Dolphin Whistles Research Articles

How humans discriminate acoustically among bottlenose dolphin signature whistles with and without masking by boat noise.

Anthropogenic noise in the world's oceans is known to impede many species' ability to perceive acoustic signals, but little research has addressed how this noise affects the perception of bioacoustic signals used for communication in marine mammals. Bottlenose dolphins (Tursiops truncatus) use signature whistles containing identification information. Past studies have used human participants to gain insight into dolphin perception, but most previous research investigated echolocation. In Experiment 1, human participants were tested on their ability to discriminate among signature whistles from three dolphins. Participants' performance was nearly errorless. In Experiment 2, participants identified signature whistles masked by five different samples of boat noise utilizing different signal-to-noise ratios. Lower signal-to-noise ratio and proximity in frequency between the whistle and noise both significantly decreased performance. Like dolphins, human participants primarily identified whistles using frequency contour. Participants reported greater use of amplitude in noise-present vs noise-absent trials, but otherwise did not vary cue usage. These findings can be used to generate hypotheses about dolphins' performance and auditory cue use for future research. This study may provide insight into how specific characteristics of boat noise affect dolphin whistle perception and may have implications for conservation and regulations.

Time-frequency modulation based mimicking dolphin whistle for covert underwater acoustic communication

For covert underwater communications, conventional bio-mimicking methods have been developed. Signal shapes of these methods are similar to real whistles, but not the same since they divide a dolphin whistle into symbols, which are modulated by conventional digital modulations. This paper proposes a dolphin whistle mimicking modulation method that transmits different consecutive whistles with the same frequency contour of the real dolphin whistles and different time-frequency locations. This paper also develops a demodulator for decoding the proposed mimicking whistles. The mimicking performance of the proposed time-frequency shift keying was compared with those of the conventional methods by using correlation coefficients, and the proposed scheme showed the better mimicking performance than the conventional chirp spread spectrum and continuously varying-carrier frequency modulation. Using computer simulations and practical ocean experiments, the bit error rate performance of the proposed demodulator was better than that of the cross-correlation method.

High‐frequency whistles of Irrawaddy dolphins (Orcaella brevirostris) recorded in Brunei Bay

AbstractStudies of the whistles of Irrawaddy dolphins (Orcaella brevirostris) have traditionally been restricted to analyses of the human audio range (below 20 kHz). In this study, broadband acoustic recordings of Irrawaddy dolphins in Brunei Bay were obtained using a sampling frequency of 512 kHz. Among the 984 whistles collected, eight (0.81%) high‐frequency whistles were detected with fundamentals above 18.4 kHz, which is the reported maximum frequency. Four (50%) of these whistles had the entire fundamentals in the ultrasonic frequency range (above 20 kHz), where 52.3 kHz was the maximum frequency. This is the first description of the ultrasonic components of Irrawaddy dolphin whistles. Our findings demonstrate that the whistles of Irrawaddy dolphins span a wider frequency range than previously published.

Matching Signature Whistles with Photo-Identification of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) in the Fremantle Inner Harbour, Western Australia

The Swan–Canning River System is home to an Indo-Pacific bottlenose dolphin (Tursiops aduncus) community of currently 17 adult and juvenile individuals. While a complete photo-identification catalogue exists, visual monitoring requires repeated boat-based surveys and is thus laborious and expensive. Bottlenose dolphins are known to emit individually distinctive signature whistles, and therefore, passive acoustic monitoring could be a reliable and more efficient tool. Archived acoustic and photographic data from the Fremantle Inner Harbour were reviewed for instances when dolphin whistles and individual identifying images were simultaneously available. As dolphin whistles are commonly used in social encounters, dolphins producing whistles in this study were always in groups. Consequently, to assess whether distinctive whistles could be attributed to individual dolphins, conditional probabilities for recording a specific whistle in the presence of certain individuals, as well as Bayesian posterior probabilities for encountering a specific individual at times of certain whistles were computed. While a larger sample size is needed to capture all individuals in diverse groupings, this study provides the first step in developing a passive acoustic program for monitoring this small dolphin community, in order to ultimately inform its conservation management.

