Vocal individual discrimination in Japanese monkeys

The present study determines which features of the coo call are used by Japanese monkeys Macaca fuscata for vocal individual discrimination. First, two female Japanese monkeys were trained to discriminate conspecific individuals vocally, using an operant conditioning. Using as stimuli three unknown individuals with 30 calls per individual, the two monkeys succeeded in discriminating new call exemplars from the three stimulus individuals. A discriminant analysis performed on calls used as stimuli indicated that start frequency of the fundamental and call duration were variables that can differentiate individuals efficiently. Then, playbacks of acoustically modified signals were used to indicate which vocal features are used by monkeys for the individual discrimination. Stimuli signals containing modified pitch or duration, or filtered so as to keep only the fundamental component, were tested. Results indicated that Japanese monkeys use multiple acoustical cues to perform vocal individual discrimination, including at least pitch, call duration, and harmonics. However, harmonics seem to be less important for discrimination than pitch and call duration.     

Vocal signature in capybara,Hydrochoerus hydrochaeris

Social groups of capybaras are stable and cohesive. The species’ vocal communication is complex and mediates social interaction. The click call is emitted in a variety of contexts by animals from all age groups, but differs among groups; its attributed function is to keep contact among animals. To evaluate the presence of individual characteristics in the click call of capybaras, we recorded the vocalizations emitted spontaneously by six adults kept either solitary or in groups. We selected and measured the acoustic parameters of 300 click call phrases, 50 per individual. The parameters were submitted to a discriminant function analysis that revealed a classification accuracy of 76.8 %. A General Linear Model analysis revealed significant differences among the six individuals, and post hoc results showed that differences between a given pair were different from those of any other pair. The acoustic parameters that most contributed to discriminate the individual calls were click interval duration and click duration, suggesting that temporal parameters are more important than frequency parameters for individuals’ discrimination. The findings of individual characteristics in the click calls indicate that these vocalizations can be used as vocal signatures during social interactions.     

Individuality in Corncrake Crex crex vocalizations

Vocal individuality has potential for the monitoring of bird species that are otherwise difficult to observe. In this study we assessed the potential of this technique for the monitoring of the globally threatened Corncrake Crex crex based on the analysis of calls recorded in Ireland and Scotland in 1993 and 1994. Discriminant function analysis correctly attributed >80% of individual calls based on five temporal parameters. A high level of individual distinctiveness was also found in the fine temporal structure of calls. When information on fine structure was added to the discriminant function analysis, classification of calls to individuals was improved to 100%. The structure of an individual's call was found to be consistent over the 2-year period. This study highlighted the problemsassociated with setting criteria for re-identifying individuals by calls. The consequences of these problems and the potential of vocal individuality for providing monitoring information in this species are discussed.     

Sex recognition in brown skuas: do acoustic signals matter?

Bird vocalisations are often essential for sex recognition, especially in species that show little morphological sex dimorphism. Brown skuas (Catharacta antarctica lonnbergi), which exhibit uniform plumage across both sexes, emit three main calls: the long call, the alarm call and the contact call. We tested the potential for sex recognition in brown skua calls of 42 genetically sexed individuals by analysing 8–12 acoustic parameters in the temporal and frequency domains of each call type. For every call type, we failed to find sex differences in any of the acoustic parameters measured. Stepwise discriminant function analysis (DFA) revealed that sexes cannot be unambiguously classified, with increasing uncertainty of correct classification from contact calls to long calls to alarm calls. Consequently, acoustic signalling is probably not the key mechanism for sex recognition in brown skuas.     

Vocal individuality,but not stability,in wild palm cockatoos (Probosciger aterrimus)

The ability to identify individuals within a population is often essential for a detailed understanding of the ecology and conservation of a species. However, some species, including large parrots, are notoriously difficult to catch and mark for individual identification. Palm cockatoos (Probosciger aterrimus) are a large, poorly understood species of parrot which are likely in severe decline within the eastern part – and possibly the western part – of their range on Cape York Peninsula, Australia. Here, we investigated whether three different palm cockatoo call types are sufficiently individually distinctive to function as a non-invasive “marker” for identifying individuals over time. Using Discriminant Function Analysis, overall identification accuracy among 12 putative individuals for all call types was 81% (i.e. 148 out of 183 calls were assigned to the correct individual) on the basis of multiple temporal, energy (amplitude) and frequency measurements on the spectrogram. For three different call types, individual identification accuracy among males and females ranged from 69 to 95%. However, based on a limited sample sizes of five putative individuals between years, our data suggest that individual call structure, as quantified by call parameters, was not stable between years. We discuss the applicability of these results for future studies of palm cockatoos and other parrot species.     

