Sounds in common: Time-frequency as the classification parameters for pulsed sounds produced by Delphinus delphis.

Sounds produced by dolphins can be grouped into tonal (whistles) and pulsed sounds (e.g., echolocation clicks and burst sounds). Clicks are broadband pulses temporarily spaced to allow echo processing between the sound source and the object. Echolocation is related mainly to prey detection and environmental recognition. Echolocation click trains tend to present a decreasing inter-click interval due to a continuous changing of the target's location when the animal approaches to capture the prey. In addition to foraging and feeding contexts, burst pulsed sounds have been associated with short social communication. Although echolocation clicks are relatively well documented, there is no consensus regarding the broad variety of the burst pulsed signals. The present study analyzed time-frequency characteristics by conducting a clustering and discrimination analysis to classify pulsed sounds. A total of 64 click trains were analyzed from short-beaked common dolphins recorded in the slope region of the western South Atlantic Ocean. Three analyses (time; frequency; and combined time-frequency parameters) were compared through k-means clustering and posterior cluster validation using Random forest analysis. The k-means clustering resulted in four clusters for all groups of analysis. The time parameters were the most accurate among the comparisons, with the first two-dimensional axis corresponding to 87 % (Dim1 = 70.2 % and Dim2 = 17.2 %). The random forest analysis showed that the time-frequency dataset was the best classification of pulsed sounds in D. delphis (Accuracy = 84.6 %; confidence interval CI = 65.1 %-95.6 %; p < 0.01). This result considers the animal an acoustical identity, emphasizing the importance of certain parameters that influence this identity and thus reflecting the energy-cost optimization for sound production.

Common dolphin whistle responses to experimental mid-frequency sonar.

Oceanic delphinids that occur in and around Navy operational areas are regularly exposed to intense military sonar broadcast within the frequency range of their hearing. However, empirically measuring the impact of sonar on the behavior of highly social, free-ranging dolphins is challenging. Additionally, baseline variability or the frequency of vocal state-switching among social oceanic dolphins during undisturbed conditions is lacking, making it difficult to attribute changes in vocal behavior to anthropogenic disturbance. Using a network of drifting acoustic buoys in controlled exposure experiments, we investigated the effects of mid-frequency (3-4 kHz) active sonar (MFAS) on whistle production in short-beaked (Delphinus delphis delphis) and long-beaked common dolphins (Delphinus delphis bairdii) in southern California. Given the complexity of acoustic behavior exhibited by these group-living animals, we conducted our response analysis over varying temporal windows (10 min- 5 s) to describe both longer-term and instantaneous changes in sound production. We found that common dolphins exhibited acute and pronounced changes in whistle rate in the 5 s following exposure to simulated Navy MFAS. This response was sustained throughout sequential MFAS exposures within experiments simulating operational conditions, suggesting that dolphins may not habituate to this disturbance. These results indicate that common dolphins exhibit brief yet clearly detectable acoustic responses to MFAS. They also highlight how variable temporal analysis windows-tuned to key aspects of baseline vocal behavior as well as experimental parameters related to MFAS exposure-enable the detection of behavioral responses. We suggest future work with oceanic delphinids explore baseline vocal rates a-priori and use information on the rate of change in vocal behavior to inform the analysis time window over which behavioral responses are measured.

Pathology of short-term dorsal fin tag-attachments in tagged and re-stranded short-beaked common dolphins Delphinus delphis on Cape Cod, MA.

