Low frequency narrow-band calls in bottlenose dolphins (Tursiops truncatus): signal properties, function, and conservation implications

Dolphins routinely use sound for social purposes, foraging and navigating. These sounds are most commonly classified as whistles (tonal, frequency modulated, typical frequencies 5-10 kHz) or clicks (impulsed and mostly ultrasonic). However, some low frequency sounds have been documented in several species of dolphins. Low frequency sounds produced by bottlenose dolphins (Tursiops truncatus) were recorded in three locations along the Gulf of Mexico. Sounds were characterized as being tonal with low peak frequencies (mean?=?990 Hz), short duration (mean?=?0.069 s), highly harmonic, and being produced in trains. Sound duration, peak frequency and number of sounds in trains were not significantly different between Mississippi and the two West Florida sites, however, the time interval between sounds within trains in West Florida was significantly shorter than in Mississippi (t?=?-3.001, p?=?0.011). The sounds were significantly correlated with groups engaging in social activity (F=8.323, p=0.005). The peak frequencies of these sounds were below what is normally thought of as the range of good hearing in bottlenose dolphins, and are likely subject to masking by boat noise.     

Discriminating features of echolocation clicks of melon-headed whales (Peponocephala electra), bottlenose dolphins (Tursiops truncatus), and Gray's spinner dolphins (Stenella longirostris longirostris)

Spectral parameters were used to discriminate between echolocation clicks produced by three dolphin species at Palmyra Atoll: melon-headed whales (Peponocephala electra), bottlenose dolphins (Tursiops truncatus) and Gray's spinner dolphins (Stenella longirostris longirostris). Single species acoustic behavior during daytime observations was recorded with a towed hydrophone array sampling at 192 and 480 kHz. Additionally, an autonomous, bottom moored High-frequency Acoustic Recording Package (HARP) collected acoustic data with a sampling rate of 200 kHz. Melon-headed whale echolocation clicks had the lowest peak and center frequencies, spinner dolphins had the highest frequencies and bottlenose dolphins were nested in between these two species. Frequency differences were significant. Temporal parameters were not well suited for classification. Feature differences were enhanced by reducing variability within a set of single clicks by calculating mean spectra for groups of clicks. Median peak frequencies of averaged clicks (group size 50) of melon-headed whales ranged between 24.4 and 29.7 kHz, of bottlenose dolphins between 26.7 and 36.7 kHz, and of spinner dolphins between 33.8 and 36.0 kHz. Discriminant function analysis showed the ability to correctly discriminate between 93% of melon-headed whales, 75% of spinner dolphins and 54% of bottlenose dolphins.     

中华白海豚和东亚窄脊江豚回声定位信号分析与比较

为了增进珍稀齿鲸物种的了解和保护,对中华白海豚(Sousa chinensis)和东亚窄脊江豚(Neophocaena asiaeorientalis sunmeri)的回声定位信号特性进行了分析和比较.通过船只观测与声学监听的方式对厦门海域中华白海豚和东亚窄脊江豚的回声定位信号进行了调查,并对其声学参数进行了统计和对...     

Geographic variations in the whistles of spinner dolphins (Stenella longirostris) of the Main Hawai'ian Islands

Geographic variations in the whistles of Hawai'ian spinner dolphins are discussed by comparing 27 spinner dolphin pods recorded in waters off the Islands of Kaua'i, O'ahu, Lana'i, and Hawai'i. Three different behavioral states, the number of dolphins observed in each pod, and ten parameters extracted from each whistle contour were considered by using clustering and discriminant function analyses. The results suggest that spinner dolphin pods in the Main Hawai'ian Islands share characteristics in approximately 48% of their whistles. Spinner dolphin pods had similar whistle parameters regardless of the island, location, and date when they were sampled and the dolphins' behavioral state and pod size. The term "whistle-specific subgroup" (WSS) was used to designate whistle groups with similar whistles parameters (which could have been produced in part by the same dolphins). The emission rate of whistles was higher when spinner dolphins were socializing than when they were traveling or resting, suggesting that whistles are mainly used during close-range interactions. Spinner dolphins also seem to vary whistle duration according to their general behavioral state. Whistle duration and the number of turns and steps of a whistle may be more important in delivering information at the individual level than whistle frequency parameters.     