The first call note of the Anhui tree frog (Rhacophorus zhoukaiya) is acoustically suited for enabling individual recognition

The effective production of acoustic signals is critically important for intraspecific communication in vocal animals; however, it is also highly time-consuming, energetically demanding and likely to increase predation risks. Thus, we hypothesized that the biological significance of each component of complex acoustic signals would differ serving specific functions and that the first component of such signals would be most important for social signalling and exhibit unique acoustic characteristics because of the precedence effect. To test this hypothesis, we measured temporal and spectral acoustic parameters for each note in the advertisement calls of the Anhui tree frog (Rhacophorus zhoukaiya), a species in which males build mud burrows and call from within these nests. Multivariate analyses including hierarchical cluster analysis and multidimensional scaling were used, based on temporal and spectral acoustic parameters for each of 10 notes/call. These results show that the first call notes form one cluster while the other notes form a second cluster in multidimensional space when the parameters measured were normalized. Furthermore, the temporal and spectral sound attributes of the first call note provide sufficient information for discrimination between different individuals. Moreover, discriminant analysis showed that the fundamental frequency of the first note is sufficient to identify individuals when the data are not normalized. Taken together, these results indicate that the first call note is poised to play an important role in Anhui tree frog vocal communication insofar as the temporal and spectral features provide sufficient information for individual recognition.     

Provisioning calls of the cooperatively breeding bell miner Manorina melanophrys encode sufficient information for individual discrimination

Acoustically-mediated individual discrimination has been the focus of much investigation in ornithology. For cooperatively breeding species, strong selection pressure favouring individual recognition during acts of altruism is predicted under many of the most interesting hypotheses proposed to account for helping behaviour. We investigated differences in 157 calls given by 12 different individuals as they provisioned nestlings, including both breeding and helper bell miners Manorina melanophrys (Meliphagidae), a cooperatively breeding honeyeater endemic to south-eastern Australia. Individual differences were apparent in all 15 call parameters analysed, many with a high level of repeatability. Moreover, the information capacity of mew calls allows as many as 515 different signatures to exist in the system. As many parameters showed strong sex differences, separate discriminant functions were used to predict individual identity within each sex. Five parameters were used in a function that correctly identified over 90% of female calls in both a training (n = 35) and test dataset (n = 11) not used to generate the function. Among males, a separate function used six parameters to correctly assign individual identity in 89.3% (n=84; training) and 77.8% (n=27; test) of cases. Three original parameters, including spectral and spatial characteristics, were highly correlated with functions predicting identity in both sexes. The accuracy of functions was not influenced by a signaller's sex, breeding status, or by sampling period which was spread over as much as two breeding seasons within individuals. Significant potential therefore exists for bell miners to use these simple provisioning calls in individual discrimination. If this is the case, call information also has the potential to be used by receivers as predicted by signalling hypotheses proposed to account for cooperative helping behaviour, such as the pay-to-stay and social prestige hypotheses.     

Individuality in trumpet calls of the crested auklet (<Emphasis Type="Italic">Aethia cristatella</Emphasis>), a highly social species

Crested auklet is a unique representative of the family Alcidae with a complex social behavior nesting in dense colonies with a size of up to one million individuals. Here we study specific individual features in their trumpet calls, loud vocalizations used by auklets as an advertising display. In 2008 and 2009, we recorded 231 calls of 24 individually marked male crested auklets from the Talan Island (Sea of Okhotsk). According to a stepwise discriminant function analysis based on seven variables in the call introduction, the rate of correct call assignment to an individual was 84.8% and that in the case of seven variables of the main part of the call was 82.3%, which considerably exceeds a random classification. Thus, a single introduction or a single main part of the call is sufficient for a reliable individual identification of a calling bird. We assume that such redundancy of information for individual recognition can enhance identification of calls of a large number of auklet conspecifics in dense colonies with a nesting density of several tens of pairs per 1 m².     

Orangutan Long Call Degradation and Individuality Over Distance: A Playback Approach

Researchers have documented individual vocal recognition in several primate species but do not know whether the changes in acoustical parameters that might occur over distance influence the informational content of a call that relates to individuality. Accordingly, we performed playback experiments using male orangutan long-distance calls (long calls) and rerecorded them at increasing distances from the source. We aimed to determine 1) which acoustical parameters changed over distance and 2) whether the percentage of calls that a discriminant analyses would assign to the correct individual would change over distance. High-frequency harmonics were attenuated and lost with increasing distance, but other parameters did not change. The percentage of calls assigned to the correct individual did not change over distance, indicating that even though there are some acoustical changes over distance the opportunity for other individuals to recognize the caller remains similar until ≤300 m, which was the maximum distance at which we rerecorded calls. Extending similar experiments to other primate species and other taxa, while subsequently conducting experiments to assess whether individual discrimination by receivers is indeed based on relatively stable acoustical parameters, would forward our understanding of acoustic communication.     

