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Formants, or resonance frequencies of the upper vocal tract, are an essential part of acoustic communication. Articulatory gestures—such as jaw, tongue, lip, and soft palate movements—shape formant structure in human vocalizations, but little is known about how nonhuman mammals use those gestures to modify formant frequencies. Here, we report a case study with an adult male harbor seal trained to produce an arbitrary vocalization composed of multiple repetitions of the sound wa. We analyzed jaw movements frame‐by‐frame and matched them to the tracked formant modulation in the corresponding vocalizations. We found that the jaw opening angle was strongly correlated with the first (F1) and, to a lesser degree, with the second formant (F2). F2 variation was better explained by the jaw angle opening when the seal was lying on his back rather than on the belly, which might derive from soft tissue displacement due to gravity. These results show that harbor seals share some common articulatory traits with humans, where the F1 depends more on the jaw position than F2. We propose further in vivo investigations of seals to further test the role of the tongue on formant modulation in mammalian sound production. [ABSTRACT FROM AUTHOR]

Variation in formant frequencies has been shown to affect social interactions and sexual competition in a range of avian species. Yet, the anatomical bases of this variation are poorly understood. Here, we investigated the morphological correlates of formants production in the vocal apparatus of African penguins. We modelled the geometry of the supra-syringeal vocal tract of 20 specimens to generate a population of virtual vocal tracts with varying dimensions. We then estimated the acoustic response of these virtual vocal tracts and extracted the centre frequency of the first four predicted formants. We demonstrate that: (i) variation in length and cross-sectional area of vocal tracts strongly affects the formant pattern, (ii) the tracheal region determines most of this variation, and (iii) the skeletal size of penguins does not correlate with the trachea length and consequently has relatively little effect on formants. We conclude that in African penguins, while the variation in vocal tract geometry generates variation in resonant frequencies supporting the discrimination of conspecifics, such variation does not provide information on the emitter's body size. Overall, our findings advance our understanding of the role of formant frequencies in bird vocal communication. [ABSTRACT FROM AUTHOR]

Animal vocalisations encode a wide range of biological information about the age, sex, body size, and social status of the emitter. Moreover, vocalisations play a significant role in signalling the identity of the emitter to conspecifics. Recent studies have shown that, in the African penguin (Spheniscus demersus), acoustic cues to individual identity are encoded in the fundamental frequency (F0) and resonance frequencies (formants) of the vocal tract. However, although penguins are known to produce vocalisations where F0 and formants vary among individuals, it remains to be tested whether the receivers can perceive and use such information in the individual recognition process. In this study, using the Habituation-Dishabituation (HD) paradigm, we tested the hypothesis that penguins perceive and respond to a shift of ± 20% (corresponding to the natural inter-individual variation observed in ex-situ colonies) of F0 and formant dispersion (ΔF) of species-specific calls. We found that penguins were more likely to look rapidly and for longer at the source of the sound when F0 and formants of the calls were manipulated, indicating that they could perceive variations of these parameters in the vocal signals. Our findings provide the first experimental evidence that, in the African penguin, listeners can perceive changes in F0 and formants, which can be used by the receiver as potential cues for the individual discrimination of the emitter. [ABSTRACT FROM AUTHOR]

Source–filter theory posits that an individual's size and vocal tract length are reflected in the parameters of their calls. In species that mate assortatively, this could result in vocal similarity. In the context of mate selection, this would mean that animals could listen in to find a partner that sounds—and therefore is—similar to them. We investigated the social calls of the little auk (Alle alle), a highly vocal seabird mating assortatively, using vocalizations produced inside 15 nests by known individuals. Source- and filter-related acoustic parameters were used in linear mixed models testing the possible impact of body size. A principal component analysis followed by a permuted discriminant function analysis tested the effect of sex. Additionally, randomization procedures tested whether partners are more vocally similar than random birds. There was a significant effect of size on the mean fundamental frequency of a simple call, but not on parameters of a multisyllable call with apparent formants. Neither sex nor partnership influenced the calls—there was, however, a tendency to match certain parameters between partners. This indicates that vocal cues are at best weak indicators of size, and other factors likely play a role in mate selection.

Features varying more between than within individuals are considered as potential cues for individual recognition. According to the source‐filter theory of vocal production, the fundamental frequency of mammals' vocalizations depends on the characteristics of vocal folds, while Formants are determined by the characteristics of the vocal tract. Goat mothers and their kids (Capra hircus) display mutual recognition, and both source‐related parameters (f0) and filter‐related ones (Formants) have been shown to be individualized. Here, we aimed to identify if f0 and Formants are used by goat kids to recognize their mother's vocalizations. To do this, we independently modified these parameters in calls of goat mothers to different degrees (within or exceeding the range of natural intra‐individual variability), and we played back these modified calls to their kids. We found no effect of f0 or Formants modification on the kids' reactions. Further analyses revealed that goat kids emitted fewer calls when modifications to f0 resulted in higher values of the first energy quartile, suggesting a role of the distribution of energy in the spectrum in maternal recognition. We propose that either: (i) f0 and Formants are not involved in goats' maternal recognition; (ii) goat kids have a tolerance for variation when recognizing their mother's calls that exceeds the performed shifts in these parameters; or (iii) goat kid maternal recognition is based on a combination of features and might be more flexible than previously thought, such that when one feature is modified, kids focus on other features. The effect of the spectral energy distribution modification on the kid responses, which depends both on f0 and Formant heights, suggests that (iii) is a likely explanation. Our findings support the hypothesis of complex individual acoustic recognition from the early stages of development in ungulates. [ABSTRACT FROM AUTHOR]

