Delphinid whistle production and call matching during playback of simulated military sonar

In 2007 and 2008, controlled exposure experiments were performed in the Bahamas to study behavioral responses to simulated mid‐frequency active sonar (MFA) by three groups of odontocetes: false killer whales, Pseudorca crassidens; short‐finned pilot whales, Globicephala macrorhynchus; and melon‐headed whales, Peponocephala electra. An individual in each group was tagged with a Dtag to record acoustic and movement data. During exposures, some individuals produced whistles that seemed similar to the experimental MFA stimulus. Statistical tests were thus applied to investigate whistle‐MFA similarity and the relationship between whistle production rate and MFA reception time. For the false killer whale group, overall whistle rate and production rate of the most MFA‐like whistles decreased with time since last MFA reception. Despite quite low whistle rates overall by the melon‐headed whales, statistical results indicated minor transient silencing after each signal reception. There were no apparent relationships between pilot whale whistle rates and MFA sounds within the exposure period. This variability of responses suggests that changes in whistle production in response to acoustic stimuli depend not only on species and sound source, but also on the social, behavioral, or environmental contexts of exposure.     

Automated identification and clustering of subunits within delphinid vocalizations

Tonal vocalizations or whistles produced by many species of delphinids range from simple tones to complex frequency contours. Whistle structure varies in duration, frequency, and composition between delphinid species, as well as between populations and individuals. Categorization of whistles may be improved by decomposition of complex calls into simpler subunits, much like the use of phonemes in classification of human speech. We identify a potential whistle decomposition scheme and normalization process to facilitate comparison of whistle subunits derived from tonal vocalizations of bottlenose dolphins (Tursiops truncatus), spinner dolphins (Stenella longirostris), and short‐beaked common dolphins (Delphinus delphis). Network analysis is then used to compare subunits within the vocal corpus of each species. By processing whistles through a series of steps including segmentation, normalization, and dynamic time warping, we are able to automatically cluster selected subunits by shape, regardless of differences in absolute frequency or moderate differences in duration. Using the clustered subunits, we demonstrate a preliminary species classification scheme based on rates of subunit occurrence in vocal repertoires. This provides a potential mechanism for comparing the structure of complex vocalizations within and between species.     

Whistle characteristics of common dolphins (Delphinus sp.) in the Hauraki Gulf,New Zealand

Quantifying the vocal repertoire of a species is critical for subsequent analysis of signal functionality, geographic variation, and social relevance. However, the vocalizations of free‐ranging common dolphins (Delphinus sp.) have not previously been described from New Zealand waters. We present the first quantitative analysis of whistle characteristics to be undertaken on the New Zealand population. Acoustic data were collected in the Hauraki Gulf, North Island from 28 independent dolphin group encounters. A total of 11,715 whistles were collected from 105.1 min of recordings. Seven whistle contours were identified containing 29 subtypes. Vocalizations spanned from 3.2 to 23 kHz, with most whistles occurring between 11 and 13 kHz. Whistle duration ranged from 0.01 to 4.00 s (mean ± SD; 0.27 ± 0.32). Of the 2,663 whistles analyzed, 82% have previously been identified within U.K. populations. An additional six contours, apparently unique to New Zealand Delphinus were also identified. Data presented here offer a first insight into the whistle characteristics of New Zealand Delphinus. Comparisons with previously studied populations reveal marked differences in the whistle frequency and modulation of the New Zealand population. Interpopulation differences suggest behavior and the local environment likely play a role in shaping the vocal repertoire of this species.     

Whistle discrimination and categorization by the Atlantic bottlenose dolphin (Tursiops truncatus): a review of the signature whistle framework and a perceptual test

Dolphin whistles vary by frequency contour, changes in frequency over time. Individual dolphins may broadcast their identities via uniquely contoured whistles, "signature whistles." A recent debate concerning categorization of these whistles has highlighted the on-going need for perceptual studies of whistles by dolphins. This article reviews research on dolphin whistles as well as presenting a study in which a captive, female, adult bottlenose dolphin performed a conditional matching task in which whistles produced by six wild dolphins in Sarasota Bay were each paired with surrogate producers, specific objects/places. The dolphin subject also categorized unfamiliar exemplars produced by the whistlers represented by the original stimuli. The dolphin successfully discriminated among the group of whistles, associated them with surrogate producers, grouped new exemplars of the same dolphin's whistle together when the contour was intact, and discriminated among same-contour whistles produced by the same dolphin. Whistle sequences that included partial contours were not categorized with the original whistlers. Categorization appeared to be based on contour rather than specific acoustic parameters or voice cues. These findings are consistent with the perceptual tenets associated with the signature whistle framework which suggests that dolphins use individualized whistle contours for identification of known conspecifics.     