Automated classification of bird and amphibian calls using machine learning: A comparison of methods

We compared the ability of three machine learning algorithms (linear discriminant analysis, decision tree, and support vector machines) to automate the classification of calls of nine frogs and three bird species. In addition, we tested two ways of characterizing each call to train/test the system. Calls were characterized with four standard call variables (minimum and maximum frequencies, call duration and maximum power) or eleven variables that included three standard call variables (minimum and maximum frequencies, call duration) and a coarse representation of call structure (frequency of maximum power in eight segments of the call). A total of 10,061 isolated calls were used to train/test the system. The average true positive rates for the three methods were: 94.95% for support vector machine (0.94% average false positive rate), 89.20% for decision tree (1.25% average false positive rate) and 71.45% for linear discriminant analysis (1.98% average false positive rate). There was no statistical difference in classification accuracy based on 4 or 11 call variables, but this efficient data reduction technique in conjunction with the high classification accuracy of the SVM is a promising combination for automated species identification by sound. By combining automated digital recording systems with our automated classification technique, we can greatly increase the temporal and spatial coverage of biodiversity data collection.     

Differences in songflight calls and social calls between two phonic types of the vespertilionid bat Pipistrellus pipistrellus

The pipistrelle ( Pipistrellus pipistrellus ) occurs as two phonic types in Britain, its echolocation calls falling into two distinct frequency bands, with mean frequencies of maximum energy at 55 kHz and 46 kHz. These are termed the 55 kHz and 45 kHz phonic types here for simplicity. Songflight calls produced by males in the mating season, probably to attract females, differed between the two phonic types in the number of components in the calls and the call parameters measured. Songflight calls of the 55 kHz phonic type, which generally consisted of three components, were of higher frequencies than those of the 45 kHz phonic type, usually of four components. There were also significant differences in call parameters among individuals. A discriminant analysis of songflight calls classified 100% of individuals to the correct phonic type. The relationships between echolocation call frequency and songflight call frequency differed significantly between phonic types. Social calls produced during flight also differed between phonic types, in the number of components and call parameters measured. Social calls were compared to songflight calls of each phonic type. Social calls of the 55 kHz phonic type did not differ significantly from songflight calls; there were small but significant differences between the two types of calls of the 45 kHz phonic type. The study provides support for the hypothesis that the phonic types should be treated as sibling species. If songflight calls are used for mate choice, the differences may allow reproductive isolation between the two phonic types. The functions of songflight calls and social calls need to be investigated through experimental studies to explain the implications of the differences between phonic types.     

Advertisement Call Variation in the Golden Rocket Frog (Anomaloglossus beebei): Evidence for Individual Distinctiveness

Individual distinctiveness in acoustic signals can allow receivers to respond appropriately to different individuals. The aim of this research was to examine signal variation and to investigate the relative importance of different acoustic properties for coding individual distinctiveness in the advertisement calls of male golden rocket frogs (Anomaloglossus beebei). We examined patterns of within‐individual and among‐individual variability in 760 advertisement calls of 40 males as well as repeatability in 16 additional males that were recorded on two different occasions. We examined eight call properties, and all properties exhibited significant among‐individual variation and moderate to high repeatability across days. We employed discriminant function analysis to examine individual distinctiveness statistically. These analyses assigned 79% of calls to the correct individual. Fine temporal properties, including pulse duration, pulse rate, and pulse interval, as well as the spectral property of dominant frequency, contributed most toward discrimination among individuals. These results indicate that individual male golden rocket frogs can be distinguished statistically by their advertisement calls. We use these findings to evaluate results of previous playback studies in A. beebei and to generate testable predictions regarding the potential for specific call properties to function in mate selection and social recognition.     

Do Zebra Finch Parents Fail to Recognise Their Own Offspring?