Simple Summary: Whether a vocalization is emitted though the mouth or nose impacts on its acoustic shape through specific vocal tract resonances. In human language, nasalized compared to oral vowels can change the meaning of words. African elephants mainly use low-frequency rumbles to communicate and can utter them either through the mouth or trunk. In this study, we used an acoustic camera to visualize the sound emission of rumbles in Asian elephants, which have been studied considerably less than African elephants. We recorded nine captive female Asian elephants and analyzed the acoustic structure of 203 calls. We found that most rumbles (64%) were emitted through the trunk, 21% through the mouth, and surprisingly, 13% simultaneously through the mouth and trunk. Some of the rumbles were combined with orally emitted roars. The nasal rumbles concentrated most spectral energy in lower frequencies, whereas the oral and mixed rumbles showed a broader spectral energy distribution and concentration in higher frequencies, and were louder. The roars were the loudest, broadest and highest in frequency. This study is the first to demonstrate coupled oral and nasal emission in a non–human animal, thereby setting an important framework to study the functions of this acoustic variability in elephant communication and the evolution of vocal flexibility across species. Sound production mechanisms set the parameter space available for transmitting biologically relevant information in vocal signals. Low–frequency rumbles play a crucial role in coordinating social interactions in elephants' complex fission–fusion societies. By emitting rumbles through either the oral or the three-times longer nasal vocal tract, African elephants alter their spectral shape significantly. In this study, we used an acoustic camera to visualize the sound emission of rumbles in Asian elephants, which have received far less research attention than African elephants. We recorded nine adult captive females and analyzed the spectral parameters of 203 calls, including vocal tract resonances (formants). We found that the majority of rumbles (64%) were nasally emitted, 21% orally, and 13% simultaneously through the mouth and trunk, demonstrating velopharyngeal coupling. Some of the rumbles were combined with orally emitted roars. The nasal rumbles concentrated most spectral energy in lower frequencies exhibiting two formants, whereas the oral and mixed rumbles contained higher formants, higher spectral energy concentrations and were louder. The roars were the loudest, highest and broadest in frequency. This study is the first to demonstrate velopharyngeal coupling in a non-human animal. Our findings provide a foundation for future research into the adaptive functions of the elephant acoustic variability for information coding, localizability or sound transmission, as well as vocal flexibility across species. [ABSTRACT FROM AUTHOR]

Eurasian deer are characterized by the extraordinary diversity of their vocal repertoires. Male sexual calls range from roars with relatively low fundamental frequency (hereafter fo) in red deer Cervus elaphus, to moans with extremely high fo in sika deer Cervus nippon, and almost infrasonic groans with exceptionally low fo in fallow deer Dama dama. Moreover, while both red and fallow males are capable of lowering their formant frequencies during their calls, sika males appear to lack this ability. Female contact calls are also characterized by relatively less pronounced, yet strong interspecific differences. The aim of this study is to examine the anatomical bases of these inter‐specific and inter‐sexual differences by identifying if the acoustic variation is reflected in corresponding anatomical variation. To do this, we investigated the vocal anatomy of male and female specimens of each of these three species. Across species and sexes, we find that the observed acoustic variability is indeed related to expected corresponding anatomical differences, based on the source‐filter theory of vocal production. At the source level, low fo is associated with larger vocal folds, whereas high fo is associated with smaller vocal folds: sika deer have the smallest vocal folds and male fallow deer the largest. Red and sika deer vocal folds do not appear to be sexually dimorphic, while fallow deer exhibit strong sexual dimorphism (after correcting for body size differences). At the filter level, the variability in formants is related to the configuration of the vocal tract: in fallow and red deer, both sexes have evolved a permanently descended larynx (with a resting position of the larynx much lower in males than in females). Both sexes also have the potential for momentary, call‐synchronous vocal tract elongation, again more pronounced in males than in females. In contrast, the resting position of the larynx is high in both sexes of sika deer and the potential for further active vocal tract elongation is virtually absent in both sexes. Anatomical evidence suggests an evolutionary reversal in larynx position within sika deer, that is, a secondary larynx ascent. Together, our observations confirm that the observed diversity of vocal behaviour in polygynous deer is supported by strong anatomical differences, highlighting the importance of anatomical specializations in shaping mammalian vocal repertoires. Sexual selection is discussed as a potential evolutionary driver of the observed vocal diversity and sexual dimorphisms. [ABSTRACT FROM AUTHOR]