Odontocetes are difficult to study in the wild, making tagging and remote tracking a valuable practice. However, evaluations of host responses at tagging sites have been primarily limited to visual observations in the field. Here we explore the macro- and microscopic pathology of dorsal fin tag attachments in 13 stranded and released short-beaked common dolphins Delphinus delphis from Cape Cod, MA that later re-stranded and died or were euthanized 1-28 d post-tagging. Tags were attached to stranded dolphins' dorsal fins using 2 methods: core biopsy or piercing. Grossly, the piercing method resulted in epidermal compression into the dermis. One tag site had a necrotic border 28 d after application. Grossly, the biopsy method resulted in minimal to no tissue reaction. Two tag sites had granulation tissue accumulation 4 and 12 d after tagging. Histopathologic findings for all tag types and animals consisted of focal epithelial loss, dermal edema, perivascular edema, inflammation and hyperplasia, and inter- and extracellular edema in the adjacent epidermis. Minor expected pathological changes given the procedure were also observed: superficial epidermal necrosis in 3 cases, and superficial bacterial colonization in 2 cases. There was no evidence of sepsis and tagging was not related to cause of re-stranding or death in any case. These gross and histopathologic findings support previous observational conclusions in small delphinids that with appropriate sterile technique, the impacts of single pin dorsal fin tagging on the animal can be minimal and localized. Of the 2 methods, core biopsy may be a better tagging method.

Species information in whistle frequency modulation patterns of common dolphins.

The most flexible communication systems are those of open-ended vocal learners that can acquire new signals throughout their lifetimes. While acoustic signals carry information in general voice features that affect all of an individual's vocalizations, vocal learners can also introduce novel call types to their repertoires. Delphinids are known for using such learned call types in individual recognition, but their role in other contexts is less clear. We investigated the whistles of two closely related, sympatric common dolphin species, Delphinus delphis and Delphinus bairdii, to evaluate species differences in whistle contours. Acoustic recordings of single-species groups were obtained from the Southern California Bight. We used an unsupervised neural network to categorize whistles and compared the resulting whistle types between species. Of the whistle types recorded in more than one encounter, 169 were shared between species and 60 were species-specific (32 D. delphis types, 28 D. bairdii types). Delphinus delphis used 15 whistle types with an oscillatory frequency contour while only one such type was found in D. bairdii. Given the role of vocal learning in delphinid vocalizations, we argue that these differences in whistle production are probably culturally driven and could help facilitate species recognition between Delphinus species. This article is part of the theme issue 'Vocal learning in animals and humans'.

Hyperbaric tracheobronchial compression in cetaceans and pinnipeds.

Assessment of the compressibility of marine mammal airways at depth is crucial to understanding vital physiological processes such as gas exchange during diving. Very few studies have directly assessed changes in cetacean and pinniped tracheobronchial shape, and none have quantified changes in volume with increasing pressure. A harbor seal, gray seal, harp seal, harbor porpoise and common dolphin were imaged promptly post mortem via computed tomography in a radiolucent hyperbaric chamber. Volume reconstructions were performed of segments of the trachea and bronchi of the pinnipeds and bronchi of the cetaceans for each pressure treatment. All specimens examined demonstrated significant decreases in airway volume with increasing pressure, with those of the harbor seal and common dolphin nearing complete collapse at the highest pressures. The common dolphin bronchi demonstrated distinctly different compression dynamics between 50% and 100% lung inflation treatments, indicating the importance of air in maintaining patent airways, and collapse occurred caudally to cranially in the 50% treatment. Dynamics of the harbor seal and gray seal airways indicated that the trachea was less compliant than the bronchi. These findings indicate potential species-specific variability in airway compliance, and cessation of gas exchange may occur at greater depths than those predicted in models assuming rigid airways. This may potentially increase the likelihood of decompression sickness in these animals during diving.

Hybridization in the wild between Tursiops truncatus (Montagu 1821) and Delphinus delphis (Linnaeus 1758).

A case of intergeneric hybridization in the wild between a female bottlenose dolphin (Tursiops truncatus) and a short-beaked common dolphin (Delphinus delphis), considered members of 'vulnerable' and 'endangered' subpopulations in the Mediterranean, respectively, by the International Union of Conservation of Nature is described in this paper. The birth of the hybrid was registered in the Bay of Algeciras (southern Spain) in August 2016, and the animal has been tracked on frequent trips aboard dolphin-watching platforms. This unique occurrence is the result of an apparent ongoing interaction (10 years) between a female bottlenose dolphin and common dolphins. The calf has a robust body with length similar to Tursiops, while its lateral striping and coloration are typical of Delphinus. It displays the common dolphin's 'criss-cross' pattern. However, the thoracic patch is lighter than in D. delphis and its dorsal area is light grey, with a 'V' shape under the dorsal fin. This paper also provides a comprehensive mini-review of hybridizations of T. truncatus with other species.

