Dorian S Houser

• Ph.D. in Biology
• Research Director at National Marine Mammal Foundation

Knowledge about species-specific hearing is vital to assessing how anthropogenic noise impacts marine mammals. Unfortunately, no empirical audiogram exists for any mysticete whale. We therefore developed a catch-and-release method to assess hearing in a small mysticete, the minke whale ( Balaenoptera acutorostrata ). Stationary lead nets were place...

Echolocating dolphins use spectral cues to discriminate complex echoes from targets with multiple reflective surfaces; however, the specific cues involved are not clear. This study investigated the role of spectral interference patterns on the dolphin’s ability to discriminate two-highlight echoes. In task one of the study, two dolphins were traine...

Frequency-modulated "chirp" stimuli that offset cochlear dispersion (i.e., input compensation) have shown promise for increasing auditory brainstem response (ABR) amplitudes relative to traditional sound stimuli. To enhance ABR methods with marine mammal species known or suspected to have low ABR signal-to-noise ratios, the present study examined t...

Using passive acoustic monitoring (PAM) and convolutional neural networks (CNN), we monitored the movements of the two endangered Amazon River dolphin species, the boto (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) from main rivers to floodplain habitats (várzea) in the Mamirauá Reserve (Amazonas, Brazil). We detected dolphin presence in...

The ability of bottlenose dolphins to detect changes in echo phase was investigated using a jittered-echo paradigm. The dolphins' task was to produce a conditioned vocalization when phantom echoes with fixed echo delay and phase changed to those with delay and/or phase alternated ("jittered") on successive presentations. Conditions included: jitter...

Rotational behaviour has been observed when dolphins track or detect targets, however, its role in echolocation is unknown. We used computed tomography data of one live and one recently deceased bottlenose dolphin, together with measurements of the acoustic properties of head tissues, to perform acoustic property reconstruction. The anatomical conf...

In his 1979 paper “Perception of echo phase information in bat sonar” [Science 204, 1336–1338], Jim Simmons introduced the “jittered-echo” paradigm. In this method, bats discriminated between electronic echoes with fixed delay (i.e., simulating fixed range) and those with delays that alternated (“jittered”) on successive echoes. The jittered-echo p...

The bottlenose dolphin, WEN (also known as “Nacho”), worked in the swimmer interdiction system of the United States Navy Marine Mammal Program (MMP) before being transferred to MMP research in the mid-2000s. During his time away from systems work, WEN participated in diverse research projects that benefited our understanding of dolphin hearing, bio...

Dolphins potentially use spectral cues to discriminate inter-highlight interval (IHI) for passively presented, simulated two-highlight echo stimuli. To investigate this potential, dolphins were trained to listen to repetitive two-highlight “background” echoes and respond upon a change to “target” echoes with increased IHI. In the first experimental...

How do dolphins attend to echos and ignore background noise? Is the process similar to how humans attend to conversations in a crowded room? Studies of human selective attention have highlighted endogenous brain processes modulating the magnitude of early responses to sounds in auditory evoked potentials (AEPs), yet these rely on task-specific inst...

Echo-delay (range) resolution is a critical feature of animal biosonar. In 1973, Jim Simmons was the first scientist to test bat range discrimination thresholds at a variety of absolute ranges. He used a two alternative-force-choice paradigm with two phantom echo generators (PEGs) and compared the results to the bats’ performance with physical targ...

Knowing the hearing range and sensitivity of a marine mammal is fundamental to determining its potential for being impacted by ocean noise. Enabling stranding responders to perform hearing tests on stranded odontocetes is the most likely means by which most odontocete species will be tested and by which population-level variability in hearing will...

All species of toothed whales studied to date can learn to reduce their hearing sensitivity when warned of an impending intense sound; however, the specific conditions under which animals will employ this technique are not well understood. The present study was focused on determining whether dolphins would reduce their hearing sensitivity in respon...

The use of auditory evoked potentials has been promoted as a means by which to collect audiometric information from odontocete cetaceans that are rarely encountered in stranding situations. This article presents the results of auditory evoked potential hearing tests collected from stranded odontocetes over nearly a decade. For six species, no audio...