AbstractBased on source-filter theory, the present research attempts to investigate between- and within-speaker variability in simple vowels of Persian using experimental phonetics tools. This research aims to discover which of the simple vowels of Persian represent more speaker-specific information and which acoustic parameters can better distinguish Persian speakers. To test between- and within-speaker variability, two types of acoustic parameters, one related to the larynx, i.e. fundamental frequency, and the other related to the vocal tract, i.e. formant frequencies, were selected. Fundamental frequency as well as formant values were extracted from the steady state point of the vowels uttered by twelve Persian-speaking male speakers. Speech data were recorded non-contemporaneously in laboratory environment on two different occasions separated by one to two weeks, thereby allowing for analyzing occasion-to occasion within-speaker variability. Speech tokens were acoustically measured with PRAAT version 5.2.34 and statistical analyses were carried out with SPSS version 21 and R version 3.3.3. Results of the study indicated that the low front vowel /a/ and the third formant frequency convey more speaker-specific information compared to the other vowels and formant frequencies. In addition, discriminatory power of fundamental frequency was reported to be stronger than formant frequencies. The results also revealed that fundamental frequency is correlated with the first formant frequency which is subsequently indicative of interdependence between the source and filter sections.Keywords: Acoustic phonetics, forensic phonetics, formant frequency, fundamental frequency, speaker identification, source-filter theory IntroductionVerbal communication is an integral part of human social interactions. Everyday experience tells us that humans are able to recognize easily familiar speakers through their voice. This indicates that speech sounds contain specific information which could be reflected in the acoustic characteristics of speech signals. Vowels are among those speech sounds which have always been the center of attention in the field of forensic voice comparison. In a study by Gold and French (2011), vowels have been reported as one of the most analyzed segments among forensic practitioners. They also reported that F0 and formant structures are two acoustic parameters which are commonly used in forensic voice comparison. Earlier studies on particularities of vowels were primarily focused on calculation of the average values of formant and fundamental frequency over a long stretches of a speech recordings. However, long term extraction of F0 and formant values represent solely the discriminatory power of formant structures without allowing us to measure the strengths of vowels in separation. In this study we aim to examine the discriminatory role of simple vowels in Persian with focus on the extent of source-filter independence or interdependence within the context of speaker identification. This study aims to determine which of the vowels in Persian can better distinguish speakers and which acoustic parameters of the source and filter sections represent more speaker-specificity. We also aim to examine whether source and filter features have potential in capturing complementary information about speakers that can be used to improve speaker discrimination. Materials & MethodsTo test between-and within-speaker variability, twelve male Persian speakers were recorded on two different sessions, separated by a time-lapse of one to two weeks. Speakers were asked to read the 54 sentences one by one, with a pause, and in a natural way, without any marked intonation. Speech tokens were analyzed using Praat (version 5.2.34, Boersma and Weenink, 2013). For this study, mean values of the fundamental frequency (F0) and the first four formants i.e. F1, F2, F3 and F4 were measured at the central points of six simple vowels in Persian. Statistical analysis of data was carried out using R (R core Team 2014) version 3.3.3 and SPSS (IBM Corp. 2012) version 21. Discussion of Results and ConclusionsIn this section, we provide the results of different acoustical models i.e. univariate analysis of variance, multinomial logistic regression and principal component analysis that were employed on the collected speech data of Persian. In the present study, we explored potential speaker-specific acoustic parameters of simple vowels in Persian based on the source-filter theory. Statistical analysis of speech data revealed that selected acoustic parameters i.e. F0, F1, F2, F3 and F4 of the all vowels, except for the F2 of the vowel /u/, were able to discriminate between Persian speakers. The current findings showed that the low front vowel /a/ appear to convey the highest between-speaker discrimination power. In terms of formant structures, for most vowels, effects of speaker were stronger on F3 and F4 compared to F1 and F2. Additionally, fundamental frequency was reported to be more discriminatory than formant frequencies. The results also revealed a significant correlation between F0 and F1 which show a considerable interdependence between the source and filter sections.

The stability of individual acoustic features is fundamental in social species, and more importantly in monogamous and territorial species, showing long-term fidelity both to the partner and the breeding site. In this study, the stability over time of two discrete vocal types was investigated in the African penguin (Spheniscus demersus), a monogamous and territorial seabird. Contact calls and ecstatic display songs were recorded from an ex situ colony in 2017 and in 2020. For each vocalisation, we measured 14 spectral and temporal acoustic parameters related to both source and filter components. Two separate leave-one-out cross-validated Discriminant Function Analyses (DFA) were then performed, generating the discriminant functions from the vocalisations collected in 2017 to classify those recorded in 2020. The DFA correctly classified 62% of the contact calls (10 subjects) and 80.9% of the ecstatic display songs (seven subjects) according to the correct emitter, showing that acoustic cues to individuality encoded in both vocal types remained unchanged over four consecutive breeding seasons. We suggest that, in this monogamous and territorial bird species, individual acoustic stability could be selected for to identify groupmates and neighbours over the years and to help couples to reunite in consecutive breeding seasons, increasing individual fitness. [ABSTRACT FROM AUTHOR]

Sound production mechanisms set the parameter space available for transmitting biologically relevant information in vocal signals. Low–frequency rumbles play a crucial role in coordinating social interactions in elephants’ complex fission–fusion societies. By emitting rumbles through either the oral or the three-times longer nasal vocal tract, African elephants alter their spectral shape significantly. In this study, we used an acoustic camera to visualize the sound emission of rumbles in Asian elephants, which have received far less research attention than African elephants. We recorded nine adult captive females and analyzed the spectral parameters of 203 calls, including vocal tract resonances (formants). We found that the majority of rumbles (64%) were nasally emitted, 21% orally, and 13% simultaneously through the mouth and trunk, demonstrating velopharyngeal coupling. Some of the rumbles were combined with orally emitted roars. The nasal rumbles concentrated most spectral energy in lower frequencies exhibiting two formants, whereas the oral and mixed rumbles contained higher formants, higher spectral energy concentrations and were louder. The roars were the loudest, highest and broadest in frequency. This study is the first to demonstrate velopharyngeal coupling in a non-human animal. Our findings provide a foundation for future research into the adaptive functions of the elephant acoustic variability for information coding, localizability or sound transmission, as well as vocal flexibility across species.

Characterizing mechanisms of vocal production provides important insight into the ecology of acoustic divergence. In this study, we characterized production mechanisms of two types of vocalizations emitted by western harvest mice (Reithrodontomys megalotis), a species uniquely positioned to inform trait evolution because it is a sister taxon to peromyscines (Peromyscus and Onychomys spp.), which use vocal fold vibrations to produce long-distance calls, but more ecologically and acoustically similar to baiomyines (Baiomys and Scotinomys spp.), which employ a whistle mechanism. We found that long-distance calls (∼10 kHz) were produced by airflow-induced vocal fold vibrations, whereas high-frequency quavers used in close-distance social interactions (∼80 kHz) were generated by a whistle mechanism. Both production mechanisms were facilitated by a characteristic laryngeal morphology. Our findings indicate that the use of vocal fold vibrations for long-distance communication is widespread in reithrodontomyines (Onychomys, Peromyscus, Reithrodontomys spp.) despite overlap in frequency content that characterizes baiomyine whistled vocalizations. The results illustrate how different production mechanisms shape acoustic variation in rodents and contribute to ecologically relevant communication distances., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists Ltd.)

