Communication masking in marine mammals: A review and research strategy

Underwater noise, whether of natural or anthropogenic origin, has the ability to interfere with the way in which marine mammals receive acoustic signals (i.e., for communication, social interaction, foraging, navigation, etc.). This phenomenon, termed auditory masking, has been well studied in humans and terrestrial vertebrates (in particular birds), but less so in marine mammals. Anthropogenic underwater noise seems to be increasing in parts of the world's oceans and concerns about associated bioacoustic effects, including masking, are growing. In this article, we review our understanding of masking in marine mammals, summarise data on marine mammal hearing as they relate to masking (including audiograms, critical ratios, critical bandwidths, and auditory integration times), discuss masking release processes of receivers (including comodulation masking release and spatial release from masking) and anti-masking strategies of signalers (e.g. Lombard effect), and set a research framework for improved assessment of potential masking in marine mammals.

A California sea lion (Zalophus californianus) can keep the beat: motor entrainment to rhythmic auditory stimuli in a non vocal mimic

Is the ability to entrain motor activity to a rhythmic auditory stimulus, that is "keep a beat," dependent on neural adaptations supporting vocal mimicry? That is the premise of the vocal learning and synchronization hypothesis, recently advanced to explain the basis of this behavior (A. Patel, 2006, Musical Rhythm, Linguistic Rhythm, and Human Evolution, Music Perception, 24, 99-104). Prior to the current study, only vocal mimics, including humans, cockatoos, and budgerigars, have been shown to be capable of motoric entrainment. Here we demonstrate that a less vocally flexible animal, a California sea lion (Zalophus californianus), can learn to entrain head bobbing to an auditory rhythm meeting three criteria: a behavioral response that does not reproduce the stimulus; performance transfer to a range of novel tempos; and entrainment to complex, musical stimuli. These findings show that the capacity for entrainment of movement to rhythmic sounds does not depend on a capacity for vocal mimicry, and may be more widespread in the animal kingdom than previously hypothesized.

Vocal learning in seals, sea lions, and walruses

The pinnipeds provide a variety of clues to those interested in the vocal learning capabilities of non-human animals. Observational and experimental studies of seals, sea lions, and walruses reveal elements of vocal development, contextual control, plasticity in expression and learning, and even imitation of complex sounds. Consideration of the factors that influence the expression of these capabilities informs understanding of the behavioral and structural mechanisms that support vocal learning in mammals and the evolutionary forces shaping these capabilities.

Algal toxin impairs sea lion memory and hippocampal connectivity, with implications for strandings

Domoic acid (DA) is a naturally occurring neurotoxin known to harm marine animals. DA-producing algal blooms are increasing in size and frequency. Although chronic exposure is known to produce brain lesions, the influence of DA toxicosis on behavior in wild animals is unknown. We showed, in a large sample of wild sea lions, that spatial memory deficits are predicted by the extent of right dorsal hippocampal lesions related to natural exposure to DA and that exposure also disrupts hippocampal-thalamic brain networks. Because sea lions are dynamic foragers that rely on flexible navigation, impaired spatial memory may affect survival in the wild.

Comparative assessment of amphibious hearing in pinnipeds

Auditory sensitivity in pinnipeds is influenced by the need to balance efficient sound detection in two vastly different physical environments. Previous comparisons between aerial and underwater hearing capabilities have considered media-dependent differences relative to auditory anatomy, acoustic communication, ecology, and amphibious life history. New data for several species, including recently published audiograms and previously unreported measurements obtained in quiet conditions, necessitate a re-evaluation of amphibious hearing in pinnipeds. Several findings related to underwater hearing are consistent with earlier assessments, including an expanded frequency range of best hearing in true seals that spans at least six octaves. The most notable new results indicate markedly better aerial sensitivity in two seals (Phoca vitulina and Mirounga angustirostris) and one sea lion (Zalophus californianus), likely attributable to improved ambient noise control in test enclosures. An updated comparative analysis alters conventional views and demonstrates that these amphibious pinnipeds have not necessarily sacrificed aerial hearing capabilities in favor of enhanced underwater sound reception. Despite possessing underwater hearing that is nearly as sensitive as fully aquatic cetaceans and sirenians, many seals and sea lions have retained acute aerial hearing capabilities rivaling those of terrestrial carnivores.