Previous studies suggested that dolphins perceive echo spectral features on coarse (macrospectrum) and fine (microspectrum) scales. This study was based on a finding that these auditory percepts are, to some degree, dependent on the dolphin's ∼250- μs auditory temporal window (i.e., “critical interval”). Here, two dolphins were trained to respond o...

Cochlear dispersion causes increasing delays between neural responses from high-frequency regions in the cochlear base and lower-frequency regions toward the apex. For broadband stimuli, this can lead to neural responses that are out-of-phase, decreasing the amplitude of farfield neural response measurements. In the present study, cochlear travelin...

No PDF available ABSTRACT Spinning is a natural and common dolphin behavior; however, its role in echolocation is unknown. We used computed tomography (CT) data of a live and a recently deceased bottlenose dolphin together with measurements of the acoustic properties of head tissues to perform acoustic property reconstrcution. The anatomical config...

No PDF available ABSTRACT In humans, understanding a voice amidst competing sounds depends on parsing the sound mixture into “streams” representing each source's content. Streaming can be influenced by top-down attentional focus, while acoustic features can affect streaming percepts through bottom-up, automatic processing of pitch, timbre, and loca...

No PDF available ABSTRACT Previous studies have suggested that dolphins perceive echo spectral features on both large (macrospectrum) and small (microspectrum) scales. The current study was based on a finding that these percepts are—to some degree—dependent on the dolphin’s ∼250-μs “critical interval” of echolocation. Two dolphins were trained to p...

Auditory neuroscience in dolphins has largely focused on auditory brainstem responses; however, such measures reveal little about the cognitive processes dolphins employ during echolocation and acoustic communication. The few previous studies of mid- and long-latency auditory-evoked potentials (AEPs) in dolphins report different latencies, polariti...

Understanding baseline hormone levels, the magnitude of intra-individual variability, and their variation as a function of life history is difficult in toothed whales (e.g. dolphins and porpoises) because of the effects of capture stress. To determine the endocrine profile of the common bottlenose dolphin (Tursiops truncatus) as a function of seaso...

Two previous studies suggest that bottlenose dolphins exhibit an “oddball” auditory evoked potential (AEP) to stimulus trains where one of two stimuli has a low probability of occurrence relative to another. However, they reported oddball AEPs at widely different latency ranges (50 vs 500 ms). The present work revisited this experiment in a single...

Evidence for synaptopathy, the acute loss of afferent auditory nerve terminals, and degeneration of spiral ganglion cells associated with temporary threshold shift (TTS) in traditional laboratory animal models (e.g., mice, guinea pigs) has brought into question whether TTS should be considered a non-injurious form of noise impact in marine mammals....

Stimulation of the thyroid with thyroid-stimulating hormone (TSH) is a potentially useful diagnostic of thyroid dysfunction, but little is known about the response of the thyroid to TSH stimulation in bottlenose dolphins (Tursiops truncatus). To better characterize the response of the dolphin thyroid to TSH stimulation, five adult dolphins particip...

The auditory brainstem response (ABR) to stimulus onset has been extensively used to investigate dolphin hearing. The mechanisms underlying this onset response have been thoroughly studied in mammals. In contrast, the ABR evoked by sound offset has received relatively little attention. To build upon previous observations of the dolphin offset ABR,...

Bottlenose dolphins ( Tursiops truncatus ) typically feed on prey that are high in lipid and protein content and nearly devoid of carbohydrate, a dietary feature shared with other marine mammals. However, unlike fasted-adapted marine mammals that predictably incorporate fasting into their life history, dolphins feed intermittently throughout the da...

Auditory brainstem responses (ABRs) to linear-enveloped, broadband noisebursts were measured in six bottlenose dolphins to examine relationships between sound onset envelope properties and the ABR peak amplitude. Two stimulus manipulations were utilized: (1) stimulus onset envelope pressure rate-of-change was held constant while plateau pressure an...

