Call diversity in the North Pacific killer whale populations: implications for dialect evolution and population history

Characteristics and spatiotemporal variation of sei whale (Balaenoptera borealis) downsweeps recorded in Atlantic Canada

The call characteristics and vocal behaviour of sei whales (Balaenoptera borealis) off eastern Canada, including potential spatiotemporal variation, is poorly understood. Such information can improve the performance of automated detector-classifiers, enhancing the accuracy and efficiency of identifying sei whales in large acoustic datasets. Ultimately, these data can be used to understand the occurrence, distribution, and population structure of sei whales in Atlantic Canada. We measured sei whale downsweep characteristics recorded from six locations off Nova Scotia (NS) and Newfoundland and Labrador (NFLD), Canada over a two-year period (2015-2017), and examined variation between call subtypes (singlets, doublets, triplets+), and seasons (Spring, Summer, Fall, Winter). We found that downsweeps had a mean duration of 1.58 s, sweeping from 75.66 to 34.22 Hz, with a peak frequency of 43.89 Hz and an intra-call interval of 2.22 s. Most call characteristics did not vary between location, subtype, or season; however, significantly longer downsweeps occurred off NS, within doublet calls, and in fall months. We also found that NFLD had a higher proportion of doublets (70%) than NS (52%). This variation may be evidence of acoustically diverging sei whale populations, as well as useful for improving detector-classifiers of sei whales in the region.

Call combination patterns in Icelandic killer whales (Orcinus orca)

Acoustic sequences have been described in a range of species and in varying complexity. Cetaceans are known to produce complex song displays but these are generally limited to mysticetes; little is known about call combinations in odontocetes. Here we investigate call combinations produced by killer whales (Orcinus orca), a highly social and vocal species. Using acoustic recordings from 22 multisensor tags, we use a first order Markov model to show that transitions between call types or subtypes were significantly different from random, with repetitions and specific call combinations occurring more often than expected by chance. The mixed call combinations were composed of two or three calls and were part of three call combination clusters. Call combinations were recorded over several years, from different individuals, and several social clusters. The most common call combination cluster consisted of six call (sub-)types. Although different combinations were generated, there were clear rules regarding which were the first and last call types produced, and combinations were highly stereotyped. Two of the three call combination clusters were produced outside of feeding contexts, but their function remains unclear and further research is required to determine possible functions and whether these combinations could be behaviour- or group-specific.

Orcas remember what to copy: a deferred and interference-resistant imitation study

Response facilitation has often been portrayed as a "low level" category of social learning, because the demonstrator's action, which is already in the observer's repertoire, automatically triggers that same action, rather than induces the learning of a new action. One way to rule out response facilitation consists of introducing a delay between the demonstrator's behavior and the observer's response to let their possible effects wear off. However, this may not rule out "delayed response facilitation" in which the subject could be continuously "mentally rehearsing" the demonstrated actions during the waiting period. We used a do-as-the-other-did paradigm in two orcas to study whether they displayed cognitive control regarding their production of familiar actions by (1) introducing a delay ranging from 60 to 150 s between observing and producing the actions and (2) interspersing distractor (non-target) actions performed by the demonstrator and by the subjects during the delay period. These two manipulations were aimed at preventing the mental rehearsal of the observed actions during the delay period. Both orcas copied the model's target actions on command after various delay periods, and crucially, despite the presence of distractor actions. These findings suggest that orcas are capable of selectively retrieving a representation of an observed action to generate a delayed matching response. Moreover, these results lend further support to the proposal that the subjects' performance relied not only on a mental representation of the specific actions that were requested to copy, but also flexibly on the abstract and domain general rule requested by the specific "copy command". Our findings strengthen the view that orcas and other cetaceans are capable of flexible and controlled social learning.