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.

Dolphin whistle repertoires around São Miguel (Azores): Are you common or spotted?

Short-beaked common dolphins (Delphinus delphis) and Atlantic spotted dolphins (Stenella frontalis) are both common in the Azores archipelago during summer. Because both species are sympatric, at least in part of their range, they may use acoustic features to recognize conspecifics and maintain school cohesion throughout their different activities. Delphinid whistles were recorded with a 96-kHz sampling rate using towed hydrophone system during surveys held in summer of 2013 and 2014 around São Miguel Island (Azores, Portugal). A total of 256 whistles attributed to either short-beaked common dolphin (n = 133) or Atlantic spotted dolphin (n = 123) were selected and processed with a contour extraction software. Elementary statistical analysis showed that duration, frequency and slope variables were significantly different for both species, although in most cases their range overlapped. We performed a discriminant analysis to test species classification: the dataset was randomly split into one calibration subset (186 whistles) and one validation subset (70 whistles). The discriminant analysis retained four variables (global slope, duration, minimal and final frequencies) as useful for classification. The discriminant function resulted in correct classification rates of 78.5% (calibration subset) and 81.4% (validation subset). Common dolphin whistles were better classified than Atlantic spotted dolphin whistles (83.4% and 74.8%) respectively. This study shows that reliable species identification can be achieved for common and spotted dolphins using their whistle repertoire characteristics.

Active contour-based detection of estuarine dolphin whistles in spectrogram images

An algorithm for detecting tonal vocalizations from estuarine dolphin (Sotalia guianensis) specimens without interference of a human operator was developed. The raw audio data collected from a passive monitoring sensor in the Cananéia underwater soundscape was converted to spectrogram images. Detection is a four-step method: first, ridge maps are obtained from the spectrogram images; second, a probabilistic Hough transform algorithm is applied to detect ridges similar to thick line segments, referred to as line-like, which are adjusted to the geometry of the whistles in the images via an active contour algorithm; third, feature vectors are built from the geometry of each detected whistle, with 9 descriptive features; and fourth, the detections are fed to a random forest classifier to parse out mistakes by the detection process. We developed a system for classifying the characteristic patterns detected as Sotalia guianensis whistles or random empty detections. We obtained accuracy of 0.977 and F1-score of 0.981.

Whistle variability of the Mediterranean short beak common dolphin

Abstract The short‐beaked common dolphin is a highly vocal species, with a wide distribution in all oceans, including the Mediterranean and the Black Seas. In the Mediterranean Sea, the short‐beaked common dolphin inhabits both pelagic and neritic waters. Osteological collections and the literature show that short‐beaked common dolphins were widespread and abundant in much of the Mediterranean Sea until the late 1960s. During recent decades the species has declined in the whole basin, and, in 2003, it was listed as Endangered in the IUCN Red List. Genetic studies strongly suggest that the Mediterranean and the Eastern North Atlantic populations are isolated from each other. Genetic differentiation within the Mediterranean Sea, between the Eastern Mediterranean (Ionian Sea) and Western Mediterranean populations, is also reported. The aim of this study was to investigate the geographical variation in the characteristics of whistles of free‐ranging short‐beaked common dolphins living in the Mediterranean Sea, and to evaluate if whistle acoustic structure is the result of adaptation to local environment characteristics or of a possible genetic diversification. Recordings were collected from 1994 to 2012 throughout the basin, employing multiple platforms. Twenty‐six independent acoustic detections were made, and 704 whistles were extracted and considered for statistical analysis. Whistle analysis enabled the identification of distinct geographical units of short‐beaked common dolphin within the Mediterranean Sea. Genetic isolation is probably the major cause of the geographic variance of the Mediterranean short‐beaked common dolphin whistle structure, which may reflect some evolutionary adaptations to particular ecological conditions or may be the by‐product of morphological evolution. The results of the present study show that intra‐Mediterranean variability of whistle structure reflects the path of genetic studies, highlighting the possible use of acoustic data in combination with other sources of data (genetic, morphological, etc.) to identify geographic areas where discrete management units occur.