Vocalisations are commonly expressed by gregarious animals, including cattle, as a form of short- and long-distance communication. They can provide conspecifics with meaningful information about the physiology, affective state and physical attributes of the caller. In cattle, calls are individually distinct meaning they assist animals to identify specific individuals in the herd. Consequently, there is potential for these vocalisations to be acoustically analysed to make inferences about how individual animals or herds are coping with their external surroundings, and then act on these signals to improve feed conversion efficiency, reproductive efficiency and welfare. In the case of dairy farming, where herd sizes are expanding and farmers are becoming more reliant on technologies to assist in the monitoring of cattle, the study of vocal behaviour could provide an objective, cost effective and non-invasive alternative to traditional measures of welfare. The vocalisations of cattle in response to calf separation, social isolation and painful husbandry procedures, alongside changes to feeding and oestrous activity are here reviewed. For future application of sound technology, research is first necessary to analyse the acoustic structure of cattle vocalisations and determine the specific information they encode. This review draws together the latest research in field of cattle bioacoustics highlighting how the source–filter theory and affective state dimensional approach can be adopted to decode this information and improve on-farm management.

Diversification of animal vocalizations plays a key role in behavioral evolution and speciation. Vocal organ morphology represents an important source of acoustic variation, yet its small size, complex shape, and absence of homologous landmarks pose major challenges to comparative analyses. Here, we use a geometric morphometric approach based on geometrically homologous landmarks to quantify shape variation of laryngeal cartilages of four rodent genera representing three families. Reconstructed cartilages of the larynx from contrast-enhanced micro-CT images were quantified by variable numbers of three-dimensional landmarks placed on structural margins and major surfaces. Landmark sets were superimposed using generalized Procrustes analysis prior to statistical analysis. Correlations among pairwise Procrustes distances were used to identify the minimum number of landmarks necessary to fully characterize shape variation. We found that the five species occupy distinct positions in morphospace, with variation explained in part by phylogeny, body size, and differences in vocal production mechanisms. Our findings provide a foundation for quantifying the contribution of vocal organ morphology to acoustic diversification. [ABSTRACT FROM AUTHOR]

Vocal individuality is a prerequisite for individual recognition, especially when visual and chemical cues are not available or effective. Vocalizations encoding information of individual identity have been reported in many social animals and should be particularly adaptive for species living in large and complexly organized societies. Here, we examined the individuality in coo calls of adult male golden snub-nosed monkeys (Rhinopithecus roxellana) living in a large and multilevel society. Coo calls are one of the most frequently occurring call types in R. roxellana and likely serve as the signals for contact maintenance or advertisement in various contexts including group movement, foraging, and resting. From April to October 2016, April to July 2017, and September to October 2017, we recorded a total of 721 coo calls from six adult males in a provisioned, free-ranging group and one adult male in captivity in Shennongjia National Park, China. We selected 162 high-quality recordings to extract 14 acoustic parameters based on the source-filter theory. Results showed that each of all parameters significantly differed among individuals, while pairwise comparisons failed to detect any parameter that was different between all pairs. Furthermore, a discriminant function analysis indicated that the correct assignment rate was 80.2% (cross-validation: 67.3%), greater than expected by chance (14.3%). In conclusion, we found evidence that coo calls of adult male R. roxellana allowed the reliable accuracy of individual discrimination complementarily enhanced by multiple acoustic parameters. The results of our study point to the selective pressures acting on individual discrimination via vocal signals in a highly gregarious forest-living primate. [ABSTRACT FROM AUTHOR]

The source-filter theory proposes that information on caller properties is communicated through acoustic qualities, as physical state and performance ability are reflected in the voice. Vocal stability, manifested through harshness, is especially intriguing and has rarely been explored although harsh sounds are prevalent in nature. Male rock hyraxes (Procavia capensis) produce loud complex calls that we term songs. Only the calls of older, socially dominant males include a harsh sound termed snort. As snorts are the rarest element in songs, we hypothesized that high-quality snorts are difficult to produce, and that their quality consists in the ability to maintain smoothness throughout this low-pitched, harsh call. We quantified harshness by measuring periodicity deviations and expected to find a link between social parameters (residence, rank, and weight) and the ability to produce longer, smoother snorts. In addition, we presumed that if calls are used as vocal contests, conspecifics would avoid answering songs that exhibit a higher acoustic ability than their own songs. We found that in wild hyrax songs, snort harshness was associated with both weight and social rank, but in opposite directions. Heavier males produced smoother snorts and higher-ranked individuals produced harsher snorts, possibly indicating aggressiveness. Playback experiments showed that longer and harsher synthetic snorts, inserted into natural songs, reduced conspecific answer rates. Snorts may communicate complex information on hyrax weight and dominance by means of element length and harshness. Our present results provide a stimulating insight into the understanding of acoustics in mammalian vocal communication. [ABSTRACT FROM AUTHOR]

Steady-state vowels are vowels that are uttered with a momentarily fixed vocal tract configuration and with steady vibration of the vocal folds. In this steady-state, the vowel waveform appears as a quasi-periodic string of elementary units called pitch periods. Humans perceive this quasi-periodic regularity as a definite pitch. Likewise, so-called pitch-synchronous methods exploit this regularity by using the duration of the pitch periods as a natural time scale for their analysis. In this work, we present a simple pitch-synchronous method using a Bayesian approach for estimating formants that slightly generalizes the basic approach of modeling the pitch periods as a superposition of decaying sinusoids, one for each vowel formant, by explicitly taking into account the additional low-frequency content in the waveform which arises not from formants but rather from the glottal pulse. We model this low-frequency content in the time domain as a polynomial trend function that is added to the decaying sinusoids. The problem then reduces to a rather familiar one in macroeconomics: estimate the cycles (our decaying sinusoids) independently from the trend (our polynomial trend function); in other words, detrend the waveform of steady-state waveforms. We show how to do this efficiently.

Source-filter theory posits that an individual's size and vocal tract length are reflected in the parameters of their calls. In species that mate assortatively, this could result in vocal similarity. In the context of mate selection, this would mean that animals could listen in to find a partner that sounds-and therefore is-similar to them. We investigated the social calls of the little auk ( Alle alle ), a highly vocal seabird mating assortatively, using vocalizations produced inside 15 nests by known individuals. Source- and filter-related acoustic parameters were used in linear mixed models testing the possible impact of body size. A principal component analysis followed by a permuted discriminant function analysis tested the effect of sex. Additionally, randomization procedures tested whether partners are more vocally similar than random birds. There was a significant effect of size on the mean fundamental frequency of a simple call, but not on parameters of a multisyllable call with apparent formants. Neither sex nor partnership influenced the calls-there was, however, a tendency to match certain parameters between partners. This indicates that vocal cues are at best weak indicators of size, and other factors likely play a role in mate selection., Competing Interests: Authors declare no competing interests., (© 2023 The Authors.)