Previous bottlenose dolphin studies suggest that the coarse envelope of an echo spectrum (“macrostructure”) has hierarchical dominance over finer-scale spectral features (“microstructure”) during synthetic echo discrimination tasks. In this study, two dolphins listened to and discriminated between underwater sound stimuli consisting of pairs of cli...

Evaluating the impacts of anthropogenic disturbance on free-ranging marine mammal populations, many of which are in decline, requires robust diagnostic markers of physiological stress and health. However, circulating levels of canonical 'stress hormones' such as glucocorticoids, which are commonly used to evaluate animal health, do not capture the...

Three bottlenose dolphins (Tursiops truncatus) participated in simulated cylinder wall thickness discrimination tasks utilizing electronic “phantom” echoes. The first experiment resulted in psychometric functions (percent correct vs wall thickness difference) similar to those produced by a dolphin performing the task with physical cylinders. In the...

Plasma glucocorticoid (CORT) levels are one measure of stress in wildlife and give us insight into natural processes relevant to conservation issues. Many studies use total CORT concentrations to draw conclusions about animals' stress state and response to their environment. However, the blood of tetrapods contains corticosteroid-binding globulin (...

Differences in odontocete biosonar emissions have been reported for animals with hearing loss compared to those with normal hearing. For example, some animals with high-frequency hearing loss have been observed to lower the dominant frequencies of biosonar signals to better match a reduced audible frequency range. However, these observations have b...

Biosonar echo delay resolution was investigated in four bottlenose dolphins (Tursiops truncatus) using a “jittered” echo paradigm, where dolphins discriminated between electronic echoes with fixed delay and those whose delay alternated (jittered) on successive presentations. The dolphins performed an echo-change detection task and produced a condit...

A previously published analytical method demonstrated the quantification of the hormone cortisol in cetacean skin. However, little is known about the transfer of hormones between blood and skin. Recognizing that such information is essential to effectively using skin samples within marine mammal stress research, the primary goals of this study were...

Light is rapidly attenuated in sea water, and the habitats of dolphins and other toothed whales are characterized by reduced visual stimuli relative to terrestrial habitats. Sound is efficiently transmitted under water; however, and these species have undergone significant modifications to head structures that facilitate both passive and active (ec...

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 consis...

Little research has been conducted on the somatosensory system of toothed whales and it remains uncertain how tactile sensitivity varies about their bodies. In this study, tactile sensitivity to high‐frequency (250‐Hz) displacement of the skin was quantified in three trained adult common bottlenose dolphins (Tursiops truncatus) using a vibratory de...

The Protocol on Environmental Protection of the Antarctic Treaty stipulates that the protection of the Antarctic environment and associated ecosystems be fundamentally considered in the planning and conducting of all activities in the Antarctic Treaty area. One of the key pollutants created by human activities in the Antarctic is noise, which is pr...

While the response to acute stress is adaptive in nature, repeated or chronic stress can impact an animal's fitness by depleting its energy stores and suppressing immune function and reproduction. This can be especially deleterious for species that rely on energy reserves to fuel key life history stages (e.g. reproduction), already experience physi...

No PDF available ABSTRACT Previous work with bottlenose dolphins [Dubrovsky et al., “Mechanisms of signal discrimination and identification in the auditory system of Tursiops truncatus,” in Marine Mammal Sensory Systems, edited by Thomas et al. (Plenum, New York, 1992), pp. 235–240] suggested that the perception of coarse envelopes of echo power sp...

No PDF available ABSTRACT Auditory signal processing in dolphins is of special interest because these animals echolocate and possess high upper-cutoffs of hearing (∼140 kHz). The few previous measurements of auditory cortical responses (ACRs) in dolphins have been performed in air or with a restrained dolphin half submerged in water. Our goal was t...

No PDF available ABSTRACT The use of auditory evoked potentials (AEPs) to test the hearing of toothed whales has produced hundreds of toothed whale audiograms over the last two decades. Unfortunately, multiple approaches to obtaining AEP audiograms have been employed, including different methods for estimating the hearing threshold, the use of diff...

