Vocal Individuality Research Articles

Vocal individuality in owls: a taxon-wide review in the context of Tinbergen’s four questions

Vocal individuality in owls: a taxon-wide review in the context of Tinbergen’s four questions

Duets convey information about pair and individual identities in a Neotropical bird

Abstract Vocal individuality is essential for social discrimination but has been poorly studied in animals that produce communal signals (duets or choruses). Song overlapping and temporal coordination make the assessment of individuality in communal signals more complex. Additionally, selection may favor the accurate identification of pairs over individuals by receivers in year-round territorial species with duetting and long-term pair bonding. Here, we studied pair and individual vocal signatures in the polyphonal duets of rufous horneros Furnarius rufus, a Neotropical bird known for its long-term pair bonds. Hornero partners engage in duets to deter territorial intruders and protect their partnership year-round, and can discern duets from neighbors versus strangers. Using a dataset of 471 duets from 43 pairs in two populations, we measured fine-scale acoustic features across different duet levels (e.g., complete duets to non-overlapping syllable parts) and analysis levels (pair or individual). Permuted linear discriminant function analyses classified pairs and individuals more accurately than expected by chance (means: 45% and 47% versus 4 and 2%). Pair identity explained more variance in the multivariate acoustic features of duets than individual or population identities. The initial frequency of the duet showed strong potential for encoding pair identity. The acoustic traits contributing most to individual vocal signatures varied between sexes, which might facilitate the simultaneous assessment of duetters’ identities by receivers. Our study indicates that vocal individuality may exist even in species with intricate and innate communal signals and elucidates the mechanisms employed by horneros in their social discrimination ability.

Using autonomous recording units for vocal individuality: insights from Barred Owl identification

Using autonomous recording units for vocal individuality: insights from Barred Owl identification

Speakers are more cooperative and less individual when interacting in larger group sizes.

Cooperation, acoustically signaled through vocal convergence, is facilitated when group members are more similar. Excessive vocal convergence may, however, weaken individual recognizability. This study aimed to explore whether constraints to convergence can arise in circumstances where interlocutors need to enhance their vocal individuality. Therefore, we tested the effects of group size (3 and 5 interactants) on vocal convergence and individualization in a social communication scenario in which individual recognition by voice is at stake. In an interactive game, players had to recognize each other through their voices while solving a cooperative task online. The vocal similarity was quantified through similarities in speaker i-vectors obtained through probabilistic linear discriminant analysis (PLDA). Speaker recognition performance was measured through the system Equal Error Rate (EER). Vocal similarity between-speakers increased with a larger group size which indicates a higher cooperative vocal behavior. At the same time, there was an increase in EER for the same speakers between the smaller and the larger group size, meaning a decrease in overall recognition performance. The decrease in vocal individualization in the larger group size suggests that ingroup cooperation and social cohesion conveyed through acoustic convergence have priority over individualization in larger groups of unacquainted speakers.

Ovine vocal individuality expression by ewes and lambs at a late (40 days) post-partum time point.

Recognizing the identity of conspecifics is important for survival and social interactions. In sheep, vocal individuality enhances postnatal recognition and strengthens the mother-offspring bond. Although previous studies report vocal individuality in an early postnatal period (3-15 days of life), scarce information exists on whether individuality occurs at a later postnatal time point. The aim of the study was to identify whether individuality in acoustic cues is expressed in ewes' and their lambs' bleats at 40 days post-partum. Dairy ewes (N = 21) and their suckling offspring (n = 30) were isolated separately without hearing or seeing each other, and vocalizations were recorded. Different approaches for estimating individuality on 18 determined acoustic parameters were implemented. All parameters showed individuality, but higher individuality appeared in those related with source and spectral characteristics. A discriminant function analysis showed similar levels of individuality between ewes and lambs, but lower than that reported in an earlier postnatal time, suggesting that ewes and lambs do not need strong individualized cues at the examined time point. In both cases, jitter was the only common parameter, suggesting its importance. Distinctive cues were noted between siblings. Ewes displayed vocal individuality at their dry season (a later time point than suckling period), which was based on amplitude-related acoustic parameters.