Underwater temporary threshold shift induced by octave-band noise in three species of pinniped

Pure-tone sound detection thresholds were obtained in water for one harbor seal (Phoca vitulina), two California sea lions (Zalophus californianus), and one northern elephant seal (Mirounga angustirostris) before and immediately following exposure to octave-band noise. Additional thresholds were obtained following a 24-h recovery period. Test frequencies ranged from 100 Hz to 2000 Hz and octave-band exposure levels were approximately 60-75 dB SL (sensation level at center frequency). Each subject was trained to dive into a noise field and remain stationed underwater during a noise-exposure period that lasted a total of 20-22 min. Following exposure, three of the subjects showed threshold shifts averaging 4.8 dB (Phoca), 4.9 dB (Zalophus), and 4.6 dB (Mirounga). Recovery to baseline threshold levels was observed in test sessions conducted within 24 h of noise exposure. Control sessions in which the subjects completed a simulated noise exposure produced shifts that were significantly smaller than those observed following noise exposure. These results indicate that noise of moderate intensity and duration is sufficient to induce TTS under water in these pinniped species.

How Animals Classify Friends and Foes

A model of stimulus equivalence, which describes how non-similarity-based categories are formed, is used to describe aspects of animal social and communicative interactions such as kinship, friendship, coalitions, territorial behavior, and referential calling. Although this model was originally designed to deal with stimulus relations in linguistic behavior, it can be readily applied to understanding the cognitive mechanisms that underlie social as well as non-social categorizations in numerous taxa. This approach provides a new, parsimonious, and experimentally based understanding of how animals without language deal with problems of classification in their environment.

The sap47 gene of Drosophila melanogaster codes for a novel conserved neuronal protein associated with synaptic terminals

Proteins expressed specifically in neurons and transported to synaptic terminals are likely to constitute important molecular elements of nervous system function. In an effort to characterize synapse-associated proteins (SAPs) of Drosophila, we have isolated from a hybridoma library several monoclonal antibodies (MABs) that selectively stain synaptic terminals in immunohistochemical preparations. MAB nc46 binds to most but not all synaptic terminals of the Drosophila nervous system, it also recognizes a protein with homologous distribution in other dipteran flies and binds to large parts of fish CNS. In Western blots the antibody labels a Drosophila brain protein of 47 kDa and cross-reacts with brain proteins from several species including insects, fish, mouse and man. From these data we conclude that the corresponding gene has been conserved in evolution at least among diptera. Using MAB nc46 and expression cloning we have identified the 'sap47' gene coding for the 'synapse-associated protein of 47 kDa' of Drosophila melanogaster. Sequence analysis of genomic and cDNA clones reveals the intron-exon structure of the gene and characterizes the complete open reading frames of two alternatively spliced transcripts. The sap47 gene is located in 89A8-B3 on chromosome 3R and codes for two almost identical inferred polypeptides of 347 and 351 amino acids with no significant sequence homology to known proteins.

Why pinnipeds don't echolocate

Odontocete cetaceans have evolved a highly advanced system of active biosonar. It has been hypothesized that other groups of marine animals, such as the pinnipeds, possess analogous sound production, reception, and processing mechanisms that allow for underwater orientation using active echolocation. Despite sporadic investigation over the past 30 years, the accumulated evidence in favor of the pinniped echolocation hypothesis is unconvincing. We argue that an advanced echolocation system is unlikely to have evolved in pinnipeds primarily because of constraints imposed by the obligate amphibious functioning of the pinniped auditory system. As a result of these constraints, pinnipeds have not developed highly acute, aquatic, high frequency sound production or reception systems required for underwater echolocation. Instead, it appears that pinnipeds have evolved enhanced visual, tactile, and passive listening skills. The evolutionary refinement of alternative sensory systems allows pinnipeds to effectively forage, navigate, and avoid predators under water despite the lack of active biosonar capabilities.

