Evaluation of anthropogenic impacts on marine turtle populations in Lençóis Maranhenses National Park

Currently, one of the most significant threats to marine turtle populations is incidental capture, which can occur on a large scale due to industrial, coastal, or artisanal fishing practices, such as gillnets, fish corrals, longlines, and trawl nets. Other threats exacerbate the situation for these animals, including environmental degradation, global warming, and noise pollution associated with navigation, oil and gas exploration and production, naval sonars, military operations, fishing, and marine seismic research. Our objective was to evaluate and identify anthropogenic impacts on marine turtle populations in the Lençóis Maranhenses National Park. Between 2015 and 2019, systematic monitoring recorded 173 marine turtle strandings, including 164 dead and nine live individuals of E. imbricata, predominantly of C. mydas (46.2 %, n = 80), L. olivacea (38.2 %, n = 66), E. imbricata (5.2 %, n = 9), C. caretta (0.6 %, n = 1), and 9.8 % (n = 17) unidentified. Among the 44 individuals with determined causes of death, 79.5 % (n = 35) were linked to anthropogenic actions, 9 % (n = 4) to natural causes, and 11.3 % (n = 5) to mixed anthropogenic/natural factors. The remaining 129 cases were classified as indeterminate. A significant increase in strandings was observed during the 3D seismic survey period (mean = 34.4 strandings/bimester) compared to periods without seismic activity (mean = 4.8 strandings/bimester). These results underscore that cumulative threats, particularly human-driven pressures, severely impact marine turtle populations. An enhanced understanding of these dynamics is critical for developing targeted conservation strategies at Lençóis Maranhenses National Park. The findings provide actionable insights to mitigate anthropogenic impacts and inform regional management plans.

Cold-blooded culture? Assessing cultural behaviour in reptiles and its potential conservation implications

It is becoming clear that the cognition of a species plays an important role in successful conservation, with cultural processes being a fundamental part of this. However, in contrast to mammals and birds, very little is known about cultural processes (and the social learning that underlies these) in reptiles. Here, we summarize the current state of knowledge, consider why this information is so limited and assess candidate behaviours observed in the wild, which warrant further investigation through the lens of cultural traditions. We then make suggestions for the fundamental next steps necessary to start to address this issue. This includes future experimental work and also consideration of how existing datasets, such as those capturing animal movement or acoustic activity, can be used to assess cultural questions. In addition, we emphasize the important role that engaging key conservation stakeholders, such as zoos, aquaria and ecotourism providers, could play in furthering our understanding of cultural behaviour in this group and the potential conservation implications of this knowledge. Whether there is cultural behaviour in reptiles and the relationship that this has with conservation remain unclear; however, the findings of this review suggest that these are areas worthy of further research.This article is part of the theme issue 'Animal culture: conservation in a changing world'.

Natural and shipping underwater sound distribution in the Northern Adriatic Sea basin and possible application on target areas

The underwater sound distribution generated by natural sources, shipping and trawling activities has been computed by the Quonops© modelling webservice for the Northern Adriatic Sea (NAS) during 2020, a year characterized by the COVID-19 lockdown restrictions. Modelling has been calibrated by using a year-long time series of field measurements covering the domain of interest. Sound levels (50th percentile) ranged between 75 and 90 dB re 1μPa for all the considered frequencies (63 Hz, 125 Hz, 250 Hz third octave bands). Noisier NAS areas match with the shipping lanes and the distribution of trawling activity. Pressure sound indices based on masking effect were computed for two Ecologically/Biologically Significant Marine Areas (EBSA) located in the NAS. Results indicated a significant contribution of vessel and fishery-generated noise to the local soundscape and provide a basis for addressing NAS underwater noise pollution, with special reference to the Marine Spatial Planning approach.