Vocal individuality measures reveal spatial and temporal variation in roding behaviour in Woodcock (Scolopax rusticola)

Use of species‐specific field methods may be required for taxa that are inherently difficult to survey, for example species with cryptic camouflage or secretive behaviour. However, these methods often require more manual effort and therefore cost. Passive acoustic monitoring (PAM) is now an established tool to reduce manual effort to monitor species, and analysis of spectrograms provides the means to discriminate individuals by call characteristics. At night, male Eurasian Woodcock Scolopax rusticola make distinct, audible ‘roding’ displays above the tree canopy to advertise to females. Studying this behaviour at an individual level while using PAM presents opportunities to improve monitoring methods of this cryptic, Red‐Listed species. This study evaluates the potential use of vocal individuality measurements in distinguishing Woodcock individuals and interpreting spatial and temporal patterns in their roding displays across woodland sites. Woodcock roding calls were recorded from woodland fragments across two regions comprising 20 sites. A random forest classifier was applied to reduce the time needed to find and manually verify calls. Principal component analysis (PCA) and hierarchical clustering algorithms were used on call measurements, describing duration and frequency differences in calls. When clusters formed, they were used to qualitatively assess supposed individual spatial and temporal variation in roding behaviour. The variance of dimensionally reduced measurements was used to interpret local Woodcock abundance and changes over time. Supposed individuals used many sites within a region, and many sites were used by multiple birds. However, sites showed clusters of calls from supposed individuals in different proportions. It was difficult to discriminate individuals using PCA with more than six birds because the degree of call overlap increased. Though the call measurement variance is associated with number of call events, it may provide a suitable method for representing population size without call count bias.

Song as a signal of male identity and quality in the Green-winged Saltator (Saltator similis)

Birdsong is commonly associated with sexual selection. It can influence mate choice through honestly signaling the emitter's quality. Such quality may be reflected, among other factors, in body condition and bite force. The Green-winged Saltator (Saltator similis) is a good model species to assess such relationships because males defend their territories throughout the year and sing a stereotyped song, which is known to be associated with sexual selection. The objective of this work is to assess the relationships among song structure, vocal individuality, and possible fitness indicators (body condition, morphology, and bite force) in captive male Green-winged Saltators. We conducted a linear discriminant analysis (LDA) to assess vocal individuality and to highlight the principal acoustic variables associated with it. We conducted Y-Aware PCAs to evaluate the relationship between the body condition index, bite force, and morphometric and bioacoustic variables. The morphometric variables, particularly head width and length, explained 23% of the bite force variation. The acoustic variation explained 36.17% of body condition index variation, but a small nonsignificant amount of bite force (12%). From the acoustic variables, peak frequency and high frequency of the last syllable, slope of the first syllable, and emission rate showed significant negative relationships with body condition. The LDA model based on acoustic variables had a 97.47% accuracy, although this result might also reflect regional song dialects. The acoustic variables most relevant to individual classification (first syllable duration, last syllable duration, whole-song slope, and whole-song duration) are not the same variables that had a stronger relationship to body condition. Besides that, there can be a weaker mixed signaling, as variables related to frequency modulations were important in both contexts. Our results suggest that the song may act as an honest signal of an individual's body condition in this species.

First description of the nest and breeding behavior of the Scarlet-and-white Tanager (Chrysothlypis salmoni) from the Colombian Chocó

The avian genus Chrysothlypis has 2 species distributed in Central and South America. We report the first description of the nest and breeding behavior of the Scarlet-and-white Tanager (Chrysothlypis salmoni) from the Colombian Chocó. A single nest was found and monitored in March 2018 during a field season in the Anchicayá Valley, Valle del Cauca, Colombia. The nest resembles the nest of congeners in the subfamily Hemithraupinae: a cup constructed with rootlets, leaves, fungal rhizomorphs, moss, and spiderwebs. It was placed 6 m above ground in a Cespedecia spathulata tree located at the forest edge alongside a gravel road. We observed only the female brooding a single chick, but the male helped feed the nestling. Groups of vocal individuals visited the nest and, on one occasion, a suspected helper fed the nestling. Our study presents the first nesting information for this species.