Amphibious hearing in spotted seals (Phoca largha): underwater audiograms, aerial audiograms and critical ratio measurements

Spotted seals (Phoca largha) inhabit Arctic regions that are facing both rapid climate change and increasing industrialization. While little is known about their sensory capabilities, available knowledge suggests that spotted seals and other ice seals use sound to obtain information from the surrounding environment. To quantitatively assess their auditory capabilities, the hearing of two young spotted seals was tested using a psychophysical paradigm. Absolute detection thresholds for tonal sounds were measured in air and under water over the frequency range of hearing, and critical ratios were determined using octave-band masking noise in both media. The behavioral audiograms show a range of best sensitivity spanning four octaves in air, from approximately 0.6 to 11 kHz. The range of sensitive hearing extends across seven octaves in water, with lowest thresholds between 0.3 and 56 kHz. Critical ratio measurements were similar in air and water and increased monotonically from 12 dB at 0.1 kHz to 30 dB at 25.6 kHz, indicating that the auditory systems of these seals are quite efficient at extracting signals from background noise. This study demonstrates that spotted seals possess sound reception capabilities different from those previously described for ice seals, and more similar to those reported for harbor seals (Phoca vitulina). The results are consistent with the amphibious lifestyle of these seals and their apparent reliance on sound. The hearing data reported herein are the first available for spotted seals and can inform best management practices for this vulnerable species in a changing Arctic.

Northern Elephant Seals Memorize the Rhythm and Timbre of Their Rivals' Voices

The evolutionary origin of rhythm perception, a cognitive ability essential to musicality, remains unresolved [1-5]. The ability to perceive and memorize rhythmic sounds is widely shared among humans [6] but seems rare among other mammals [7, 8]. Although the perception of temporal metrical patterns has been found in a few species, this ability has only been demonstrated through behavioral training [9] (but see [10] for an example of spontaneous tempo coordination in a bonobo), and there is no experimental evidence to indicate its biological function. Furthermore, there is no example of a non-human mammal able to remember and recognize auditory rhythmic patterns among a wide range of tempi. In the northern elephant seal Mirounga angustirostris, the calls of mature males comprise a rhythmic series of pulses, with the call of each individual characterized by its tempo and timbre; these individual vocal signatures are stable over years and across contexts [11]. Here, we report that northern elephant seal males routinely memorize and recognize the unique tempo and timbre of their rivals' voices and use this rhythmic information to individually identify competitors, which facilitates navigation within the social network of the rookery. By performing playbacks with natural and modified vocalizations, we show that males are sensitive to call rhythm disruption independently of modification of spectral features and that they use both temporal and spectral cues to identify familiar rivals. While spectral features of calls typically encode individual identity in mammalian vocalizations [12], this is the first example of this phenomenon involving sound rhythm.

Beat Keeping in a Sea Lion As Coupled Oscillation: Implications for Comparative Understanding of Human Rhythm

Human capacity for entraining movement to external rhythms-i.e., beat keeping-is ubiquitous, but its evolutionary history and neural underpinnings remain a mystery. Recent findings of entrainment to simple and complex rhythms in non-human animals pave the way for a novel comparative approach to assess the origins and mechanisms of rhythmic behavior. The most reliable non-human beat keeper to date is a California sea lion, Ronan, who was trained to match head movements to isochronous repeating stimuli and showed spontaneous generalization of this ability to novel tempos and to the complex rhythms of music. Does Ronan's performance rely on the same neural mechanisms as human rhythmic behavior? In the current study, we presented Ronan with simple rhythmic stimuli at novel tempos. On some trials, we introduced "perturbations," altering either tempo or phase in the middle of a presentation. Ronan quickly adjusted her behavior following all perturbations, recovering her consistent phase and tempo relationships to the stimulus within a few beats. Ronan's performance was consistent with predictions of mathematical models describing coupled oscillation: a model relying solely on phase coupling strongly matched her behavior, and the model was further improved with the addition of period coupling. These findings are the clearest evidence yet for parity in human and non-human beat keeping and support the view that the human ability to perceive and move in time to rhythm may be rooted in broadly conserved neural mechanisms.