圈养瓶鼻海豚对20 kHz连续声信号的行为响应研究

将20 kHz连续声信号作为刺激信号,测试了厦门某海湾圈养的两只瓶鼻海豚对该信号的行为变化。通过对比信号发射期与间歇期海豚相对声源的水面距离、露出水面的次数以及水下发出的click定位声信号的数目等变化,判断发射信号对海豚行为的影响。给出了瓶鼻海豚对测试信号产生躲避行为的声压级门限(154±2 dB re 1μPa,rms),并与鼠海豚的躲避声压门限级进行了对比。结果表明:信号发射期,瓶鼻海豚移离了声源位置,增加了露出水面的次数,水下发出click声信号的次数明显减少。因此,瓶鼻海豚对20kHz连续信号产生了行为改变。      

Echolocation signals of Heaviside's dolphins (Cephalorhynchus heavisidii)

Field recordings of echolocation signals produced by Heaviside's dolphins (Cephalorhynchus heavisidii) were made off the coast of South Africa using a hydrophone array system. The system consisted of three hydrophones and an A-tag (miniature stereo acoustic data-logger). The mean centroid frequency was 125 kHz, with a -3 dB bandwidth of 15 kHz and -10 dB duration of 74 μs. The mean back-calculated apparent source level was 173 dB re 1 μPa(p.-p.). These characteristics are very similar to those found in other Cephalorhynchus species, and such narrow-band high-frequency echolocation clicks appear to be a defining characteristic of the Cephalorhynchus genus. Click bursts with very short inter-click intervals (up to 2 ms) were also recorded, which produced the "cry" sound reported in other Cephalorhynchus species. Since inter-click intervals correlated positively to click duration and negatively to bandwidth, Heaviside's dolphins may adjust their click duration and bandwidth based on detection range. The bimodal distribution of the peak frequency and stable bimodal peaks in spectra of individual click suggest a slight asymmetry in the click production mechanism.     

Click sounds produced by cod (Gadus morhua)

Conspicuous sonic click sounds were recorded in the presence of cod (Gadus morhua), together with either harp seals (Pagophilus groenlandicus), hooded seals (Cystophora cristata) or a human diver in a pool. Similar sounds were never recorded in the presence of salmon (Salmo salar) together with either seal species, or from either seal or fish species when kept separately in the pool. It is concluded that cod was the source of these sounds and that the clicks were produced only when cod were approached by a swimming predatorlike body. The analyzed click sounds (n = 377) had the following characteristics (overall averages +/- S.D.): peak frequency = 5.95 +/- 2.22 kHz; peak-to-peak duration = 0.70 +/- 0.45 ms; sound pressure level (received level) = 153.2 +/- 7.0 dB re 1 microPa at 1 m. At present the mechanism and purpose of these clicks is not known. However, the circumstances under which they were recorded and some observations on the behavior of the seals both suggest that the clicks could have a predator startling function.     

Linking the sounds of dolphins to their locations and behavior using video and multichannel acoustic recordings

It is difficult to attribute underwater animal sounds to the individuals producing them. This paper presents a system developed to solve this problem for dolphins by linking acoustic locations of the sounds of captive bottlenose dolphins with an overhead video image. A time-delay beamforming algorithm localized dolphin sounds obtained from an array of hydrophones dispersed around a lagoon. The localized positions of vocalizing dolphins were projected onto video images. The performance of the system was measured for artificial calibration signals as well as for dolphin sounds. The performance of the system for calibration signals was analyzed in terms of acoustic localization error, video projection error, and combined acoustic localization and video error. The 95% confidence bounds for these were 1.5, 2.1, and 2.1 m, respectively. Performance of the system was analyzed for three types of dolphin sounds: echolocation clicks, whistles, and burst-pulsed sounds. The mean errors for these were 0.8, 1.3, and 1.3 m, respectively. The 95% confidence bound for all vocalizations was 2.8 m, roughly the length of an adult bottlenose dolphin. This system represents a significant advance for studying the function of vocalizations of marine animals in relation to their context, as the sounds can be identified to the vocalizing dolphin and linked to its concurrent behavior.     

Testing the odontocete acoustic prey debilitation hypothesis: no stunning results

The hypothesis that sounds produced by odontocetes can debilitate fish was examined. The effects of simulated odontocete pulsed signals on three species of fish commonly preyed on by odontocetes were examined, exposing three individuals of each species as well as groups of four fish to a high-frequency click of a bottlenose dolphin [peak frequency (PF) 120 kHz, 213-dB peak-to-peak exposure level (EL)], a midfrequency click modeled after a killer whale's signal (PF 55 kHz, 208-dB EL), and a low-frequency click (PF 18 kHz, 193-dB EL). Fish were held in a 50-cm diameter net enclosure immediately in front of a transducer where their swimming behavior, orientation, and balance were observed with two video cameras. Clicks were presented at constant rates and in graded sweeps simulating a foraging dolphin's "terminal buzz." No measurable change in behavior was observed in any of the fish for any signal type or pulse modulation rate, despite the fact that clicks were at or near the maximum source levels recorded for odontocetes. Based on the results, the hypothesis that acoustic signals of odontocetes alone can disorient or "stun" prey cannot be supported.     