Vocalisations are commonly expressed by gregarious animals, including cattle, as a form of short- and long-distance communication. They can provide conspecifics with meaningful information about the physiology, affective state and physical attributes of the caller. In cattle, calls are individually distinct meaning they assist animals to identify specific individuals in the herd. Consequently, there is potential for these vocalisations to be acoustically analysed to make inferences about how individual animals or herds are coping with their external surroundings, and then act on these signals to improve feed conversion efficiency, reproductive efficiency and welfare. In the case of dairy farming, where herd sizes are expanding and farmers are becoming more reliant on technologies to assist in the monitoring of cattle, the study of vocal behaviour could provide an objective, cost effective and non-invasive alternative to traditional measures of welfare. The vocalisations of cattle in response to calf separation, social isolation and painful husbandry procedures, alongside changes to feeding and oestrous activity are here reviewed. For future application of sound technology, research is first necessary to analyse the acoustic structure of cattle vocalisations and determine the specific information they encode. This review draws together the latest research in field of cattle bioacoustics highlighting how the source–filter theory and affective state dimensional approach can be adopted to decode this information and improve on-farm management. [ABSTRACT FROM AUTHOR]

Some rodents produce ultrasonic vocalizations (USVs) for social communication using an aerodynamic whistle, a unique vocal production mechanism not found in other animals. The functional anatomy and evolution of this sound production mechanism remains unclear. Using laryngeal airway reconstruction, we identified anatomical specializations critical for USV production. A robust laryngeal cartilaginous framework supports a narrow supraglottal airway. An intralaryngeal airsac-like cavity termed the ventral pouch was present in three muroid rodents (suborder Myomorpha), but was absent in a heteromyid rodent (suborder Castorimorpha) that produces a limited vocal repertoire and no documented USVs. Small lesions to the ventral pouch in laboratory rats caused dramatic changes in USV production, supporting the hypothesis that an interaction between a glottal exit jet and the alar edge generates ultrasonic signals in rodents. The resulting undulating airflow around the alar edge interacts with the resonance of the ventral pouch, which may function as a Helmholtz resonator. The proposed edge-tone mechanism requires control of intrinsic laryngeal muscles and sets the foundation for acoustic variation and diversification among rodents. Our work highlights the importance of anatomical innovations in the evolution of animal sound production mechanisms.

Functional changes in vocal organ morphology and motor control facilitate the evolution of acoustic signal diversity. Although many rodents produce vocalizations in a variety of social contexts, few studies have explored the underlying production mechanisms. Here, we describe mechanisms of audible and ultrasonic vocalizations (USVs) produced by grasshopper mice (genus Onychomys). Grasshopper mice are predatory rodents of the desert that produce both loud, long-distance advertisement calls and USVs in closedistance mating contexts. Using live-animal recording in normal air and heliox, laryngeal and vocal tract morphological investigations, and biomechanical modelling, we found that grasshopper mice employ two distinct vocal production mechanisms. In heliox, changes in higher-harmonic amplitudes of long-distance calls indicate an airflow-induced tissue vibration mechanism, whereas changes in fundamental frequency of USVs support a whistle mechanism. Vocal membranes and a thin lamina propria aid in the production of long-distance calls by increasing glottal efficiency and permitting high frequencies, respectively. In addition, tuning of fundamental frequency to the second resonance of a bell-shaped vocal tract increases call amplitude. Our findings indicate that grasshopper mice can dynamically adjust motor control to suit the social context and have novel morphological adaptations that facilitate long-distance communication. [ABSTRACT FROM AUTHOR]

Summary The traditional source-filter theory of voice production describes a linear relationship between the source (glottal flow pulse) and the filter (vocal tract). Such a linear relationship does not allow for nor explain how changes in the filter may impact the stability and regularity of the source. The objective of this experiment was to examine what effect unpredictable changes to vocal tract dimensions could have on fo stability and individual harmonic intensities in situations in which low frequency harmonics cross formants in a fundamental frequency glide. To determine these effects, eight human subjects (five male, three female) were recorded producing fo glides while their vocal tracts were artificially lengthened by a section of vinyl tubing inserted into the mouth. It was hypothesized that if the source and filter operated as a purely linear system, harmonic intensities would increase and decrease at nearly the same rates as they passed through a formant bandwidth, resulting in a relatively symmetric peak on an intensity-time contour. Additionally, fo stability should not be predictably perturbed by formant/harmonic crossings in a linear system. Acoustic analysis of these recordings, however, revealed that harmonic intensity peaks were asymmetric in 76% of cases, and that 85% of fo instabilities aligned with a crossing of one of the first four harmonics with the first three formants. These results provide further evidence that nonlinear dynamics in the source-filter relationship can impact fo stability as well as harmonic intensities as harmonics cross through formant bandwidths. [ABSTRACT FROM AUTHOR]

• Considerable independence of acoustic measures of source and filter. • Both source and filter encode speaker-specific information. • Degree of source-filter independence is speaker-specific. • Combining source and filter can produce optimal speaker recognition performance. This study examines the extent to which speaker-specific information is encoded in different features of vocal output and the relationships between those features. A range of acoustic features, grouped as source (laryngeal voice quality measures and fundamental frequency) and filter features (formants and Mel-frequency cepstral coefficients; MFCCs), were extracted from the vocalic portion of the hesitation marker um for 90 male speakers of Standard Southern British English. Little overall correlation between the sets of features was observed, suggesting no strong interdependence between source and filter in our data. Although filter features were consistently better at discriminating between same- and different-speaker pairs compared with source features, combining source and filter has the potential of producing the lowest error rates and the strongest speaker discrimination scores. Taken together, results show that source and filter provide complementary speaker-specific information. However, the extent of the improvements in speaker discrimination performance when combining source and filter varied across speakers. We explore potential explanations for this finding and discuss the implications for source-filter theory, and for applied fields such as speaker recognition and forensic speech science. [ABSTRACT FROM AUTHOR]

