Echolocation is a closed-loop active sensing modality in which animals not only choose how they move to acquire information, but also actively modulate incoming sensory (echo) information by shaping the acoustic signals they emit to probe the environment. While many models describe how echolocating animals react to prior echoes by adjusting subsequent behavior, few explicitly model how they cognitively reason about information embedded in echoes when determining future actions. Here, we extend "infotaxis," an information-greedy algorithm originally developed for olfactory search, to sonar sensing by formulating an echolocating agent searching for a single target under sensory uncertainty characterized by probabilities of miss and false alarm. Through analytical and computational analyses, we show that the characteristic exploration-exploitation balance of infotaxis is conserved across sensory modalities, and that the efficiency and reliability of infotaxis search depend strongly on sensory information quality. Compared with a maximum a posteriori agent that always directs the beam to the most probable target location, the infotaxis agent consistently completes searches with fewer pings and greater robustness to sensory uncertainty. These results highlight information as a powerful concept for understanding active sensing and for revealing principles of sonar-guided autonomy in both biological and engineered systems. ### Competing Interest Statement The authors have declared no competing interest. Office of Naval Research, https://ror.org/00rk2pe57, N00014-18-1-2069, N00014-20-1-2709, N00014-23-12065

Cetaceans spend considerable periods of time underwater, but much about the physiological response of these animals during a dive remains unknown. We present an approach that combines near-infrared spectroscopy (NIRS) with kinematic measurements to investigate bottlenose dolphin hemodynamics in managed settings during voluntary breathing at the surface, diving to a submerged target and free swimming. For all conditions, oxygenated hemoglobin initially increased following respiration as oxygen saturation increased and then decreased until the following breath. Deoxygenated hemoglobin remained relatively steady during shorter respiration periods (near surface and swimming dives) but declined during the longer breath-holds and swimming dives. This was also seen in the calculated tissue saturation index, with up to an 8.9% decrease during extended breath-holds. The approach presented here provides new insights into the physiological responses of free-swimming animals and is an important step towards making these measurements on animals in the wild.

We analyze ocean ambient sound from a global network of hydrophones installed and maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization. We process acoustic data from nine hydrophones across six hydroacoustic stations distributed across five oceans: North Pacific, South Pacific, South Atlantic, Indian, and Southern oceans, for up to 19 consecutive years between 2003 and 2021. We identify dominant natural and anthropogenic sources for all six stations and observe long-term trends at four of them. Out of 20 statistical tests, 15 identified a significant downward trend in sound pressure level. Possible causes of these decreasing trends include a global recession in 2016, the COVID-19 pandemic in 2020, five major earthquakes between 2004 and 2012 (with none in 2013-2021), and a steadily increasing sea surface temperature, which decreases the sea surface critical angle and hence the contribution from near-surface sources (e.g., shipping) to sound pressure level.

We have built a unique library of sounds produced by known individual common bottlenose dolphins ( Tursiops truncatus ), by recording them non-invasively with suction cup hydrophones during brief catch and release health assessments and with digital acoustic tags (DTAGs). We have catalogued the name-like signature whistles (SWs) of most animals in this resident community of 170 dolphins, which has enabled us to begin studying little known “non-signature whistles” (NSW). We have so far identified 22 shared NSW types, of which two, NSWA and NSWB, are known to have been produced by at least 25 and 35 different dolphins respectively. We are studying the functions of shared NSWs with playback experiments to free-swimming dolphins. We provide background on past playback studies and how they have informed our current research; in particular, received level (RL) of playbacks was found to significantly influence strength of response. Varied responses to playbacks reflect the complexity of dolphin communication, and highlight the need for larger sample sizes to be able to correctly interpret NSW functions. However, results so far have provided support for both the referential nature of SW and the affiliative nature of SW copies (SWCs), because a majority of control playbacks of a dolphin’s own signature whistle (self playbacks) elicited positive responses. NSWA elicited a majority of negative responses, suggesting an alarm-type function, and NSWB elicited varying responses, supporting our suggested function of this whistle type as a “query,” produced when something unexpected or unfamiliar is heard. Given that SW and SWC are known to be learned and appear to be referential signals, it is likely that shared, stereotyped NSW are both learned and referential as well, an idea that is supported by the fact that dolphins are flexible, life-long vocal production learners, unlike most other non-human mammals. Our study provides the first evidence in dolphins for a wider repertoire of shared, context-specific signals, which could form the basis for a language-like communication system. ### Competing Interest Statement The authors have declared no competing interest.