The response of sea turtles to vocalizations opens new perspectives to reduce their bycatch

Incidental capture of non-target species poses a pervasive threat to many marine species, with sometimes devastating consequences for both fisheries and conservation efforts. Because of the well-known importance of vocalizations in cetaceans, acoustic deterrents have been extensively used for these species. In contrast, acoustic communication for sea turtles has been considered negligible, and this question has been largely unexplored. Addressing this challenge therefore requires a comprehensive understanding of sea turtles' responses to sensory signals. In this study, we scrutinized the avenue of auditory cues, specifically the natural sounds produced by green turtles (Chelonia mydas) in Martinique, as a potential tool to reduce bycatch. We recorded 10 sounds produced by green turtles and identified those that appear to correspond to alerts, flight or social contact between individuals. Subsequently, these turtle sounds-as well synthetic and natural (earthquake) sounds-were presented to turtles in known foraging areas to assess the behavioral response of green turtles to these sounds. Our data highlighted that the playback of sounds produced by sea turtles was associated with alert or increased the vigilance of individuals. This therefore suggests novel opportunities for using sea turtle sounds to deter them from fishing gear or other potentially harmful areas, and highlights the potential of our research to improve sea turtles populations' conservation.

Anthropogenic noise predicts sea turtle behavioural responses

Anthropogenic noise is a pollutant of global concern. While the effects of underwater noise pollution have been frequently studied in fish and mammals, our understanding of how this anthropogenic stressor affects marine reptiles is scant. Using a multichannel data logger equipped with a camera and hydrophone, we quantified behavioural responses of a free-ranging green turtle (Chelonia mydas) to vessel noise in the Galapagos Archipelago, an important nesting site in the eastern Pacific. We found that while travelling the turtle increased its vigilance with increasing vessel noise. However, when on the seabed the turtle did not increase its vigilance with increasing noise levels. Our findings illustrate that noise pollution has the potential to alter overall time budgets of animals. Identifying real-time responses of wild animals illustrate how in situ approaches allow to assess the effects of human activities on marine systems.

Soundscape of a Mediterranean seashore during loggerhead sea turtle (Caretta caretta) spawning season

The soundscape is an intrinsic property of an ecosystem and influences the species that live in it. Here, we examined for the first time the soundscape of a beach, one of the most dynamic ecosystems on Earth, where every year the loggerhead sea turtle Caretta caretta lays eggs. The aim of this work was to analyze the acoustic components (biophony, anthropophony and geophony) to which turtles embryos were exposed throughout the development and the post-hatching period. The acoustic monitoring was carried out on the volcanic island of Linosa (central Mediterranean Sea, Strait of Sicily), during the months of July and August 2022, close to two turtle nests. Results revealed that all the acoustic levels (octave bands from 4 Hz to 16 kHz, and total 1-24,000 Hz band) showed lower values in July, and during the night. Furthermore, above 1 kHz the levels decreased and had very little variability. Anthropogenic noise was the main component of the soundscape and consisted of marine and land traffic, that affected sound levels directly or via seismic tremors. When the beach was exposed to the breaking waves, the latters were the first contributor to the noise up to 1 kHz. The only recognized biophony was represented by the shearwater choruses in July (at the frequency band 700-1500 Hz), but they had a negligible weight on the soundscape. Finally, human speech contributed to the soundscape at higher frequencies (1-8 kHz). These outcomes show that the embryos and the post-hatching turtles are exposed to a high anthropogenic noise level, which the effects of are still unknown.

Managing long-term wellness in captive sea turtles

Circumstances surrounding advances in stranding response and veterinary care have created a growing need for the long-term housing of captive sea turtles. However, the difficulty in recreating natural conditions in captive settings places a responsibility on caregivers to offset wild-type behavioural deficits with enrichment programming that is, preferably, commensurate with the limitations of each enclosure. Though standardised, multi-institutional behavioural monitoring programmes are currently lacking for marine turtles, facilities housing (or planning to house) sea turtles for the long-term are strongly advised to include 'wellness' as a fundamental part of their animal care protocol. Here, concepts of wellness and enrichment in sea turtles are reviewed, and a framework for developing longterm behavioural monitoring programming is provided.