Bone-conducted sound in a dolphin's mandible: Experimental investigation of elastic waves mediating information on sound source position.

Mammals use binaural or monaural (spectral) cues to localize acoustic sources. While the sensitivity of terrestrial mammals to changes in source elevation is relatively poor, the accuracy achieved by the odontocete cetaceans' biosonar is high, independently of where the source is. Binaural/spectral cues are unlikely to account for this remarkable skill. In this paper, bone-conducted sound in a dolphin's mandible is studied, investigating its possible contribution to sound localization. Experiments are conducted in a water tank by deploying, on the horizontal and median planes of the skull, ultrasound sources that emit synthetic clicks between 45 and 55 kHz. Elastic waves propagating through the mandible are measured at the pan bones and used to localize source positions via either binaural cues or a correlation-based full-waveform algorithm. Exploiting the full waveforms and, most importantly, reverberated coda, it is possible to enhance the accuracy of source localization in the vertical plane and achieve similar resolution of horizontal- vs vertical-plane sources. The results noted in this paper need to be substantiated by further experimental work, accounting for soft tissues and making sure that the data are correctly mediated to the internal ear. If confirmed, the results would favor the idea that dolphin's echolocation skills rely on the capability to analyze the coda of biosonar echoes.

Comparative anatomical study of sound production and reception systems in the common dolphin (Delphinus delphis) and the harbour porpoise (Phocoena phocoena) heads.

Magnetic resonance imaging (MRI) and computed tomography (CT) scans were used to analyse, respectively, the soft tissues and the bones of the heads of four common dolphins and three harbour porpoises. This imaging study was completed by an examination of anatomical sections performed on two odontocete heads (a subadult common dolphin and a subadult harbour porpoise). The three complementary approaches allowed to illustrate anatomical differences in the echolocation systems of the common dolphin and the harbour porpoise. We captured images confirming strong differences of symmetry of the melon and of its connexions to the MLDB (Monkeys Lips/Dorsal Bursae) between the common dolphin and the harbour porpoise. The melon of the common dolphin is asymmetrically directly connected to the right bursae cantantes at its right side, whereas the melon of the harbour porpoise is symmetrical, and separated from the two bursae cantantes by a set of connective tissues. Another striking difference comes from the bursae cantantes themselves, less deeply located in the head of the common dolphin than in the harbour porpoise.

Dolphin blubber/axial muscle shear: implications for rigid transdermal intramuscular tracking tag trauma in whales.

Whale tracking tags often penetrate semi-rigid blubber, with intramuscular sharp tips and toggling barbs under the subdermal sheath to reduce premature shedding. Tag sites can show persistent regional swellings or depressions. Fibroelastic blubber grips a tag, so if muscle shears relative to blubber during locomotion, the tag tip could cavitate the muscle within overall shearing distance. We modeled shearing of blubber relative to muscle, within the dorsal-ventral peduncular movement range of four common dolphin (Delphinus delphis) cadavers (mean length 186 cm). The net change in angle and hence tip distance moved was calculated with dorsal and ventral flexion, and compared between 1.5 mm diameter needles inserted into blubber only and through blubber into muscle. The greatest shearing value was 3.6 cm, and shearing was most pronounced in the areas ventral and caudal to the dorsal fin. Scaled dummy tags were also inserted and the animal cyclically flexed dorsally and ventrally for 18 h. Tag sites were dissected and cavities around the tag tips documented. If this shearing is comparable in large whales, depressions and regional swellings observed with intramuscular tracking tags are likely the result of tissue loss and repair, respectively. Placing tags para-sagittally anterior to the dorsal fin would cause the least trauma, but pain from such tags remains a concern.