Automated classification of dolphin whistles based on the convolutional neural network

A number of classification systems for dolphin whistles are applied to study the relation between dolphin whistles and their behaviors. Traditional approaches require prior knowledge, numerous pre-processing and manually extracting the features by conventional signal processing methods. In this study, deep convolutional neural networks are used to classify whistles and automatically learn the sound characteristics from training data with less pre-processing. The classification system is trained by using a database of 9 sorts of measured dolphin signals with nearly 4000 samples, and whistle contours in testing dataset are divided into six types, including constant frequency, upsweep, downsweep, concave, convex, and multiple. Finally, more than 90% for the classification accuracy rate is reached, and results show insensitivity to background noise. Therefore, the algorithm can be employed to study the potential relation between dolphin whistle signals and behaviors, and then facilitate future studies on dolphin habits.

Addressing challenges in automated whistle extraction

Automated extraction of dolphin whistles from field recordings presents a challenging problem, as typically these recordings are noisy, contain multiple overlapping whistles and interfering signals. This results in many spurious detections, partial extraction and fragmentation of the whistle contours, which could cause problems for applications such as whistle-based localization. This paper presents the probability hypothesis density (PHD) filter, a non-traditional whistle tracking method based on multi-target Bayesian framework. Two implementations of the PHD filter, one based on Gaussian mixtures and one based on particle methods, are adapted for this task and tested on a large real-world dataset consisting of six dolphin species. The proposed filters successfully track simultaneous whistles, and compare favourably to standard methods. Moreover, it is shown how the proposed methods can be used to enhance the localization efforts with linear towed arrays.

A biomimetic communication method based on time shift using dolphin whistle

A biomimetic communication method based on time shift using dolphin whistle

Influence of environmental, social and behavioural variables on the whistling of the common bottlenose dolphin (Tursiops truncatus)

Bottlenose dolphin communication has been studied extensively. Nevertheless, the combined effect of several variables on whistle structure has yet to be clearly understood. Here, we investigated the influence of underwater noise, boat presence, behaviour, group size and calf presence on the whistles of a small population of common bottlenose dolphins (Tursiops truncatus). Surface behaviour and acoustic data were collected in Sardinia (Italy-Western Mediterranean Sea) between 2015 and 2017. Whistle frequency parameters (minimum, maximum, start, end frequencies and frequency range), duration, number of inflection points and number of harmonics were correlated with the ambient noise levels recorded prior to the whistle display and to behavioural and social factors by means of GAM models. Whistle frequencies varied as a function of the interaction between noise and boat presence, mainly when the noise increased at lower frequencies. Dolphin whistles also varied according to group size, which was further affected by the presence of calves, and behaviours characterised by high levels of arousal such as feeding. The study provides sufficient evidence to support that multiple factors need to be considered when investigating whistle structure as the species is capable of adopting different strategies depending on environmental, social and behavioural context. Finally, this data indicates that communication within a small population of bottlenose dolphins is vulnerable to acoustic pollution, thereby highlighting the need for further investigation into the effects of different noises on whistle structure, as well as the consistency of patterns across different populations. Exploring a dolphin’s ability to adapt its communication to environmental context is crucial to understanding the consequences of ocean noise increase, especially in highly anthroposised areas such as the Mediterranean Sea. This is one of the few studies that investigate how noise and boat traffic affect bottlenose dolphin whistles whilst considering the concomitant effect of other variables such as behaviour, group size and composition. We demonstrated that dolphins vary their whistles as a function of increasing noise and boat presence, group size and calf presence, and during specific behaviour which requires intense contact between individuals. Finally, the results showed that other variables should be considered to fully understand those which influence whistle structure and adaptability.