Previous studies in dolphins suggests that, like humans, dolphins display an “oddball” response in the auditory cortex when one auditory stimulus has a low probability of occurrence relative to others in a stimulus train. However, the previous dolphin studies used stimuli and sequences that were not biologically relevant to the animal. Additionally, human experiments showed that the magnitude of the auditory cortical response scales with attention when the subjects must respond to a “target” sound. The present study compared auditory cortical responses in four dolphins (two juvenile and two geriatric) to three whistle-like stimuli. We contrasted auditory cortical responses when the stimuli were presented in a randomized sequence to those in a predictable sequence where an oddball was present based on fixed probabilities. Two different conditions were tested: (1) when the dolphins were passively listening to the stimuli and (2) when the dolphins were trained to listen to the stimuli and produce a whistle response to one of the three stimuli designated as the target. This paper will discuss the differences in auditory cortical responses between the two sequences and conditions, as well as notable differences between the juvenile and geriatric subjects. [Work funded by ONR.]

Voluntary movements of echolocating animals with respect to targets in natural foraging or laboratory target discrimination tasks have long been interpreted as a closed-loop sensorimotor feedback driven by information in previously received echoes. However, what can we infer about sensorimotor integration and auditory information-gathering from animal movement trajectories? In this work, we use unsupervised clustering to analyze the movement trajectories of a free-swimming, echolocating harbor porpoise trained to select a sphere against prolate spheroids of varying aspect ratios presented at different angles, and show that the animal's discrimination performance and overall trajectory can be explained by task difficulty based on the similarity of target echo spectra received during its initial approach. The porpoise continued to evaluate its target selection via incoming echoes throughout the trials, and reversed its decision at very close ranges in a subset of trials. In more challenging scenarios, the animal engaged in prolonged, focused ensonification of a single target, sometimes via buzzes, which we interpret as an evidence accumulation process toward decision making. Our findings highlight movement, in addition to acoustic emissions, as a key behavioral readout in the active information acquisition process embodied in echolocation.

We report the first application of diffusion tractography to a mysticete, which was analyzed alongside three odontocete brains, allowing the first direct comparison of strength and laterality of auditory pathways in echolocating and non-echolocating whales. Brains were imaged post-mortem at high resolution with a specialized steady state free precession diffusion sequence optimized for dead tissue. We conducted probabilistic tractography to compare the qualitative features, tract strength, and lateralization of potential ascending and descending auditory paths in the mysticete versus odontocetes. Tracts were seeded in the inferior colliculi (IC), a nexus for ascending auditory information, and the cerebellum, a center for sensorimotor integration. Direct IC to temporal lobe pathways were found in all animals, replicating previous cetacean tractography and suggesting conservation of the primary auditory projection path in the cetacean clade. Additionally, odontocete IC-cerebellum pathways exhibited higher overall tract strength than in the mysticete, suggesting they may play a role in supporting the rapid sensorimotor integration demands of echolocation. Further, in the mysticete, contralateral right IC to left cerebellum pathways were 17x stronger than those between left IC and right cerebellum, while in odontocetes, the laterality was reversed, and left IC to right cerebellum pathways were 2-4x stronger than those between right IC and left cerebellum. This lateralization may also relate to echolocation. Right cerebellum is responsible for integrating sensory and motor signals from the left cortical hemisphere, and in odontocetes, this hemisphere likely controls the contralateral right-side phonic lips, which have been empirically implicated in the production of echolocation clicks. We also found differences in the specific subregions of cerebellum targeted by the IC between the mysticete and odontocetes, some of which may also bear on hearing and vocal production. This study establishes foundational knowledge on mysticete brain connectivity and extends knowledge on pathways supporting hearing and auditory-motor integration across the order Cetacea.

Ultrasonic cavitation noise from fast vessels overlaps spectrally with echolocation clicks of toothed whales and therefore has the potential to degrade echolocation performance through auditory masking of returning echoes. Here, we tested that hypothesis by exposing two trained echolocating porpoises carrying DTAGs to two different levels of decidecade noise centered on 2 kHz (non-masking) and 125 kHz (masking) during an active target discrimination task. We found no click level adjustments or effects on discrimination performance in trials with non-masking noise or low-level masking noise. However, when exposed to high-level masking noise of 113±3 dB re. 1 µPa root mean square (RMS), the porpoises increased their mean click source levels by 7-17 dB. Despite this Lombard response of 0.2-0.5 dBsignal/dBnoise, and longer time and more clicks used by the porpoises to perform the task in noise, both animals were still significantly poorer at discriminating the targets (64-85% success rate) than in the other treatments (94-100%), thus demonstrating adverse masking effects. When the porpoises were offered spatial release from masking by relocating the noise source off-axis relative to the animal-to-target axis, echolocation performance was regained. We conclude that moderate levels of high-frequency noise, such as from cavitating vessel propellers several hundred meters from a vessel, can mask porpoise echolocation in a way that cannot be fully compensated for. As biosonar is vital for foraging and navigation around hazards such as gillnets for porpoises and other toothed whales, this study highlights that masking effects should be considered in impact assessments of cavitating vessels around echolocating toothed whales.