Convolutional Neural Networks for the Identification of African Lions from Individual Vocalizations.

The classification of vocal individuality for passive acoustic monitoring (PAM) and census of animals is becoming an increasingly popular area of research. Nearly all studies in this field of inquiry have relied on classic audio representations and classifiers, such as Support Vector Machines (SVMs) trained on spectrograms or Mel-Frequency Cepstral Coefficients (MFCCs). In contrast, most current bioacoustic species classification exploits the power of deep learners and more cutting-edge audio representations. A significant reason for avoiding deep learning in vocal identity classification is the tiny sample size in the collections of labeled individual vocalizations. As is well known, deep learners require large datasets to avoid overfitting. One way to handle small datasets with deep learning methods is to use transfer learning. In this work, we evaluate the performance of three pretrained CNNs (VGG16, ResNet50, and AlexNet) on a small, publicly available lion roar dataset containing approximately 150 samples taken from five male lions. Each of these networks is retrained on eight representations of the samples: MFCCs, spectrogram, and Mel spectrogram, along with several new ones, such as VGGish and stockwell, and those based on the recently proposed LM spectrogram. The performance of these networks, both individually and in ensembles, is analyzed and corroborated using the Equal Error Rate and shown to surpass previous classification attempts on this dataset; the best single network achieved over 95% accuracy and the best ensembles over 98% accuracy. The contributions this study makes to the field of individual vocal classification include demonstrating that it is valuable and possible, with caution, to use transfer learning with single pretrained CNNs on the small datasets available for this problem domain. We also make a contribution to bioacoustics generally by offering a comparison of the performance of many state-of-the-art audio representations, including for the first time the LM spectrogram and stockwell representations. All source code for this study is available on GitHub.

Acoustic individuality in the hissing calls of the male black grouse (Lyrurus tetrix).

Acoustic individuality may well play a big role during the mating season of many birds. Black grouse (Lyrurus tetrix) produces two different long-distance calls during mating on leks: rookooing and hissing calls. The first one represents low frequency series of bubbling sounds and the second one represents hissing sound. This hissing represents a signal not produced by the syrinx. We analyzed 426 hissing calls from 24 individuals in Finland and Scotland. We conducted cross-validated discrimination analyses (DFA). The discrimination model classified each call with almost 78% accuracy (conventional result) and the validated DFA revealed 71% output, that is much higher than classification by chance (4%). The most important variables were Frequency 95%, 1st Quartile Frequency, Aggregate Entropy and Duration 90%. We also tested whether between individual variation is higher than within individual variation using PIC (Potential for individual coding) and we found that all acoustic parameters had PIC > 1. We confirmed that hissing call of black grouse is individually distinct. In comparison to the signals produced by the syrinx, non-vocal sounds have been studied rarely and according to our knowledge, this is the second evidence of vocal individuality in avian hissing sounds which are not produced by syrinx. Individuality in the vocalization of the male black grouse may aid females in mating partner selection, and for males it may enable competitor recognition and assessment. Individually distinct hissing calls could be of possible use to monitor individuals on leks. Such a method could overcome problems during traditional monitoring methods of this species, when one individual can be counted multiple times, because catching and traditional marking is problematic in this species.

The energetics of social signaling during roost location in Spix's disc-winged bats.