Chimpanzee pant-hoots encode individual information more reliably than group differences

Vocal learning, the ability to modify the acoustic structure of vocalizations based on social experience, is a fundamental feature of speech in humans (Homo sapiens). While vocal learning is common in taxa such as songbirds and whales, the vocal learning capacities of nonhuman primates appear more limited. Intriguingly, evidence for vocal learning has been reported in chimpanzees (Pan troglodytes), for example, in the form of regional variation ("dialects") in the "pant-hoot" calls. This suggests that some capacity for vocal learning may be an ancient feature of the Pan-Homo clade. Nonetheless, reported differences have been subtle, with intercommunity variation representing only a small portion of the total acoustic variation. To gain further insights into the extent of regional variation in chimpanzee vocalizations, we performed an analysis of pant-hoots from chimpanzees in the neighboring Kasekela and Mitumba communities at Gombe National Park, Tanzania, and the geographically distant Kanyawara community at Kibale National Park, Uganda. We did not find any statistically significant differences between the neighboring communities at Gombe or among geographically distant communities. Furthermore, we found differences among individuals in all communities. Hence, the variation in chimpanzee pant-hoots reflected individual differences, rather than group differences. Thus, we did not find evidence of dialects in this population, suggesting that extensive vocal learning emerged only after the lineages of Homo and Pan diverged.

Evidence from sperm whale clans of symbolic marking in non-human cultures

Culture, a pillar of the remarkable ecological success of humans, is increasingly recognized as a powerful force structuring nonhuman animal populations. A key gap between these two types of culture is quantitative evidence of symbolic markers-seemingly arbitrary traits that function as reliable indicators of cultural group membership to conspecifics. Using acoustic data collected from 23 Pacific Ocean locations, we provide quantitative evidence that certain sperm whale acoustic signals exhibit spatial patterns consistent with a symbolic marker function. Culture segments sperm whale populations into behaviorally distinct clans, which are defined based on dialects of stereotyped click patterns (codas). We classified 23,429 codas into types using contaminated mixture models and hierarchically clustered coda repertoires into seven clans based on similarities in coda usage; then we evaluated whether coda usage varied with geographic distance within clans or with spatial overlap between clans. Similarities in within-clan usage of both "identity codas" (coda types diagnostic of clan identity) and "nonidentity codas" (coda types used by multiple clans) decrease as space between repertoire recording locations increases. However, between-clan similarity in identity, but not nonidentity, coda usage decreases as clan spatial overlap increases. This matches expectations if sympatry is related to a measurable pressure to diversify to make cultural divisions sharper, thereby providing evidence that identity codas function as symbolic markers of clan identity. Our study provides quantitative evidence of arbitrary traits, resembling human ethnic markers, conveying cultural identity outside of humans, and highlights remarkable similarities in the distributions of human ethnolinguistic groups and sperm whale clans.

Passive acoustic monitoring of killer whales (Orcinus orca) reveals year-round distribution and residency patterns in the Gulf of Alaska

Killer whales (Orcinus orca) are top predators throughout the world's oceans. In the North Pacific, the species is divided into three ecotypes-resident (fish-eating), transient (mammal-eating), and offshore (largely shark-eating)-that are genetically and acoustically distinct and have unique roles in the marine ecosystem. In this study, we examined the year-round distribution of killer whales in the northern Gulf of Alaska from 2016 to 2020 using passive acoustic monitoring. We further described the daily acoustic residency patterns of three killer whale populations (southern Alaska residents, Gulf of Alaska transients, and AT1 transients) for one year of these data. Highest year-round acoustic presence occurred in Montague Strait, with strong seasonal patterns in Hinchinbrook Entrance and Resurrection Bay. Daily acoustic residency times for the southern Alaska residents paralleled seasonal distribution patterns. The majority of Gulf of Alaska transient detections occurred in Hinchinbrook Entrance in spring. The depleted AT1 transient killer whale population was most often identified in Montague Strait. Passive acoustic monitoring revealed that both resident and transient killer whales used these areas much more extensively than previously known and provided novel insights into high use locations and times for each population. These results may be driven by seasonal foraging opportunities and social factors and have management implications for this species.

