Colleen Reichmuth Kastak

Underwater temporary threshold shift in pinnipeds : Effects of noise level and duration

Behavioral psychophysical techniques were used to evaluate the residual effects of underwater noise on the hearing sensitivity of three pinnipeds: a California sea lion (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris). Temporary threshold shift (TTS), defined as the difference between auditory thresholds obtained before and after noise exposure, was assessed. The subjects were exposed to octave-band noise centered at 2500 Hz at two sound pressure levels: 80 and 95 dB SL (re: auditory threshold at 2500 Hz). Noise exposure durations were 22, 25, and 50 min. Threshold shifts were assessed at 2500 and 3530 Hz. Mean threshold shifts ranged from 2.9-12.2 dB. Full recovery of auditory sensitivity occurred within 24 h of noise exposure. Control sequences, comprising sham noise exposures, did not result in significant mean threshold shifts for any subject. Threshold shift magnitudes increased with increasing noise sound exposure level (SEL) for two of the three subjects. The results underscore the importance of including sound exposure metrics (incorporating sound pressure level and exposure duration) in order to fully assess the effects of noise on marine mammal hearing.

Age‐related hearing loss in sea lions and their scientists

Interest in the hearing capabilities of California sea lions (Zalophus californianus) was first stimulated by the echolocation hypothesis and more recently by rising concern about coastal noise pollution. During a series of audiometric tests, we measured the absolute hearing sensitivity of two sea lions and two of their human investigators. Aerial hearing curves for each subject were obtained with a go/no‐go procedure and standard psychophysics. Additionally, underwater hearing curves were obtained for the sea lions using the same procedures. Underwater, the older sea lion (22–25 years of age) showed hearing losses relative to the younger sea lion (13–16 years) that ranged from 10 dB at lower frequencies to 50 dB near the upper frequency limit. The older sea lions’ hearing losses in air were consistent with those measured underwater. The older human (69 years) tested also showed losses relative to the younger human (22 years). These differences ranged from 15 dB at lower frequencies up to 35 dB at the hig...

Reliability of underwater hearing thresholds in pinnipeds

Repeated measures of low-frequency underwater hearing sensi- tivity in individuals of three pinniped species tested over 4-7 years are pre- sented. Despite changes in the experience of the subjects and certain testing parameters (e.g., equipment and research personnel), measured underwater hearing thresholds within subjects over relatively long periods of time were quite similar at the frequencies tested (0.2-6.4 kHz). These data address the reliability of acoustic signal detection measurements over time in pinnipeds using psychophysical techniques. They are also relevant in considering the cumulative effects of aging, experience, and noise exposure on pinniped hear- ing in certain frequency bands.

Noise‐induced temporary threshold shifts in pinnipeds: Effects of noise energy

Auditory pure‐tone thresholds were obtained in air and in water from three pinnipeds before and immediately after exposure to octave‐band noise. Noise exposure durations were 1.5, 12, 22, 25, or 50 min, and noise levels were 65, 80, or 95 dB referenced to each subjects pure‐tone threshold. In air and in water, pre‐ and postnoise thresholds were obtained at the center frequency of the octave band. In water, thresholds were also obtained at a frequency octave higher than the octave‐band center frequencies. Maximum threshold shifts for each species were about 15 dB in air and in water. Under all exposure conditions hearing sensitivity recovered within 24 h. In both media, a curvilinear function best predicted the magnitude of threshold shift from noise energy flux density. At TTS magnitudes greater than 10 dB, this function predicts a 2‐dB increase in threshold shift for a 1‐dB increase in noise energy, agreeing well with data collected from other mammals.

Temporal integration in a California sea lion and a harbor seal: Estimates of aerial auditory sensitivity as a function of signal duration

Stimulus durations shorter than some critical value result in elevated signal‐detection thresholds due to temporal integration (or temporal summation) properties of the auditory system. These properties are important from a theoretical perspective in terms of the trade‐offs of stimulus duration and intensity on sensitivity. From a methodological perspective, temporal integration is important because absolute detection thresholds measured using signal durations shorter than the temporal integration period may underestimate hearing sensitivity. In this study, aerial sound‐detection thresholds were estimated at 2500 and 3530 Hz in a California sea lion (Zalophus californianus) and a harbor seal (Phoca vitulina). Thresholds were measured for each frequency at seven stimulus durations ranging from 100 to 500 ms using behavioral psychophysics in a hemianechoic chamber. In general, thresholds increased as tone duration decreased for durations shorter than approximately 300 ms. For tone durations longer than 300 ...

