Bottlenose dolphin mothers modify signature whistles in the presence of their own calves.

Human caregivers interacting with children typically modify their speech in ways that promote attention, bonding, and language acquisition. Although this "motherese," or child-directed communication (CDC), occurs in a variety of human cultures, evidence among nonhuman species is very rare. We looked for its occurrence in a nonhuman mammalian species with long-term mother-offspring bonds that is capable of vocal production learning, the bottlenose dolphin (Tursiops truncatus). Dolphin signature whistles provide a unique opportunity to test for CDC in nonhuman animals, because we are able to quantify changes in the same vocalizations produced in the presence or absence of calves. We analyzed recordings made during brief catch-and-release events of wild bottlenose dolphins in waters near Sarasota Bay, Florida, United States, and found that females produced signature whistles with significantly higher maximum frequencies and wider frequency ranges when they were recorded with their own dependent calves vs. not with them. These differences align with the higher fundamental frequencies and wider pitch ranges seen in human CDC. Our results provide evidence in a nonhuman mammal for changes in the same vocalizations when produced in the presence vs. absence of offspring, and thus strongly support convergent evolution of motherese, or CDC, in bottlenose dolphins. CDC may function to enhance attention, bonding, and vocal learning in dolphin calves, as it does in human children. Our data add to the growing body of evidence that dolphins provide a powerful animal model for studying the evolution of vocal learning and language.

Long-distance vocalizations of spotted hyenas contain individual, but not group, signatures.

In animal societies, identity signals are common, mediate interactions within groups, and allow individuals to discriminate group-mates from out-group competitors. However, individual recognition becomes increasingly challenging as group size increases and as signals must be transmitted over greater distances. Group vocal signatures may evolve when successful in-group/out-group distinctions are at the crux of fitness-relevant decisions, but group signatures alone are insufficient when differentiated within-group relationships are important for decision-making. Spotted hyenas are social carnivores that live in stable clans of less than 125 individuals composed of multiple unrelated matrilines. Clan members cooperate to defend resources and communal territories from neighbouring clans and other mega carnivores; this collective defence is mediated by long-range (up to 5 km range) recruitment vocalizations, called whoops. Here, we use machine learning to determine that spotted hyena whoops contain individual but not group signatures, and that fundamental frequency features which propagate well are critical for individual discrimination. For effective clan-level cooperation, hyenas face the cognitive challenge of remembering and recognizing individual voices at long range. We show that serial redundancy in whoop bouts increases individual classification accuracy and thus extended call bouts used by hyenas probably evolved to overcome the challenges of communicating individual identity at long distance.

A practical guide for generating unsupervised, spectrogram-based latent space representations of animal vocalizations.

BACKGROUND: The manual detection, analysis and classification of animal vocalizations in acoustic recordings is laborious and requires expert knowledge. Hence, there is a need for objective, generalizable methods that detect underlying patterns in these data, categorize sounds into distinct groups and quantify similarities between them. Among all computational methods that have been proposed to accomplish this, neighbourhood-based dimensionality reduction of spectrograms to produce a latent space representation of calls stands out for its conceptual simplicity and effectiveness. Goal of the study/what was done: Using a dataset of manually annotated meerkat Suricata suricatta vocalizations, we demonstrate how this method can be used to obtain meaningful latent space representations that reflect the established taxonomy of call types. We analyse strengths and weaknesses of the proposed approach, give recommendations for its usage and show application examples, such as the classification of ambiguous calls and the detection of mislabelled calls. What this means: All analyses are accompanied by example code to help researchers realize the potential of this method for the study of animal vocalizations.

Dynamic biosonar adjustment strategies in deep-diving Risso's dolphins driven partly by prey evasion.