Bottlenose dolphins exchange signature whistles when meeting at sea

The bottlenose dolphin, Tursiops truncatus, is one of very few animals that, through vocal learning, can invent novel acoustic signals and copy whistles of conspecifics. Furthermore, receivers can extract identity information from the invented part of whistles. In captivity, dolphins use such signature whistles while separated from the rest of their group. However, little is known about how they use them at sea. If signature whistles are the main vehicle to transmit identity information, then dolphins should exchange these whistles in contexts where groups or individuals join. We used passive acoustic localization during focal boat follows to observe signature whistle use in the wild. We found that stereotypic whistle exchanges occurred primarily when groups of dolphins met and joined at sea. A sequence analysis verified that most of the whistles used during joins were signature whistles. Whistle matching or copying was not observed in any of the joins. The data show that signature whistle exchanges are a significant part of a greeting sequence that allows dolphins to identify conspecifics when encountering them in the wild.     

Day and night sounds of the Guiana dolphin, <Emphasis Type="Italic">Sotalia guianensis</Emphasis> (Cetacea: Delphinidae) in southeastern Brazil

Many cetaceans are known to be acoustically active at night. However, for most dolphin species, there is little information about their nocturnal acoustic activities. To study the acoustic repertoire of Sotalia guianensis, diurnal and nocturnal sounds (whistles, burst pulses, low-frequency narrowband (LFN) sounds, and clicks) were identified in the Cananéia estuary (25° 01′ S–25° 13′ S/47° 52′ W–48° 06′ W), south of the state of São Paulo, southeastern Brazil, during April, June, and November of 2012. The emission rate of these sounds was compared between daytime and nighttime using the chi-squared statistical test. The mean values of the acoustic parameters of whistles, burst pulses, LFN sounds, and clicks were compared using the t test. Whistles, burst pulses, and LFN sounds were more frequent at night, as these individuals require greater acoustic communication in the absence of light, mainly for social communication. Echolocation emission rates were similar in both day and nighttime. Dolphin sound structure also varied throughout the day, with dolphins emitting lower-frequency sounds at night. Low-frequency sounds, with longer wavelengths, provide many advantages for dolphins active at night because such sounds propagate greater distances. This study demonstrates that the sounds produced by S. guianensis are dependent on the time of day, with social communication sounds being more influenced by the presence of light.     

EFFECTS OF WATERCRAFT NOISE ON THE ACOUSTIC BEHAVIOR OF BOTTLENOSE DOLPHINS, TURSIOPS TRUNCATUS, IN SARASOTA BAY, FLORIDA

Watercraft may provide the greatest source of anthropogenic noise for bottlenose dolphins living in coastal waters. A resident community of about 140 individuals near Sarasota, Florida, are exposed to a vessel passing within 100 m approximately every six minutes during daylight hours. I investigated the circumstances under which watercraft traffic may impact the acoustic behavior of this community, specifically looking for short-term changes in whistle frequency range, duration, and rate of production. To analyze whistles and received watercraft noise levels, acoustic recordings were made using two hydrophones towed from an observation vessel during focal animal follows of 14 individual dolphins. The duration and frequency range of signature whistles did not change significantly relative to vessel approaches. However, dolphins whistled significantly more often at the onset of approaches compared to during and after vessel approaches. Whistle rate was also significantly greater at the onset of a vessel approach than when no vessels were present. Increased whistle repetition as watercraft approach may simply reflect heightened arousal, an increased motivation for animals to come closer together, with whistles functioning to promote reunions. It may also be an effective way to compensate for signal masking, maintaining communication in a noisy environment.     