A review of the history, development and application of auditory weighting functions in humans and marine mammals

This document reviews the history, development, and use of auditory weighting functions for noise impact assessment in humans and marine mammals. Advances from the modern era of electroacoustics, psychophysical studies of loudness, and other related hearing studies are reviewed with respect to the development and application of human auditory weighting functions, particularly A-weighting. The use of auditory weighting functions to assess the effects of environmental noise on humans-such as hearing damage-risk criteria-are presented, as well as lower-level effects such as annoyance and masking. The article also reviews marine mammal auditory weighting functions, the development of which has been fundamentally directed by the objective of predicting and preventing noise-induced hearing loss. Compared to the development of human auditory weighting functions, the development of marine mammal auditory weighting functions have faced additional challenges, including a large number of species that must be considered, a lack of audiometric information on most species, and small sample sizes for nearly all species for which auditory data are available. The review concludes with research recommendations to address data gaps and assumptions underlying marine mammal auditory weighting function design and application.

Amphibious hearing in ringed seals (Pusa hispida): underwater audiograms, aerial audiograms and critical ratio measurements

Ringed seals are semi-aquatic marine mammals with a circumpolar Arctic distribution. In this study, we investigate the amphibious hearing capabilities of ringed seals to provide auditory profiles for this species across the full range of hearing. Using psychophysical methods with two trained ringed seals, detection thresholds for narrowband signals were measured under quiet, carefully controlled environmental conditions to generate aerial and underwater audiograms. Masked underwater thresholds were measured in the presence of octave-band noise to determine critical ratios. Results indicate that ringed seals possess hearing abilities comparable to those of spotted and harbor seals, and considerably better than previously reported for ringed and harp seals. Best sensitivity was 49 dB re 1 µPa (12.8 kHz) in water, and -12 dB re 20 µPa (4.5 kHz) in air, rivaling the acute hearing abilities of some fully aquatic and terrestrial species in their respective media. Critical ratio measurements ranged from 14 dB at 0.1 kHz to 31 dB at 25.6 kHz, suggesting that ringed seals—like other true seals—can efficiently extract signals from background noise across a broad range of frequencies. The work described herein extends similar research on amphibious hearing in spotted seals, the results of which were recently published in this journal [Sills et al., J. Exp. Biol., 217, 726-734 (2014)]. These parallel studies enhance our knowledge of the auditory capabilities of ice-living seals, and inform effective management strategies for these and related species in a rapidly changing Arctic environment.

The diagnostic potential of the chromosome translocation t(2;13) in rhabdomyosarcoma: a Pcr study of fresh-frozen and paraffin-embedded tumour samples

The chromosomal translocation t(2;13)(q35;q14) has been reported in alveolar paediatric rhabdomyosarcoma. The rearrangement leads to the juxtaposition of the PAX-3 and FORKHEAD genes and the production of a fusion protein with putative transcriptional regulatory activity. The diagnostic potential of this translocation has been examined using a reverse transcription polymerase chain reaction (RT-PCR) assay to detect translocations in both fresh-frozen and archival formalin-fixed, paraffin-embedded rhabdomyosarcoma. A total of 25 tumours and one cell line were examined. PAX-3-FORKHEAD chimeric mRNAs were amplified by PCR in 8 of 15 cases of alveolar rhabdomyosarcoma. Translocations were detectable in both fresh-frozen tissues (4 of 7) and paraffin-embedded tumours (3 of 7) and in the alveolar rhabdomyosarcoma cell line. Our study confirms that the t(2;13) translocation is not present in embryonal rhabdomyosarcomas, but can be detected in nearly half of alveolar rhabdomyosarcomas, whether fresh-frozen or paraffin-embedded. The PCR-based t(2;13) translocation assay can aid in the diagnosis of rhabdomyosarcoma, but cannot replace a careful histopathological evaluation. It may contribute in further characterizing an otherwise undifferentiated small cell tumour, where it may be indicative of clinical behaviour.