Toothed whales have evolved flexible biosonar systems to find, track and capture prey in diverse habitats. Delphinids, phocoenids and iniids adjust inter-click intervals and source levels gradually while approaching prey. In contrast, deep-diving beaked and sperm whales maintain relatively constant inter-click intervals and apparent output levels during the approach followed by a rapid transition into the foraging buzz, presumably to maintain a long-range acoustic scene in a multi-target environment. However, it remains unknown whether this rapid biosonar adjustment strategy is shared by delphinids foraging in deep waters. To test this, we investigated biosonar adjustments of a deep-diving delphinid, the Risso's dolphin (Grampus griseus). We analyzed inter-click interval and apparent output level adjustments recorded from sound recording tags to quantify in situ sensory adjustment during prey capture attempts. Risso's dolphins did not follow typical (20logR) biosonar adjustment patterns seen in shallow-water species, but instead maintained stable repetition rates and output levels up to the foraging buzz. Our results suggest that maintaining a long-range acoustic scene to exploit complex, multi-target prey layers is a common strategy amongst deep-diving toothed whales. Risso's dolphins transitioned rapidly into the foraging buzz just like beaked whales during most foraging attempts, but employed a more gradual biosonar adjustment in a subset (19%) of prey approaches. These were characterized by higher speeds and minimum specific acceleration, indicating higher prey capture efforts associated with evasive prey. Thus, tracking and capturing evasive prey using biosonar may require a more gradual switch between multi-target echolocation and single-target tracking.

Whistling is metabolically cheap for communicating bottlenose dolphins (Tursiops truncatus).

Toothed whales depend on sound for communication and foraging, making them potentially vulnerable to acoustic masking from increasing anthropogenic noise. Masking effects may be ameliorated by higher amplitudes or rates of calling, but such acoustic compensation mechanisms may incur energetic costs if sound production is expensive. The costs of whistling in bottlenose dolphins (Tursiops truncatus) have been reported to be much higher (20% of resting metabolic rate, RMR) than theoretical predictions (0.5-1% of RMR). Here, we address this dichotomy by measuring the change in the resting O2 consumption rate (V̇O2 ), a proxy for RMR, in three post-absorptive bottlenose dolphins during whistling and silent trials, concurrent with simultaneous measurement of acoustic output using a calibrated hydrophone array. The experimental protocol consisted of a 2-min baseline period to establish RMR, followed by a 2-min voluntary resting surface apnea, with or without whistling as cued by the trainers, and then a 5-min resting period to measure recovery costs. Daily fluctuations in V̇O2  were accounted for by subtracting the baseline RMR from the recovery costs to estimate the cost of apnea with and without whistles relative to RMR. Analysis of 52 sessions containing 1162 whistles showed that whistling did not increase metabolic cost (P>0.1, +4.2±6.9%) as compared with control trials (-0.5±5.9%; means±s.e.m.). Thus, we reject the hypothesis that whistling is costly for bottlenose dolphins, and conclude that vocal adjustments such as the Lombard response to noise do not represent large direct energetic costs for communicating toothed whales.

Signal-specific amplitude adjustment to noise in common bottlenose dolphins (Tursiops truncatus).

Anthropogenic underwater noise has increased over the past century, raising concern about the impact on cetaceans that rely on sound for communication, navigation and locating prey and predators. Many terrestrial animals increase the amplitude of their acoustic signals to partially compensate for the masking effect of noise (the Lombard response), but it has been suggested that cetaceans almost fully compensate with amplitude adjustments for increasing noise levels. Here, we used sound-recording DTAGs on pairs of free-ranging common bottlenose dolphins (Tursiops truncatus) to test (i) whether dolphins increase signal amplitude to compensate for increasing ambient noise and (ii) whether adjustments are identical for different signal types. We present evidence of a Lombard response in the range 0.1-0.3 dB per 1 dB increase in ambient noise, which is similar to that of terrestrial animals, but much lower than the response reported for other cetaceans. We found that signature whistles tended to be louder and with a lower degree of amplitude adjustment to noise compared with non-signature whistles, suggesting that signature whistles may be selected for higher output levels and may have a smaller scope for amplitude adjustment to noise. The consequence of the limited degree of vocal amplitude compensation is a loss of active space during periods of increased noise, with potential consequences for group cohesion, conspecific encounter rates and mate attraction.

Heaviside's dolphins (Cephalorhynchus heavisidii) relax acoustic crypsis to increase communication range.

