelle: Retraction of the larynx during rutting barks in male Mongolian gazelle (Procapra gutturosa Pallas, 1777)

This study provides the first evidence of pronounced temporary laryngeal descent in a bovid species. An elaborate acoustic display is prominent in male courtship behavior of polygynous Mongolian gazelle. During rut, rounding up of females is accompanied by continuous head‐up barking by dominant males. Throughout the rut their evolutionarily enlarged larynx descends to a low mid‐neck resting position. In the course of each bark the larynx is additionally retracted toward the sternum by 30% of the resting vocal tract length. A geometric model of active larynx movements was constructed by combining results of video documentation, dissection, skeletonization, and behavioral observation. The considerable distance between resting position and maximal laryngeal descent suggests a backward tilting of the hyoid apparatus and an extension of the thyrohyoid connection during the retraction phase. Return to the resting position is effected by strap muscles and by the elastic recoil of the pharynx and the thyrohyoid connection. An intrapharyngeal inflation of the peculiar palatinal pharyngeal pouch of adult males is inferred from a short‐time expansion of the ventral neck region rostral to the laryngeal prominence. The neck of adult dominant males is accentuated by long gray guard hairs during the rut. The passive swinging of the heavy larynx of adult males during locomotion gives the impression of a handicap imposed on rutting males. Apparently, this disadvantage becomes outweighed by the profits for reproductive success. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Retraction of the mobile descended larynx during groaning enables fallow bucks (Dama dama) to lower their formant frequencies

A permanently descended larynx is found in humans and several other species of mammals. In addition to this, the larynx of species such as fallow deer is mobile and in males it can be retracted during vocalization. The most likely explanation for the lowered retractable larynx in mammals is that it serves to exaggerate perceived body size (size exaggeration hypothesis) by decreasing the formant frequencies of calls. In this study, we quantified for the first time the elongation of the vocal tract in fallow bucks during vocalization. We also measured the effect of this vocal tract length (VTL) increase on formant frequencies (vocal tract resonances) and formant dispersion (spacing of formants). Our results show that fallow bucks increase their VTL on average by 52% during vocalization. This elongation resulted in strongly lowered formant frequencies and decreased formant dispersion. There were minimal changes to formants 1 and 2 (−0.91 and +1.9%, respectively) during vocal tract elongation, whereas formants 3, 4 and 5 decreased substantially: 18.9, 10.3 and 13.6%, respectively. Formant dispersion decreased by 12.4%. Formants are prominent in deer vocalizations and are used by males to gain information on the competitive abilities of signallers. It remains to be seen whether females also use the information that formants contain for assessing male quality before mating.     

Monkeys and humans share a common computation for face/voice integration

Speech production involves the movement of the mouth and other regions of the face resulting in visual motion cues. These visual cues enhance intelligibility and detection of auditory speech. As such, face-to-face speech is fundamentally a multisensory phenomenon. If speech is fundamentally multisensory, it should be reflected in the evolution of vocal communication: similar behavioral effects should be observed in other primates. Old World monkeys share with humans vocal production biomechanics and communicate face-to-face with vocalizations. It is unknown, however, if they, too, combine faces and voices to enhance their perception of vocalizations. We show that they do: monkeys combine faces and voices in noisy environments to enhance their detection of vocalizations. Their behavior parallels that of humans performing an identical task. We explored what common computational mechanism(s) could explain the pattern of results we observed across species. Standard explanations or models such as the principle of inverse effectiveness and a "race" model failed to account for their behavior patterns. Conversely, a "superposition model", positing the linear summation of activity patterns in response to visual and auditory components of vocalizations, served as a straightforward but powerful explanatory mechanism for the observed behaviors in both species. As such, it represents a putative homologous mechanism for integrating faces and voices across primates.     

On human speech,syntax, and language

The evolution of human speech and syntax, which appear to be the defining characteristics of modern human beings, is discussed. Speech depends on the morphology of the mouth, tongue, and larynx which yield the human «vocal tract», and neural mechanisms that facilitate the perception of speech and make possible the control of the articulatory gestures that underly speech. The neural mechanisms that underly human syntax may have derived by means of the Darwinian process of preadaption from the structures of the brain that first evolved to facilitate speech motor control. Recent data consistent with this theory are presented; deficits in the comprehension of syntax of normal aged people are correlated with a slowdown in speech rate.     