Mimicking dolphin whistles with continuously varying carrier frequency modulation for covert underwater acoustic communication

This paper proposes a covert underwater acoustic communication scheme to mimic dolphin whistles by continuously varying the carrier frequency and by using antipodal symbol modulation. The continuously varying carrier frequency precisely mimics dolphin whistles, while the conventional method utilizes discrete carrier-frequencies. The antipodal symbol modulation increases bit error rate (BER) performance by a larger Euclidean distance between symbols than conventional chirp spread spectrum (CSS). BER performance was tested by computer simulations and practical ocean experiments, and the degree of mimic (DoM) was evaluated by correlation coefficients and the mean opinion score (MOS) test. BER of the proposed method attains about a 3 dB SNR gain compared with that of conventional CSS, and the DoM of the proposed scheme demonstrates a larger correlation coefficient and a 39% greater MOS score than that of conventional CSS.

Whistles emitted by Indo-Pacific humpback dolphins (Sousa chinensis) in Zhanjiang waters, China.

Whistles emitted by Indo-Pacific humpback dolphins in Zhanjiang waters, China, were collected by using autonomous acoustic recorders. A total of 529 whistles with clear contours and signal-to-noise ratio higher than 10 dB were extracted for analysis. The fundamental frequencies and durations of analyzed whistles were in ranges of 1785-21 675 Hz and 30-1973 ms, respectively. Six tonal types were identified: constant, downsweep, upsweep, concave, convex, and sine whistles. Constant type was the most dominant tonal type, accounting for 32.51% of all whistles, followed by sine type, accounting for 19.66% of all whistles. This paper examined 17 whistle parameters, which showed significant differences among the six tonal types. Whistles without inflections, gaps, and stairs accounted for 62.6%, 80.6%, and 68.6% of all whistles, respectively. Significant intraspecific differences in all duration and frequency parameters of dolphin whistles were found between this study and the study in Malaysia. Except for start frequency, maximum frequency and the number of harmonics, all whistle parameters showed significant differences between this study and the study conducted in Sanniang Bay, China. The intraspecific differences in vocalizations for this species may be related to macro-geographic and/or environmental variations among waters, suggesting a potential geographic isolation among populations of Indo-Pacific humpback dolphins.

Human auditory discrimination of bottlenose dolphin signature whistles masked by noise: Investigating perceptual strategies for anthropogenic noise pollution

Anthropogenic masking noise in the world’s oceans is known to impede many species ability to perceive acoustic signals, but little research has addressed how this noise pollution affects the detection of bioacoustic signals used for communication. Bottlenose dolphins use signature whistles which contain identification information. Past studies have shown that human participants can be used as models for dolphin hearing, but most previous research investigated echolocation. In experiment 1, human participants were tested on their ability to discriminate among signature whistles from three dolphins. Participants’ performance was nearly errorless (M = 98.8%). In experiment 2, participants identified signature whistles masked by five different samples of boat noise, with different signals to noise ratios. Preliminary results suggest that participants perform worse in lower ratios of signal to noise, that some signature whistles are easier to identify in the presence of noise, and that some noises have more detrimental impacts on whistle recognition. The presence of boat noise may cause participants to use more auditory cues in order to identify whistles, although participants always relied most heavily on frequency contour and duration. This study may provide insight into the impacts of different types of boat noise on dolphin whistle perception.

