Little information exists on endocrine responses to noise exposure in marine mammals. In the present study, cortisol, aldosterone, and epinephrine levels were measured in 30 bottlenose dolphins (Tursiops truncatus) before and after exposure to simulated U.S. Navy mid-frequency sonar signals (3250–3450 Hz). Control and exposure sessions, each consisting of ten trials, were performed sequentially with each dolphin. While swimming across the experimental enclosure during exposure trials, each dolphin received a single 1-s exposure with received sound pressure levels (SPLs, dB re 1 μPa) of 115, 130, 145, 160, 175, or 185 dB. Blood samples were collected through behaviorally conditioned, voluntary participation of the dolphins approximately one week prior to, immediately following, and approximately one week after exposure were analyzed for hormones via radioimmunoassay. Aldosterone was below detection limits in all samples. Neither cortisol nor epinephrine showed a consistent relationship with received SPL, even though dolphins abandoned trained behaviors after exposure to the highest SPLs and the severity of behavioral changes scaled with SPL. It remains unclear if dolphins interpret high-level anthropogenic sound as stressful, annoying, or threatening and whether behavioral responses to sound can be equated to a physiological (endocrine) response.
I. INTRODUCTION
The use of an animal's behavioral response to anthropogenic stressors may be an ineffective means of predicting population consequences of disturbance when used alone. For example, factors such as habitat quality and the availability of suitable habitat potentially affect an animal's decision to abandon the habitat during disturbance, which may or may not have subsequent consequences at the population level (Gill et al., 2001). It follows that a lack of response to anthropogenic stimuli does not necessarily indicate that an animal is undisturbed either, e.g., an animal with only a limited amount of suitable habitat may resist behaviorally responding to disturbances because of the cost of habitat loss. This outward “tolerance” of disturbance can potentially come at a physiological cost which, if sufficient to affect survivorship or reproduction, may ultimately have consequences at the population level.
From an endocrine perspective, the stress response is classically characterized by the release of catecholamines from the adrenal medulla or glucocorticoids from the adrenal cortex. The former is the basis for the fight-or-flight response, which rapidly mobilizes glucose for utilization and primes the cardiovascular system for a potential increase in the demand for oxygen by the body. The latter response, mediated by activation of the hypothalamic-pituitary-adrenal (HPA) hormone axis, serves to enable a more sustained response to the stressor and affects endogenous fuel substrate use, immune function, and potentially influences growth and reproduction. Collectively, these are adaptive responses that permit an organism to cope with stressors of relatively short duration but which, over sustained periods, can become deleterious to the health and reproduction of an organism (Romero and Butler, 2007).
Anthropogenic sound has been implicated as a causative factor in a number of marine mammal strandings (Frantzis, 1998; Jepson et al., 2003; Fernández et al., 2005; D'amico et al., 2009; Fernández et al., 2012; Jepson et al., 2013), and it has received attention as potentially having sublethal behavioral and physiological impacts [National Research Council (NRC), 2005]. However, relatively little information has been obtained that links acoustic disturbance to concomitant endocrine responses in cetaceans. Captive belugas (Delphinapterus leucas) demonstrated no catecholamine response to oil drilling sound playbacks (Thomas et al., 1990), but did show a statistically significant elevation in catecholamines following exposure to sounds from a seismic water gun (Romano et al., 2004). A bottlenose dolphin (Tursiops truncatus) exposed to the same water gun signals did not have a measurable catecholamine response but did have an increase in aldosterone, a hormone suggested as being a significant stress indicator in odontocetes (St. Aubin and Dierauf, 2001; Champagne et al., 2018). Heart rate has been used in a couple of instances to look at the responses of odontocetes to acoustic playbacks of conspecific signals (e.g., whistles and jaw claps) and playbacks of bandpass signals within the region of best hearing (Miksis et al., 2001; Lyamin et al., 2011). Within all of these studies, limitations on interpretation exist due to extremely limited sample sizes and experimental design constraints.