Body size is key to many life-history processes, including reproduction. Across species, climate change and other stressors have caused reductions in the body size to which animals can grow, called asymptotic size, with consequences for demography. A reduction in mean asymptotic length was documented for critically endangered North Atlantic right whales, in parallel with declines in health and vital rates resulting from human activities and environmental changes. Here, we tested whether smaller body size was associated with lower reproductive output, using a state-space model for individual health, survival and reproduction that quantifies the mechanistic links between these processes. Body size (as represented by the cube of length) was strongly associated with a female's calving probability at each reproductive opportunity. This relationship explained 62% of the variation in calving among reproductive females, along with their decreasing health (20%). The effects of decreasing mean body size on reproductive performance are another concerning indication of the worsening prospects for this species and many others affected by environmental change, requiring a focus of conservation and management interventions on improving conditions that affect reproduction as well as reducing mortality.

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.

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.

Some studies of how human activities can affect wild free-ranging animals may be considered to have potential negative outcomes too severe to be ethically studied. This creates a societal dilemma involving choices between continuing risky activities with high uncertainty about their potential effects on wildlife, often with considerable associated precaution or undertaking focused research to reduce uncertainty, but with some risk of harm from either strong response leading to potential stranding or direct physical injury from sound exposure. Recent and ongoing field experiments have measured the conditions in which wild cetaceans respond to military sonar, and provided insight into the nature of responses. Here mitigation measures are reported for one of the first such experiments designed to measure fine-scale behavioural responses to controlled exposures of midfrequency (3–4 kHz) active sonar. The objective was to do so without causing the kinds of physical harm that have been previously observed (e.g. stranding events) and that motivated the study. A critical goal of this experimental study was to identify a response that was safe but that could be used as an indicator of the probability of risk from more extreme or sustained exposure from real military operations. A monitoring and mitigation protocol was developed using a feedback control procedure for real-time mitigation of potential harm. Experimental protocols were modulated relative to indicators of potential risk with the explicit objective of detecting potentially harmful consequences of sound exposure and taking appropriate corrective action. Three categories of mitigation methods were developed and integrated within the experimental protocol incorporating designed, engineered, and operational mitigation measures. Controlled exposure experiments involving free-ranging animals were conducted without any evident harm to the experimental subjects, while successfully eliciting behavioural responses that provided meaningful results to inform management decisions. This approach demonstrates the importance of careful design of protocols in exposure-response experiments, particularly in pioneering studies assessing response where both the potential for harm and level of uncertainty may be high.

Most auditory evoked potential (AEP) studies in echolocating toothed whales measure neural responses to outgoing clicks and returning echoes using short-latency auditory brainstem responses (ABRs) arising a few ms after acoustic stimuli. However, little is known about longer-latency cortical AEPs despite their relevance for understanding echo processing and auditory stream segregation. Here, we used a non-invasive AEP setup with low click repetition rates on a trained harbor porpoise to test the long-standing hypothesis that echo information from distant targets is completely processed before the next click is emitted. We reject this hypothesis by finding reliable click-related AEP peaks with latencies of 90 and 160 ms, which are longer than 99% of click intervals used by echolocating porpoises, demonstrating that some higher-order echo processing continues well after the next click emission even during slow clicking. We propose that some of the echo information, such as range to evasive prey, is used to guide vocal-motor responses within 50-100 ms, but that information used for discrimination and auditory scene analysis is processed more slowly, integrating information over many click-echo pairs. We conclude by showing theoretically that the identified long-latency AEPs may enable hearing sensitivity measurements at frequencies ten times lower than current ABR methods.

