Signature whistles in wild bottlenose dolphins: long-term stability and emission rates

Whistles are key elements in the acoustic repertoire of bottlenose dolphins. In this species, the frequency contours of whistles are used as individual signatures. Assessing the long-lasting stability of such stereotyped signals, and the abundant production of non-stereotyped whistles in the wild, is relevant to a more complete understanding of their biological function. Additionally, studying the effects of group size and activity patterns on whistle emission rate may provide insights into the use of these calls. In this study, we document the decades-long occurrence of whistles with stereotyped frequency contours in a population of wild bottlenose dolphins, resident in the region of the Sado estuary, Portugal. Confirmed stereotypy throughout more than 20 years, and positive identification using the signature identification (SIGID) criteria, suggests that the identified stereotyped whistles are in fact signature whistles. The potential roles of non-stereotyped whistles, which represent 68 % of all whistles recorded, are still unclear and should be further investigated. Emission rates were significantly higher during food-related events. Finally, our data show a comparatively high overall whistle production for this population, and no positive correlation between group size and emission rates, suggesting social or environmental restriction mechanisms in vocal production.     

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.     

Biphonal calls in Atlantic spotted dolphins (Stenella frontalis): bitonal and burst-pulse whistles

Biphonation, the simultaneous production of two sounds by a single animal, is found in the vocalizations of a range of mammalian species. Its prevalence suggests it plays an important role in acoustic communication. Concurrent vocal and behavioural recordings were made of Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas. The occurrence of two types of biphonal signals is reported: burst-pulse whistles with combined tonal and burst-pulse elements, and bitonal whistles. Biphonal whistles are rarely described in reports of dolphin acoustic repertoires, but were common in these dolphins: of all whistles analysed (n = 1211), 26.84% were burst-pulse whistles and 4.71% were bitonal whistles. A subset of whistles (n = 397) were attributed to dolphins of specific age classes, and used to compare prevalence of biphonation across age. Biphonation occurred in 61.54% of sexually mature and 48.32% of sexually immature dolphins’ whistles. Sexually immature dolphins emitted more burst-pulse whistles than older dolphins: 44.13% of sexually immature dolphins’ whistles were burst-pulse whistles, while 15.38% of adult whistles were burst-pulse whistles. Bitonal whistle production was more prevalent in sexually mature dolphins: 41.03% of adult whistles were bitonal, while only 4.19% of sexually immature dolphins’ whistles were bitonal. The prevalence of a biphonal component in specific repeated, stereotyped whistle contours suggests that these acoustic features could be important components of contact calls, or signature whistles. The biphonal components of spotted dolphin whistles may serve to convey additional information as to identity, age or other factors to conspecifics.     

Signature whistles in free‐ranging populations of Indo‐Pacific bottlenose dolphins,Tursiops aduncus

Common bottlenose dolphins (Tursiops truncatus) use individually distinctive signature whistles which are highly stereotyped and function as contact calls. Here we investigate whether Indo‐Pacific bottlenose dolphins (T. aduncus) use signature whistles. The frequency trace of whistle contours recorded from three genetically distinct free‐ranging populations was extracted and sorted into whistle types of similar shape using automated categorization. A signature whistle identification method based on the temporal patterns in signature whistle sequences of T. truncatus was used to identify signature whistle types (SWTs). We then compared the degree of variability in SWTs for several whistle parameters to determine which parameters are likely to encode identity information. Additional recordings from two temporarily isolated T. aduncus made during natural entrapment events in 2008 and 2009 were analyzed for the occurrence of SWTs. All populations were found to produce SWTs; 34 SWTs were identified from recordings of free‐ranging T. aduncus and one SWT was prevalent in each recording of the two temporarily isolated individuals. Of the parameters considered, mean frequency and maximum frequency were the least variable and therefore most likely to reflect identity information encoded in frequency modulation patterns. Our results suggest that signature whistles are commonly used by T. aduncus.     