In the present study, we test whether exposure to a simulated U.S. Navy mid-frequency sonar signal results in an endocrine response consistent with either the fight-or-flight response or HPA activation. Specifically, changes in epinephrine (fight-or-flight) and cortisol or aldosterone (HPA activation) levels were measured in bottlenose dolphins exposed to sonar signals ranging in received sound pressure level (SPL) from 115 to 185 dB re 1 μPa (hereafter designated as “dB SPL”). The goal of the study, which capitalized on a prior study of dolphin behavioral responses to mid-frequency active sonar exposure (Houser et al., 2013), was to determine whether the received level of a sonar signal was related to an endocrine response.
II. MATERIALS AND METHODS
A. Subjects
Thirty bottlenose dolphins (T. truncatus) participated in a controlled exposure study. All procedures of the controlled exposure study were approved by the Institutional Animal Care and Use Committee of the Naval Information Warfare Center Pacific in San Diego, CA, and the Department of the Navy Bureau of Medicine and Surgery. All procedures used in the study followed applicable USA Department of Defense guidelines for the care of laboratory animals.
Dolphins were maintained by the U.S. Navy Marine Mammal Program (MMP) in open-water, netted enclosures (9 × 9 to 9 × 18 m) located within San Diego Bay. Each dolphin received a daily allotment of fish required to maintain target weights established by MMP veterinary staff. Dolphins ranged from 6.6 to 45.5 years of age at the time of the study (Table I). There was a bias toward participation by male dolphins (21 male/9 female) due to the distribution of animals available for the study. None of the subjects had previously participated in any research related to noise-induced hearing threshold shifts or had, to our knowledge, been exposed to high-intensity sonar signals.
Animal identification (ID), age, and gender for animals participating in the sonar exposure study.
Subject ID . | Age (yr) . | Gender . |
---|---|---|
D1 | 6.6 | M |
D2 | 8.5 | M |
D3 | 9.5 | M |
D4 | 7.5 | M |
D5 | 39.5 | M |
D6 | 29.5 | M |
D7 | 11.5 | M |
D8 | 29.5 | M |
D9 | 39.5 | M |
D10 | 31.5 | F |
D11 | 9 | F |
D12 | 18.3 | M |
D13 | 29.5 | M |
D14 | 27.5 | M |
D15 | 28.8 | M |
D16 | 29.5 | M |
D17 | 32.5 | F |
D18 | 17.7 | M |
D19 | 35.5 | F |
D20 | 10 | M |
D21 | 21 | F |
D22 | 45.5 | M |
D23 | 17.7 | M |
D24 | 25.5 | M |
D25 | 27.5 | F |
D26 | 26 | F |
D27 | 27.5 | F |
D28 | 25.5 | F |
D29 | 28.5 | M |
D30 | 28.5 | M |
Subject ID . | Age (yr) . | Gender . |
---|---|---|
D1 | 6.6 | M |
D2 | 8.5 | M |
D3 | 9.5 | M |
D4 | 7.5 | M |
D5 | 39.5 | M |
D6 | 29.5 | M |
D7 | 11.5 | M |
D8 | 29.5 | M |
D9 | 39.5 | M |
D10 | 31.5 | F |
D11 | 9 | F |
D12 | 18.3 | M |
D13 | 29.5 | M |
D14 | 27.5 | M |
D15 | 28.8 | M |
D16 | 29.5 | M |
D17 | 32.5 | F |
D18 | 17.7 | M |
D19 | 35.5 | F |
D20 | 10 | M |
D21 | 21 | F |
D22 | 45.5 | M |
D23 | 17.7 | M |
D24 | 25.5 | M |
D25 | 27.5 | F |
D26 | 26 | F |
D27 | 27.5 | F |
D28 | 25.5 | F |
D29 | 28.5 | M |
D30 | 28.5 | M |
B. Experimental setup
The experimental setup is described in detail elsewhere (Houser et al., 2013); only those details required to understand the setup in relation to the endocrine study are described here. The study was performed in a 9 × 18 m floating, netted enclosure located within San Diego Bay (Fig. 1). Station A, the location at which a dolphin started a trial, was located to the east side of the enclosure closest to San Diego Bay, and station B was opposite of station A and closest to the shore. A target paddle was mounted to the side of the enclosure at station B and extended just below the water surface. A transducer for generating the acoustic stimulus (designated as T in Fig. 1) was located 1 m underwater and 1 m behind the station B target paddle. Cameras were mounted above (designated as V in Fig. 1) the midpoint of the enclosure and to the sides of the enclosure to record animal behavior during the session (see the description and analysis in Houser et al., 2013). A hydrophone (TC-4013, Reson, Slangerup, Denmark) was placed at a depth of 1 m midway between the sound source and the target paddle at station B but slightly offset to the side. The hydrophone monitored underwater sound during the sessions.