Long-term social aggregations are maintained by multiple mechanisms, including the use of acoustic signals, which may nonetheless entail significant energetic costs. To date, however, no studies have gauged whether there are significant energetic costs to social call production in bats, which heavily rely on acoustic communication for a diversity of social tasks. We measured energetic expenditure during acoustic signaling in Spix's disc-winged bats (Thyroptera tricolor), a species that commonly uses contact calls to locate the ephemeral furled leaves that they use for roosting. To determine the cost of sound production, we measured oxygen consumption using intermittent-flow respirometry methods, with and without social signaling. Our results show that the emission of contact calls significantly increases oxygen consumption; vocal individuals spent, on average, 12.42kJ more during social signaling trials than they spent during silent trials. We also found that as resting metabolic rate increased in males, there was a decreasing probability that they would emit response calls. These results provide support to the 'allocation model', which predicts that only individuals with lower self-maintenance costs can afford to spend energy in additional activities. Our results provide a step forward in our understanding of how physiology modulates behavior, specifically how the costs of call production and resting metabolic rate may explain the differences in vocal behavior among individuals.

Long-term stability of vocal individuality cues in a territorial and monogamous seabird.

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.

Individual-based acoustic variation of the alarm calls in the long-tailed ground squirrel.

Based on their phylogenetic position, Nearctic ground squirrels are closest relatives to the long-tailed ground squirrel Urocitellus undulates even though it has Palaearctic distribution. We aimed to investigate the variability of alarm calls of the long-tailed ground squirrel to test the individual variation in alarm calls. This species is known to produce two types of alarm calls: whistle alarms and wideband calls. Although ground squirrels are a model group for the study of vocal individuality, this phenomenon has not yet been studied in a species producing two such completely different types of alarms. Most of ground squirrel species produce either whistle or wideband alarms and this species represents a unique model for testing the degree of individual variability depending on completely different acoustic structures. We analysed 269 whistle alarms produced by 13 individuals and 591 wideband alarms from 25 individuals at the western part of Lake Baikal. A discriminant function analysis (DFA) assigned 93.5% (88.9%, cross-validated result) of whistle alarms to the correct individual and 91.4% (84%) of wideband alarms. This is the first evidence of individual variation in wideband alarms compared with whistle alarms and occurrence of vocal individuality in two warning signals of a completely different acoustic structure produced by a ground squirrel.

Acoustic preadaptation to transmit vocal individuality of savanna nightjars in noisy urban environments

As urbanization has expanded dramatically, the impacts of urban noise on wildlife have drawn increasing attention. However, previous studies have focused primarily on diurnal songbirds and much less on nocturnal nonpasserines such as nightjars. The savanna nightjar has recently successfully colonized urban areas in Taiwan. Using 1925 calls recorded from 67 individuals, we first investigated the individual differences of the acoustic structures; and, for those acoustic variables with significant individual differences, we examined the correlation between the acoustic structures and the ambient noise levels. We then compared the transmission efficacy of vocal individuality among three sets of acoustic variables: all acoustic variables, noise-related variables, and noise-unrelated variables. Using seven artificial frequency-shifted calls to represent seven different individuals in playback-recording experiments, we also investigated the transmission efficacy of vocal individuality and variable accuracy in three different urban noise levels (high, medium, low). We found that all 30 acoustic variables derived from the acoustic structures demonstrated significant individual differences, and 14 frequency-based variables were negatively correlated with ambient noise levels. Although transmission efficacy was significantly affected by urban noise, individuality information was still transmitted with high accuracy. Furthermore, the noise-unrelated structures (which included the maximum frequency, the maximum amplitude frequency, and the mean frequency of the call) had a significantly higher transmission efficacy of vocal individuality than the noise-related variables (which included the minimum frequency, the frequency at the start and the end of the call) in both field observation and playback-recording experiments. We conclude that these noise-unrelated acoustic features may be one of the key preadaptations for this nocturnal nonpasserine to thrive so successfully in its newly adopted urban environment.