Vocal universals and geographic variations in the acoustic repertoire of the common bottlenose dolphin

Acoustical geographic variation is common in widely distributed species and it is already described for several taxa, at various scales. In cetaceans, intraspecific variation in acoustic repertoires has been linked to ecological factors, geographical barriers, and social processes. For the common bottlenose dolphin (Tursiops truncatus), studies on acoustic variability are scarce, focus on a single signal type-whistles and on the influence of environmental variables. Here, we analyze the acoustic emissions of nine bottlenose dolphin populations across the Atlantic Ocean and the Mediterranean Sea, and identify common signal types and acoustic variants to assess repertoires' (dis)similarity. Overall, these dolphins present a rich acoustic repertoire, with 24 distinct signal sub-types including: whistles, burst-pulsed sounds, brays and bangs. Acoustic divergence was observed only in social signals, suggesting the relevance of cultural transmission in geographic variation. The repertoire dissimilarity values were remarkably low (from 0.08 to 0.4) and do not reflect the geographic distances among populations. Our findings suggest that acoustic ecology may play an important role in the occurrence of intraspecific variability, as proposed by the 'environmental adaptation hypothesis'. Further work may clarify the boundaries between neighboring populations, and shed light into vocal learning and cultural transmission in bottlenose dolphin societies.

Separating overlapping bat calls with a bi-directional long short-term memory network

Acquiring clear acoustic signals is critical for the analysis of animal vocalizations. Bioacoustics studies commonly face the problem of overlapping signals, which can impede the structural identification of vocal units, but there is currently no satisfactory solution. This study presents a bi-directional long short-term memory network to separate overlapping echolocation-communication calls of 6 different bat species and reconstruct waveforms. The separation quality was evaluated using 7 temporal-spectrum parameters. All the echolocation pulses and syllables of communication calls in the overlapping signals were separated and parameter comparisons showed no significant difference and negligible deviation between the extracted and original calls. Clustering analysis was conducted with separated echolocation calls from each bat species to provide an example of practical application of the separated and reconstructed calls. The result of clustering analysis showed high corrected rand index (82.79%), suggesting the reconstructed waveforms could be reliably used for species classification. These results demonstrate a convenient and automated approach for separating overlapping calls. The study extends the application of deep neural networks to separate overlapping animal sounds.

Australian long-finned pilot whales (Globicephala melas) emit stereotypical, variable, biphonic, multi-component, and sequenced vocalisations, similar to those recorded in the northern hemisphere

While in the northern hemisphere, many studies have been conducted on the vocal repertoire of long-finned pilot whales (Globicephala melas), no such study has been conducted in the southern hemisphere. Presented here, is the first study on the vocalisations of long-finned pilot whales along the southern coast of mainland Australia. Multiple measures were taken of 2028 vocalisations recorded over five years in several locations. These vocalisations included tonal sounds with and without overtones, sounds of burst-pulse character, graded sounds, biphonations, and calls of multiple components. Vocalisations were further categorised based on spectrographic features into 18 contour classes. Altogether, vocalisations ranged from approximately 200 Hz to 25 kHz in fundamental frequency and from 0.03 s to 2.07 s in duration. These measures compared well with those from northern hemisphere pilot whales. Some call types were almost identical to northern hemisphere vocalisations, even though the geographic ranges of the two populations are far apart. Other call types were unique to Australia. Striking similarities with calls of short-finned pilot whales (Globicephala macrorhynchus) and sometimes sympatric killer whales (Orcinus orca) were also found. Theories for call convergence and divergence are discussed.

Independent acoustic variation of the higher- and lower-frequency components of biphonic calls can facilitate call recognition and social affiliation in killer whales

Each resident-type (R-type) killer whale pod has a set of stereotyped calls that are culturally transmitted from mother to offspring. The functions of particular call types are not yet clearly understood, but it is believed that calls with two independently modulated frequency components (biphonic calls) play an important role in pod communication and cohesion at long ranges. In this study we examined the possible functions of biphonic calls in R-type killer whales. First, we tested the hypothesis that the additional component enhances the potential of a call to identify the family affiliation. We found that the similarity patterns of the lower- and higher frequency components across the families were largely unrelated. Calls were classified more accurately to their respective family when both lower- and higher-frequency components were considered. Second, we tested the long-range detectability of the lower- and higher-frequency components. After adjusting the received levels by the killer whale hearing sensitivity to different frequency ranges, the sensation level of the higher-frequency component was higher than the amplitude of the lower-frequency component. Our results suggest that the higher-frequency component of killer whale biphonic calls varies independently of the lower-frequency component, which enhances the efficiency of these calls as family markers. The acoustic variation of the higher-frequency component allows the recognition of family identity of a caller even if the shape of the lower-frequency component accidentally becomes similar in unrelated families. The higher-frequency component can also facilitate family recognition when the lower-frequency component is masked by low-frequency noise.