(Color online) Overhead layout of the enclosure used in the controlled exposure study. The enclosure measured 9 × 18 m. Station A (indicated by A on the figure) was the location at which each trial began. Station B (indicated by B on the figure) was the location of the response paddle touched by the dolphin on each trial. The transducer (indicated by T on the figure) was located approximately 1 m behind station B. The midpoint of the enclosure, designated as V for the location of the overhead video camera, is the location at which each dolphin received the simulated sonar exposure, i.e., during the test sessions the dolphin was exposed when crossing the gray shaded area.
(Color online) Overhead layout of the enclosure used in the controlled exposure study. The enclosure measured 9 × 18 m. Station A (indicated by A on the figure) was the location at which each trial began. Station B (indicated by B on the figure) was the location of the response paddle touched by the dolphin on each trial. The transducer (indicated by T on the figure) was located approximately 1 m behind station B. The midpoint of the enclosure, designated as V for the location of the overhead video camera, is the location at which each dolphin received the simulated sonar exposure, i.e., during the test sessions the dolphin was exposed when crossing the gray shaded area.
The dolphin trainers and the primary investigator (PI), who controlled acoustic playbacks, utilized headset intercoms (XO-1, Telex, Burnsville, MN) to communicate with one another during a session. Communications were relayed to an audio mixer where they were mixed with the hydrophone signal, integrated with the video camera signals, and recorded to computer disk (StreamPix, NorPix, Montreal, Canada).
C. Behavioral task
The behavioral task and experimental design are described below. The rationale and a more detailed discussion of the experimental design, which focused on quantification of behavioral responses to sonar exposure, can be found in Houser et al. (2013).
Each dolphin was trained to leave station A (Fig. 1), travel to the opposite end of the enclosure to touch the target paddle at station B with its rostrum, and then return to station A (hereafter referred to as the ABA behavior). An animal trainer located at station A initiated each trial by providing a visual signal to the dolphin. The dolphin then had 30 s to complete the ABA behavior. Another trainer, located at station B, verified that the dolphin touched the target paddle. Completion of the ABA behavior within the 30-s trial duration resulted in a fish reward being given to the dolphin. The reward was a fixed amount of Icelandic capelin (Mallotus villosus) equal to 1% of the dolphin's daily allotment of capelin. The next trial began immediately upon conclusion of the prior 30-s trial. Ten trials were conducted in sequence to complete a session. Sessions, whether control or experimental, had a duration of 5 min. Each dolphin was trained on the ABA behavior until it performed ten repetitions of the task (i.e., one session) without error. The dolphin was then tested within several days of reaching performance criterion.