Transcriptome Analyses Provide Insights into the Auditory Function in Trachemys scripta elegans

An auditory ability is essential for communication in vertebrates, and considerable attention has been paid to auditory sensitivity in mammals, birds, and frogs. Turtles were thought to be deaf for a long time; however, recent studies have confirmed the presence of an auditory ability in Trachemys scripta elegans as well as sex-related differences in hearing sensitivity. Earlier studies mainly focused on the morphological and physiological functions of the hearing organ in turtles; thus, the gene expression patterns remain unclear. In this study, 36 transcriptomes from six tissues (inner ear, tympanic membrane, brain, eye, lung, and muscle) were sequenced to explore the gene expression patterns of the hearing system in T. scripta elegans. A weighted gene co-expression network analysis revealed that hub genes related to the inner ear and tympanic membrane are involved in development and signal transduction. Moreover, we identified six differently expressed genes (GABRA1, GABRG2, GABBR2, GNAO1, SLC38A1, and SLC12A5) related to the GABAergic synapse pathway as candidate genes to explain the differences in sexually dimorphic hearing sensitivity. Collectively, this study provides a critical foundation for genetic research on auditory functions in turtles.

Effect of Temperature on the Plasticity of Peripheral Hearing Sensitivity to Airborne Sound in the Male Red-Eared Slider Trachemys scripta elegans

Chelonians are considered the least vocally active group of extant reptiles and known as “low-frequency specialists” with a hearing range of <1.0 kHz. As they are ectothermic organisms, most of their physiological and metabolic processes are affected by temperature, which may include the auditory system responses. To investigate the influence of temperature on turtle hearing, Trachemys scripta elegans was chosen to measure the peripheral hearing sensitivity at 10, 20, 30, and 40°C (close to the upper limit of heat resistance) using the auditory brainstem response (ABR) test. An increase in temperature (from 10 to 30°C) resulted in improved hearing sensitivity (a wider hearing sensitivity bandwidth, lower threshold, and shorter latency) in T. scripta elegans. At 40°C, the hearing sensitivity bandwidth continued to increase and the latency further shortened, but the threshold sensitivity reduced in the intermediate frequency range (0.5–0.8 kHz), increased in the high-frequency range (1.0–1.3 kHz), and did not significantly change in the low-frequency range (0.2–0.4 kHz) compared to that at 30°C. Our results suggest that although the hearing range of turtles is confined to lower frequencies than that in other animal groups, turtle hearing showed exceptional thermal regulation ability, especially when the temperature was close to the upper limit of heat resistance. Temperature increases that are sensitive to high frequencies imply that the males turtles’ auditory system adapts to a high-frequency sound environment in the context of global warming. Our study is expected to spur further research on the high-temperature plasticity of hearing sensitivity in diverse taxa or in the same group with different temperature ranges. Moreover, it facilitates forecasting the adaptive evolution of the auditory system to global warming.

Influence of environmental and anthropogenic acoustic cues in sea-finding of hatchling leatherback (Dermochelys coriacea) sea turtles

Although the visual and geomagnetic orientation cues used by sea turtle hatchlings during sea-finding have been well studied, the potential for auditory stimuli to act as an orientation cue has not been explored. We investigated the response of sea turtle hatchlings to natural and anthropogenic noises present on their nesting beaches during sea-finding. The responses of hatchling leatherback sea turtles, Dermochelys coriacea, collected from the Sandy Point National Wildlife Refuge, St. Croix, were measured in the presence of aerial acoustic sounds within hatchlings' hearing range of 50 to 1600 Hz. The highest sound energy produced by beach waves occurs at frequencies 50-1000 Hz, which overlaps with the most sensitive hearing range of hatchling leatherbacks (50-400 Hz). Natural beach wave sounds, which have highest sound energy at frequencies of 50-1000 Hz, may be masked by human conversations (85-650 Hz) and vehicle traffic noise (60-8000 Hz). In the presence of three stimuli, a) beach wave sounds (72.0 dB re: 20 μPa), b) human conversation (72.4 dB re: 20 μPa), and c) vehicle traffic noise (71.1 dB re: 20 μPa), hatchlings exhibited no phonotaxic response (wave sounds: mean angle = 152.1°, p = 0.645; human conversation: mean angle = 67.4°, p = 0.554; traffic noise: mean angle = 125.7°, p = 0.887). These results may be due to the hatchlings being unable to localize sounds in the experimental arena. Visual and auditory cues may also converge to affect sea-finding orientation. Future studies should focus on the localization ability of sea turtles and on the role that sound may play in orientation when combined with other sensory and environmental cues.