No PDF available ABSTRACT Previous studies have reported changes in biosonar emissions in a few odontocete subjects as audible frequency range is reduced with increasing age (i.e., presbycusis). For example, some animals have been observed to lower the dominant frequencies of their biosonar clicks to better match their reduced audible frequency ran...

No PDF available ABSTRACT Prior research suggests that dolphins can echo-discriminate targets by eavesdropping on conspecific echolocation or by listening to the pulse-echo sequence of an electronic sonar system. To evaluate dolphins as bistatic receivers/classifiers, a three-year old, male bottlenose dolphin was trained to produce a conditioned ph...

No PDF available ABSTRACT Match-to-sample (MTS) has been demonstrated in dolphins using echolocation and across sensory modalities (e.g., visual to echolocation and vice-versa). To assess potential for bistatic MTS, a pilot study was conducted in which a dolphin listened to one of three recorded playbacks of echoes obtained from targets of differen...

Previous studies suggested that frequency-modulated tonal stimuli where the frequency sweeps upward (up-chirps) may enhance auditory brainstem response (ABR) amplitudes in mammals. In this study, ABRs were measured in response to up-chirps in three killer whales (Orcinus orca) and compared to ABRs evoked by broadband clicks. Chirp durations ranged...

Natural and anthropogenic stressors have been reported to impact the health of marine mammals. Therefore, investigation of quantifiable biomarkers in response to stressors is required. We hypothesized that stress protein expression would be associated with biological and health variables in wild and managed-care bottlenose dolphins (Tursiops trunca...

A collaborative effort was undertaken to delineate underwater noise levels within holding enclosures at marine mammal facilities. Ambient noise levels were measured under normal operating conditions in the enclosures of 14 participating facilities. Facility habitats varied from ocean environments to fully enclosed pools. The means and standard erro...

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echoloc...

Chronic physiological stress impacts animal fitness by catabolizing metabolic stores and suppressing reproduction. This can be especially deleterious for capital breeding carnivores such as marine mammals, with potential for ecosystem-wide effects. However, the impacts and indicators of chronic stress in animals are currently poorly understood. To...

Psychophysical methods similar to those employed with bats were used to examine jittered echo-delay resolution in bottlenose dolphins (Tursiops truncatus). Two dolphins were trained to produce echolocation clicks and report a change from electronic echoes with a fixed delay of ~ 12.6 ms (~ 9.4 m simulated range) to echoes with delays that alternate...

The frequency range of hearing is important for assessing the potential impact of anthropogenic noise on marine mammals. Auditory evoked potentials (AEPs) are commonly used to assess toothed whale hearing, but measurement methods vary across researchers and laboratories. In particular, estimates of the upper-frequency limit of hearing (UFL) can var...

Echolocating toothed whales generally adjust click intensities and rates according to target range to ensure that echoes from targets of interest arrive before a subsequent click is produced, presumably facilitating range estimation from the delay between clicks and returning echoes. However, this click-echo-click paradigm for dolphin biosonar is m...

ABSTRACT Fine-scale echo delay resolution has been investigated using a “jittered” echo paradigm, where animals discriminate between electronic echoes with fixed delay (i.e., simulating fixed range) and echoes with delays that alternate (jitter) on successive echoes. The data consist of the animals’ discrimination performance (e.g., error rate) as...

No consensus on the stimuli used in toothed whale hearing tests using evoked potential methods currently exists. However, stimulus bandwidth should directly affect determination of the upper-frequency limit (UFL) of hearing because of the steep reduction in hearing sensitivity at frequencies immediately below the UFL—the broader the stimulus bandwi...

Echolocation in bottlenose dolphins (Tursiops truncatus) is an active process involving outgoing signal transmission and echo reception. Transmitted echolocation “clicks” interact with a target to produce echoes that contain representative information about the target. Echolocation allows dolphins to detect, discriminate, and identify complex targe...