Pinniped hearing and anthropogenic noise

Our behavioral studies of pinniped auditory processing, including amphibious hearing, auditory masking, and temporary threshold shift (TTS), have provided comparative data on hearing and the potential effects of anthropogenic noise for three pinniped species. Differences for one harbor seal between aerial thresholds measured using headphones in a noisy environment and those measured without headphones in a hemi‐anechoic chamber are quite large. While additional data are required, the preliminary data suggest that it will be necessary to re‐evaluate pinniped aerial hearing capabilities and their susceptibility to aerial anthropogenic noise impacts. Assessments of simultaneous (masking) and residual (TTS) effects of controlled noise exposure have also provided insight into the potential effects of anthopogenic noise on free‐ranging pinnipeds. Our underwater masking studies indicated relatively low critical ratios for low‐frequency tonal sounds, while underwater TTS studies demonstrated that auditory fatigue...

The Role of Learning in the Production and Comprehension of Auditory Signals by Pinnipeds

The aim of this paper is to discuss the important role that behavioral mechanisms, such as contingency learning and equivalence class formation, play in the production and comprehension of auditory signals in the context of mammalian social communication. Observations and experiments on vocal communication in mammals have often emphasized the importance of learning either from the perspective of the signaler or from the perspective of the receiver. It is our goal to discuss the roles and potential mechanisms of learning on both sides of communication. While marine mammals are notable in their capacity for complex learning in their vocal communication, until now, the major emphasis has been on the study of cetaceans. In the current paper, we focus primarily on the pinnipeds (seals, sea lions, and walruses) as a source for insight into how the learned aspects of auditory signaling and receiving may be acquired. We find that the results from carefully designed laboratory experiments can aid in the interpretation of field observations of communicative behavior. The complementary use of both of these approaches improves our understanding of the cognitive operations being carried out by animals in their natural environment.

Towards a predictive model of noise‐induced temporary threshold shift for an amphibious marine mammal, the California sea lion (Zalophuscalifornianus)

A California sea lion that had previously been tested under water was assessed for noise‐induced temporary threshold shift (TTS) in air. One hundred ninety‐two controlled exposures of octave‐band noise centered at 2.5 kHz were conducted over a 3‐year period. The noise was varied in level (to 133 dB SPL re: 20 μPa) and duration (to 50 min) to generate a variety of equal sound exposure levels (SELs). Behavioral psychophysics was used to measure hearing sensitivity at 2.5 kHz before, immediately following, and 24 h following noise exposure. The levels of threshold shifts obtained ranged up to 30 dB. In cases where TTS exceeded 20 dB, thresholds were obtained at regular intervals until recovery occurred. The average slope of the long‐term recovery function was 10 dB per log(minute). Results show that the threshold shifts correlated with SEL; however, the equal‐energy trading rule did not apply in all circumstances, with exposure duration contributing more than exposure level. Repeated testing showed no eviden...

Underwater hearing thresholds in pinnipeds measured over a 6‐year period

While absolute hearing thresholds have been obtained for some marine mammals, few published data are available on how measurements of individual auditory sensitivity may change over relatively long periods of time. Studies that have investigated temporal changes in sensitivity have typically focused on animals in which differences in hearing are anticipated (age‐related hearing loss). This study investigated the replicability of underwater hearing thresholds in prime‐aged individuals of three pinniped species over a 6‐year period. Aside from their age and experience with behavioral signal detection tasks, test subjects were of similar physical condition throughout this experiment. They were tested in the same enclosure at similar test frequencies (0.1–6.4 kHz) using identical methodology and criteria. Underwater hearing thresholds obtained throughout this testing period were not significantly different. These data indicate that underwater hearing sensitivity may remain relatively stable over long periods ...

How acoustic signals become meaningful to listeners: An experimental approach

Most models of animal acoustic communication describe how vocal cues produced by a signaler influence the behavior of a listener. The response made by a listener depends in large part on the perceived meaning of the signal. But, how do signals become meaningful to listeners? In some cases, such as imprinting, signal meaning can be attributed to structural cues that are perceived and acted upon through an innate releasing mechanism. In other instances, signals may be arbitrarily related to objects, individuals, or species. Equivalence theory provides a model describing how some arbitrary signals may acquire meaning. Here, we describe theory and experimental evidence in the form of cross‐modal matching‐to‐sample tasks showing how acoustic signals can become referents for visual stimuli. The subject of these behavioral experiments is a California sea lion with extensive experience in performing associative learning tasks. The aim of the experiments is to establish multiple auditory‐visual discriminations and...