Geographic variation in the time‐frequency characteristics of high‐frequency whistles produced by killer whales (Orcinus orca)

Investigating intraspecific variation in acoustic signals can indicate the extent of isolation and divergence between populations and adaptations to local environments. Here we analyze the variation in killer whale high‐frequency (>17 kHz) whistles recorded off Norway, Iceland, and in the North Pacific. We used a combination of methods including multivariate comparisons of spectral and temporal parameters and categorization of contours to types. Our results show that spectral and temporal characteristics of high‐frequency whistles recorded in the North Pacific show significant differences from whistles recorded in the Northeast Atlantic, being generally stereotyped, lower in frequency, and slightly longer in duration. Most high‐frequency whistles from the North Pacific were downsweeps, whereas this was one of the least common types recorded in the Northeast Atlantic. The repertoire of whistles recorded in Norway was similar to Iceland, but whistles produced in Norway had significantly lower maximum frequency and frequency range. Most methods were able to discriminate between whistles of the North Pacific and the Northeast Atlantic, but were unable to consistently distinguish whistles from Iceland and Norway. This suggests that macro‐ and microgeographic differences in high‐frequency whistles of killer whales may reflect historical geographic isolation between ocean basins and more recent divergence between adjacent populations.     

Recognition of Frequency Modulated Whistle-Like Sounds by a Bottlenose Dolphin (Tursiops truncatus) and Humans with Transformations in Amplitude,Duration and Frequency

Bottlenose dolphins (Tursiops truncatus) use the frequency contour of whistles produced by conspecifics for individual recognition. Here we tested a bottlenose dolphin’s (Tursiops truncatus) ability to recognize frequency modulated whistle-like sounds using a three alternative matching-to-sample paradigm. The dolphin was first trained to select a specific object (object A) in response to a specific sound (sound A) for a total of three object-sound associations. The sounds were then transformed by amplitude, duration, or frequency transposition while still preserving the frequency contour of each sound. For comparison purposes, 30 human participants completed an identical task with the same sounds, objects, and training procedure. The dolphin’s ability to correctly match objects to sounds was robust to changes in amplitude with only a minor decrement in performance for short durations. The dolphin failed to recognize sounds that were frequency transposed by plus or minus ½ octaves. Human participants demonstrated robust recognition with all acoustic transformations. The results indicate that this dolphin’s acoustic recognition of whistle-like sounds was constrained by absolute pitch. Unlike human speech, which varies considerably in average frequency, signature whistles are relatively stable in frequency, which may have selected for a whistle recognition system invariant to frequency transposition.     

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.     

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.     

Stand dynamics and spatial patterns across varying sites in the invasive Lantana camara L. (Verbenaceae)

As with many invasive plant species, little is known of the population spatial patterns and stand dynamics of Lantana camara L. (Verbenaceae)??a thicket-forming weed of worldwide significance in managed and conservation lands, including coastal and inland habitats of Eastern Australia. Consequently, we mapped and followed annually for 3?years the demographic fate of more than 2000 Lantana individuals at sites with four land-uses (hoop pine plantation, cattle farm, and two eucalyptus forests with occasional grazing and periodic burning regime, respectively) in Queensland, SE Australia. Populations exhibited plant size distributions that were continuous (i.e., of L or symmetric type) and unimodal, except the farm population where bimodality was observed. Newly established plants could be reproductive within one growing season at ~50?cm in height, especially where environmental resources were not limiting. Density had an appreciable effect on the weed??s reproductive capacity and growth, but not on survival. Established and newly recruited individuals were aggregated but the degree of aggregation decreased with plant size. However, in the sites that had experienced burning or mechanical clearing, Lantana seedling/juvenile recruitment assumed negative association (spatial displacement) in relation to established individuals. The findings of this study agree with the notion that ecological processes often leave characteristic spatial signatures, which if interpreted using appropriate hypotheses can help to ascertain factors responsible for the observed spatial patterns and stand dynamics.     