Understanding the physiological response of marine mammals to anthropogenic stressors can inform marine ecosystem conservation strategies. Stress stimulates the activation of the hypothalamic–pituitary–adrenal (HPA) axis and synthesis of glucocorticoid (GC) hormones, which increase energy substrate availability while suppressing energy-intensive pr...

Several hormones are potential indicators of stress in free-ranging animals and provide information on animal health in managed-care settings. In response to stress, glucocorticoids (GC, e.g. cortisol) first appear in circulation but are later incorporated into other tissues (e.g. adipose) or excreted in feces or urine. These alternative matrices c...

Although the auditory brainstem response (ABR) is known to be an onset response, specific features of acoustic stimuli that affect the morphology of the ABR are not well understood. In this study, the effects of stimulus onset properties were investigated by measuring ABRs in seven bottlenose dolphins while systematically manipulating stimulus rise...

Systemic steroid hormone measurements are often used in the assessment of reproductive, developmental, and stress physiology in vertebrates. In protected wildlife, such as the common bottlenose dolphin (Tursiops truncatus), these measures can provide critical information about health and fitness to aid in effective conservation and management. Circ...

Unlike terrestrial mammals that have unambiguous aerial sound transmission pathways via the outer ear and tympanum, sound reception pathways in most odontocetes are not well understood. Recent studies have used auditory brainstem response (ABR) measurements to examine sound reception pathways. This study sought to determine how sound source placeme...

The auditory brainstem response to a dolphin’s own emitted biosonar click can be measured by averaging epochs of the instantaneous electroencephalogram (EEG) that are time-locked to the emitted click. In this study, averaged EEGs were measured using surface electrodes placed on the head in six different configurations while dolphins performed an ec...

The United States Navy Marine Mammal Program (MMP) has been in existence for over 50 years. Following its inception, the program quickly became involved in the study of marine mammal sensory systems and bioacoustics. Early studies included the pioneering work of C. Scott Johnson in obtaining the first behavioral audiogram in a dolphin and Sam Ridgw...

Vibratory pile drivers, used for marine construction, can produce sustained, high sound pressure levels (SPLs) in areas that overlap with dolphin habitats. Dolphins rely on echolocation for navigation, detecting predators and prey, and to coordinate group behavior. This study examined the effects of vibratory pile driver noise on dolphin sustained...

Many species undergo natural fasts as part of their life histories. Extended fasting is associated with increased β-oxidation of fatty acids and reduced oxidation of glucose to minimize commitment of body protein to gluconeogenesis. However, the metabolic strategies used to sustain extended fasts simultaneous with high rates of energy expenditure a...

The primary sense in odontocetes is hearing and a large portion of the odontocete brain is devoted to the auditory processing of echolocation signals. Hearing deficits in odontocetes potentially compromise the ability to forage, navigate, socialize, and evade predators. This presents a challenge to survival and reproduction in wild odontocetes and...

The National Marine Fisheries service regulates the impact of anthropogenic ocean noise to marine mammals. Other government and commercial groups that produce ocean noise are subject to regulation. Both groups desire greater knowledge of hearing in marine mammals to better predict, mitigate, and regulate noise exposure. Auditory evoked potential (A...

No PDF available ABSTRACT Experiments with bats show echo delay discrimination capabilities on sub-microsecond scales, which is an acuity that is much higher than that predicted by the auditory nervous system’s ability to directly encode high-frequency phase information (i.e., through neural phase locking). To provide a cross-species comparison, ec...

Knowledge of the emitted click or chirp in echolocating dolphins and bats is believed to be critical to using echo delays to reveal target range and shape. In dolphins, the neural representation of the “self-heard” click should not equate to that of a click measured in the far-field due to sound propagation path differences and temporal dispersion...

The auditory brainstem response (ABR) to a dolphin’s own emitted biosonar click may be measured by averaging epochs of the instantaneous electroencephalogram (EEG) that are time-locked to the emitted click. In this study, waves in the averaged EEG preceding the biosonar click-evoked ABR were measured using surface electrodes placed on the head in s...