Acoustic Pressure, Particle Motion, and Induced Ground Motion Signals from a Commercial Seismic Survey Array and Potential Implications for Environmental Monitoring

An experimental marine seismic source survey off the northwest Australian coast operated a 2600 cubic inch (41.6 l) airgun array, every 5.88 s, along six lines at a northern site and eight lines at a southern site. The airgun array was discharged 27,770 times with 128,313 pressure signals, 38,907 three-axis particle motion signals, and 17,832 ground motion signals recorded. Pressure and ground motion were accurately measured at horizontal ranges from 12 m. Particle motion signals saturated out to 1500 m horizontal range (50% of signals saturated at 230 and 590 m at the northern and southern sites, respectively). For unsaturated signals, sound exposure levels (SEL) correlated with measures of sound pressure level and water particle acceleration (r2= 0.88 to 0.95 at northern site and 0.97 at southern) and ground acceleration (r2= 0.60 and 0.87, northern and southern sites, respectively). The effective array source level was modelled at 247 dB re 1µPa m peak-to-peak, 231 dB re 1 µPa2 m mean-square, and 228 dB re 1 µPa2∙m2 s SEL at 15° below the horizontal. Propagation loss ranged from −29 to −30log10 (range) at the northern site and −29 to −38log10(range) at the southern site, for pressure measures. These high propagation losses are due to near-surface limestone in the seabed of the North West Shelf.

Ontogenetic development of hearing sensitivity to airborne sound in the female red-eared slider, Trachemys scripta elegans

Ontogenetic development of hearing sensitivity has been verified in many groups of vertebrates, but not turtles. Turtles exhibit sexual dimorphism in hearing. To examine the development of hearing in female turtles, auditory brainstem responses (ABR) were compared by assessing the hearing-sensitivity bandwidth, ABR threshold, and latency of female Trachemys scripta elegans aged 1 week, 1 month, 1 yr, and 5 yr. The hearing-sensitivity bandwidths were 0.2-1.1, 0.2-1.1, 0.2-1.3, and 0.2-1.4 kHz in each age group, respectively. Below 0.6 kHz, the ABR threshold decreased from the 1-week to 1-yr age group, with a significant difference between age groups. No significant difference was detected between the 1- and 5-yr age groups (within a stimulus frequency of 0.2-0.6 kHz). Above 0.6 kHz, ABR thresholds decreased significantly from the 1-yr to 5-yr age group (within a stimulus frequency of 0.7-1.0 kHz). There was no significant difference between the 1-month and 1-yr age groups (within a stimulus frequency of 0.7-1.0 kHz), or between the 1-week and 1-month age groups (within a stimulus frequency of 0.7-1.0 kHz, except 0.9 kHz). Thus, female turtle hearing shows frequency-segmented development.

Three auditory brainstem response (ABR) methods tested and compared in two anuran species

Hearing sensitivity has been extensively investigated, often by measuring the auditory brainstem response (ABR). ABR measurements are relatively non-invasive, easy to reproduce, and allow the assessment of sensitivity when psychophysical data are difficult to obtain. However, the experimental methods differ greatly in respect to stimulation, which may result in different audiograms. We used three different methods in the same individual frogs: stimulating with brief tone bursts (tABR), long-duration tones (ltABR) and masked ABR (mABR), where transients are masked by a long-duration sinusoid, and the sensitivity is assessed by the difference between unmasked and masked ABR. We measured sensitivity in a range from 100 to 3500 Hz, and the resulting audiograms show two sensitivity peaks at 400-600 Hz and 1500-1600 Hz (both sensitive down to 30 dB re. 20 µPa). We found similar results below 1000 Hz, but when stimulating with long-duration tones, the sensitivity decreased more rapidly above this frequency. We showed that the frequency specificity of tone bursts becomes poorly defined with shorter duration at low frequencies. Comparisons between subjectively (visual inspection by researchers) and objectively (thresholds defined by signal-to-noise ratio) defined audiograms showed very little variation. In conclusion, the mABR method gave the most sensitive audiograms. The tABR method showed a similar audiogram when using relatively long-duration tone bursts (25 ms). The ltABR method is not a good choice for studying hearing thresholds above 1000 Hz because of the bias introduced by spike rate saturation in the nerve fibers and their inability to phase lock.