ACOUSTIC IDENTIFICATION OF NINE DELPHINID SPECIES IN THE EASTERN TROPICAL PACIFIC OCEAN

Acoustic methods may improve the ability to identify cetacean species during shipboard surveys. Whistles were recorded from nine odontocete species in the eastern tropical Pacific to determine how reliably these vocalizations can be classified to species based on simple spectrographic measurements. Twelve variables were measured from each whistle ( n = 908). Parametric multivariate discriminant function analysis (DFA) correctly classified 41.1% of whistles to species. Non-parametric classification and regression tree (CART) analysis resulted in 51.4% correct classification. Striped dolphin whistles were most difficult to classify. Whistles of bottlenose dolphins, false killer whales, and pilot whales were most distinctive. Correct classification scores may be improved by adding prior probabilities that reflect species distribution to classification models, by measuring alternative whistle variables, using alternative classification techniques, and by localizing vocalizing dolphins when collecting data for classification models.     

A New Quantitative Technique for Categorizing Whistles Using Simulated Signals and Whistles from Captive Bottlenose Dolphins (Delphinidae,Tursiops truncatus)

A widespread problem in the study of animal vocalizations is evaluating the acoustic similarity of signals both between individuals of a social group and between social groups. This problem becomes especially salient when classifying the narrow-band frequency-modulated signals, such as whistles, found in many avian and mammalian species. Whistles are usually characterized by their relative change in frequency over time, known as whistle ‘contour’. Measuring such a characteristic is difficult as it is not a single measurement, such as the mean frequency or duration of a signal, but several associated measurements of frequency across time. This paper reports on a new quantitative technique for determining whistle types based on whistle contour similarity and an application of this technique to the whistles of bottlenose dolphins to demonstrate its utility. This ‘contour similarity’ technique (CS technique) uses cluster analysis to group the correlation coefficients of frequency measurements from a data set of signals. To demonstrate the efficacy of this CS technique, three data sets were analysed, two using computer-generated signals and a third using adult bottlenose dolphin whistles, to (1) examine the efficacy of correlation coefficients for grouping signals by their similarity in whistle contour and (2) determine the viability of this technique for categorizing bottlenose dolphin whistles. Measured actual frequencies and correlation matrices from the four simulated signal types and a correlation matrix from the whistles of five captive adult bottlenose dolphins were each subjected to K-means cluster analysis and the resulting signal types were evaluated. Results indicated that the technique grouped actual frequencies according to the amount of shared actual frequencies and grouped correlation coefficients successfully according to signal contour. This result endured even if contours differed in overall duration or actual frequency or were expanded or compressed with respect to frequency or time. The results suggest that this approach is a viable method for assigning whistle contours to categories in bottlenose dolphins or any other species with narrow-band, frequency-modulated signals.     

Whistle variation in Mediterranean common bottlenose dolphin: The role of geographical,anthropogenic, social,and behavioral factors

The studies on the variation of acoustic communication in different species have provided insight that genetics, geographic isolation, and adaptation to ecological and social conditions play important roles in the variability of acoustic signals. The dolphin whistles are communication signals that can vary significantly among and within populations. Although it is known that they are influenced by different environmental and social variables, the factors influencing the variation between populations have received scant attention. In the present study, we investigated the factors associated with the acoustic variability in the whistles of common bottlenose dolphin (Tursiops truncatus), inhabiting two Mediterranean areas (Sardinia and Croatia). We explored which factors, among (a) geographical isolation of populations, (b) different environments in terms of noise and boat presence, and (c) social factors (including group size, behavior, and presence of calves), were associated with whistle characteristics. We first applied a principal component analysis to reduce the number of collinear whistle frequency and temporal characteristics and then generalized linear mixed models on the first two principal components. The study revealed that both geographic distance/isolation and local environment are associated with whistle variations between localities. The prominent differences in the acoustic environments between the two areas, which contributed to the acoustic variability in the first principal component (PC1), were found. The calf's presence and foraging and social behavior were also found to be associated with dolphin whistle variation. The second principal component (PC2) was associated only with locality and group size, showing that longer and more complex tonal sound may facilitate individual recognition and cohesion in social groups. Thus, both social and behavioral context influenced significantly the structure of whistles, and they should be considered when investigating acoustic variability among distant dolphin populations to avoid confounding factors.     