Penguins produce contact calls to maintain social relationships and group cohesion. Such vocalisations have recently been demonstrated to encode individual identity information in the African penguin. Using a source-filter theory approach, we investigated whether acoustic cues of individuality can also be found in other Spheniscus penguins and the acoustic features of contact calls have diverged within this genus. We recorded vocalisations from two ex-situ colonies of Humboldt penguin and Magellanic penguin (sympatric and potentially interbreeding in the wild) and one ex-situ group of African penguins (allopatric although capable of interbreeding with the other two species in captivity). We measured 14 acoustic parameters from each vocalisation. These included temporal (duration), source-related (fundamental frequency, f 0 ), and filter-related (formants) parameters. They were then used to carry out a series of stepwise discriminant function analyses (with cross-validation) and General Linear Model comparisons. We showed that contact calls allow individual discrimination in two additional species of the genus Spheniscus . We also found that calls can be classified according to species in a manner far greater than that attributable by chance, even though there is limited genetic distance among African, Humboldt, and Magellanic penguins. Our results provide further evidence that the source-filter theory is a valuable framework for investigating the biologically meaningful information contained in bird vocalisations. Our findings also provide novel insights into penguin vocal communication and suggest that contact calls of the penguin family are affected by selection for individuality. [ABSTRACT FROM AUTHOR]

An unresolved issue in comparative approaches to speech evolution is the apparent absence of an intermediate vocal communication system between human speech and the less flexible vocal repertoires of other primates. We argue that humans’ ability to modulate nonverbal vocal features evolutionarily linked to expression of body size and sex (fundamental and formant frequencies) provides a largely overlooked window into the nature of this intermediate system. Recent behavioral and neural evidence indicates that humans’ vocal control abilities, commonly assumed to subserve speech, extend to these nonverbal dimensions. This capacity appears in continuity with context-dependent frequency modulations recently identified in other mammals, including primates, and may represent a living relic of early vocal control abilities that led to articulated human speech. [ABSTRACT FROM AUTHOR]

The linear source-filter model of speech production assumes that the source of the speech sounds is independent of the filter. However, acoustic simulations based on the physical speech production models show that when the fundamental frequency of the source harmonics approaches the first formant of the vocal tract filter, the filter has significant effects on the source due to the nonlinear coupling between them. In this study, two interactive system models are proposed under the quasi steady Bernoulli flow and linear vocal tract assumptions. An algorithm is developed to estimate the model parameters. Glottal flow and the linear vocal tract parameters are found by conventional methods. Rosenberg model is used to synthesize the glottal waveform. A recursive optimization method is proposed to find the parameters of the interactive model. Finally, glottal flow produced by the nonlinear interactive system is computed. The experimental results show that the interactive system model produces fine details of glottal flow source accurately. [ABSTRACT FROM AUTHOR]

Voice is a major means of communication for humans, non-human mammals and many other vertebrates like birds and anurans. The physical and physiological principles of voice production are described by two theories: the MyoElastic-AeroDynamic (MEAD) theory and the Source-Filter Theory (SFT). While MEAD employs a multiphysics approach to understand the motor control and dynamics of self-sustained vibration of vocal folds or analogous tissues, SFT predominantly uses acoustics to understand spectral changes of the source via linear propagation through the vocal tract. Because the two theories focus on different aspects of voice production, they are often applied distinctly in specific areas of science and engineering. Here, we argue that the MEAD and the SFT are linked integral aspects of a holistic theory of voice production, describing a dynamically coupled system. The aim of this manuscript is to provide a comprehensive review of both the MEAD and the source-filter theory with its nonlinear extension, the latter of which suggests a number of conceptual similarities to sound production in brass instruments. We discuss the application of both theories to voice production of humans as well as of animals. An appraisal of voice production in the light of non-linear dynamics supports the notion that voice production can best be described with a systems view, considering coupled systems rather than isolated contributions of individual sub-systems., (Copyright © 2022 The Voice Foundation. Published by Elsevier Inc. All rights reserved.)

Crocodilians are among the most vocal non-avian reptiles. Adults of both sexes produce loud vocalizations known as 'bellows' year round, with the highest rate during the mating season. Although the specific function of these vocalizations remains unclear, they may advertise the caller's body size, because relative size differences strongly affect courtship and territorial behaviour in crocodilians. In mammals and birds, a common mechanism for producing honest acoustic signals of body size is via formant frequencies (vocal tract resonances). To our knowledge, formants have to date never been documented in any non-avian reptile, and formants do not seem to play a role in the vocalizations of anurans. We tested for formants in crocodilian vocalizations by using playbacks to induce a female Chinese alligator (Alligator sinensis) to bellow in an airtight chamber. During vocalizations, the animal inhaled either normal air or a helium/ oxygen mixture (heliox) in which the velocity of sound is increased. Although heliox allows normal respiration, it alters the formant distribution of the sound spectrum. An acoustic analysis of the calls showed that the source signal components remained constant under both conditions, but an upward shift of high-energy frequency bands was observed in heliox. We conclude that these frequency bands represent formants. We suggest that crocodilian vocalizations could thus provide an acoustic indication of body size via formants. Because birds and crocodilians share a common ancestor with all dinosaurs, a better understanding of their vocal production systems may also provide insight into the communication of extinct Archosaurians. [ABSTRACT FROM AUTHOR]