Soundscape of green turtle foraging habitats in Fiji, South Pacific

The soundscape features of the marine environment provide crucial information about ecosystem health for many species, and they are defined by the local biological, geophysical, and anthropogenic components. In this study, we investigated the soundscape at green turtle neritic foraging habitats in Fiji, South Pacific, with the aims of characterizing the contribution of each component and of comparing the levels of acoustic pressure among sites with different abundances of sea turtles. Four sites were selected at two islands, and one hydrophone was deployed at each site. Generalized additive models highlighted that sound pressure levels (SPLs) at low frequencies (125–250 Hz) were especially affected by wind conditions, while at higher frequencies (>250 Hz) SPLs were mostly influenced by fish and crustacean acoustic activity. Higher abundances of green turtles were found at sites with the highest levels of SPLs and the highest number of acoustic emissions by fishes and crustaceans but were not related to maximum seagrass and macroalgae coverage, or the highest number of fish. The selected coastal habitats have negligible anthropogenic noise, thus this study informs physiological and behavioral studies of the acoustic signatures that sea turtles might target and provides a baseline against which potential impact of soundscape changes on sea turtle spatial abundance and distribution can be evaluated.

Auditory brainstem responses in the red-eared slider Trachemys scripta elegans (Testudoformes: Emydidae) reveal sexually dimorphic hearing sensitivity

Hearing sensitivity is of general interest from the perspective of understanding the functionality and evolution of vertebrate auditory systems. Sexual dimorphism of auditory systems has been reported in several species of vertebrates, but little is known about this phenomenon in turtles. Some morphological characteristics, such as middle ear and tympanic membrane that influence the hearing sensitivity of animals can result in hearing sexual dimorphism. To examine whether sexual dimorphism in hearing sensitivity occurs in turtles and to compare hearing characteristics with respect to the shape of the tympanic membrane, we measured the hearing sensitivity and tympanum diameter in both sexes of Trachemys scripta elegans. The results showed that, with the exception of 0.9 kHz, auditory brainstem response thresholds were significantly lower in females than in males for frequencies in the 0.2-1.1 kHz range, indicating that the hearing of females shows greater sensitivity. No significant differences were detected in the tympanum diameter of both sexes. These results showed that sexually dimorphic hearing sensitivity has evolved in turtles; however, this difference does not appear to be related to differences in the size of the tympanic membrane. The possible origin and function of the sexual differences in auditory characteristic are discussed.

Underwater hearing in sea snakes (Hydrophiinae): first evidence of auditory evoked potential thresholds

The viviparous sea snakes (Hydrophiinae) are a secondarily aquatic radiation of more than 60 species that possess many phenotypic adaptations to marine life. However, virtually nothing is known of the role and sensitivity of hearing in sea snakes. This study investigated the hearing sensitivity of the fully marine sea snake Hydrophis stokesii by measuring auditory evoked potential (AEP) audiograms for two individuals. AEPs were recorded from 40 Hz (the lowest frequency tested) up to 600 Hz, with a peak in sensitivity identified at 60 Hz (163.5 dB re. 1 µPa or 123 dB re. 1 µm s^-2). Our data suggest that sea snakes are sensitive to low-frequency sounds but have relatively low sensitivity compared with bony fishes and marine turtles. Additional studies are required to understand the role of sound in sea snake life history and further assess these species' vulnerability to anthropogenic noise.

Measuring behavioral responses of sea turtles, saltwater crocodiles, and crested terns to drone disturbance to define ethical operating thresholds

Drones are being increasingly used in innovative ways to enhance environmental research and conservation. Despite their widespread use for wildlife studies, there are few scientifically justified guidelines that provide minimum distances at which wildlife can be approached to minimize visual and auditory disturbance. These distances are essential to ensure that behavioral and survey data have no observer bias and form the basis of requirements for animal ethics and scientific permit approvals. In the present study, we documented the behaviors of three species of sea turtle (green turtles, Chelonia mydas, flatback turtles, Natator depressus, hawksbill turtles, Eretmochelys imbricata), saltwater crocodiles (Crocodylus porosus), and crested terns (Thalasseus bergii) in response to a small commercially available (1.4 kg) multirotor drone flown in Northern Territory and Western Australia. Sea turtles in nearshore waters off nesting beaches or in foraging habitats exhibited no evasive behaviors (e.g. rapid diving) in response to the drone at or above 20-30 m altitude, and at or above 10 m altitude for juvenile green and hawksbill turtles foraging on shallow, algae-covered reefs. Adult female flatback sea turtles were not deterred by drones flying forward or stationary at 10 m altitude when crawling up the beach to nest or digging a body pit or egg chamber. In contrast, flyovers elicited a range of behaviors from crocodiles, including minor, lateral head movements, fleeing, or complete submergence when a drone was present below 50 m altitude. Similarly, a colony of crested terns resting on a sand-bank displayed disturbance behaviors (e.g. flight response) when a drone was flown below 60 m altitude. The current study demonstrates a variety of behavioral disturbance thresholds for diverse species and should be considered when establishing operating conditions for drones in behavioral and conservation studies.