Individual recognition systems require the sender to be individually distinctive and the receiver to be able to perceive differences between individuals and react accordingly. Many studies have demonstrated that acoustic signals of almost any species contain individualized information. However, fewer studies have tested experimentally if those signals are used for individual recognition by potential receivers. While laboratory studies using zebra finches have shown that fledglings recognize their parents by their "distance call", mutual recognition using the same call type has not been demonstrated yet. In a laboratory study with zebra finches, we first quantified between-individual acoustic variation in distance calls of fledglings. In a second step, we tested recognition of fledgling calls by parents using playback experiments. With a discriminant function analysis, we show that individuals are highly distinctive and most measured parameters show very high potential to encode for individuality. The response pattern of zebra finch parents shows that they do react to calls of fledglings, however they do not distinguish between own and unfamiliar offspring, despite individual distinctiveness. This finding is interesting in light of the observation of a high percentage of misdirected feedings in our communal breeding aviaries. Our results demonstrate the importance of adopting a receiver's perspective and suggest that variation in fledgling contact calls might not be used in individual recognition of offspring.     

The stability of the vocal signature in phee calls of the common marmoset,Callithrix jacchus

Phee calls were recorded from five captive common marmosets on three occasions. An initial recording session was followed by further sessions 1–12 days later, and finally, 12 months after the initial sample. Sonograms from the first recordings were measured using one duration and five frequency parameters, and significant differences between individuals were found for all six parameters. Discriminant function analysis was then applied to classify each call to a particular individual, witn a resulting classification accuracy of 97.27%. Analysis of the second and third recordings demonstrated accurate classification to the same caller using the measurements obtained from the initial sample. The accuracy remained high despite intra-individual differences in acoustic structure among the three recording periods. Such differences may well reflect proximate changes in the underlying arousal state of the caller. Stability over time in the vocal signature of the phee call supports the view that this vocalization may be important in signalling individual identity over long distances, in a habitat where visual contact is limited. © 1993 Wiley-Liss, Inc.     

Individual and Species Differences in Quail Calls (Coturnix c. japonica,c. c. coturnix and a Hybrid)

Male European and Japanese male quail have very stereotyped calls. This study examined which parameters best discriminated between individuals, and which between the two sub-species and a hybrid of the two (European father x Japanese mother). Recordings were made of several calls from individual Japanese, European and hybrid quail, from which sonograms were made and analyzed. We found that parameters describing the time structure of the call discriminated best between individuals. Time structure also showed the greatest differences between the two sub-species and the hybrid. Hybrid calls were extremely variable, individual hybrids often producing more than one call type, occasionally of both sub-species types. However, individuals could still be identified from the call parameters. In addition, we investigated responses of males to playbacks of calls from all three quail types. European and Japanese males responded most strongly, with calling, to the playbacks of the calls of their own sub-species, intermediately to playbacks of the hybrid and least to those of the other sub-species.     

A comparison of individual distinctiveness in three vocalizations of the dwarf mongoose (Helogale parvula)

Individual specificity can be found in the vocalizations of many avian and mammalian species. However, it is often difficult to determine whether these vocal cues to identity rise from “unselected” individual differences in vocal morphology or whether they have been accentuated by selection for the purposes of advertising caller identity. By comparing the level of acoustic individuality of different vocalizations within the repertoire of a single species, it is possible to ascertain whether selection for individual recognition has modified the vocal cues to identity in particular contexts. We used discriminant function analyses to determine the level of accuracy with which calls could be classified to the correct individual caller, for three dwarf mongoose (Helogale parvula) vocalizations: contact, snake, and isolation calls. These calls were similar in acoustic structure but divergent in context and function. We found that all three call types showed individual specificity but levels varied with call type (increasing from snake to contact to isolation call). The individual distinctiveness of each call type appeared to be directly related to the degree of benefit that signalers were likely to accrue from advertising their identity within that call context. We conclude that dwarf mongoose signalers have undergone selection to facilitate vocal individual recognition, particularly in relation to the species’ isolation call.     

IDENTIFICATION OF BRITISH BAT SPECIES BY MULTIVARIATE ANALYSIS OF ECHOLOCATION CALL PARAMETERS

ABSTRACT

Previous studies have found variability and individual distinctiveness in the echolocation calls of bats. We consider two implications of individually distinct echolocation calls: 1) whether bats may be able to use such variation to recognise familiar conspecifics, and 2) whether investigators could use such variation to identify known individuals or to census populations. We compared the discriminability of the echolocation calls of big brown bats (Eptesicus fuscus) recorded in three situations: (a) while held in the hand, (b) while perched on a platform, and (c) while flying in an anechoic chamber. Using variables describing each sonar call, we employed discriminant function analysis (DFA) to assign calls to recording situation or to bat. Discrimination of calls by recording situation was largely unsuccessful, although flying calls could be distinguished from platform calls. Assignment of calls to individual bat across recording situations yielded 72% success, and, within a given recording situation, yielded 87% success. Stepwise DFA reduced the number of variables needed to discriminate between individuals with only a slight decrease in correct classification. These results suggest that bats (or researchers) may be able to use the information contained in the echolocation calls for individual recognition. Individual distinctiveness raises the possibility of censusing bats by sound. We used cluster analysis in an attempt to determine whether, given a sample of calls from an unknown number of bats, a reasonable estimate of the number of bats could be obtained. Results were unsatisfactory, suggesting that cluster analysis probably will not permit acoustic censusing of bats in the field.     