A review of Canadian Arctic killer whale (Orcinus orca) ecology

The killer whale (Orcinus orca (Linnaeus, 1758)) is a widely distributed marine predator with a broad ecological niche at the species level with evidence of specialization and narrow ecological niches among populations. Their occurrence in Canadian Arctic waters is limited by sea ice and it has been suggested that climate warming, which has caused increases in the area of ice-free water and duration of the ice-free season, has led to an increased killer whale presence during the open-water period. In this review, we summarize our knowledge of Canadian Arctic killer whale demographics and ecology, synthesizing published and previously unpublished information in a single document. More specifically, we summarize our knowledge of killer whale population size and trends, distribution and seasonality (including results from recent satellite-tracking studies), feeding ecology, and threats, and identify research priorities in the Canadian Arctic. Despite increased research efforts during the past decade, our demographic and ecological knowledge remains incomplete. An improved ecological understanding is necessary for effective management of killer whales and their prey, species of ecological, economic, and cultural importance to Canadian Inuit and the marine ecosystem. This knowledge will allow us to better understand the ecological consequences of a changing Arctic climate.

Comparing context-dependent call sequences employing machine learning methods: an indication of syntactic structure of greater horseshoe bats

For analysis of vocal syntax, accurate classification of call sequence structures in different behavioural contexts is essential. However, an effective, intelligent program for classifying call sequences from numerous recorded sound files is still lacking. Here, we employed three machine learning algorithms (logistic regression, support vector machine and decision trees) to classify call sequences of social vocalizations of greater horseshoe bats (Rhinolophus ferrumequinum) in aggressive and distress contexts. The three machine learning algorithms obtained highly accurate classification rates (logistic regression 98%, support vector machine 97% and decision trees 96%). The algorithms also extracted three of the most important features for the classification: the transition between two adjacent syllables, the probability of occurrences of syllables in each position of a sequence, and the characteristics of a sequence. The results of statistical analysis also supported the classification of the algorithms. The study provides the first efficient method for data mining of call sequences and the possibility of linguistic parameters in animal communication. It suggests the presence of song-like syntax in the social vocalizations emitted within a non-breeding context in a bat species.

Computational geometric tools for quantitative comparison of locomotory behavior

A fundamental challenge for behavioral neuroscientists is to accurately quantify (dis)similarities in animal behavior without excluding inherent variability present between individuals. We explored two new applications of curve and shape alignment techniques to address this issue. As a proof-of-concept we applied these methods to compare normal or alarmed behavior in pairs of medaka (Oryzias latipes). The curve alignment method we call Behavioral Distortion Distance (BDD) revealed that alarmed fish display less predictable swimming over time, even if individuals incorporate the same action patterns like immobility, sudden changes in swimming trajectory, or changing their position in the water column. The Conformal Spatiotemporal Distance (CSD) technique on the other hand revealed that, in spite of the unpredictability, alarmed individuals exhibit lower variability in overall swim patterns, possibly accounting for the widely held notion of "stereotypy" in alarm responses. More generally, we propose that these new applications of established computational geometric techniques are useful in combination to represent, compare, and quantify complex behaviors consisting of common action patterns that differ in duration, sequence, or frequency.