Vocalizations and Behaviour of Pacific Humpback Dolphins Sousa chinensis

Very little is known about the acoustic repertoire of the Pacific humpback dolphin Sousa chinensis . This study, off eastern Australia, used concurrent observations of surface behaviour and acoustic recordings to gain an insight into the behavioural significance of humpback dolphin vocalizations. Humpback dolphins exhibit five different vocalization categories: broad band clicks; barks; quacks; grunts; and whistles. Broad band clicks were high in frequency (8 kHz to > 22 kHz), were directly related to foraging behaviour and may play a role in social behaviour. Barks and quacks were burst pulse sounds (frequency: 0.6 kHz to > 22 kHz, duration: 0.1–8 s) and were associated with both foraging and social behaviour. The grunt vocalization is a low frequency narrow band sound (frequency 0.5–2.6 kHz, duration 0.06–2 s) and was only heard during socializing. There were 17 different types of whistles, ranging widely in frequency (0.9–22 kHz) and vocal structure (n=329). The predominant whistle types used by the groups were type 1 (46%) and type 2 (17%). Most whistles were heard during both socializing and foraging. The number of whistles recorded in a group increased significantly as the number of mother–calf pairs increased, suggesting that whistles may be used as contact calls. Few vocalizations were heard during either travelling or milling behaviours. Broad band clicks, barks and whistle type 1 were the only vocalizations recorded during either travelling or milling.     

Identifying signature whistles from recordings of groups of unrestrained bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) have individually distinctive signature whistles. Each individual dolphin develops its own unique frequency modulation pattern and uses it to broadcast its identity. However, underwater sound localization is challenging, and researchers have had difficulties identifying signature whistles. The traditional method to identify them involved isolating individuals. In this context, the signature whistle is the most commonly produced whistle type of an animal. However, most studies on wild dolphins cannot isolate animals. We present a novel method, SIGnature IDentification (SIGID), that can identify signature whistles in recordings of groups of dolphins recorded via a single hydrophone. We found that signature whistles tend to be delivered in bouts with whistles of the same type occurring within 1–10 s of each other. Nonsignature whistles occur with longer or shorter interwhistle intervals, and this distinction can be used to identify signature whistles in a recording. We tested this method on recordings from wild and captive bottlenose dolphins and show thresholds needed to identify signature whistles reliably. SIGID will facilitate the study of signature whistle use in the wild, signature whistle diversity between different populations, and potentially allow signature whistles to be used in mark‐recapture studies.     

Variability and context specificity of narwhal (Monodon monoceros) whistles and pulsed calls

The behavioral and environmental context of animal calls provides insights into their functions. Narwhals are a highly vocal species and, like other social cetaceans, rely on acoustic signals to communicate. We characterize and categorize narwhal whistles and pulsed calls, as well as investigate variation in these calls under different contexts (behavior, herd, and year) using recordings made during the month of August 2006–2008, in Koluktoo Bay (72°04′N, 80°32′W). We detected similarities among whistles but not pulsed calls that were produced under a similar behavioral context. Both whistles and pulsed calls recorded within the same herd were more similar than whistles and pulsed calls recorded within different herds. We did not find any type of whistle to be associated with a specific behavior although some acoustical features might be behavior specific. Both whistles and pulsed calls show properties that are consistent with the hypothesis that narwhals produce group‐ or individual‐specific calls.     