Audiograms of three subterranean rodent species (genus Fukomys) determined by auditory brainstem responses reveal extremely poor high-frequency hearing

Life underground has shaped the auditory sense of subterranean mammals, shifting their hearing range to low frequencies. Mole-rats of the genus Fukomys have, however, been suggested to hear at frequencies up to 18.5 kHz, unusually high for a subterranean rodent. We present audiograms of three mole-rat species, Fukomys anselli, Fukomys micklemi and the giant mole-rat Fukomys mechowii, based on evoked auditory brainstem potentials. All species showed low sensitivity and restricted hearing ranges at 60 dB SPL extending from 125 Hz to 4 kHz (5 octaves) with most-sensitive hearing between 0.8 kHz and 1.4 kHz. The high-frequency cut-offs are the lowest found in mammals to date. In contrast to predictions from middle ear morphology, F. mechowii did not show higher sensitivity than F. anselli in the low-frequency range. These data suggest that the hearing range of Fukomys mole-rats is highly restricted to low frequencies and similar to that of other subterranean mammals.

Modelos animais para avaliação auditiva: revisão de literatura

RESUMO O objetivo dessa revisão é delinear os modelos animais viáveis para a pesquisa pré-clínica auditiva, considerando suas características anatômicas, fisiológicas, vantagens e desvantagens. Foram consultadas as bases de dados Scielo, Pubmed e Periódicos Capes, utilizando descritores envolvendo audição, testes auditivos e espécies animais, individualmente e cruzados entre si. Foram lidos os resumos dos artigos encontrados nas bases de dados, com posterior seleção baseada nos critérios: artigos disponíveis em sua integridade, uso de modelos animais em procedimentos audiológicos que incluísse a descrição dos métodos de avaliação, as vantagens e/ou desvantagens do uso da espécie, publicados entre 1995 e 2016. Apesar da existência de modelos alternativos, os mamíferos são ainda amplamente utilizados em pesquisa. Constatou-se que os ratos, camundongos e cobaios são frequentemente utilizados e, além destes, ovelhas, coelhos e chinchilas. Os métodos para avaliação auditiva contemplam principalmente emissões otoacústicas por produto de distorção, potencial evocado auditivo de tronco encefálico e avaliação histológica, principalmente em roedores. A escolha do animal de experimentação para avaliação do sistema auditivo depende de fatores anatômicos, fisiológicos, econômicos, espaciais, psicossociais e do objetivo da avaliação.

Audiogram of the kea parrot, Nestor notabilis

Vocal communication requires the sender to produce a sound, which transmits through the environment and is perceived by the receiver. Perception is dependent on the quality of the received signal and the receiver's frequency and amplitude sensitivity; hearing sensitivity of animals can be tested using behavioural detection tasks, showing the physical limitations of sound perception. Kea parrots (Nestor notabilis) were tested for their ability to hear sounds that varied in terms of both frequency and amplitude by means of a simple auditory detection task. Audiograms for three kea were similar, with the region of highest sensitivity (1-5 kHz) corresponding to the frequency of the highest amplitude in kea calls. Compared with other parrots and other bird taxa, the overall shape of the kea audiogram follows a similar pattern. However, two potentially interesting differences to the audiograms of other birds were found: an increase in sensitivity at approximately 12 kHz and a decreased sensitivity to frequencies below 1 kHz.