Spatiotemporal Dynamics of the Local Population of Black Sea Bottlenose Dolphins (Tursiops truncatus ponticus Barabash, 1940): Visual and Acoustic Description Methods

The local Sudak–Novyi Svet (Crimea) population of Black Sea bottlenose dolphins was studied. Observations and acoustic recordings were made throughout 2014 and 2015. This study is the first in Russia to use acoustic identification of individuals based on a “signature whistle” catalog in addition to visual identification. Bottlenose dolphin signatures consisted of tonal signals (whistles) with a frequency contour shape unique for each dolphin. These whistles predominated in the acoustic repertoire of each individual. In this aspect, the signatures can be regarded as acoustic markers of individuals. Analysis of the entire set of dolphin whistle recordings (nearly 30 000 signals) revealed 206 dominant types, i.e., signature whistles. Visual and acoustic data were compared to characterize groups constituting the studied population, to describe the seasonal pattern of visits of different groups to the water area, and to identify transient and resident groups.

2P6-1 Time-Delay based Mimicking Dolphin Whistle for Covert Underwater Communication

2P6-1 Time-Delay based Mimicking Dolphin Whistle for Covert Underwater Communication

Whistle variability of Guiana dolphins in South America: Latitudinal variation or acoustic adaptation?

AbstractA previous comparison of whistles using data sampled at 48 kHz suggested that certain frequency parameters vary along a latitudinal gradient. This geographical pattern may be biased because whistles sampled at higher frequencies could potentially have very different frequency contents. The current study compared the acoustic parameters of Guiana dolphin (Sotalia guianensis) whistles recorded at a higher sampling rate (96 kHz) and from groups occupying two never before sampled sites, Benevente Bay, Espírito Santo, Brazil, and Formosa Bay, Rio Grande do Norte, Brazil, with recordings of other populations in South America. By only considering data sampled at a rate of at least 96 kHz, we aimed to detect differences in whistles across locations. Contrary to previous findings, our analyses do not indicate any clear separation between northern and southern populations based on whistles, and do not corroborate the hypothesis of latitudinal acoustic variation in this species. The variation in Guiana dolphin whistle parameters found here appears to be influenced by latitude to some extent, but several other factors, including sampling method, environmental fluctuations, and social influence on vocal learning, may be confounding the detection of a geographic pattern in these whistle samples.

Dolphins simplify their vocal calls in response to increased ambient noise

Ocean noise varies spatially and temporally and is driven by natural and anthropogenic processes. Increased ambient noise levels can cause signal masking and communication impairment, affecting fitness and recruitment success. However, the effects of increasing ambient noise levels on marine species, such as marine mammals that primarily rely on sound for communication, are not well understood. We investigated the effects of concurrent ambient noise levels on social whistle calls produced by bottlenose dolphins (Tursiops truncatus) in the western North Atlantic. Elevated ambient noise levels were mainly caused by ship noise. Increases in ship noise, both within and below the dolphins' call bandwidth, resulted in higher dolphin whistle frequencies and a reduction in whistle contour complexity, an acoustic feature associated with individual identification. Consequently, the noise-induced simplification of dolphin whistles may reduce the information content in these acoustic signals and decrease effective communication, parent–offspring proximity or group cohesion.

Can whistles be used to estimate dolphin abundance?

Dolphin whistles have been studied extensively for both wild and captive animals. They are used in the communication between individuals, to maintain contact within individuals of a herd, and to coordinate herd movements. However, little is still known on how whistles can be used to ascribe individuals. Therefore, the present study is focused on determining the stereotypy of the most frequently emitted whistles by captive bottlenose dolphins,Tursiops truncatus, housed in two different marine parks. Stereotypy was computed by changing the similarity index while classifying whistles into whistle types using Matlab BELUGA and ArtWARP. Signature whistles are an individual distinction and have a similarity index greater than 95%, therefore, allowing to assess the minimum number of whistling dolphins in a pod that emit signature whistles. This is specially important in the wild, where in some occasions, it is very difficult to assess how many individuals are present. Thus this very simple method will be useful to quantify the number of signature whistles from underwater recordings, and to relate it with the possible number of dolphins present. It is necessary to implement methods like this one to better understand how dolphins are using whistles, since acoustic communication is the most important sense in dolphin species. [Work supported by PAPIIT&PASPA-UNAM.]