Characteristics and microgeographic variation of whistles from the vocal repertoire of beluga whales (Delphinapterus leucas) from the White Sea

The beluga whale (Delphinapterus leucas) produces a variety of sounds, including whistles as well as pulsed, noisy, and biphonic vocalizations. This study presents the fine-scale, microgeographic characteristics of beluga whistles from data collected in four locations across Onega Bay and Dvina Bay in the White Sea, Russia. Ten parameters were measured from 1232 whistles. The whistles had a fundamental frequency from 0.4 to 25.0 kHz and duration from 0.04 to 3.93 s. Although two distinct types could be recognized in the White Sea beluga's whistle repertoire, other whistles formed a graded continuum. Among them, "flat" whistle contours with no inflection points appear to be the most common (39.7%), to be followed by frequency-modulated whistles: ascending (27.1%) and descending (15.6%). Non-linear phenomena detected in the whistles included frequency jumps (23.1%), biphonations (13.2%), sidebands (5.2%), and subharmonics (0.5%). The whistles varied in frequency parameters and duration across the locations, while those recorded in the same location in different years showed minimal difference. Beluga whistles appear to be an extremely diverse class of vocalizations. This, together with the lack of clear correlations between the behavior of whales and whistle production suggests whistles may perform multiple functions within the beluga whale communication system.

The newly described Araguaian river dolphins, Inia araguaiaensis (Cetartiodactyla, Iniidae), produce a diverse repertoire of acoustic signals

The recent discovery of the Araguaian river dolphin (Inia araguaiaensis) highlights how little we know about the diversity and biology of river dolphins. In this study, we described the acoustic repertoire of this newly discovered species in concert with their behaviour. We analysed frequency contours of 727 signals (sampled at 10 ms temporal resolution). These contours were analyzed using an adaptive resonance theory neural network combined with dynamic time-warping (ARTwarp). Using a critical similarity value of 96%, frequency contours were categorized into 237 sound-types. The most common types were emitted when calves were present suggesting a key role in mother-calf communication. Our findings show that the acoustic repertoire of river dolphins is far from simple. Furthermore, the calls described here are similar in acoustic structure to those produced by social delphinids, such as orcas and pilot whales. Uncovering the context in which these signals are produced may help understand the social structure of this species and contribute to our understanding of the evolution of acoustic communication in whales.

Two unit analysis of Sri Lankan pygmy blue whale song over a decade

Sri Lankan pygmy blue whale song consists of three repeated units: (1) low frequency pulsive unit, (2) frequency modulated (FM) upsweep, and (3) long tonal downsweep. The Unit 2 FM unit has up to three visible upsweeps with energy concentrated at approximately 40, 50, and 60 Hz, while the Unit 3 (∼100 Hz) tonal downsweep is the most distinct unit lasting 20-30 s. Spectral characteristics of the Units 2 and 3 song elements, along with ocean sound levels, were analyzed in the Indian Ocean from 2002 to 2013. The peak frequency of the tonal Unit 3 calls decreased from approximately 106.5 to 100.7 Hz over a decade corresponding to a 5.4% decrease. Over the same time period, the frequency content of the Unit 2 upsweeps did not change as dramatically with only a 3.1% change. Ambient sound levels in the vocalization bands did not exhibit equivalent patterns in amplitude trends. Analysis showed no increase in the ambient sound or compensated peak amplitude levels of the tonal downsweeps, eliminating the presence of a Lombard effect. Here it is proposed that each song unit may convey different information and thus may be responding to different selective pressures.

Colonizing the Wild West: Low Diversity of Complete Mitochondrial Genomes in Western North Pacific Killer Whales Suggests a Founder Effect

In the North Pacific, fish-eating R-type "resident" and mammal-eating T-type "transient" killer whales do not interbreed and differ in ecology and behavior. Full-length mitochondrial genomes (about 16.4 kbp) were sequenced and assembled for 12 R-type and 14 T-type killer whale samples from different areas of the western North Pacific. All R-type individuals had the same haplotype, previously described for R-type killer whales from both eastern and western North Pacific. However, haplotype diversity of R-type killer whales was much lower in the western North Pacific than in the Aleutian Islands and the eastern North Pacific. T-type whales had 3 different haplotypes, including one previously undescribed. Haplotype diversity of T-type killer whales in the Okhotsk Sea was also much lower than in the Aleutian Islands and the eastern North Pacific. The highest haplotype diversity for both R- and T-type killer whales was observed in the Aleutian Islands. We discuss how the environmental conditions during the last glacial period might have shaped the history of killer whale populations in the North Pacific. Our results suggest the recent colonization or re-colonization of the western North Pacific by small groups of killer whales originating from the central or eastern North Pacific, possibly due to favorable environmental changes after the Last Glacial Maximum.