The rise and fall of dialects in northern elephant seals

Vocal dialects are fundamental to our understanding of the transmission of social behaviours between individuals and populations, however few accounts trace this phenomenon among mammals over time. Northern elephant seals (Mirounga angustirostris) provide a rare opportunity to examine the trajectory of dialects in a long-lived mammalian species. Dialects were first documented in the temporal patterns of the stereotyped vocal displays produced by breeding males at four sites in the North Pacific in 1968 and 1969, as the population recovered from extreme exploitation. We evaluated the longevity of these geographical differences by comparing these early recordings to calls recently recorded at these same locations. While the presence of vocal dialects in the original recordings was re-confirmed, geographical differences in vocal behaviour were not found at these breeding rookeries nearly 50 years later. Moreover, the calls of contemporary males displayed more structural complexity after approximately four generations, with substantial between-individual variation and call features not present in the historical data. In the absence of measurable genetic variation in this species-owing to an extreme population bottleneck-a combination of migration patterns and cultural mutation are proposed as factors influencing the fall of dialects and the dramatic increase in call diversity.

Onset, growth, and recovery of in-air temporary threshold shift in a California sea lion (Zalophus californianus)

A California sea lion (Zalophus californianus) was tested in a behavioral procedure to assess noise-induced temporary threshold shift (TTS) in air. Octave band fatiguing noise was varied in both duration (1.5-50 min) and level (94-133 dB re 20 muPa) to generate a variety of equal sound exposure level conditions. Hearing thresholds were measured at the center frequency of the noise (2500 Hz) before, immediately after, and 24 h following exposure. Threshold shifts generated from 192 exposures ranged up to 30 dB. Estimates of TTS onset [159 dB re (20 muPa)(2) s] and growth (2.5 dB of TTS per dB of noise increase) were determined using an exponential function. Recovery for threshold shifts greater than 20 dB followed an 8.8 dB per log(min) linear function. Repeated testing indicated possible permanent threshold shift at the test frequency, but a later audiogram revealed no shift at this frequency or higher. Sea lions appear to be equally susceptible to noise in air and in water, provided that the noise exposure levels are referenced to absolute sound detection thresholds in both media. These data provide a framework within which to consider effects arising from more intense and/or sustained exposures.

Rival assessment among northern elephant seals: evidence of associative learning during male-male contests

Specialized signals emitted by competing males often convey honest information about fighting ability. It is generally believed that receivers use these signals to directly assess their opponents. Here, we demonstrate an alternative communication strategy used by males in a breeding system where the costs of conflict are extreme. We evaluated the acoustic displays of breeding male northern elephant seals (Mirounga angustirostris), and found that social knowledge gained through prior experience with signallers was sufficient to maintain structured dominance relationships. Using sound analysis and playback experiments with both natural and modified signals, we determined that males do not rely on encoded information about size or dominance status, but rather learn to recognize individual acoustic signatures produced by their rivals. Further, we show that behavioural responses to competitors' calls are modulated by relative position in the hierarchy: the highest ranking (alpha) males defend their harems from all opponents, whereas mid-ranking (beta) males respond differentially to familiar challengers based on the outcome of previous competitive interactions. Our findings demonstrate that social knowledge of rivals alone can regulate dominance relationships among competing males within large, spatially dynamic social groups, and illustrate the importance of combining descriptive and experimental methods when deciphering the biological relevance of animal signals.

Molecular analysis of E-cadherin and cadherin-11 in Wilms' tumours

Different studies of Wilms' tumours have demonstrated a loss of heterozygosity (LOH) of chromosome 16q ranging from 17 to 25%. In order to search for a potential tumour suppressor gene on 16q, we chose the calcium-dependent cell adhesion molecules E-cadherin and cadherin-11 as candidate genes, which are both located on the long arm of chromosome 16. E-cadherin is known to be expressed in epithelial structures, whereas cadherin-11 is supposed to be expressed in mesenchymal structures and developing epithelium, including renal tubules. For the present study, fresh frozen tissue from 30 Wilms' tumours and corresponding non-tumour tissues were analysed. Single nucleotide polymorphisms of the E-cadherin and cadherin-11 genes were chosen and analysed for allelic inactivation by polymerase chain reaction (PCR) amplification and sequence analysis. Loss of expression of one E-cadherin allele was seen in 10% (2/20) of the informative cases. Two out of 11 informative cases (18%) showed loss of expression of one cadherin-11 allele. No length alterations of either the E-cadherin or the cadherin-11 messenger RNAs were identified using reverse transcription PCR and agarose gel electrophoresis in tumour tissue. Sequencing of the entire E-cadherin coding region in seven cases showed the wild-type sequence. These data imply that E-cadherin and cadherin-11 are not likely to play typical tumour suppressor roles in Wilms' tumour. Interestingly, the E-cadherin immunohistochemistry showed a deviation from the normal reaction pattern in 50% of the cases, with 27% (8/30) showing an apical or cytoplasmic reaction and 23% (7/30) being completely negative. Northern blot analysis revealed that the overall expression of cadherin-11 is much stronger than that of E-cadherin. In several cases, the expression levels of the two genes were inversely correlated, suggesting the existence of a regulatory mechanism. Analysis of differential expression of the various cadherins and their subsequent signal transduction pathways might contribute to a better understanding of the complexity of Wilms' tumour formation.