Hearing in the Juvenile Green Sea Turtle (Chelonia mydas): A Comparison of Underwater and Aerial Hearing Using Auditory Evoked Potentials

Sea turtles spend much of their life in aquatic environments, but critical portions of their life cycle, such as nesting and hatching, occur in terrestrial environments, suggesting that it may be important for them to detect sounds in both air and water. In this study we compared underwater and aerial hearing sensitivities in five juvenile green sea turtles (Chelonia mydas) by measuring auditory evoked potential responses to tone pip stimuli. Green sea turtles detected acoustic stimuli in both media, responding to underwater stimuli between 50 and 1600 Hz and aerial stimuli between 50 and 800 Hz, with maximum sensitivity between 200 and 400 Hz underwater and 300 and 400 Hz in air. When underwater and aerial hearing sensitivities were compared in terms of pressure, green sea turtle aerial sound pressure thresholds were lower than underwater thresholds, however they detected a wider range of frequencies underwater. When thresholds were compared in terms of sound intensity, green sea turtle sound intensity level thresholds were 2–39 dB lower underwater particularly at frequencies below 400 Hz. Acoustic stimuli may provide important environmental cues for sea turtles. Further research is needed to determine how sea turtles behaviorally and physiologically respond to sounds in their environment.

Underwater Hearing in Turtles

The hearing of turtles is poorly understood compared with the other reptiles. Although the mechanism of transduction of sound into a neural signal via hair cells has been described in detail, the rest of the auditory system is largely a black box. What is known is that turtles have higher hearing thresholds than other reptiles, with best frequencies around 500 Hz. They also have lower underwater hearing thresholds than those in air, owing to resonance of the middle ear cavity. Further studies demonstrated that all families of turtles and tortoises share a common middle ear cavity morphology, with scaling best suited to underwater hearing. This supports an aquatic origin of the group. Because turtles hear best under water, it is important to examine their vulnerability to anthropogenic noise. However, the lack of basic data makes such experiments difficult because only a few species of turtles have published audiograms. There are also almost no behavioral data available (understandable due to training difficulties). Finally, few studies show what kinds of sounds are behaviorally relevant. One notable paper revealed that the Australian snake-necked turtle (Chelodina oblonga) has a vocal repertoire in air, at the interface, and under water. Findings like these suggest that there is more to the turtle aquatic auditory scene than previously thought.

Amphibious auditory evoked potentials in four North American Testudines genera spanning the aquatic-terrestrial spectrum

Animals exhibit unique hearing adaptations in relation to the habitat media in which they reside. This study was a comparative analysis of auditory specialization in relation to habitat medium in Testudines, a taxon that includes both highly aquatic and fully terrestrial members. Evoked potential audiograms were collected in four species groups representing diversity along the aquatic-terrestrial spectrum: terrestrial and fossorial Gopherus polyphemus, terrestrial Terrapene carolina carolina, and aquatic Trachemys scripta and Sternotherus (S. odoratus and S. minor). Additionally, underwater sensitivity was tested in T. c. carolina, T. scripta, and Sternotherus with tympana submerged just below the water surface. In aerial audiograms, T. c. carolina were most sensitive, with thresholds 18 dB lower than Sternotherus. At 100-300 Hz, thresholds in T. c. carolina, G. polyphemus, and T. scripta were similar to each other. At 400-800 Hz, G. polyphemus thresholds were elevated to 11 dB above T. c. carolina. The underwater audiograms of T. c. carolina, T. scripta, and Sternotherus were similar. The results suggest aerial hearing adaptations in emydids and high-frequency hearing loss associated with seismic vibration detection in G. polyphemus. The underwater audiogram of T. c. carolina could reflect retention of ancestral aquatic auditory function.

Ontogenetic investigation of underwater hearing capabilities in loggerhead sea turtles (Caretta caretta) using a dual testing approach