Selective reactions to different killer whale call categories in two delphinid species

The risk of predation is often invoked as an important factor influencing the evolution of social organization in cetaceans, but little direct information is available about how these aquatic mammals respond to predators or other perceived threats. We used controlled playback experiments to examine the behavioral responses of short-finned pilot whales (Globicephala macrorhynchus) off Cape Hatteras, NC, USA, and Risso's dolphins (Grampus griseus) off the coast of Southern California, USA, to the calls of a potential predator, mammal-eating killer whales. We transmitted calls of mammal-eating killer whales, conspecifics and baleen whales to 10 pilot whales and four Risso's dolphins equipped with multi-sensor archival acoustic recording tags (DTAGs). Only playbacks of killer whale calls resulted in significant changes in tagged animal heading. The strong responses observed in both species occurred only following exposure to a subset of killer whale calls, all of which contained multiple non-linear properties. This finding suggests that these structural features of killer whale calls convey information about predatory risk to pilot whales and Risso's dolphins. The observed responses differed between the two species; pilot whales approached the sound source while Risso's dolphins fled following playbacks. These divergent responses likely reflect differences in anti-predator response mediated by the social structure of the two species.

Imitation of novel conspecific and human speech sounds in the killer whale (Orcinus orca)

Vocal imitation is a hallmark of human spoken language, which, along with other advanced cognitive skills, has fuelled the evolution of human culture. Comparative evidence has revealed that although the ability to copy sounds from conspecifics is mostly uniquely human among primates, a few distantly related taxa of birds and mammals have also independently evolved this capacity. Remarkably, field observations of killer whales have documented the existence of group-differentiated vocal dialects that are often referred to as traditions or cultures and are hypothesized to be acquired non-genetically. Here we use a do-as-I-do paradigm to study the abilities of a killer whale to imitate novel sounds uttered by conspecific (vocal imitative learning) and human models (vocal mimicry). We found that the subject made recognizable copies of all familiar and novel conspecific and human sounds tested and did so relatively quickly (most during the first 10 trials and three in the first attempt). Our results lend support to the hypothesis that the vocal variants observed in natural populations of this species can be socially learned by imitation. The capacity for vocal imitation shown in this study may scaffold the natural vocal traditions of killer whales in the wild.

Vocal repertoire of long-finned pilot whales (Globicephala melas) in northern Norway

The knowledge of the vocal repertoire of pilot whales is very limited. In this paper, the vocal repertoire of long-finned pilot whales recorded during different encounters in the Vestfjord in northern Norway between November 2006 and August 2010 are described. Sounds were analysed using two different methods: (1) an observer-based audio-visual inspection of FFT-derived spectrograms, with which, besides a general variety of clicks, buzzes, nonharmonic sounds, and whistles, 129 different distinct call types and 25 subtypes were distinguished. These call types included pulsed calls and discrete structured whistles varying from simple to highly complex structures composed of several segments and elements. In addition, ultrasonic whistles previously not described for pilot whales were found. In addition to the diversity of single calls, call sequences consisting of repetitions and combinations of specific call types were recorded and (2) a parametric approach that permitted the confirmation of the high variability in pilot whale call structures was developed. It is concluded that the pilot whale vocal repertoire is among the most complex for the mammalian species and the high structural variability, along with call repetitions and combinations, require a closer investigation to judge their importance for vocal communication.