Quantifying the cumulative effects of stressors on individuals and populations can inform the development of effective management and conservation strategies. We developed a Bayesian state–space model to assess the effects of multiple stressors on individual survival and reproduction. In the model, stressor effects on vital rates are mediated by changes in underlying health, allowing for the comparison of effect sizes while accounting for intrinsic factors that might affect an individual's vulnerability and resilience. We applied the model to a 50‐year dataset of sightings, calving events and stressor exposure of critically endangered North Atlantic right whales Eubalaena glacialis. The viability of this population is threatened by a complex set of stressors, including vessel strikes, entanglement in fishing gear and fluctuating prey availability. We estimated that blunt and deep vessel strike injuries and severe entanglement injuries had the largest effect on the health of exposed individuals, reinforcing the urgent need for mitigation measures. Prey abundance had a smaller but protracted effect on health across individuals, and estimated long‐term trends in survival and reproduction followed the trend of the prey index, highlighting that long‐term ecosystem‐based management strategies are also required. Our approach can be applied to quantify the effects of multiple stressors on any long‐lived species where suitable indicators of health and long‐term monitoring data are available.

How do dolphins attend to echos and ignore background noise? Is the process similar to how humans attend to conversations in a crowded room? Studies of human selective attention have highlighted endogenous brain processes modulating the magnitude of early responses to sounds in auditory evoked potentials (AEPs), yet these rely on task-specific instructions difficult to employ in animal studies. We trained an adult male dolphin (Tursiops truncatus) to attend to a stream of rapidly presented tones, and provide a whistle response to a “target” tone while withholding responses to “background” tones in an amplitude discrimination task. The background sounds were designed to function as a tonal mismatch negativity paradigm with two frequencies: a standard tone presented 80% of the time, and a deviant presented 20% of the time. Depending on condition, the target could be the same frequency as the standard or the deviant. We observed an enhanced AEP response to a deviant of the same frequency as the target, while the AEP response to the standard-target condition was reduced. This was not predicted by previous human experiments, and we are currently following up with a pilot a human task modeled after the dolphin implicit target detection task.

Toothed whales routinely discriminate and select prey via echolocation. Over a decade, the harbor porpoise Freja in Fjord and Bælt, Kerteminde, Denmark participated in a series of target discrimination experiments involving metal objects. Different from experiments in which the subjects were constrained to be stationary, Freja’s free and voluntary movements offered insights into the tightly coupled nature of acoustic sampling and movements in echolocation-based target discrimination. Compared with observations from wild animals with unknown prey, the controlled experimental task and stimuli made it possible to interpret her behavior based on predictable target echo features. Here, we test the hypothesis that an echolocating animal plans its movement based on prior echo returns to gather better target discrimination information. We trained Freja to approach and select a sphere against a spheroid in a two-alternative forced-choice discrimination task, and show that her performance and movement trajectory varied depending on the aspect ratio and presentation angle of the spheroid. Consistent with our hypothesis, we further show that Freja’s movement patterns were likely driven by echo information received earlier during the approach that is correlated with the discrimination difficulty. These results highlight movements as an important behavioral readout of the dynamic, closed-loop sensorimotor feedback in echolocation. [Work supported by ONR.]

Vocalisation rates were measured from North Atlantic right whales (Eubalaena glacialis) in spring 1999-2000 in the Great South Channel and off Cape Cod, USA, and in summer 1999-2000 in the Bay of Fundy, Canada. Vocalisations were classed as either ‘moans’, ‘low-frequency (LF) calls’ or ‘gunshots’. Towed hydrophone recordings (36.1 hours) were made in 21 encounters where loose aggregations of right whales were within about 1,000m. Recordings were also made using acoustic tags attached by suction cups to ten different whales (29.5 hours). Tags also recorded depth data. Moan rates (sounds per aggregation per hour) were correlated with size of whale aggregation. Individual whales produced moans at ~ 0-10 per hour (recorded from tags and the towed hydrophone). Small aggregations (2-10) gave higher moan rates (usually < ~ 60 per hr) and larger aggregations ( > 10) higher still ( ~ 70-700 per hr) (recorded from towed hydrophone). Results from the Bay of Fundy indicate high moan rates at night. Moans were usually produced in clusters. Tag data showed that moans were usually produced when whales were within about 10m of the surface. A passive acoustic system could potentially provide supplementary information on the distribution of aggregations of right whales. This could be useful for management (1) in the long term, by aiding the prediction of right whale distribution, or (2) as a real-time tool for helping to route shipping away from concentrations of right whales. The empirical evidence presented here on vocalisation rates will assist in assessing feasibility. The clustering of moans and the tendency to produce them near the surface could hamper detection and localisation efforts. Further research is underway to investigate other important practical issues such as detectability and source levels.