Control and experimental sessions were conducted on the same day. Control sessions were always conducted first with a rest of several minutes between the control session and the exposure session. Exposure sessions and control sessions were performed in the same manner except that the dolphin was exposed to a playback of a mid-frequency sonar signal as it crossed the midpoint of the enclosure swimming from A to B. Playbacks occurred once per trial and only if the dolphin swam past the midpoint of the enclosure. If the dolphin refused to participate on a trial and did not cross the enclosure, no playback occurred for that trial. If a dolphin refused to participate in a trial, it was given an opportunity to participate in the following trial through a trainer recall (a hand slap of the water surface). The recall was given 25 s into the trial for which the dolphin refused to participate, and then again at 5 and 10 s into the next trial. If the dolphin returned to the trainer before 10 s into the subsequent trial, it was given the cue to perform the task. If the dolphin returned after 10 s into the trial, it was kept on station by the trainer until the next trial.
Received levels for each individual were kept at the same level within a session. SPLs at the midpoint of the enclosure (where the animal received the exposure) were 115, 130, 145, 160, 175, or 185 dB SPL. Five dolphins were tested at each of the six received SPLs, but each dolphin was tested at only one SPL (i.e., N = 5 for each received SPL condition). Each dolphin was assigned a study identifier (D1–D30) so that its age, history, and prior experience could not be identified except by the PI. A co-investigator (CI), who was blind to which dolphin a particular study identifier was assigned, randomly assigned an acoustic exposure level to each study identifier. On the day of testing for a given dolphin, the PI requested the acoustic exposure level for that dolphin's study identifier from the CI. Thus, only the PI knew the exposure level for the dolphin, but the level was not known until the day that the control and experimental sessions took place.
Twenty-seven of the dolphins were housed far from the pier complex with the experimental enclosure. Three dolphins lived at the pier complex where testing occurred. These dolphins were tested first so as to avoid habituation or sensitization to the playbacks. Each of the three dolphins was tested while the other two dolphins were working or training in the open ocean away from the test site.
D. Sound source and acoustic stimulus
A piezoelectric cylinder located approximately 1 m behind station B and placed at 1 m of depth was used for stimulus generation. The electronic stimulus waveform was played from a laptop computer, amplified (LVC 5050, AE Techron, Elkhart, IN), and sent to the piezoelectric cylinder. The maximum source level of the projector was ∼202 dB SPL at 1 m, which resulted in a maximum received level of ∼185 dB SPL at the midpoint between A and B. The acoustic stimulus was based on that of mid-frequency active sonar waveforms commonly used by the U.S. Navy and chosen because of its relevance to broadscale Navy activities and its putative relationship to prior stranding events (Jepson et al., 2003; Fernández et al., 2005; D'amico et al., 2009). The waveform consisted of a 0.5-s frequency modulated (FM) upsweep (center frequency ∼3250 Hz) with a 50 ms rise time followed immediately by a 0.5-s CW (∼3450 Hz). The stimulus SPL was calibrated at the midpoint of the enclosure prior to each control and following each experimental session for a dolphin. Instantaneous sound pressure was measured with four hydrophones (Reson TC-4013) stretched across the width of the enclosure at approximately equal increments (at the level of the “V” designation in Fig. 1; see Houser et al., 2013, for a description of variation in received levels across the enclosure). Hydrophone signals were high pass filtered (100 Hz) and amplified (Reson VP1000) prior to being digitized (USB-6259, National Instruments Inc., Austin, TX). All signals were calibrated from the pressure waveform except the lowest level signals (∼115 dB SPL). The received levels of these signals were determined from the frequency domain because the signal was indistinguishable from the ambient noise in the time domain. The duty cycle of the signal if a dolphin completed every trial on time was ∼3.3%, which is similar to the duty cycle employed by the U.S. Navy during typical mid-frequency active sonar operations in open water (e.g., one ping every 20–30 s).