Previous studies have demonstrated that increasing-frequency chirp stimuli (up-chirps) can enhance human auditory brainstem response (ABR) amplitudes by compensating for temporal dispersion occurring along the cochlear partition. In this study, ABRs were measured in two bottlenose dolphins (Tursiops truncatus) in response to spectrally white clicks...

OBJECTIVE To evaluate the impact of oral megestrol acetate (MA) administration on adrenal function in male bottlenose dolphins ( Tursiops truncatus ). DESIGN Serial cross-sectional study. ANIMALS 8 adult male dolphins, all of which were receiving MA at various daily doses (range, 0 to 60 mg, PO) for the control of reproductive behavior. PROCEDURES...

The marginal value theorem models patch departure decisions for foraging animals when resources are unevenly distributed. A key component of these models is the decelerating energy gain function used to represent patch depletion. However, the within-patch gain function has rarely been assessed in marine predators. We evaluated the gain functions in...

Marine mammals may be negatively affected by anthropogenic noise. Behavioural response studies (BRS) aim to establish a relationship between noise exposure conditions (dose) from a potential stressor and associated behavioural responses of animals. A recent series of BRS have focused on the effects of naval sonar sounds on cetaceans. Here, we revie...

Immune and endocrine responses play a critical role in allowing animals to adjust to environmental perturbations. We measured immune and endocrine related markers in multiple samples from individuals from two managed-care care dolphin groups (n = 82 samples from 17 dolphins and single samples collected from two wild dolphin populations: Indian Rive...

The neural representation of the dolphin broadband biosonar click was investigated by measuring auditory brainstem responses (ABRs) to “self-heard” clicks masked with noise bursts having various high-pass cutoff frequencies. Narrowband ABRs were obtained by sequentially subtracting responses obtained with noise having lower high-pass cutoff frequen...

Previous studies have demonstrated that increasing-frequency chirp (up-chirp) stimuli can enhance auditory brainstem response (ABR) amplitudes by compensating for temporal dispersion occurring along the cochlear partition. In this study, ABRs were measured in two bottlenose dolphins in response to 5-µs, spectrally “white” clicks, up-chirps, and dec...

Killer whales (Orcinus orca) are one of the most cosmopolitan marine mammal species with potential widespread exposure to anthropogenic noise impacts. Previous audiometric data on this species were from two adult females [Szymanski, Bain, Kiehl, Pennington, Wong, and Henry (1999). J. Acoust. Soc. Am. 108, 1322–1326] and one sub-adult male [Hall and...

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 weigh...

Cochlear place specificity of the auditory brainstem response (ABR) was investigated in five bottlenose dolphins by measuring ABRs to broadband clicks presented simultaneously with masking noise having various high-pass cutoff frequencies. Click and noise stimuli were digitally compensated to account for the transmitting response of the piezoelectr...

Two experiments were performed that investigated the effects of (1) click level and (2) continuous broadband noise on the binaural auditory brainstem response (ABR) of normal-hearing and hearing-impaired bottlenose dolphins (Tursiops truncatus). In addition to spectrally uncompensated clicks and noise, stimuli were digitally compensated to achieve...

The echolocation beam of toothed whales has been studied ever since it was first discovered in 1960. Recent studies have focused on the frequency distributions across the cross sections of the beams. Other studies have focussed on describing the entire acoustic field around the animal. However, no one has yet described the timing of each frequency...

The work of Whitlow Au and colleagues has demonstrated that dolphin biosonar forms a highly directional, forward-facing beam. In our recent studies, we have expanded upon previous work by making biosonar beam measurements using high-resolution hydrophone arrays with up to 48 hydrophones. Bottlenose dolphins were trained to echolocate on both physic...

The echolocation beam of the bottlenose dolphin was first carefully described by Au and colleagues (1978)[“Propagation of Atlantic bottlenose dolphin echolocation signals,” J. Acoust. Soc. Am. 64, 411-422] using various hydrophone array configurations and targets located in front of the dolphin and along its longitudinal axis. Measured beams were d...