Cadherin-11 is highly expressed in rhabdomyosarcomas and during differentiation of myoblasts in vitro

Rhabdomyosarcomas bear a morphological and genetic resemblance to developing skeletal muscle. Apart from myogenic marker genes (bHLH factors, myosin, actin), cell adhesion molecules such as N-cadherin and N-CAM have been reported to be expressed both in rhabdomyosarcomas and during myogenesis. The present study demonstrates the expression of another cadherin, cadherin-11, in rhabdomyosarcomas and during differentiation of myoblasts in vitro: cadherin-11, a predominantly mesenchymal cell adhesion molecule, is highly expressed in embryonal rhabdomyosarcomas and alveolar rhabdomyosarcomas, which do not bear the Pax-3-FKHR fusion previously described. Cadherin-11 is down-regulated in normal skeletal muscle and after myotube formation in vitro. The results of this study suggest that cadherin-11 might be involved in myogenesis and that rhabdomyosarcomas may re-express or fail to down-regulate cadherin-11. Since alveolar rhabdomyosarcomas bearing the t(2;13) translocation do not express cadherin-11, it is postulated that Pax-3 and cadherin-11 might be linked and involved in the same myogenic pathway.

Rapid behavioural diagnosis of domoic acid toxicosis in California sea lions

Domoic acid is a neurotoxic metabolite of widely occurring algal blooms that has caused multiple marine animal stranding events. Exposure to high doses of domoic acid, a glutamate agonist, may lead to persistent medial temporal seizures and damage to the hippocampus. California sea lions (Zalophus californianus) are among the most visible and frequent mammalian victims of domoic acid poisoning, but rapid, reliable diagnosis in a clinical setting has proved difficult owing to the fast clearance of the toxin from the blood stream. Here, we show that the behavioural orienting responses of stranded sea lions diagnosed with domoic acid toxicosis habituate more slowly to a series of non-aversive auditory stimuli than do those of sea lions with no apparent neurological deficits. A signal detection analysis based on these habituation measures was able to correctly identify 50 per cent of subjects with domoic acid toxicosis while correctly rejecting approximately 93 per cent of controls, suggesting potential diagnostic merit.

Active touch in sea otters: in-air and underwater texture discrimination thresholds and behavioral strategies for paws and vibrissae

Sea otters (Enhydra lutris) are marine predators that forage on a wide array of cryptic, benthic invertebrates. Observational studies and anatomical investigations of the sea otter somatosensory cortex suggest that touch is an important sense for detecting and capturing prey. Sea otters have two well-developed tactile structures: front paws and facial vibrissae. In this study, we use a two-alternative forced choice paradigm to investigate tactile sensitivity of a sea otter subject's paws and vibrissae, both in air and under water. We corroborate these measurements by testing human subjects with the same experimental paradigm. The sea otter showed good sensitivity with both tactile structures, but better paw sensitivity (Weber fraction, c=0.14) than vibrissal sensitivity (c=0.24). The sea otter's sensitivity was similar in air and under water for paw (cair=0.12, cwater=0.15) and for vibrissae (cair=0.24, cwater=0.25). Relative to the human subjects we tested, the sea otter achieved similar sensitivity when using her paw and responded approximately 30-fold faster regardless of difficulty level. Relative to non-human mammalian tactile specialists, the sea otter achieved similar or better sensitivity when using either her paw or vibrissae and responded 1.5- to 15-fold faster near threshold. Our findings suggest that sea otters have sensitive, rapid tactile processing capabilities. This functional test of anatomy-based hypotheses provides a mechanistic framework to interpret adaptations and behavioral strategies used by predators to detect and capture cryptic prey in aquatic habitats.