Range extension for the common dolphin (Delphinus sp.) to the Colombian Caribbean, with taxonomic implications from genetic barcoding and phylogenetic analyses.

The nearest known population of common dolphins (Delphinus sp.) to the Colombian Caribbean occurs in a fairly restricted range in eastern Venezuela. These dolphins have not been previously reported in the Colombian Caribbean, likely because of a lack of study of the local cetacean fauna. We collected cetacean observations in waters of the Guajira Department, northern Colombia (~11°N, 73°W) during two separate efforts: (a) a seismic vessel survey (December 2009-March 2010), and (b) three coastal surveys from small boats (May-July 2012, May 2013, and May 2014). Here we document ten sightings of common dolphins collected during these surveys, which extend the known range of the species by ~1000 km into the southwestern Caribbean. We also collected nine skin biopsies in 2013 and 2014. In order to determine the taxonomic identity of the specimens, we conducted genetic barcoding and phylogenetic analyses using two mitochondrial markers, the Control Region (mtDNA) and Cytochrome b (Cytb). Results indicate that these specimens are genetically closer to the short-beaked common dolphin (Delphinus delphis) even though morphologically they resemble a long-beaked form (Delphinus sp.). However, the specific taxonomic status of common dolphins in the Caribbean and in the Western Atlantic remains unresolved. It is also unclear whether the distribution of the species between northern Colombia and eastern Venezuela is continuous or disjoined, or whether they can be considered part of the same stock.

Predicting Interactions between Common Dolphins and the Pole-and-Line Tuna Fishery in the Azores.

Common dolphins (Delphinus delphis) are responsible for the large majority of interactions with the pole-and-line tuna fishery in the Azores but the underlying drivers remain poorly understood. In this study we investigate the influence of various environmental and fisheries-related factors in promoting the interaction of common dolphins with this fishery and estimate the resultant catch losses. We analysed 15 years of fishery and cetacean interaction data (1998-2012) collected by observers placed aboard tuna fishing vessels. Dolphins interacted in less than 3% of the fishing events observed during the study period. The probability of dolphin interaction varied significantly between years with no evident trend over time. Generalized additive modeling results suggest that fishing duration, sea surface temperature and prey abundance in the region were the most important factors explaining common dolphin interaction. Dolphin interaction had no impact on the catches of albacore, skipjack and yellowfin tuna but resulted in significantly lower catches of bigeye tuna, with a predicted median annual loss of 13.5% in the number of fish captured. However, impact on bigeye catches varied considerably both by year and fishing area. Our work shows that rates of common dolphin interaction with the pole-and-line tuna fishery in the Azores are low and showed no signs of increase over the study period. Although overall economic impact was low, the interaction may lead to significant losses in some years. These findings emphasize the need for continued monitoring and for further research into the consequences and economic viability of potential mitigation measures.

Behavioural effects of tourism on oceanic common dolphins, Delphinus sp., in New Zealand: the effects of Markov analysis variations and current tour operator compliance with regulations.

Common dolphins, Delphinus sp., are one of the marine mammal species tourism operations in New Zealand focus on. While effects of cetacean-watching activities have previously been examined in coastal regions in New Zealand, this study is the first to investigate effects of commercial tourism and recreational vessels on common dolphins in an open oceanic habitat. Observations from both an independent research vessel and aboard commercial tour vessels operating off the central and east coast Bay of Plenty, North Island, New Zealand were used to assess dolphin behaviour and record the level of compliance by permitted commercial tour operators and private recreational vessels with New Zealand regulations. Dolphin behaviour was assessed using two different approaches to Markov chain analysis in order to examine variation of responses of dolphins to vessels. Results showed that, regardless of the variance in Markov methods, dolphin foraging behaviour was significantly altered by boat interactions. Dolphins spent less time foraging during interactions and took significantly longer to return to foraging once disrupted by vessel presence. This research raises concerns about the potential disruption to feeding, a biologically critical behaviour. This may be particularly important in an open oceanic habitat, where prey resources are typically widely dispersed and unpredictable in abundance. Furthermore, because tourism in this region focuses on common dolphins transiting between adjacent coastal locations, the potential for cumulative effects could exacerbate the local effects demonstrated in this study. While the overall level of compliance by commercial operators was relatively high, non-compliance to the regulations was observed with time restriction, number or speed of vessels interacting with dolphins not being respected. Additionally, prohibited swimming with calves did occur. The effects shown in this study should be carefully considered within conservation management plans, in order to reduce the risk of detrimental effects on common dolphins within the region.