Acoustic differentiation of Shiho- and Naisa-type short-finned pilot whales in the Pacific Ocean

Divergence in acoustic signals used by different populations of marine mammals can be caused by a variety of environmental, hereditary, or social factors, and can indicate isolation between those populations. Two types of genetically and morphologically distinct short-finned pilot whales, called the Naisa- and Shiho-types when first described off Japan, have been identified in the Pacific Ocean. Acoustic differentiation between these types would support their designation as sub-species or species, and improve the understanding of their distribution in areas where genetic samples are difficult to obtain. Calls from two regions representing the two types were analyzed using 24 recordings from Hawai'i (Naisa-type) and 12 recordings from the eastern Pacific Ocean (Shiho-type). Calls from the two types were significantly differentiated in median start frequency, frequency range, and duration, and were significantly differentiated in the cumulative distribution of start frequency, frequency range, and duration. Gaussian mixture models were used to classify calls from the two different regions with 74% accuracy, which was significantly greater than chance. The results of these analyses indicate that the two types are acoustically distinct, which supports the hypothesis that the two types may be separate sub-species.

Physical constraints of cultural evolution of dialects in killer whales

Odontocete sounds are produced by two pairs of phonic lips situated in soft nares below the blowhole; the right pair is larger and is more likely to produce clicks, while the left pair is more likely to produce whistles. This has important implications for the cultural evolution of delphinid sounds: the greater the physical constraints, the greater the probability of random convergence. In this paper the authors examine the call structure of eight killer whale populations to identify structural constraints and to determine if they are consistent among all populations. Constraints were especially pronounced in two-voiced calls. In the calls of all eight populations, the lower component of two-voiced (biphonic) calls was typically centered below 4 kHz, while the upper component was typically above that value. The lower component of two-voiced calls had a narrower frequency range than single-voiced calls in all populations. This may be because some single-voiced calls are homologous to the lower component, while others are homologous to the higher component of two-voiced calls. Physical constraints on the call structure reduce the possible variation and increase the probability of random convergence, producing similar calls in different populations.

Vast assembly of vocal marine mammals from diverse species on fish spawning ground

Observing marine mammal (MM) populations continuously in time and space over the immense ocean areas they inhabit is challenging but essential for gathering an unambiguous record of their distribution, as well as understanding their behaviour and interaction with prey species. Here we use passive ocean acoustic waveguide remote sensing (POAWRS) in an important North Atlantic feeding ground to instantaneously detect, localize and classify MM vocalizations from diverse species over an approximately 100,000 km(2) region. More than eight species of vocal MMs are found to spatially converge on fish spawning areas containing massive densely populated herring shoals at night-time and diffuse herring distributions during daytime. We find the vocal MMs divide the enormous fish prey field into species-specific foraging areas with varying degrees of spatial overlap, maintained for at least two weeks of the herring spawning period. The recorded vocalization rates are diel (24 h)-dependent for all MM species, with some significantly more vocal at night and others more vocal during the day. The four key baleen whale species of the region: fin, humpback, blue and minke have vocalization rate trends that are highly correlated to trends in fish shoaling density and to each other over the diel cycle. These results reveal the temporospatial dynamics of combined multi-species MM foraging activities in the vicinity of an extensive fish prey field that forms a massive ecological hotspot, and would be unattainable with conventional methodologies. Understanding MM behaviour and distributions is essential for management of marine ecosystems and for accessing anthropogenic impacts on these protected marine species.

Killer whale call frequency is similar across the oceans, but varies across sympatric ecotypes

Killer whale populations may differ in genetics, morphology, ecology, and behavior. In the North Pacific, two sympatric populations ("resident" and "transient") specialize on different prey (fish and marine mammals) and retain reproductive isolation. In the eastern North Atlantic, whales from the same populations have been observed feeding on both fish and marine mammals. Fish-eating North Pacific "residents" are more genetically related to eastern North Atlantic killer whales than to sympatric mammal-eating "transients." In this paper, a comparison of frequency variables in killer whale calls recorded from four North Pacific resident, two North Pacific transient, and two eastern North Atlantic populations is reported to assess which factors drive the large-scale changes in call structure. Both low-frequency and high-frequency components of North Pacific transient killer whale calls have significantly lower frequencies than those of the North Pacific resident and North Atlantic populations. The difference in frequencies could be related to ecological specialization or to the phylogenetic history of these populations. North Pacific transient killer whales may have genetically inherited predisposition toward lower frequencies that may shape their learned repertoires.