Bottlenose dolphins that forage with artisanal fishermen whistle differently

Acoustic communication is a taxonomically widespread phenomenon, crucial for social animals. We evaluate social sounds from bottlenose dolphins (Tursiops truncatus) of Laguna, southern Brazil, whose social structure is organized around a cooperative foraging tactic with artisanal fishermen. This tactic involves stereotyped and coordinated behaviour by dolphins and fishermen and is performed by a subset of the dolphin population, splitting it into two distinct social communities. We compared the acoustic parameters and type of whistles emitted by dolphins of the “non‐cooperative” and “cooperative” communities, both during their interactions with fishermen and in times where dolphins were engaged in other types of foraging. Our findings show how dolphins’ social sounds differ between foraging tactics and social communities. The frequencies of six whistle types (ascending, descending, concave, convex, multiple, flat) were significantly dependent on tactics and communities. Ascending whistles were more common than expected during foraging without fishermen, and among dolphins of the non‐cooperative community. Whistle acoustic parameters (duration, number of inclination changes and inflection points, and initial, final, maximum, minimum frequencies) also varied between social communities. In general, whistles emitted by cooperative dolphins, mainly when not interacting with fishermen, tended to be shorter, had higher frequency and more inflections than those emitted by non‐cooperative dolphins. These results suggest that different whistles may convey specific information among dolphins related to foraging, which we hypothesize promote social cohesion among members of the same social community. These differences in acoustic repertoires add a new dimension of complexity to this unique human–animal interaction.     

Spatial and temporal patterns in the calling behavior of beluga whales,Delphinapterus leucas,in Cook Inlet,Alaska

Cook Inlet beluga whales (CIBs) are an endangered population residing in Cook Inlet, Alaska. We characterized the calling behavior of CIBs to improve our understanding of sounds produced by this population. Bottom‐moored hydrophones were deployed at Eagle Bay in summer 2009 and at Trading Bay in summer and winter 2009. CIB sounds were qualitatively analyzed and categorized as a whistle, pulsed call, or click train. A total of 4,097 calls were analyzed, and 66 unique whistle contours were identified. Whistles were quantitatively analyzed using a custom Matlab program. A chi‐square test showed the call category usage at Eagle Bay during summer 2009 and those at Trading Bay during summer 2009 and winter 2009–2010 differed significantly (P < 0.001). Pulsed calls were more common during summer months, and click trains within the frequency band (12.5 kHz) were more common in Eagle Bay. The variation in calling behavior suggests differences in habitat usage or in the surrounding environment, including background noise. With the proposed development projects in Cook Inlet and the potential increase in ambient noise level due to ocean acidification, it is important to understand how this endangered population uses sound, and what anthropogenic factors may influence that use.     

Dolphin whistles: a functional misnomer revealed by heliox breathing

Delphinids produce tonal whistles shaped by vocal learning for acoustic communication. Unlike terrestrial mammals, delphinid sound production is driven by pressurized air within a complex nasal system. It is unclear how fundamental whistle contours can be maintained across a large range of hydrostatic pressures and air sac volumes. Two opposing hypotheses propose that tonal sounds arise either from tissue vibrations or through actual whistle production from vortices stabilized by resonating nasal air volumes. Here, we use a trained bottlenose dolphin whistling in air and in heliox to test these hypotheses. The fundamental frequency contours of stereotyped whistles were unaffected by the higher sound speed in heliox. Therefore, the term whistle is a functional misnomer as dolphins actually do not whistle, but form the fundamental frequency contour of their tonal calls by pneumatically induced tissue vibrations analogous to the operation of vocal folds in terrestrial mammals and the syrinx in birds. This form of tonal sound production by nasal tissue vibrations has probably evolved in delphinids to enable impedance matching to the water, and to maintain tonal signature contours across changes in hydrostatic pressures, air density and relative nasal air volumes during dives.     

Communication in bottlenose dolphins: 50 years of signature whistle research

Bottlenose dolphins (Tursiops truncatus) produce individually distinctive signature whistles that broadcast the identity of the caller. Unlike voice cues that affect all calls of an animal, signature whistles are distinct whistle types carrying identity information in their frequency modulation pattern. Signature whistle development is influenced by vocal production learning. Animals use a whistle from their environment as a model, but modify it, and thus invent a novel signal. Dolphins also copy signature whistles of others, effectively addressing the whistle owner. This copying occurs at low rates and the resulting copies are recognizable as such by parameter variations in the copy. Captive dolphins can learn to associate novel whistles with objects and use these whistles to report on the presence or absence of the object. If applied to signature whistles, this ability would make the signature whistle a rare example of a learned referential signal in animals. Here, we review the history of signature whistle research, covering definitions, acoustic features, information content, contextual use, developmental aspects, and species comparisons with mammals and birds. We show how these signals stand out amongst recognition calls in animals and how they contribute to our understanding of complexity in animal communication.     