Language out of Music: The Four Dimensions of Vocal Learning

A growing consensus drawing on research in a wide variety of disciplines has, over the last fifteen years or so, argued the need to revisit Darwin's conjecture of 1871 that language may be descended from an existing, musical medium of communication that developed from animal calls. This paper seeks to examine, in an extension of Hockett's analysis of the design features required for linguistic communication, the nature of the acoustic information produced and perceived in human vocalisation, and to consider the anatomical and neural mechanisms on which these depend. An attempt is made to sketch an evolutionary chronology for key prerequisites of human orality. Cross‐species comparisons are employed to illuminate the role of four acoustic variables (pitch, duration, amplitude and timbre), viewing the potential for human vocal productivity from the perspective of animal communication. Although humans are the only species to combine entrainment to pulse with attunement to precisely‐tracked pitches, we also depend both for musical interaction and the production and perception of vowel sounds on precise and conscious control of the property of timbre. Drawing on, amongst others, Scherer's analyses of emotionally triggered sounds in a variety of species, and Fernald's presentation of the similarities of infant cries and adult production of infant‐directed speech in a variety of cultures and languages, a case is made for the instinctive components of human communication being more music‐like than language‐like. In conclusion, historical and comparative data are employed to outline the adaptive and exaptive sequence by which human vocal communication evolved. The roles of selective pressures that conform to different adaptive models are compared—natural selection, sexual selection, group selection—leading to the proposal that all of these must have played their part at different stages in the process in a ‘mosaic’ model consistent with the development of other human traits.     

La parole à la portée du conduit vocal de l'homme de Neandertal. Nouvelles recherches,nouvelles perspectives

Investigations about the origin of articulated language result in different interpretations dealing with the phonetic capacity of our ancestors, namely after the discovery of the Neandertals. According to anatomic arguments now called to question, principally the position of the larynx as regard to the basis of skull, some authors claimed that these fossil humans could not be endowed with speech. From a new reconstruction of the estimated position of the larynx and the vocal tract, articulatory simulations were undertaken in order to propose some potential vocalic [i a u] prototypes for Neandertals. And we can show Neandertals could pronounce vowels as differentiated as those of modern humans. To cite this article: J.-L. Heim et al., C. R. Palevol 1 (2002) 129–134.     

Speech motor control

The development of new measurement techniques and improved models of the larynx and the vocal tract have significantly advanced our understanding of speech motor control. Recently, several groups have been using electromagnetic transduction techniques to record tongue movements. The laryngeal vibrations have been modeled and studied using techniques from non-linear dynamics. Computational models of supraglottal movements have been proposed and tested. A connectionist model that synthesizes the results obtained from observing the effects of variations in rate, stress, and phonetic context on speech kinematics has recently been proposed.     

The anatomy of vocal divergence in North American Elk and European red deer

Loud and frequent vocalizations play an important role in courtship behavior in Cervus species. European red deer (Cervus elaphus) produce low‐pitched calls, whereas North American elk (Cervus canadensis) produce high‐pitched calls, which is remarkable for one of the biggest land mammals. Both species engage their vocal organs in elaborate maneuvers but the precise mechanism is unknown. Vocal organs were compared by macroscopic and microscopic dissection. The larynx is sexually dimorphic in red deer but not in elk. The laryngeal lumen is more constricted in elk, and narrows further during ontogeny. Several elements of the hyoid skeleton and two of four vocal tract segments are longer in red deer than in elk allowing greater vocal tract expansion and elongation. We conclude that elk submit the larynx and vocal tract to much higher tension than red deer, whereby, enormously stressed vocal folds of reduced effective length create a high resistance glottal source. The narrow, high impedance laryngeal vestibulum matches glottal and vocal tract impedance allowing maximum power transfer. In red deer longer and relaxed vocal folds create a less resistant glottal source and a wider vestibulum matches the low glottal impedance to the vocal tract, thereby also ensuring maximum power transfer. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.