Vocal discrimination of African lions and its potential for collar-free tracking

ABSTRACT Previous research has shown that African lions (Panthera leo) have the ability to discriminate between conspecific vocalisations, but little is known about how individual identity is conveyed in the spectral structure of roars. Using acoustic – accelerometer biologgers that allow vocalisations to be reliably associated with individual identity, we test for vocal individuality in the fundamental frequency (f0) of roars from 5 male lions, firstly by comparing simple f0 summary features and secondly by modelling the temporal pattern of the f0 contour. We then assess the application of this method for discriminating between individuals using passive acoustic monitoring. Results indicate that f0 summary features only allow for vocal discrimination with 70.7% accuracy. By comparison, vocal discrimination can be achieved with an accuracy of 91.5% based on individual differences in the temporal pattern of the f0 sequence. We further demonstrate that passively recorded lion roars can be localised and differentiated with similar accuracy. The existence of individually unique f0 contours in lion roars and their relatively lower attenuation indicates a likely mechanism enabling individual lions to identify conspecifics over long distances. These differences can be exploited by researchers to track individuals across the landscape and thereby supplement conventional lion monitoring approaches.

The reliability of individual vocal signature varies across the bonobo's graded repertoire

Vocalizations often contain both ‘dynamic’ information, related to short-term fluctuations in the individual's emotional states, and ‘static’ information, related to long-term attributes such as age, sex, weight and body size which define an ‘individual vocal signature’. While both types of information may be of functional value to receivers, dynamic information requires acoustic versatility, while static information depends on acoustic stability. Here we investigated whether an individual vocal signature is present across the vocal repertoire of the bonobo, Pan paniscus. First, the analysis of the acoustic structure of its five most common tonal vocalizations emphasized the highly graded structure of its repertoire. We then evaluated the reliability of identity information across these call types. The results show that, while all call types supported information related to identity, the reliability of these vocal signatures was not consistent along the graded vocal continuum. Caller identity was strongly encoded at one end of the acoustic gradation (high-hoot) and decreased from bark, soft bark, peep-yelp to peep calls. Strikingly, the reliability of the individual signature thus decreased from calls used in high-arousal contexts to those used in low-arousal contexts. To the best of our knowledge, this is the first demonstration that an acoustic gradation that codes for ‘dynamic’ information can be accompanied by variation in the ‘static’ information that supports vocal individuality.

High-level individuality in vocalizations of a well-known avian brood parasite: Asian koel (Eudynamys scolopacea L.)

ABSTRACT The Asian koel (Eudynamys scolopacea) is a well-known brood parasitic bird that visits Pakistan every summer from India. This species produces at least five loud call types that are presumed to be used in territorial and mating behaviour. We noticed vocal differences among individuals in the wild and hypothesised that Asian koels produce individually distinct vocalizations. To test this, we recorded type 1 cooee calls, coe calls, type 1 coegh calls, type 2 coegh call and wurroo calls from 46 adult birds in the wild, and measured 15 different acoustic parameters, both spectral and temporal, for each vocalization type. We also measured the Potential of Individual Coding (PIC). Parameters having PIC values ≥1.1 were further analysed through stepwise cross-validated discriminant function analysis (DFA). The DFA accurately categorised 86.5% of the type 1 cooee calls, 95.7% of coe calls, 91.5% of the type 1 coegh calls, 82.2% of type 2 coegh calls and 83.3% of wurroo calls to the individuals from which these calls were recorded. Thus, consistent with our hypothesis, our results indicate that Asian koels produce individually distinct calls.

Individuality in the vocalizations of infant and adult coppery titi monkeys (Plecturocebus cupreus).