The costs of predation may exert significant pressure on the mode of communication used by an animal, and many species balance the benefits of communication (e.g. mate attraction) against the potential risk of predation. Four groups of toothed whales have independently evolved narrowband high-frequency (NBHF) echolocation signals. These signals help NBHF species avoid predation through acoustic crypsis by echolocating and communicating at frequencies inaudible to predators such as mammal-eating killer whales. Heaviside's dolphins (Cephalorhynchus heavisidii) are thought to exclusively produce NBHF echolocation clicks with a centroid frequency around 125 kHz and little to no energy below 100 kHz. To test this, we recorded wild Heaviside's dolphins in a sheltered bay in Namibia. We demonstrate that Heaviside's dolphins produce a second type of click with lower frequency and broader bandwidth in a frequency range that is audible to killer whales. These clicks are used in burst-pulses and occasional click series but not foraging buzzes. We evaluate three different hypotheses and conclude that the most likely benefit of these clicks is to decrease transmission directivity and increase conspecific communication range. The expected increase in active space depends on background noise but ranges from 2.5 (Wenz Sea State 6) to 5 times (Wenz Sea State 1) the active space of NBHF signals. This dual click strategy therefore allows these social dolphins to maintain acoustic crypsis during navigation and foraging, and to selectively relax their crypsis to facilitate communication with conspecifics.

Echolocation click source parameters of Australian snubfin dolphins (Orcaella heinsohni).

The Australian snubfin dolphin (Orcaella heinsohni) is endemic to Australian waters, yet little is known about its abundance and habitat use. To investigate the feasibility of Passive Acoustic Monitoring for snubfin dolphins, biosonar clicks were recorded in Cygnet Bay, Australia, using a four-element hydrophone array. Clicks had a mean source level of 200 ± 5 dB re 1 µPa pp, transmission directivity index of 24 dB, mean centroid frequency of 98 ± 9 kHz, and a root-mean-square bandwidth of 31 ± 3 kHz. Such properties lend themselves to passive acoustic monitoring, but are comparable to similarly-sized delphinids, thus requiring additional cues to discriminate between snubfins and sympatric species.

Single-click beam patterns suggest dynamic changes to the field of view of echolocating Atlantic spotted dolphins (Stenella frontalis) in the wild.

Echolocating animals exercise an extensive control over the spectral and temporal properties of their biosonar signals to facilitate perception of their actively generated auditory scene when homing in on prey. The intensity and directionality of the biosonar beam defines the field of view of echolocating animals by affecting the acoustic detection range and angular coverage. However, the spatial relationship between an echolocating predator and its prey changes rapidly, resulting in different biosonar requirements throughout prey pursuit and capture. Here, we measured single-click beam patterns using a parametric fit procedure to test whether free-ranging Atlantic spotted dolphins (Stenella frontalis) modify their biosonar beam width. We recorded echolocation clicks using a linear array of receivers and estimated the beam width of individual clicks using a parametric spectral fit, cross-validated with well-established composite beam pattern estimates. The dolphins apparently increased the biosonar beam width, to a large degree without changing the signal frequency, when they approached the recording array. This is comparable to bats that also expand their field of view during prey capture, but achieve this by decreasing biosonar frequency. This behaviour may serve to decrease the risk that rapid escape movements of prey take them outside the biosonar beam of the predator. It is likely that shared sensory requirements have resulted in bats and toothed whales expanding their acoustic field of view at close range to increase the likelihood of successfully acquiring prey using echolocation, representing a case of convergent evolution of echolocation behaviour between these two taxa.

Echolocation parameters of Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the wild.