Separating overlapping click trains originating from multiple individuals in echolocation recordings

Recordings of the acoustic activity of free-swimming groups of echolocating dolphins increase the likelihood of collecting overlapping click trains, originating from multiple individuals, in the same set of data. In order to evaluate the click properties of each individual based on such recordings it is necessary to identify which clicks originate from which animal. This paper suggests a computationally efficient strategy to separate overlapping click trains originating from multiple free-swimming bottlenose dolphins, enabling echolocation analysis at an individual level on several animals. This technique is based on sequential matching of the frequency spectra of successive clicks. The clicks are grouped together as individual click trains if the correlation coefficients between clicks are higher than a pre-set threshold level. The robustness of the algorithm is tested by adding artificially generated white Gaussian noise and comparing the results with other comparable commonly used methods based on inter-click intervals, centroid frequencies, and amplitude levels. The described method is applicable to a variety of experimental and observational contexts, e.g., those regarding echolocation development of calves, the hypothesized acoustic "etiquette" among dolphins when investigating the same object, and the possible occurrence of eavesdropping in large dolphin pods.     

Male sperm whale acoustic behavior observed from multipaths at a single hydrophone

Sperm whales generate transient sounds (clicks) when foraging. These clicks have been described as echolocation sounds, a result of having measured the source level and the directionality of these signals and having extrapolated results from biosonar tests made on some small odontocetes. The authors propose a passive acoustic technique requiring only one hydrophone to investigate the acoustic behavior of free-ranging sperm whales. They estimate whale pitch angles from the multipath distribution of click energy. They emphasize the close bond between the sperm whale's physical and acoustic activity, leading to the hypothesis that sperm whales might, like some small odontocetes, control click level and rhythm. An echolocation model estimating the range of the sperm whale's targets from the interclick interval is computed and tested during different stages of the whale's dive. Such a hypothesis on the echolocation process would indicate that sperm whales echolocate their prey layer when initiating their dives and follow a methodic technique when foraging.     

Source levels of clicks from free-ranging white-beaked dolphins (Lagenorhynchus albirostris Gray 1846) recorded in Icelandic waters

This study reports the source levels of clicks recorded from free-ranging white-beaked dolphins (Lagenorhynchus albirostris Gray 1846). A four-hydrophone array was used to obtain sound recordings. The hydrophone signals were digitized on-line and stored in a portable computer. An underwater video camera was used to visualize dolphins to help identify on-axis recordings. The range to a dolphin was calculated from differences in arrival times of clicks at the four hydrophones, allowing for calculations of source levels. Source levels in a single click train varied from 194 to 211 dB peak-to-peak (p-p) re: 1 microPa. The source levels varied linearly with the log of range. The maximum source levels recorded were 219 dB (p-p) re: 1 microPa.     

The energy ratio mapping algorithm: a tool to improve the energy-based detection of odontocete echolocation clicks

The energy ratio mapping algorithm (ERMA) was developed to improve the performance of energy-based detection of odontocete echolocation clicks, especially for application in environments with limited computational power and energy such as acoustic gliders. ERMA systematically evaluates many frequency bands for energy ratio-based detection of echolocation clicks produced by a target species in the presence of the species mix in a given geographic area. To evaluate the performance of ERMA, a Teager-Kaiser energy operator was applied to the series of energy ratios as derived by ERMA. A noise-adaptive threshold was then applied to the Teager-Kaiser function to identify clicks in data sets. The method was tested for detecting clicks of Blainville's beaked whales while rejecting echolocation clicks of Risso's dolphins and pilot whales. Results showed that the ERMA-based detector correctly identified 81.6% of the beaked whale clicks in an extended evaluation data set. Average false-positive detection rate was 6.3% (3.4% for Risso's dolphins and 2.9% for pilot whales).     

Classification of echolocation clicks from odontocetes in the Southern California Bight

This study presents a system for classifying echolocation clicks of six species of odontocetes in the Southern California Bight: Visually confirmed bottlenose dolphins, short- and long-beaked common dolphins, Pacific white-sided dolphins, Risso's dolphins, and presumed Cuvier's beaked whales. Echolocation clicks are represented by cepstral feature vectors that are classified by Gaussian mixture models. A randomized cross-validation experiment is designed to provide conditions similar to those found in a field-deployed system. To prevent matched conditions from inappropriately lowering the error rate, echolocation clicks associated with a single sighting are never split across the training and test data. Sightings are randomly permuted before assignment to folds in the experiment. This allows different combinations of the training and test data to be used while keeping data from each sighting entirely in the training or test set. The system achieves a mean error rate of 22% across 100 randomized three-fold cross-validation experiments. Four of the six species had mean error rates lower than the overall mean, with the presumed Cuvier's beaked whale clicks showing the best performance (<2% error rate). Long-beaked common and bottlenose dolphins proved the most difficult to classify, with mean error rates of 53% and 68%, respectively.     