The trunk-like nose of the saiga antelope Saiga tatarica is a striking example of an exaggerated trait, assumed to having evolved as a dust filter for inhaled air. In addition, it functions to elongate the vocal tract in harem saiga males for producing low-formant calls that serve as a cue to body size for conspecifics. This study applies the source--filter theory to the acoustics of nasal, oral and nasal-and-oral calls that were recorded from a captive herd of 24 mother and 32 neonate saigas within the first 10 days postpartum. Anatomical measurements of the nasal and oral vocal tracts of two specimens (one per age class) helped to establish the settings for the analysis of formants. In both mother and young, the lower formants of nasal calls/call parts were in agreement with the source--filter theory, which suggests lower formants for the longer nasal vocal tract than for the shorter oral vocal tract. Similar fundamental frequencies of the nasal and oral parts of nasal-and-oral calls were also in agreement with the source-filter theory, which postulates the independence of source and filter. However, the fundamental frequency was higher in oral than in nasal calls, probably due to the higher emotional arousal during the production of oral calls. We discuss production mechanisms and the ontogeny of formant patterns of oral and nasal calls among bovid and cervid species with and without a trunk-like nose. [ABSTRACT FROM AUTHOR]

We conducted a study of the male rut vocalizations (groans) of two closely related species, Persian and European fallow deer. Persian fallow deer are endangered, restricted to Iran and Israel, and their rut vocalizations have never been studied. By contrast, European fallow deer are one of the most common deer species in the world, and have been the subject of numerous detailed studies. Persian bucks are approximately 16% larger than European bucks, and this can have important implications for vocalizations. Persian bucks were recorded in Israel, and European bucks were recorded in the UK and Ireland. We measured temporal, fundamental frequency-related and formant-related parameters of groans and determined which acoustic parameters differed among species and populations. The comparisons revealed important structural similarities and differences, with the differences more strongly related to temporal than spectral vocal parameters. Persian buck groans were relatively long, pulsed calls of almost 1-s duration, with low fundamental frequencies, and relatively weak formant modulation. European buck groans were much shorter (0.38 s), but with similarly low fundamental frequencies and clearer formant modulation. We found some minor differences in the formant frequencies ( F4 and F5) of calls of the two European fallow populations. Given the length of time since Persian and European fallow deer diverged, and that both their mitochondrial and nuclear genomes are very different, it is notable that the structure of their groans is still so similar. Our findings suggest that the factors influencing the evolution of these vocalizations (e.g. sensory system characteristics, environment and mate choice) have probably been similar for both species. [ABSTRACT FROM AUTHOR]

Source-filter coupling is the rarest acoustic phenomenon not only in Iberian red deer, but in any mammal. In most mammals, sound production can be well described in the framework of source-filter theory. The vocal output is the result of combined work of the larynx (the source) and of the supralaryngeal vocal tract (the filter). The source-filter theory suggests the independence of source and filter. Thus, vocal tract filtering should not affect the fundamental frequency (f0) of the sound created in the larynx. Spectrographically, the source is mostly characterized by the f0 and its harmonics, while the filter by the vocal tract resonances, i.e., formant frequencies. Nevertheless, a non-independent (coupled) source and filter can be proposed when the vocal folds start oscillating at one of the formant frequencies. Coupling between source and filter has been found in human singers and predicted for red deer Cervus elaphus by a computer modeling approach. This study describes different modes of phonation in a natural bout of rutting calls of Iberian red deer Cervus elaphus hispanicus and the transition from a chaotic mode to a probable source-filter coupling mode. This phenomenon might be involved in the production of extremely high-frequency bugles of North American and Asian subspecies of C. elaphus. [ABSTRACT FROM AUTHOR]

Vocal production in songbirds requires the control of the respiratory system, the syrinx as sound source and the vocal tract as acoustic filter. Vocal tract movements consist of beak, tongue and hyoid movements, which change the volume of the oropharyngeal-esophageal cavity (OEC), glottal movements and tracheal length changes. The respective contributions of each movement to filter properties are not completely understood, but the effects of this filtering are thought to be very important for acoustic communication in birds. One of the most striking movements of the upper vocal tract during vocal behavior in songbirds involves the OEC. This study measured the acoustic effect of OEC adjustments in zebra finches by comparing resonance acoustics between an utterance with OEC expansion (calls) and a similar utterance without OEC expansion (respiratory sounds induced by a bilateral syringeal denervation). X-ray cineradiography confirmed the presence of an OEC motor pattern during song and call production, and a custom-built Hall-effect collar system confirmed that OEC expansion movements were not present during respiratory sounds. The spectral emphasis during zebra finch call production ranging between 2.5 and 5 kHz was not present during respiratory sounds, indicating strongly that it can be attributed to the OEC expansion. [ABSTRACT FROM AUTHOR]

Source-filter theory provides a framework to interpret the acoustic structure of vertebrate vocalizations in relation to biophysical production, and it predicts that specific acoustic parameters can encode information about callers. Because formant frequencies are determined by vocal tract dimensions, with longer vocal tracts producing lower formants, they can be reliable indicators of body size, as well as other important traits. In polygynous species, reliable acoustic cues to fitness-related traits are expected to be under strong sexual selection pressure through male competition and/or female choice. This study investigates whether formant frequencies of male North American bison bellow vocalizations encode information about fitness-related caller attributes. Bison exhibit male-dominance female-defence polygyny, with dominance displays involving bellows. We hypothesized that physical attributes (mass, age) would predict formants and that formants would in turn predict quality indices (dominance, copulations, offspring sired). Our results showed that heavier bulls produced lower formants and that lower formants predicted higher mating success (copulations), even when controlling for mass. Given positive associations between mating success, dominance and reproductive success (offspring sired) in bison, we conclude that bellows with lower formants reflect greater fitness in bulls. We discuss the importance of reliable acoustic cues to size and quality indices in sexual selection contexts. [Copyright &y& Elsevier]