Development of an artificial sensor for hydrodynamic detection inspired by a seal's whisker array

Nature has shaped effective biological sensory systems to receive complex stimuli generated by organisms moving through water. Similar abilities have not yet been fully developed in artificial systems for underwater detection and monitoring, but such technology would enable valuable applications for military, commercial, and scientific use. We set out to design a fluid motion sensor array inspired by the searching performance of seals, which use their whiskers to find and follow underwater wakes. This sensor prototype, called the Wake Information Detection and Tracking System (WIDTS), features multiple whisker-like elements that respond to hydrodynamic disturbances encountered while moving through water. To develop and test this system, we trained a captive harbor seal (Phoca vitulina) to wear a blindfold while tracking a remote-controlled, propeller-driven submarine. After mastering the tracking task, the seal learned to carry the WIDTS adjacent to its own vibrissal array during active pursuit of the target. Data from the WIDTS sensors describe changes in the deflection angles of the whisker elements as they pass through the hydrodynamic trail left by the submarine. Video performance data show that these detections coincide temporally with WIDTS-wake intersections. Deployment of the sensors on an actively searching seal allowed for the direct comparison of our instrument to the ability of the biological sensory system in a proof-of-concept demonstration. The creation of the WIDTS provides a foundation for instrument development in the field of biomimetic fluid sensor technology.

High-frequency hearing in seals and sea lions

Existing evidence suggests that some pinnipeds (seals, sea lions, and walruses) can detect underwater sound at frequencies well above the traditional high-frequency hearing limits for their species. This phenomenon, however, is not well studied: Sensitivity patterns at frequencies beyond traditional high-frequency limits are poorly resolved, and the nature of the auditory mechanism mediating hearing at these frequencies is unknown. In the first portion of this study, auditory sensitivity patterns in the 50-180 kHz range were measured for one California sea lion (Zalophus californianus), one harbor seal (Phoca vitulina), and one spotted seal (Phoca largha). Results show the presence of two distinct slope-regions at the high-frequency ends of the audiograms of all three subjects. The first region is characterized by a rapid decrease in sensitivity with increasing frequency-i.e. a steep slope-followed by a region of much less rapid sensitivity decrease-i.e. a shallower slope. In the second portion of this study, a masking experiment was conducted to investigate how the basilar membrane of a harbor seal subject responded to acoustic energy from a narrowband masking noise centered at 140 kHz. The measured masking pattern suggests that the initial, rapid decrease in sensitivity on the high-frequency end of the subject's audiogram is not due to cochlear constraints, as has been previously hypothesized, but rather to constraints on the conductive mechanism.

Drivers of the dive response in pinnipeds; apnea, submergence or temperature?

Long and deep dives in marine mammals are enabled by high mass-specific oxygen stores and the dive response (DR), which reduces oxygen consumption in concert with increased peripheral vasoconstriction and a lowered heart rate during dives. Diving heart rates of pinnipeds are highly variable and modulated by many factors, such as breath holding (apnea), pressure, swimming activity, temperature, and even cognitive control. However, the individual effects of these factors on diving heart rate are poorly understood due to the difficulty of parsing their relative contributions in diving pinnipeds. Here, we examined the effects of apnea and external sensory inputs as autonomic drivers of bradycardia. Specifically, we hypothesized that 1) water stimulation of facial receptors would—as is the case for terrestrial mammals—enhance the dive response, 2) increasing the facial area stimulated would lead to a more intense bradycardia, and 3) cold water would elicit a more pronounced bradycardia than warm water. Three harbor seals (Phoca vitulina) and a California sea lion (Zalophus californianus) were trained to breath-hold in air and with their heads submerged in a basin with variable water level and temperature. We show that bradycardia occurs during apnea without immersion. We also demonstrate that bradycardia is strengthened with both increasing area of facial submersion and colder water. Thus, we conclude that initiation of the DR in pinnipeds is more strongly related to breath holding than in terrestrial mammals, but the degree of the DR is potentiated autonomically via stimulation of facial mechano- and thermoreceptors upon submergence.