Disproportionate emission of bubble streams with killer whale biphonic calls: perspectives on production and function

Stereotyped pulsed calls were attributed to 11 killer whales (Orcinus orca) with and without synchronous bubble streams in three datasets collected from two facilities from 1993 to 2012. Calls with and without synchronous bubble streams and divergent overlapping high frequency components ("biphonic" vs "monophonic") were compared. Subjects produced bubbles significantly more often when calls had divergent high frequency components. However, acoustic features in one biphonic call shared by five subjects provided little evidence for an acoustic effect of synchronous bubble flow. Disproportionate bubbling supported other evidence that biphonic calls form a distinct category, but suggested a function in short-range communication.

Cultural evolution of killer whale calls: background, mechanisms and consequences

Cultural evolution is a powerful process shaping behavioural phenotypes of many species including our own. Killer whales are one of the species with relatively well-studied vocal culture. Pods have distinct dialects comprising a mix of unique and shared call types; calves adopt the call repertoire of their matriline through social learning. We review different aspects of killer whale acoustic communication to provide insights into the cultural transmission and gene-culture co-evolution processes that produce the extreme diversity of group and population repertoires. We argue that the cultural evolution of killer whale calls is not a random process driven by steady error accumulation alone: temporal change occurs at different speeds in different components of killer whale repertoires, and constraints in call structure and horizontal transmission often degrade the phylogenetic signal. We discuss the implications from bird song and human linguistic studies, and propose several hypotheses of killer whale dialect evolution.

Longitudinal recordings of the vocalizations of immature Gombe chimpanzees for developmental studies

Many researchers are interested in chimpanzee vocal communication, both as an important aspect of chimpanzee social behavior and as a source of insights into the evolution of human language. Nonetheless, very little is known about how chimpanzee vocal communication develops from infancy to adulthood. The largest dataset of audiorecordings from free-living immature chimpanzees was collected by the late Hetty van de Rijt-Plooij and Frans X. Plooij at Gombe National Park, Tanzania (1971-1973). These recordings have not yet been analysed. Therefore, the most extensive effort to study the development of chimpanzee vocalizations remains unfinished. The audiospecimens total over 10 h on 28 tapes, including 20 tapes focusing on 17 specific immature individuals with a total of 1,136 recordings. In order to make this dataset available to more researchers, the analogue sound recordings were digitized and stored in the Macaulay Library and the Dryad Repository. In addition, the original notes on the contexts of the calls were translated and transcribed from Dutch into English.

Constraints and flexibility in mammalian social behaviour: introduction and synthesis

This paper introduces a Theme Issue presenting the latest developments in research on the interplay between flexibility and constraint in social behaviour, using comparative datasets, long-term field studies and experimental data from both field and laboratory studies of mammals. We first explain our focus on mammals and outline the main components of their social systems, focusing on variation within- and among-species in numerous aspects of social organization, mating system and social structure. We then review the current state of primarily ultimate explanations of this diversity in social behaviour. We approach the question of how and why the balance between behavioural flexibility and continuity is achieved by discussing the genetic, developmental, ecological and social constraints on hypothetically unlimited behavioural flexibility. We introduce the other contributions to this Theme Issue against this background and conclude that constraints are often crucial to the evolution and expression of behavioural flexibility. In exploring these issues, the enduring relevance of Tinbergen's seminal paper 'On aims and methods in ethology', with its advocacy of an integrative, four-pronged approach to studying behaviour becomes apparent: an exceptionally fitting tribute on the 50th anniversary of its publication.

Ultrasonic whistles of killer whales (Orcinus orca) recorded in the North Pacific (L)

Ultrasonic whistles were previously found in North Atlantic killer whales and were suggested to occur in eastern North Pacific killer whales based on the data from autonomous recorders. In this study ultrasonic whistles were found in the recordings from two encounters with the eastern North Pacific offshore ecotype killer whales and one encounter with the western North Pacific killer whales of unknown ecotype. All ultrasonic whistles were highly stereotyped and all but two had downsweep contours. These results demonstrate that specific sound categories can be shared by killer whales from different ocean basins.