Combining whistle acoustic parameters to discriminate Mediterranean odontocetes during passive acoustic monitoring.

Acoustic observation can complement visual observation to more effectively monitor occurrence and distribution of marine mammals. For effective acoustic censuses, calibration methods must be determined by joint visual and acoustic studies. Research is still needed in the field of acoustic species identification, particularly for smaller odontocetes. From 1994 to 2012, whistles of four odontocete species were recorded in different areas of the Mediterranean Sea to determine how reliably these vocalizations can be classified to species. Recordings were attributed to species by simultaneous visual observation. The results of this study highlight that the frequency parameters, which are linked to physical features of animals, show lower variability than modulation parameters, which are likely to be more dependent on complex eco-ethological contexts. For all the studied species, minimum and maximum frequencies were linearly correlated with body size. DFA and Classification Tree Analysis (CART) show that these parameters were the most important for classifying species; however, both statistical methods highlighted the need for combining them with the number of contour minima and contour maxima for correct classification. Generally, DFA and CART results reflected both phylogenetic distance (especially for common and striped dolphins) and the size of the species.

Immunofluorescence evidence of melanotrophs in the pituitary of four odontocete species. An immunohistochemical study and a critical review of the literature.

Cetaceans share peculiar features of their pituitary glands, with a complete separation of pars distalis and pars nervosa by a dural septum and the absence of an intermediate lobe and cleft. In most mammals the pars intermedia is the main source of circulating α-melanocyte stimulating hormone (α-MSH), derived from a large precursor called proopiomelanocortin (POMC), which also generates adrenocorticotropic hormone (ACTH) in the adenohypophysis. The lack of an intermediate lobe in cetaceans led us to investigate whether their glands are able to produce α-MSH, and if this hormone is secreted by a distinct population of melanotrophs or by corticotrophs in the pars distalis. Immunofluorescence evidences seem to support the first assumption, with ACTH-immunoreactive (-ir) elements rarely overlapping with α-MSH-ir ones. The discovery of a population of true melanotrophs in the hypophysis of some odontocetes underscores the need for further research on the melanocortin system of cetaceans.

Automated segmentation of linear time-frequency representations of marine-mammal sounds.

Many marine mammals produce highly nonlinear frequency modulations. Determining the time-frequency support of these sounds offers various applications, which include recognition, localization, and density estimation. This study introduces a low parameterized automated spectrogram segmentation method that is based on a theoretical probabilistic framework. In the first step, the background noise in the spectrogram is fitted with a Chi-squared distribution and thresholded using a Neyman-Pearson approach. In the second step, the number of false detections in time-frequency regions is modeled as a binomial distribution, and then through a Neyman-Pearson strategy, the time-frequency bins are gathered into regions of interest. The proposed method is validated on real data of large sequences of whistles from common dolphins, collected in the Bay of Biscay (France). The proposed method is also compared with two alternative approaches: the first is smoothing and thresholding of the spectrogram; the second is thresholding of the spectrogram followed by the use of morphological operators to gather the time-frequency bins and to remove false positives. This method is shown to increase the probability of detection for the same probability of false alarms.

History of expansion and anthropogenic collapse in a top marine predator of the Black Sea estimated from genetic data.