E. Blood sampling for hormone analysis
A blood sample was collected from each dolphin approximately one week prior to the control and exposure session (PRE), immediately following the exposure session (TEST), and approximately one week after the exposure session (POST). Blood draws were only collected through behaviorally conditioned, voluntary participation of the dolphins. Each dolphin was conditioned through positive reinforcement (e.g., fish reward) to present the ventral surface of its fluke to a trainer such that blood could be collected from an arteriovenous plexus (Venn-Watson et al., 2007). Voluntary participation was critical to avoiding possible increases in serum hormones associated with handling stress (Joyce-Zuniga et al., 2016). Blood collections made after the exposure session were typically completed within 2–3 min, but no more than 5 min, after the end of the session. Blood collections were made into chilled serum and ethylenediamine tetra-acetic acid (EDTA) blood collection tubes and centrifuged immediately after collection at 1090 g for 10 min. Aliquots of the supernatant were collected immediately following centrifugation and frozen at −80 °C until further processing.
Serum samples were analyzed in duplicate for cortisol and aldosterone via radioimmunoassay (kits TKCO1 and TKAL1, respectively, Siemens Diagnostics, Livonia, MI). EDTA samples were assayed in duplicate for epinephrine using an ELISA assay (17-EPIHU-E01, Alpco, Salem, NH). These kits were previously validated for use in bottlenose dolphins (Ortiz and Worthy, 2000; Houser et al., 2011; Champagne et al., 2018). Cortisol levels were below the limit of detection for the assay in 29 samples. In these circumstances, the lower detection limit of the assay (0.2 μg/dL) was used as the value for the hormone level. Aldosterone levels were below the detection limit of the assay (6.3 pg/mL) for all samples (but see Sec. III for results and discussion). The average coefficient of variation for cortisol was 1.5%, and the average coefficient of variation for epinephrine was 3.4%.
F. Statistical analysis
Differences between treatments were tested for using a linear mixed model in which hormone concentration was the dependent variable, and the treatment category (PRE, TEST, POST) and exposure level were treated as main effects with an interaction term. Individual subject identification (ID) was included in the model as a random effect. Model residuals were visually assessed for approximate normality and homoscedasticity. All statistical analyses were conducted with JMP 11.2 (sas Institute Inc., Cary, NC).
III. RESULTS AND DISCUSSION
Relative to the PRE and POST conditions, no changes in either cortisol (Fig. 2; F1,28.19 = 0.10, p = 0.75) or epinephrine (Fig. 3; F1,28.47 = 1.50, p = 0.23) concentrations were observed for any of the exposure (TEST) conditions. Group means and measures of variability were similar across all treatments for cortisol and epinephrine, although a high level of variance in epinephrine concentration between treatment intervals in one animal (PRE = 212 pg/mL, TEST = 703 pg/mL, POST = 64.4 pg/mL) contributed to a much larger standard deviation in the 175 dB SPL exposure group (see below). Mean hormone levels were otherwise consistent with observations of hormone levels collected from Tursiops spp. under voluntary, non-stressed conditions (Funasaka et al., 2011; Champagne et al., 2017; Champagne et al., 2018).
Changes in cortisol as a function of the received level of a mid-frequency sonar exposure. Levels measured immediately following the exposure (TEST) are compared to levels measured approximately one week prior (PRE) and one week following (POST) the exposure. Symbols represent mean values, and error bars represent the standard deviation from the mean. (N = 5 for each of the received SPL groupings.)
Changes in cortisol as a function of the received level of a mid-frequency sonar exposure. Levels measured immediately following the exposure (TEST) are compared to levels measured approximately one week prior (PRE) and one week following (POST) the exposure. Symbols represent mean values, and error bars represent the standard deviation from the mean. (N = 5 for each of the received SPL groupings.)
Changes in epinephrine as a function of the received level of a mid-frequency sonar exposure. Levels measured immediately following the exposure (TEST) are compared to levels measured approximately one week prior (PRE) and one week following (POST) the exposure. Symbols represent mean values, and error bars represent the standard deviation from the mean. (N = 5 for each of the received SPL groupings.)