Echolocation is a key sensory modality for toothed whale orientation, navigation, and foraging. However, a more comparative understanding of the biosonar properties of toothed whales is necessary to understand behavioral and evolutionary adaptions. To address this, two free-ranging sympatric delphinid species, Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus), were studied. Biosonar clicks from both species were recorded within the same stretch of coastal habitat in Exmouth Gulf, Western Australia, using a vertical seven element hydrophone array. S. sahulensis used biosonar clicks with a mean source level of 199 ± 3 dB re 1 µPa peak-peak (pp), mean centroid frequency of 106 ± 11 kHz, and emitted at interclick intervals (ICIs) of 79 ± 33 ms. These parameters were similar to click parameters of sympatric T. aduncus, characterized by mean source levels of 204 ± 4 dB re 1 µPa pp, centroid frequency of 112 ± 9 kHz, and ICIs of 73 ± 29 ms. These properties are comparable to those of other similar sized delphinids and suggest that biosonar parameters are independent of sympatric delphinids and possibly driven by body size. The dynamic biosonar behavior of these delphinids may have, consequently, allowed for adaptations to local environments through high levels of control over sonar beam properties.

Caller identification and characterization of individual humpback whale acoustic behaviour.

Acoustic recording tags provide fine-scale data linking acoustic signalling with individual behaviour; however, when an animal is in a group, it is challenging to tease apart calls of conspecifics and identify which individuals produce each call. This, in turn, prohibits a robust assessment of individual acoustic behaviour including call rates and silent periods, call bout production within and between individuals, and caller location. To overcome this challenge, we simultaneously instrumented small groups of humpback whales on a western North Atlantic feeding ground with sound and movement recording tags. This approach enabled a comparison of the relative amplitude of each call across individuals to infer caller identity for 97% of calls. We recorded variable call rates across individuals (mean = 23 calls/h) and groups (mean = 55 calls/h). Calls were produced throughout dives, and most calls were produced in bouts with short inter-call intervals of 2.2 s. Most calls received a likely response from a conspecific within 100 s. This caller identification (ID) method facilitates studying both individual- and group-level acoustic behaviour, yielding novel results about the nature of sequence production and vocal exchanges in humpback whale social calls. Future studies can expand on these caller ID methods for understanding intra-group communication across taxa.

Low hunting costs in an expensive marine mammal predator.

Many large terrestrial mammalian predators use energy-intensive, high-risk, high-gain strategies to pursue large, high-quality prey. However, similar-sized marine mammal predators with even higher field metabolic rates (FMRs) consistently target prey three to six orders of magnitude smaller than themselves. Here, we address the question of how these active and expensive marine mammal predators can gain sufficient energy from consistently targeting small prey during breath-hold dives. Using harbor porpoises as model organisms, we show that hunting small aquatic prey is energetically cheap (<20% increase in FMR) for these marine predators, but it requires them to spend a large proportion (>60%) of time foraging. We conclude that this grazing foraging strategy on small prey is viable for marine mammal predators despite their high FMR because they can hunt near continuously at low marginal expense. Consequently, cessation of foraging due to human disturbance comes at a high cost, as porpoises must maintain their high thermoregulation costs with a reduced energy intake.

Acoustic behavior of humpback whale calves on the feeding ground: Comparisons across age and implications for vocal development.

Studying sound production at different developmental stages can provide insight into the processes involved in vocal ontogeny. Humpback whales (Megaptera novaeangliae) are a known vocal learning species, but their vocal development is poorly understood. While studies of humpback whale calves in the early stages of their lives on the breeding grounds and migration routes exist, little is known about the behavior of these immature, dependent animals by the time they reach the feeding grounds. In this study, we used data from groups of North Atlantic humpback whales in the Gulf of Maine in which all members were simultaneously carrying acoustic recording tags attached with suction cups. This allowed for assignment of likely caller identity using the relative received levels of calls across tags. We analyzed data from 3 calves and 13 adults. There were high levels of call rate variation among these individuals and the results represent preliminary descriptions of calf behavior. Our analysis suggests that, in contrast to the breeding grounds or on migration, calves are no longer acoustically cryptic by the time they reach their feeding ground. Calves and adults both produce calls in bouts, but there may be some differences in bout parameters like inter-call intervals and bout durations. Calves were able to produce most of the adult vocal repertoire but used different call types in different proportions. Finally, we found evidence of immature call types in calves, akin to protosyllables used in babbling in other mammals, including humans. Overall, the sound production of humpback whale calves on the feeding grounds appears to be already similar to that of adults, but with differences in line with ontogenetic changes observed in other vocal learning species.