Description of whistles of Irrawaddy dolphins (Orcaella brevirostris) from the waters of Matang,Peninsular Malaysia

The whistles of Irrawaddy dolphins (Orcaella brevirostris) from the waters of Matang, western Peninsular Malaysia are described. Duration, frequency and frequency modulation variables were measured from 163 whistles recorded using a broadband towed hydrophone. Irrawaddy dolphins produced whistles with a mean duration of 0.366 s (S.D. ± 0.217 s). The fundamental frequency of whistles extended from 3040 to 17,123 Hz with low levels of frequency modulation. These dolphins produced whistles that were comparable to those of conspecifics recorded from the waters of Kalimantan, but were generally different from the related Australian snubfin dolphin (O. heinsohni). They also differed from the whistles of the sympatric Indo-Pacific humpback dolphin (Sousa chinensis). Characteristics of Irrawaddy dolphin whistles may be useful in future passive acoustic monitoring studies to investigate differences in sympatric species and their habitat.     

Exchange of “signature” calls in captive belugas (Delphinapterus leucas)

Belugas (Delphinapterus leucas) produce echolocation clicks, burst pulses, and whistles. The sounds of 3 captive belugas were recorded using 2 hydrophones at the Port of Nagoya Public Aquarium. There were stable individual differences in the pulse patterning of one type of pulsed sounds (PS1 call), suggesting that belugas use these as “signature” calls. Eighty-eight percent of PS1 calls initiated PS1 calls from other animals within 1 s. PS1 calls repeated by the same individual occurred primarily when other belugas did not respond within 1 s of the first call. Belugas delayed successive PS1 calls when other belugas responded with a PS1 call within 1 s. There was no clear temporal pattern for whistles. It appears that the time limit for responding to calls is 1 s after the initial call. If other individuals do not respond to the PS1 call of a beluga within 1 s, belugas tend to repeat the call and wait for a response. The results of this study suggest that the belugas exchange their individual signatures by using PS1 calls, in a manner similar to that of signature whistles used by bottlenose dolphins.     

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.     

Vocalizations of Belding's ground squirrels (Spermophilus beldingi)

Vocalizations of Belding's ground squirrels (Spermophilus beldingi) were recorded during the summer of 1982 near Tioga Pass in the central Sierra Nevada of California. Sonagrams were made and call parameters were measured. Discriminant function analyses revealed that multiple-note calls (‘trills’) differed acoustically depending on whether they occurred in response to a predator, or were given by males following copulations. The post-copulatory trills of males were individually distinctive. Among anti-predator trills there was no evidence of predator-specificity within the narrow range of predators tested: trills given to two species of (stuffed) weasels (Mustela), to dogs and to humans were statistically indistinguishable. Sonagrams of trills occurring in agonistic contexts suggest that a third general category of trills may exist, but agonistic trills were more variable than either anti-predator or post-copulatory trills. The ground squirrels also gave single-note calls in the three contexts described above, either repetitively (‘chirps’) or singly (‘whistles’). Neither chirps nor whistles encoded any obvious situation-specific information, except that whistles were typically associated with rapidly-moving predators, usually raptors. The post-copulatory chirps of males were individually distinctive. Vocalizations of Belding's ground squirrels may not vary among contexts as much as do the analogous calls of California ground squirrels (S. beecheyi).     

Signatures of adaptation and genetic structure among the mainland populations of Pinus radiata (D. Don) inferred from SNP loci

Insights into the relative contributions of locus specific and genome-wide effects on population genetic diversity can be gained through separation of their resulting genetic signals. Here we explore patterns of adaptive and neutral genetic diversity in the disjunct natural populations of Pinus radiata (D. Don) from mainland California. A first-generation common garden of 447 individuals revealed significant differentiation of wood phenotypes among populations (P _ST), possibly reflecting local adaptation in response to environment. We subsequently screened all trees for genetic diversity at 149 candidate gene single nucleotide polymorphism (SNP) loci for signatures of adaptation. Ten loci were identified as being possible targets of diversifying selection following F _ST outlier tests. Multivariate canonical correlation performed on a data set of 444 individuals identified significant covariance between environment, adaptive phenotypes and outlier SNP diversity, lending support to the case for local adaptation suggested from F _ST and P _ST tests. Covariation among discrete sets of outlier SNPs and adaptive phenotypes (inferred from multivariate loadings) with environment are supported by existing studies of candidate gene function and genotype–phenotype association. Canonical analyses failed to detect significant correlations between environment and 139 non-outlier SNP loci, which were applied to estimate neutral patterns of genetic differentiation among populations (F _ST 4.3 %). Using this data set, significant hierarchical structure was detected, indicating three populations on the mainland. The hierarchical relationships based on neutral SNP markers (and SSR) were in contrast with those inferred from putatively adaptive loci, potentially highlighting the independent action of selection and demography in shaping genetic structure in this species.     