As social animals, many primates use acoustic communication to maintain relationships. Vocal individuality has been documented in a diverse range of primate species and call types, many of which have presumably different functions. Auditory recognition of one's neighbors may confer a selective advantage if identifying conspecifics decreases the need to participate in costly territorial behaviors. Alternatively, vocal individuality may be nonadaptive and the result of a unique combination of genetics and environment. Pair-bonded primates, in particular, often participate in coordinated vocal duets that can be heard over long distances by neighboring conspecifics. In contrast to adult calls, infant vocalizations are short-range and used for intragroup communication. Here, we provide two separate but complementary analyses of vocal individuality in distinct call types of coppery titi monkeys (Plecturocebus cupreus) to test whether individuality occurs in call types from animals of different age classes with presumably different functions. We analyzed 600 trill vocalizations from 30 infants and 169 pulse-chirp duet vocalizations from 30 adult titi monkeys. We predicted that duet contributions would exhibit a higher degree of individuality than infant trills, given their assumed function for long-distance, intergroup communication. We estimated 7 features from infant trills and 16 features from spectrograms of adult pulse-chirps, then used discriminant function analysis with leave-one-out cross-validation to classify individuals. We correctly classified infants with 48% accuracy and adults with 83% accuracy. To further investigate variance in call features, we used a multivariate variance components model to estimate variance partitioning in features across two levels: within- and between-individuals. Between-individual variance was the most important source of variance for all features in adults, and three of four features in infants. We show that pulse-chirps of adult titi monkey duets are individually distinct, and infant trills are less individually distinct, which may be due to the different functions of the vocalizations.

Acoustic Identification of Wild Gray Wolves, Canis lupus, Using Low Quality Recordings

Invasive trapping and radio-collaring techniques are currently used by conservation biologists to study the population dynamics of gray wolves (Canis lupus). Previous research has found wolf howls can be used to determine individual identity on high quality recordings from captive animals, offering an opportunity for non-invasive monitoring of packs.We recorded wild wolves in Central Wisconsin to determine the effectiveness of these features in determining individuality in low quality recordings. The wolf howls analyzed were from two adult individuals from separate packs. Using a principle component analysis, maximum frequency and end frequency of the calls were determined to be most individualistic. Using these features in a discriminant function analysis, howls were able to be identified from individuals with 100% accuracy. Gray wolves play an important role in ecosystem maintenance, however, the current monitoring techniques are costly and invasive. The creation of an easily accessible, non-invasive technique that can be used by individuals with a variety of technical backgrounds is necessary to address concerns faced by conservation efforts. To address these issues, all analyses performed usedfree or low-cost software, making this method of individual identification a useful alternative for conservation biologists. KEYWORDS: Canis lupus lycaon; Gray Wolf; Acoustic Signatures; Howls; Tracking Method; Conservation; Vocal Individuality

Vocal individuality of Holstein-Friesian cattle is maintained across putatively positive and negative farming contexts

Cattle mother-offspring contact calls encode individual-identity information; however, it is unknown whether cattle are able to maintain individuality when vocalising to familiar conspecifics over other positively and negatively valenced farming contexts. Accordingly, we recorded 333 high-frequency vocalisations from 13 Holstein-Friesian heifers during oestrus and anticipation of feed (putatively positive), as well as denied feed access and upon both physical and physical & visual isolation from conspecifics (putatively negative). We measured 21 source-related and nonlinear vocal parameters and stepwise discriminant function analyses (DFA) were performed. Calls were divided into positive (n = 170) and negative valence (n = 163) with each valence acting as a ‘training set’ to classify calls in the oppositely valenced ‘test set’. Furthermore, MANOVAs were conducted to determine which vocal parameters were implicated in individual distinctiveness. Within the putatively positive ‘training set’, the cross-validated DFA correctly classified 68.2% of the putatively positive calls and 52.1% of the putatively negative calls to the correct individual, respectively. Within the putatively negative ‘training set’, the cross-validated DFA correctly assigned 60.1% of putatively negative calls and 49.4% of putatively positive calls to the correct individual, respectively. All DFAs exceeded chance expectations indicating that vocal individuality of high-frequency calls is maintained across putatively positive and negative valence, with all vocal parameters except subharmonics responsible for this individual distinctiveness. This study shows that cattle vocal individuality of high-frequency calls is stable across different emotionally loaded farming contexts. Individual distinctiveness is likely to attract social support from conspecifics, and knowledge of these individuality cues could assist farmers in detecting individual cattle for welfare or production purposes.