Classification of Risso's and Pacific white-sided dolphins using spectral properties of echolocation clicks

The spectral and temporal properties of echolocation clicks and the use of clicks for species classification are investigated for five species of free-ranging dolphins found offshore of southern California: short-beaked common (Delphinus delphis), long-beaked common (D. capensis), Risso's (Grampus griseus), Pacific white-sided (Lagenorhynchus obliquidens), and bottlenose (Tursiops truncatus) dolphins. Spectral properties are compared among the five species and unique spectral peak and notch patterns are described for two species. The spectral peak mean values from Pacific white-sided dolphin clicks are 22.2, 26.6, 33.7, and 37.3 kHz and from Risso's dolphins are 22.4, 25.5, 30.5, and 38.8 kHz. The spectral notch mean values from Pacific white-sided dolphin clicks are 19.0, 24.5, and 29.7 kHz and from Risso's dolphins are 19.6, 27.7, and 35.9 kHz. Analysis of variance analyses indicate that spectral peaks and notches within the frequency band 24-35 kHz are distinct between the two species and exhibit low variation within each species. Post hoc tests divide Pacific white-sided dolphin recordings into two distinct subsets containing different click types, which are hypothesized to represent the different populations that occur within the region. Bottlenose and common dolphin clicks do not show consistent patterns of spectral peaks or notches within the frequency band examined (1-100 kHz).     

Intrinsic echolocation capability of Hector's dolphin, Cephalorhynchus hectori.

A sonar system's echolocation capabilities can be inferred from the ambiguity distribution (defined here in terms of the conventional signal response function) of each of its transmitted signals. Several records of sounds emitted by Hector's dolphin are analyzed. The computed ambiguity distributions indicate that the sonar clicks of Hector's dolphins should be capable of resolving the ranges of targets as close together as 2 cm apart, but that target velocities cannot be resolved to any useful degree from a single echo.     

The spatial context of free-ranging Hawaiian spinner dolphins (Stenella longirostris) producing acoustic signals

To improve our understanding of how dolphins use acoustic signals in the wild, a three-hydrophone towed array was used to investigate the spatial occurrence of Hawaiian spinner dolphins (Stenella longirostris) relative to each other as they produced whistles, burst pulses, and echolocation clicks. Groups of approximately 30 to 60 animals were recorded while they traveled and socialized in nearshore waters off Oahu, Hawaii. Signaling animals were localized using time of arrival difference cues on the three channels. Sequences of whistles occurred between dolphins separated by significantly greater distances than animals producing burst pulses. Whistles typically originated from dolphins spaced widely apart (median = 23 m), supporting the hypothesis that whistles play a role in maintaining contact between animals in a dispersed group. Burst pulses, on the other hand, usually came from animals spaced closer to one another (median = 14 m), suggesting they function as a more intimate form of signaling between adjacent individuals. The spacing between echolocating animals was more variable and exhibited a bimodal distribution. Three quarters of echolocating animals were separated by 10 m or more, suggesting that the task of vigilance in a pod may not be shared equally by all members at all times.     

Behavioral responses of two captive bottlenose dolphins (Tursiops truncatus) to a continuous 50 kHz tone

At present, the fundamental frequencies of signals of most commercially available acoustic alarms to deter small cetaceans are below 20 kHz, but it is not well ascertained whether higher frequencies have a deterrent effect on bottlenose dolphins (Tursiops truncatus). Two captive bottlenose dolphins housed in a floating pen were subjected to a continuous pure tone at 50 kHz with a source level of 160 ± 2 dB (re 1 μPa, rms). The behavioral responses of dolphins were judged by comparing surfacing distance relative to the sound source, number of surfacings, and number of echolocation clicks produced, during forty 15 min baseline periods with forty 15 min test periods (four sessions per day, 40 sessions in total). On all 10 study days, surfacing distance and the number of surfacings increased while click production decreased during broadcasts of test sound. The avoidance threshold sound pressure level for a continuous 50 kHz tone for the bottlenose dolphins, in the context of this study, was estimated to be 144 ± 2 dB (re 1 μPa, rms). The results indicated that a continuous 50 kHz tonal signal can deter bottlenose dolphins from an area.