Two major ecological transitions marked the history of the Black Sea after the last Ice Age. The first was the postglacial transition from a brackish-water to a marine ecosystem dominated by porpoises and dolphins once this basin was reconnected back to the Mediterranean Sea (ca. 8,000 y B.P.). The second occurred during the past decades, when overfishing and hunting activities brought these predators close to extinction, having a deep impact on the structure and dynamics of the ecosystem. Estimating the extent of this decimation is essential for characterizing this ecosystem's dynamics and for formulating restoration plans. However, this extent is poorly documented in historical records. We addressed this issue for one of the main Black Sea predators, the harbor porpoise, using a population genetics approach. Analyzing its genetic diversity using an approximate Bayesian computation approach, we show that only a demographic expansion (at most 5,000 y ago) followed by a contemporaneous population collapse can explain the observed genetic data. We demonstrate that both the postglacial settlement of harbor porpoises in the Black Sea and the recent anthropogenic activities have left a clear footprint on their genetic diversity. Specifically, we infer a strong population reduction (~90%) that occurred within the past 5 decades, which can therefore clearly be related to the recent massive killing of small cetaceans and to the continuing incidental catches in commercial fisheries. Our study thus provides a quantitative assessment of these demographically catastrophic events, also showing that two separate historical events can be inferred from contemporary genetic data.

Sonar-induced pressure fields in a post-mortem common dolphin.

Potential physical effects of sonar transmissions on marine mammals were investigated by measuring pressure fields induced in a 119-kg, 211-cm-long, young adult male common dolphin (Delphinus delphis) cadaver. The specimen was instrumented with tourmaline acoustic pressure gauges used as receiving sensors. Gauge implantation near critical tissues was guided by intraoperative, high-resolution, computerized tomography (CT) scanning. Instrumented structures included the melon, nares, ear, thoracic wall, lungs, epaxial muscle, and lower abdomen. The specimen was suspended from a frame equipped with a standard 50.8-mm-diameter spherical transducer used as the acoustic source and additional receiving sensors to monitor the transmitted and external, scattered field. Following immersion, the transducer transmitted pulsed sinusoidal signals at 5, 7, and 10 kHz. Quantitative internal pressure fields are reported for all cases except those in which the gauge failed or no received signal was detected. A full necropsy was performed immediately after the experiment to examine instrumented areas and all major organs. No lesions attributable to acoustic transmissions were found, consistent with the low source level and source-receiver distances.

The dolphin cochlear nucleus: topography, histology and functional implications.

Despite the outstanding auditory capabilities of dolphins, there is only limited information available on the cytology of the auditory brain stem nuclei in these animals. Here, we investigated the cochlear nuclei (CN) of five brains of common dolphins (Delphinus delphis) and La Plata dolphins (Pontoporia blainvillei) using cell and fiber stain microslide series representing the three main anatomical planes. In general, the CN in dolphins comprise the same set of subnuclei as in other mammals. However, the volume ratio of the dorsal cochlear nucleus (DCN) in relation to the ventral cochlear nucleus (VCN) of dolphins represents a minimum among the mammals examined so far. Because, for example, in cats the DCN is necessary for reflexive orientation of the head and pinnae towards a sound source, the massive restrictions in head movability in dolphins and the absence of outer ears may be correlated with the reduction of the DCN. Moreover, the same set of main neuron types were found in the dolphin CN as in other mammals, including octopus and multipolar cells. Because the latter two types of neurons are thought to be involved in the recognition of complex sounds, including speech, we suggest that, in dolphins, they may be involved in the processing of their communication signals. Comparison of the toothed whale species studied here revealed that large spherical cells were present in the La Plata dolphin but absent in the common dolphin. These neurons are known to be engaged in the processing of low-frequency sounds in terrestrial mammals. Accordingly, in the common dolphin, the absence of large spherical cells seems to be correlated with a shift of its auditory spectrum into the high-frequency range above 20 kHz. The existence of large spherical cells in the VCN of the La Plata dolphin, however, is enigmatic asthis species uses frequencies around 130 kHz.