Cranial asymmetry in odontocetes: a facilitator of sonic exploration?

Directional cranial asymmetry is an intriguing condition that has evolved in all odontocetes which has mostly been associated with sound production for echolocation. In this study, we investigated how cranial asymmetry varies across odontocete species both in terms of quality (i.e., shape), and quantity (magnitude of deviation from symmetry). We investigated 72 species across all ten families of Odontoceti using two-dimensional geometric morphometrics. The average asymmetric shape was largely consistent across odontocetes - the rostral tip, maxillae, antorbital notches and braincase, as well as the suture crest between the frontal and interparietal bones were displaced to the right, whereas the nasal septum and premaxillae showed leftward shifts, in concert with an enlargement of the right premaxilla and maxilla. A clear phylogenetic signal related to asymmetric shape variation was identified across odontocetes using squared-change parsimony. The magnitude of asymmetry was widely variable across Odontoceti, with greatest asymmetry in Kogiidae, Monodontidae and Globicephalinae, followed by Physeteridae, Platanistidae and Lipotidae, while the asymmetry was lowest in Lissodelphininae, Phocoenidae, Iniidae and Pontoporiidae. Ziphiidae presented a wide spectrum of asymmetry. Generalized linear models explaining magnitude of asymmetry found associations with click source level while accounting for cranial size. Using phylogenetic generalized least squares, we reconfirm that source level and centroid size significantly predict the level of cranial asymmetry, with more asymmetric marine taxa generally consisting of bigger species emitting higher output sonar signal, i.e. louder sounds. Both characteristics theoretically support foraging at depth, the former by allowing extended diving and the latter being adaptive for prey detection at longer distances. Thus, cranial asymmetry seems to be an evolutionary pathway that allows odontocetes to devote more space for sound-generating structures associated with echolocation and thus increases biosonar search range and foraging efficiency beyond simple phylogenetic scaling predictions.

Anthropogenic noise impairs cooperation in bottlenose dolphins.

Understanding the impact of human disturbance on wildlife populations is of societal importance,1 with anthropogenic noise known to impact a range of taxa, including mammals,2 birds,3 fish,4 and invertebrates.5 While animals are known to use acoustic and other behavioral mechanisms to compensate for increasing noise at the individual level, our understanding of how noise impacts social animals working together remains limited. Here, we investigated the effect of noise on coordination between two bottlenose dolphins performing a cooperative task. We previously demonstrated that the dolphin dyad can use whistles to coordinate their behavior, working together with extreme precision.6 By equipping each dolphin with a sound-and-movement recording tag (DTAG-37) and exposing them to increasing levels of anthropogenic noise, we show that both dolphins nearly doubled their whistle durations and increased whistle amplitude in response to increasing noise. While these acoustic compensatory mechanisms are the same as those frequently used by wild cetaceans,8,9,10,11,12,13 they were insufficient to overcome the effect of noise on behavioral coordination. Indeed, cooperative task success decreased in the presence of noise, dropping from 85% during ambient noise control trials to 62.5% during the highest noise exposure. This is the first study to demonstrate in any non-human species that noise impairs communication between conspecifics performing a cooperative task. Cooperation facilitates vital functions across many taxa and our findings highlight the need to account for the impact of disturbance on functionally important group tasks in wild animal populations.

Accelerometer-based predictions of behaviour elucidate factors affecting the daily activity patterns of spotted hyenas.