Changes in epinephrine as a function of the received level of a mid-frequency sonar exposure. Levels measured immediately following the exposure (TEST) are compared to levels measured approximately one week prior (PRE) and one week following (POST) the exposure. Symbols represent mean values, and error bars represent the standard deviation from the mean. (N = 5 for each of the received SPL groupings.)
The lack of change in circulating cortisol with increased levels of sound exposure is surprising given that behavioral responses to the sonar exposure were profound at the highest received levels and scaled with increasing received level (Houser et al., 2013); behavioral changes statistically related to the sonar exposure were changes in respiration rate, the slapping of the fluke on the water surface, and abandonment of trained behaviors with the severity of the behavioral response scaling in direct relation to the received SPL of the exposure. Animals were found to habituate rapidly to received levels ≤ 160 dB SPL as determined by their willingness to complete trials following the initial exposure trials in which behavioral reactions occurred (see Houser et al., 2013). This could possibly be related to why no change in cortisol was observed (i.e., if the stimulus was not perceived as threatening), but neither were changes in cortisol observed at received levels (175 dB) where no habituation was detected nor where abandonment of trained behaviors was ubiquitous (received levels of 185 dB SPL). One of two explanations for the observed lack of a cortisol response in light of the profound behavioral responses is likely: either the animals exhibited profound behavioral reactions but without perceiving the acoustic stimulus as a “stressor” (e.g., it was annoying but insufficient to upregulate the HPA axis), or the time course of sampling was inappropriate for measuring the cortisol response.
Few studies have investigated changes in circulating cortisol in response to a controlled stressor in the bottlenose dolphin. Mean blood cortisol values from wild-caught bottlenose dolphins and related toothed whales are typically 2–3 times those observed when bottlenose dolphins voluntarily participate in blood draws (St. Aubin and Geraci, 1989, 1992; St. Aubin et al., 1996; Ortiz and Worthy, 2000; St. Aubin et al., 2001; Funasaka et al., 2011; St. Aubin et al., 2013; Fair et al., 2014; Champagne et al., 2017; Champagne et al., 2018), indicating that capture and restraint are stressful events. Elevations in cortisol in these types of events are related to the time since the onset of the negative stimulus (e.g., chase and encirclement; Hart et al., 2015). More recently, a detailed study of the time-course of serum corticosteroid changes following physical restraint was documented (Champagne et al., 2018). In that study, cortisol increased between 135% and 221% in physically restrained animals (N = 5) within the first 15 min of a 2-h restraint period. Since blood samples collected in the present study were collected within 2–3 min of the end of a session, the maximum time from the first sonar exposure to collection of the blood sample was approximately 8 min. Given prior observations of cortisol increase in response to physical restraint (Champagne et al., 2018), it should be reasonably expected that if elevations in cortisol followed a similar pattern and time-course after the onset of the stressor, levels should have increased by 67%–110% and been detected following the end of the session. It is therefore possible that the dolphins, although perceiving the exposure as annoying and something to avoid, did not inherently perceive it as a sufficient threat or stressor to mount a cortisol response. Conversely, the stressor applied by Champagne et al. (2018) was a continuous restraint, and cortisol levels quickly dropped following the removal of the restraint. The acoustic stimulus in the present study was intermittent (1 s every 30 s), which might preclude a continuous corticosteroid response similar to that observed during physical restraint. Furthermore, many of the animals exposed to the highest SPL refused to participate in any trials following the first exposure, thus providing time after the initial exposure for any immediate increase in cortisol to subside before the end of the session. A similar argument could be made that animals who quickly habituated might have had only a brief cortisol response to the first (or first several) exposure(s), which elicited a behavioral response, and quickly recovered during the habituation period.