Individual Variation of Whinnies Reflects Differences in Membership Between Spider Monkey (Ateles geoffroyi) Communities

Contact calls, which function to coordinate group movement and maintain contact between conspecifics, are predicted to show high levels of acoustic variability and individual distinctiveness. We investigated interindividual variation in whinnies, a contact call, between two geographically distinct communities of wild Geoffroyi’s spider monkeys (Ateles geoffroyi), which were experiencing different degrees of stability in membership due to immigration. We recorded whinnies from 18 subjects, including 9 females ranging within the Otoch Ma’ax Yetal Kooh Reserve, Punta Laguna, Mexico, and 9 females ranging within the Santa Rosa Sector, Area de Conservación Guanacaste, Costa Rica. We examined 13 acoustic parameters of female whinnies using principal component analysis and discriminant function analysis. Individual acoustic variability was significantly different between the two communities. A higher percentage of the whinnies of females were assigned to the correct caller in the community with only 3 individuals immigrating within 36 mo before and during data collection than in the community with 15 immigrant individuals during the same period. We suggest that the variation in interindividual distinctiveness for each community was influenced by the stability of the vocal environment, which was quantitatively different between communities because of changes in membership.     

The Complex Acoustic Design of an Advertisement Call in Male Mouse Lemurs (Microcebus murinus,Prosimii, Primates) and Sources of its Variation

During the breeding season sexually active male mouse lemurs produce a trill call used for advertising. The trill call displays a variable, high pitched, complex frequency and amplitude modulated acoustical structure and is given with a high sound intensity. The different sources of its variation in structure and usage have to be determined to understand its complex acoustical design and function. We investigated the vocal morphology of the trill call of males with known age and weight as well as the behavioral context, in order to evaluate sources of its variation. Cluster and discriminant analyses revealed that the structure of trill calls showed a high degree of individual stereotypy in contrast to a high interindividual variability. A set of three distinct variables (fundamental frequency of call onset, bandwidth of 3rd frequency modulation and end frequency of 7th modulation) was sufficient to identify individual senders with almost 90% probability. The structure of the trill call of adult males was independent of age, weight and context and was stable over at least one breeding season. Its individually distinct call structure and use in relation to social status provide the potential for individual recognition and for assessing the “quality” of a male.     

Geographic Variation in the Long Calls of Male Orangutans (Pongo spp.)

Relatively few data exist on population differences in the vocal behavior of mammals. Geographic variation in calls is of special interest because of the implications for resolving evolutionary and behavioral questions. For example, information on geographic variation in vocalizations complements morphological and molecular data used to infer phylogenetic relationships and provides evidence for the mechanisms underlying call development. A quantitative acoustic analysis of orangutan long calls was undertaken, comparing flanged adult males from four geographically distinct sites across Borneo and Sumatra, revealing consistent differences among the calls of individuals. Long calls produced by orangutans from the four sites in Borneo and Sumatra differ in quantitative acoustic measures. Discriminant function analysis reveals that acoustic variables can be used in combination to assign calls to the correct individual, site and island at rates higher than that expected by chance. Specifically, four acoustic parameters proved reliable for distinguishing among the individuals, between sites, and across the two islands that arguably represent populations from separate species or subspecies. Although Bornean and Sumatran long calls share a repetitive structure and show similar call rates (0.100–0.500 LCs/h) and maximum frequency bands (0.400–1.500 kHz), they differ significantly in the number of pulses per call, call speed, call duration, bandwidth, pulse duration, and dominant frequency. Strong consistency in these acoustic parameters is also seen among males within sites and the observed variation may allow for individual recognition. Individual identification by call structure presumably benefits dispersed orangutans, where individuals characteristically forage independently and both encounters and interactions with signaling males are highly variable and largely dependent on context. Acoustic recognition of callers facilitates the choice of which males to join or avoid, thus allowing receivers to manipulate potential costs and benefits of association.