Identification and Characteristics of Signature Whistles in Wild Bottlenose Dolphins (Tursiops truncatus) from Namibia

A signature whistle type is a learned, individually distinctive whistle type in a dolphin''s acoustic repertoire that broadcasts the identity of the whistle owner. The acquisition and use of signature whistles indicates complex cognitive functioning that requires wider investigation in wild dolphin populations. Here we identify signature whistle types from a population of approximately 100 wild common bottlenose dolphins (Tursiops truncatus) inhabiting Walvis Bay, and describe signature whistle occurrence, acoustic parameters and temporal production. A catalogue of 43 repeatedly emitted whistle types (REWTs) was generated by analysing 79 hrs of acoustic recordings. From this, 28 signature whistle types were identified using a method based on the temporal patterns in whistle sequences. A visual classification task conducted by 5 naïve judges showed high levels of agreement in classification of whistles (Fleiss-Kappa statistic, κ = 0.848, Z = 55.3, P<0.001) and supported our categorisation. Signature whistle structure remained stable over time and location, with most types (82%) recorded in 2 or more years, and 4 identified at Walvis Bay and a second field site approximately 450 km away. Whistle acoustic parameters were consistent with those of signature whistles documented in Sarasota Bay (Florida, USA). We provide evidence of possible two-voice signature whistle production by a common bottlenose dolphin. Although signature whistle types have potential use as a marker for studying individual habitat use, we only identified approximately 28% of those from the Walvis Bay population, despite considerable recording effort. We found that signature whistle type diversity was higher in larger dolphin groups and groups with calves present. This is the first study describing signature whistles in a wild free-ranging T. truncatus population inhabiting African waters and it provides a baseline on which more in depth behavioural studies can be based.     

Dolphins Adjust Species-Specific Frequency Parameters to Compensate for Increasing Background Noise

An increase in ocean noise levels could interfere with acoustic communication of marine mammals. In this study we explored the effects of anthropogenic and natural noise on the acoustic properties of a dolphin communication signal, the whistle. A towed array with four elements was used to record environmental background noise and whistles of short-beaked common-, Atlantic spotted- and striped-dolphins in the Canaries archipelago. Four frequency parameters were measured from each whistle, while Sound Pressure Levels (SPL) of the background noise were measured at the central frequencies of seven one-third octave bands, from 5 to 20 kHz. Results show that dolphins increase the whistles’ frequency parameters with lower variability in the presence of anthropogenic noise, and increase the end frequency of their whistles when confronted with increasing natural noise. This study provides the first evidence that the synergy among SPLs has a role in shaping the whistles'' structure of these three species, with respect to both natural and anthropogenic noise.     

Differences in the sounds of Guiana dolphin Sotalia guianensis (Cetacea: Delphinidae) between two areas of south-eastern and southern Brazil

Cetacean populations can adjust their sound repertoire depending on the environment they are in, their population structure and the activities they are performing. Our goal was to characterize and compare, qualitatively and quantitatively, the sound repertoire of Sotalia guianensis in two areas of south-eastern and southern Brazil. We expected to find similar sound repertoires between the two regions, as they are geographically close and are part of the same complex estuarine. Acoustical parameters of the whistles, burst pulses and clicks were recorded during both daytime and night-time hours. They were compared between areas through Chi-square and Mann–Whitney tests. The samples resulted in 3,630 recorded whistles, 631 burst pulses and 44 low-frequency narrow-band sounds, with echolocation clicks present in 50.98% of the total minutes analysed. The occurrence rate of all sounds and the acoustic parameters of the whistles and clicks differed between the two areas, so our initial hypothesis was rejected. We highlighted environmental differences, behaviour exhibited by animals, number of individuals and group size and low exchange of individuals between areas as possible explanations that might account for these results, based on our knowledge of the species and areas of study, as well as a substantial literature on the physical and biological characteristics of the sounds.