An inability to detect aldosterone in any of the samples also supports the hypotheses that either there was no activation of the HPA axis in association with observed behavioral responses or that the time course of sampling was inappropriate to measure the response (as described above for cortisol). Aldosterone levels are typically very low in dolphin populations under human care and are difficult to detect and quantify (St. Aubin et al., 1996; St. Aubin and Dierauf, 2001; Champagne et al., 2017). An extraction procedure has been validated that allows low levels of aldosterone to be measured in dolphin serum (Champagne et al., 2018), but, unfortunately, the procedure was validated after the samples of this study were processed. Nevertheless, under the stress of physical restraint, aldosterone increases rapidly, like cortisol, and has been observed to increase from 175% to 567% within 15 min of the onset of continuous restraint (Champagne et al., 2018). If the HPA axis was upregulated following the sonar playback and in a manner observed during restraint, then it would be expected that measurable levels of aldosterone would be detected in the blood samples collected after the exposure. This was not the case, but since both cortisol and aldosterone share a common pathway via the HPA axis, the arguments for why no changes in aldosterone were observed are the same as those presented for cortisol.
The timing of blood sampling was established to best enable changes in circulating epinephrine to be captured following sampling, and there was an a priori expectation that epinephrine levels would vary as a function of the received level. However, mean epinephrine levels did not change in response to sonar exposure, regardless of the level of exposure received. The highest level of epinephrine observed occurred after a 175 dB SPL exposure—the level (703 pg/mL) was approximately nine times the mean value of all of the exposure samples (78 pg/mL). This animal refused to participate in any trials after the first exposure and was therefore only exposed one time. Two other dolphins exposed to 175 dB SPL signals refused to participate on some of the trials, but showed no substantial changes in epinephrine levels as a function of the exposure. It is possible that the dolphin with the high epinephrine level experienced an increase in epinephrine after the first exposure. However, such an interpretation should be made cautiously as this dolphin had the most variable epinephrine levels of all of the dolphins (PRE = 212 pg/mL, TEST = 703 pg/mL, POST = 64 pg/mL), and the response was not consistent with observations across the other 29 subjects.
Epinephrine is a more difficult hormone to understand with respect to the fight-or-flight response in dolphins as it likely also serves a role in cardiovascular control during diving (Atkinson et al., 2015), as has been observed in pinnipeds (Hochachka et al., 1995; Hurford et al., 1996). Variations in epinephrine following physical restraint were equivocal (Champagne et al., 2018)—although there appeared to be a brief increase in epinephrine at the onset of restraint, levels quickly dropped to baseline values, and high levels of individual variation in epinephrine levels prior to the onset of the restraint complicated interpretation and resolution of any patterns. Thus, based on the results in the present study, and those of prior studies, it remains inconclusive as to whether epinephrine is a good endocrine indicator of an acute stress response in dolphins.
Few studies have investigated the endocrine response to sound exposure in bottlenose dolphins. Those that have investigated the endocrine response to sound exposure in bottlenose dolphins reported no change in cortisol in response to sound exposure (Romano et al., 2004), negligible changes in epinephrine following sound exposure (Thomas et al., 1990), or at least changes in epinephrine that for an individual dolphin fell within the normal pre-exposure variation observed in the present study (Romano et al., 2004). Therefore, in context of prior studies, the patterns in hormone concentrations as a function of sonar received level, given the sampling constraints of this study, remain unresolved. As of yet, it cannot be determined whether intermittent, high-level acoustic stimuli elicit changes in circulating epinephrine, cortisol, and aldosterone consistent with a “stress response.” Indeed, the relationship between behavioral responses and endocrine responses to acoustic stressors requires additional study, and it should not be concluded, at this time, that the display of a behavioral response equates to a physiological response to the same stressor.
ACKNOWLEDGMENTS
The authors would like to thank the training staff and management of the U.S. Navy MMP for access to the dolphins used in this study and for assistance in animal training and experimental setup. Special thanks are also given to L. Yeates, who assisted in data collection during the study. This project was sponsored by the Office of Naval Research (Award No. N0001409WX20853). This is scientific contribution No. 258 of the National Marine Mammal Foundation.