Animal activity patterns are highly variable and influenced by internal and external factors, including social processes. Quantifying activity patterns in natural settings can be challenging, as it is difficult to monitor animals over long time periods. Here, we developed and validated a machine-learning-based classifier to identify behavioural states from accelerometer data of wild spotted hyenas (Crocuta crocuta), social carnivores that live in large fission-fusion societies. By combining this classifier with continuous collar-based accelerometer data from five hyenas, we generated a complete record of activity patterns over more than one month. We used these continuous behavioural sequences to investigate how past activity, individual idiosyncrasies, and social synchronization influence hyena activity patterns. We found that hyenas exhibit characteristic crepuscular-nocturnal daily activity patterns. Time spent active was independent of activity level on previous days, suggesting that hyenas do not show activity compensation. We also found limited evidence for an effect of individual identity on activity, and showed that pairs of hyenas who synchronized their activity patterns must have spent more time together. This study sheds light on the patterns and drivers of activity in spotted hyena societies, and also provides a useful tool for quantifying behavioural sequences from accelerometer data.

Estimated communication range and energetic cost of bottlenose dolphin whistles in a tropical habitat.

Bottlenose dolphins (Tursiops sp.) depend on frequency-modulated whistles for many aspects of their social behavior, including group cohesion and recognition of familiar individuals. Vocalization amplitude and frequency influences communication range and may be shaped by many ecological and physiological factors including energetic costs. Here, a calibrated GPS-synchronized hydrophone array was used to record the whistles of bottlenose dolphins in a tropical shallow-water environment with high ambient noise levels. Acoustic localization techniques were used to estimate the source levels and energy content of individual whistles. Bottlenose dolphins produced whistles with mean source levels of 146.7 ± 6.2 dB re. 1 µPa(RMS). These were lower than source levels estimated for a population inhabiting the quieter Moray Firth, indicating that dolphins do not necessarily compensate for the high noise levels found in noisy tropical habitats by increasing their source level. Combined with measured transmission loss and noise levels, these source levels provided estimated median communication ranges of 750 m and maximum communication ranges up to 5740 m. Whistles contained less than 17 mJ of acoustic energy, showing that the energetic cost of whistling is small compared to the high metabolic rate of these aquatic mammals, and unlikely to limit the vocal activity of toothed whales.

Pile driving repeatedly impacts the giant scallop (Placopecten magellanicus).

Large-scale offshore wind farms are a critical component of the worldwide climate strategy. However, their developments have been opposed by the fishing industry because of concerns regarding the impacts of pile driving vibrations during constructions on commercially important marine invertebrates, including bivalves. Using field-based daily exposure, we showed that pile driving induced repeated valve closures in different scallop life stages, with particularly stronger effects for juveniles. Scallops showed no acclimatization to repetitive pile driving across and within days, yet quickly returned to their initial behavioral baselines after vibration-cessation. While vibration sensitivity was consistent, daily pile driving did not disrupt scallop circadian rhythm, but suggests serious impacts at night when valve openings are greater. Overall, our results show distance and temporal patterns can support future mitigation strategies but also highlight concerns regarding the larger impact ranges of impending widespread offshore wind farm constructions on scallop populations.

Allied male dolphins use vocal exchanges to "bond at a distance".

Vocal interactions are intrinsic features of social groups and can play a pivotal role in social bonding.1,2 Dunbar's social bonding hypothesis posits that vocal exchanges evolved to "groom at a distance" when social groups became too large or complex for individuals to devote time to physical bonding activities.1,3 Tests of this hypothesis in non-human primates, however, suggest that vocal exchanges occur between more strongly bonded individuals that engage in higher grooming rates4-7 and thus do not provide evidence for replacement of physical bonding. Here, we combine data on social bond strength, whistle exchange frequency, and affiliative contact behavior rates to test this hypothesis in wild male Indo-Pacific bottlenose dolphins, who form multi-level alliances that cooperate over access to females.8-10 We show that, although whistle exchanges are more likely to occur within the core alliance, they occur more frequently between those males that share weaker social bonds, i.e., between core allies that spend less time together, while the opposite occurs for affiliative physical contact behavior. This suggests that vocal exchanges function as a low-cost mechanism for male dolphins that spend less time in close proximity and engage in fewer affiliative contact behaviors to reinforce and maintain their valuable alliance relationships. Our findings provide new evidence outside of the primate lineage that vocal exchanges serve a bonding function and reveal that, as the social bonding hypothesis originally suggested, vocal exchanges can function as a replacement of physical bonding activities for individuals to maintain their important social relationships.