Whistle characteristics of a newly recorded Indo-Pacific humpback dolphin (Sousa chinensis) population in waters southwest of Hainan Island,China, differ from other humpback dolphin populations

Indo-Pacific humpback dolphins (Sousa chinensis) use whistles to communicate with their conspecifics. Little is known about the acoustic repertoire of Indo-Pacific humpback dolphins in waters southwest of Hainan Island, a newly recorded population in 2014. In this study, whistles of Hainan humpback dolphin population were collected by using autonomous acoustic recorders. The fundamental frequencies and durations of whistles were in ranges of 0.71–21.35 kHz and 0.06–2.22 s, respectively. Significant intraspecific differences in duration and frequency of whistles were found between the Hainan population and the other geographically neighboring populations (in Chinese waters) or the population in Malaysia waters. Compared with other Sousa species, significant interspecific differences were also observed. Based on clustering analysis, the whistle parameters of neighboring populations were likely similar to each other. Significant differences were found between humpback dolphins in waters southwest of Hainan Island and those dolphins in the neighboring areas, supporting the hypothesis that this population may be independent. Ambient noise measurements in waters of Hainan Island, Zhanjiang, and Sanniang Bay showed that humpback dolphin populations may use whistles with longer duration, lower frequency, and fewer inflection points for more effective communication to adapt to a noisier environment.     

Plasticity of diel patterns in the diet and habitat use of feral, non-native fathead minnow Pimephales promelas (Actinopterygii, Cyprinidae) in a pond-dwelling population in England

The aim of the study was to test for diel patterns in the diet and habitat use in a feral, pond-dwelling, non-native fathead minnow Pimephales promelas population in England. Fish were collected in June 2009 using traps set in four habitat types (open waters, rushes??Juncus effusus, weeds??Potamogeton natans and mixed vegetation), then sexed and measured for total length and eviscerated weight in the laboratory. Data were analysed at 6-h intervals, with Fulton??s condition index and three dietary parameters (frequency of occurrence, number and weight) calculated. Minor diurnal differences in habitat use were observed in males, females and immatures, and these may be due to predator avoidance. Body condition varied greatly in rushes during daytime, probably due to shifts in habitat suitability (e.g. food, refuge). Detritus dominates the diet of native fathead minnow; however, planktonic crustaceans were the most important food resource for this population, with a clear ontogenetic shift, irrespective of habitat, towards greater proportion of ingested detritus in larger individuals. Overall, the results demonstrate that feral fathead minnows display substantial trophic plasticity and a wide range of habitat use, which is normally associated with invasive species. However, established fathead minnow populations in Europe are rare despite its wide-spread ornamental and scientific use.     

Vocal exchanges of signature whistles in bottlenose dolphins (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>)

Bottlenose dolphins (Tursiops truncatus) produce individually distinctive vocalizations—referred to as “signature whistles”—that are thought to function as an individual and conspecific recognition system for maintenance of consistent contact between individuals. Observations and playback experiments were conducted at aquariums to study these whistle–vocal exchanges in bottlenose dolphins. Temporal patterns of vocalization were examined by analyzing the intercall intervals between two consecutive whistles. When a second individual produced a call that was different from the first individual’s vocalization, most of these calls were shorter than 1 s. However, when two consecutive calls were produced by the same individual, the second call rarely occurred within 1 s of the first. These results suggest that a second whistle may be produced by a different caller in response to the first whistle; however, in the case of an absence of a response, the first caller is likely to give further whistles. The results of this acoustic analysis show that the dolphins used in this study mostly used signature whistles during the recorded vocal exchanges.     

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.