Diversifications in primate vocalization, including human speech, are believed to reflect evolutionary modifications in vocal anatomy and physiology. Gibbon song is acoustically unique, comprising loud, melodious, penetrating pure tone-like calls. In a white-handed gibbon, Hylobates lar, the fundamental frequency ( f0) of song sounds is amplified distinctively from the higher harmonics in normal air. In a helium-enriched atmosphere, f0 does not shift, but it is significantly suppressed and 2 f0 is emphasized. This implies that the source is independent of the resonance filter of the supralaryngeal vocal tract (SVT) in gibbons, in contrast to musical wind instruments, in which the filter primarily determines f0. Acoustic simulation further supported that gibbons' singing is produced analogously to professional human soprano singing, in which a precise tuning of the first formant ( F1) of the SVT to f0 amplifies exclusively the f0 component of the source. Thus, in gibbons, as in humans, dynamic control over the vocal tract configuration, rather than anatomical modifications, has been a dominant factor in determining call structure. The varied dynamic movements were adopted in response to unique social and ecological pressures in gibbons, allowing monogamous gibbons to produce pure-tonal melodious songs in the dense tropical forests with poor visibility. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The field of animal vocal communication has benefited greatly from improved understanding of vocal production mechanisms and specifically from the generalization of the source–filter theory of speech production to non-human mammals. The application of the source–filter theory has enabled researchers to decompose the acoustic structure of vocal signals according to their mode of production and thereby to predict the acoustic variation that is caused by anatomical or physiological attributes of the caller. The source–filter theory states that vocal signals result from a two-stage production, with the glottal wave generated in the larynx (the source), being subsequently filtered in the supralaryngeal vocal tract (the filter). This theory predicts that independent indexical information such as body size, weight, age and sex can be contained in both the glottal wave (mostly characterized by its fundamental frequency), and the spectral envelope of the radiated vocalization (mostly characterized by the vocal tract resonances or formant frequencies). Additionally, physiological fluctuations in emotional or motivational state have been found to influence the acoustic characteristics of signals in a reliable and predictable manner that is perceptually available to receivers. While animal vocalizations contain some dynamic attributes, their static attributes are sufficient to provide an effective means of acoustic individual discrimination both within and across call types. In this paper, we draw together a wealth of experimental work conducted within the source–filter framework over the last decade and we review how such experiments have elucidated the communicative value of animal vocalizations. [ABSTRACT FROM AUTHOR]

Purpose: Maximum flow declination rate (MFDR) in the glottis is known to correlate strongly with vocal intensity in voicing. This declination, or negative slope on the glottal airflow waveform, is in part attributable to the maximum area declination rate (MADR) and in part to the overall inertia of the air column of the vocal tract (lungs to lips). The purpose of this theoretical study was to show the possible contributions of air inertance and MADR to MFDR. Method: A simplified computational model of the kinematics of vocal fold movement was utilized to compute a glottal area function. The glottal flow was computed interactively with lumped vocal tract parameters in the form of resistance and inertive reactance. Results: It was shown that MADR depends almost entirely on the ratio of vibrational amplitudes of the lower to upper margins of the vocal fold tissue. Adduction, vertical phase difference, and prephonatory convergence of the glottis have a lesser effect on MADR. A relatively simple rule was developed that relates MFDR to a vibrational amplitude ratio and vocal tract inertance. Conclusion: It was concluded that speakers and singers have multiple options for control of intensity, some of which involve more source-filter interaction than others. [ABSTRACT FROM AUTHOR]

Ring doves vocalize with their beaks and nostrils closed, exhaling into inflatable chambers in the head and neck region. The source sound produced at the syrinx contains a fundamental frequency with prominent second and third harmonic overtones, but these harmonics are filtered out of the emitted signal. We show by cineradiography that the upper esophagus, oral and nasal cavities collect the expired air during vocalization and that the inflated esophagus becomes part of the suprasyringeal vocal tract. The level of the second and third harmonics, relative to the fundamental frequency (f0), is reduced in the esophagus and emitted vocalization compared with in the trachea, although these harmonics are still considerably higher in the esophagus than in the emitted signal. When the esophagus is prevented from fully inflating, there is a pronounced increase in the level of higher harmonics in the emitted vocalization. Our data suggest that the trachea and esophagus act in series as acoustically separate compartments attenuating harmonics by different mechanisms. We hypothesize that the trachea behaves as a tube closed at the syringeal end and with a variable, restricted opening at the glottal end that lowers the tracheal first resonance to match the f0 of the coo. The inflated esophagus may function as a Helmholtz resonator in which the elastic walls form the vibrating mass. Such a resonator could support the f0 over a range of inflated volumes. [ABSTRACT FROM AUTHOR]

Rodent diversification is associated with a large diversity of species-specific social vocalizations generated by two distinct laryngeal sound production mechanisms: whistling and airflow-induced vocal fold vibration. Understanding the relative importance of each modality to context-dependent acoustic interactions requires comparative analyses among closely related species. In this study, we used light gas experiments, acoustic analyses and laryngeal morphometrics to identify the distribution of the two mechanisms among six species of deer mice (Peromyscus spp.). We found that high frequency vocalizations (simple and complex sweeps) produced in close-distance contexts were generated by a whistle mechanism. In contrast, lower frequency sustained vocalizations (SVs) used in longer distance communication were produced by airflow-induced vocal fold vibrations. Pup isolation calls, which resemble adult SVs, were also produced by airflow-induced vocal fold vibrations. Nonlinear phenomena (NLP) were common in adult SVs and pup isolation calls, suggesting irregular vocal fold vibration characteristics. Both vocal production mechanisms were facilitated by a characteristic laryngeal morphology, including a two-layered vocal fold lamina propria, small vocal membrane-like extensions on the free edge of the vocal fold, and a singular ventral laryngeal air pocket known as the ventral pouch. The size and composition of vocal folds (rather than total laryngeal size) appears to contribute to species-specific acoustic properties. Our findings suggest that dual modes of sound production are more widespread among rodents than previously appreciated. Additionally, the common occurrence of NLP highlights the nonlinearity of the vocal apparatus, whereby small changes in anatomy or physiology trigger large changes in behavior. Finally, consistency in mechanisms of sound production used by neonates and adults underscores the importance of considering vocal ontogeny in the diversification of species-specific acoustic signals., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)