Sea turtles reside in different acoustic environments with each life history stage and may have different hearing capacity throughout ontogeny. For this study, two independent yet complementary techniques for hearing assessment, i.e. behavioral and electrophysiological audiometry, were employed to (1) measure hearing in post-hatchling and juvenile loggerhead sea turtles Caretta caretta (19-62 cm straight carapace length) to determine whether these migratory turtles exhibit an ontogenetic shift in underwater auditory detection and (2) evaluate whether hearing frequency range and threshold sensitivity are consistent in behavioral and electrophysiological tests. Behavioral trials first required training turtles to respond to known frequencies, a multi-stage, time-intensive process, and then recording their behavior when they were presented with sound stimuli from an underwater speaker using a two-response forced-choice paradigm. Electrophysiological experiments involved submerging restrained, fully conscious turtles just below the air-water interface and recording auditory evoked potentials (AEPs) when sound stimuli were presented using an underwater speaker. No significant differences in behavior-derived auditory thresholds or AEP-derived auditory thresholds were detected between post-hatchling and juvenile sea turtles. While hearing frequency range (50-1000/1100 Hz) and highest sensitivity (100-400 Hz) were consistent in audiograms pooled by size class for both behavior and AEP experiments, both post-hatchlings and juveniles had significantly higher AEP-derived than behavior-derived auditory thresholds, indicating that behavioral assessment is a more sensitive testing approach. The results from this study suggest that post-hatchling and juvenile loggerhead sea turtles are low-frequency specialists, exhibiting little differences in threshold sensitivity and frequency bandwidth despite residence in acoustically distinct environments throughout ontogeny.

Sedation and anesthesia of hatchling leatherback sea turtles (Dermochelys coriacea) for auditory evoked potential measurement in air and in water

Sedation or anesthesia of hatchling leatherback sea turtles was employed to acquire auditory evoked potential (AEP) measurements in air and in water to assess their hearing sensitivity in relation to potential consequences from anthropogenic noise. To reduce artifacts in AEP collection caused by muscle movement, hatchlings were sedated with midazolam 2 or 3 mg/kg i.v. for in-air (n = 7) or in-water (n = 11) AEP measurements; hatchlings (n = 5) were anesthetized with ketamine 6 mg/kg and dexmedetomidine 30 microg/kg i.v. reversed with atipamezole 300 microg/kg, half i.m. and half i.v. for in-air AEP measurements. Midazolam-sedated turtles were also physically restrained with a light elastic wrap. For in-water AEP measurements, sedated turtles were brought to the surface every 45-60 sec, or whenever they showed intention signs for breathing, and not submerged again until they took a breath. Postprocedure temperature-corrected venous blood pH, pCO2, pO2, and HCO3- did not differ among groups, although for the midazolam-sedated in-water group, pCO2 trended lower, and in the ketamine-dexmedetomidine anesthetized group there was one turtle considered clinically acidotic (temperature-corrected pH = 7.117). Venous blood lactate was greater for hatchlings recently emerged from the nest than for turtles sedated with midazolam in air, with the other two groups falling intermediate between, but not differing significantly from the high and low lactate groups. Disruptive movements were less frequent with anesthesia than with sedation in the in-air group. Both sedation with midazolam and anesthesia with ketamine-dexmedetomidine were successful for allowing AEP measurements in hatchling leatherback sea turtles. Sedation allowed the turtle to protect its airway voluntarily while limiting flipper movement. Midazolam or ketamine-dexmedetomidine (and reversal with atipamezole) would be useful for other procedures requiring minor or major restraint in leatherback sea turtle hatchlings and other sea turtles, although variable susceptibilities may require dose adjustments.

Auditory brainstem responses in Cope's gray treefrog (Hyla chrysoscelis): effects of frequency, level, sex and size

Our knowledge of the hearing abilities of frogs and toads is largely defined by work with a few well-studied species. One way to further advance comparative work on anuran hearing would be greater use of minimally invasive electrophysiological measures, such as the auditory brainstem response (ABR). This study used the ABR evoked by tones and clicks to investigate hearing in Cope's gray treefrog (Hyla chrysoscelis). The objectives were to characterize the effects of sound frequency, sound pressure level, and subject sex and body size on ABRs. The ABR in gray treefrogs bore striking resemblance to ABRs measured in other animals. As stimulus level increased, ABR amplitude increased and latency decreased, and for responses to tones, these effects depended on stimulus frequency. Frequency-dependent differences in ABRs were correlated with expected differences in the tuning of two sensory end organs in the anuran inner ear (the amphibian and basilar papillae). The ABR audiogram indicated two frequency regions of increased sensitivity corresponding to the expected tuning of the two papillae. Overall, there was no effect of subject size and only small effects related to subject sex. Together, these results indicate the ABR is an effective method to study audition in anurans.