The acoustic environment of the bottlenose dolphin often consists of noise where energy across frequency regions is coherently modulated in time (e.g., ambient noise from snapping shrimp). However, most masking studies with dolphins have employed random Gaussian noise for estimating patterns of masked thresholds. The current study demonstrates a pattern of masking where temporally fluctuating comodulated noise produces lower masked thresholds (up to a 17 dB difference) compared to Gaussian noise of the same spectral density level. Noise possessing wide bandwidths, low temporal modulation rates, and across-frequency temporal envelope coherency resulted in lower masked thresholds, a phenomenon known as comodulation masking release. The results are consistent with a model where dolphins compare temporal envelope information across auditory filters to aid in signal detection. Furthermore, results suggest conventional models of masking derived from experiments using random Gaussian noise may not generalize well to environmental noise that dolphins actually encounter.

1Sound is the primary sensory cue for most marine mammals, and this is especially true for 2 cetaceans. To passively and actively acquire information about their environment, cetaceans 3 have perhaps the most derived ears of all mammals, capable of sophisticated, sensitive hearing 4 and auditory processing. These capabilities have developed for survival in an underwater world 5 where sound travels five times faster than in air, and where light is quickly attenuated and often 6 limited at depth, at night, and in murky waters. Cetacean auditory evolution has capitalized on 7 the ubiquity of sound cues and the efficiency of underwater acoustic communication. The sense 8 of hearing is central to cetacean sensory ecology, enabling vital behaviors such as locating prey, 9 detecting predators, identifying conspecifics, and navigating. Increasing levels of anthropogenic 10 ocean noise appears to influence many of these activities. 11Here we describe the historical progress of investigations on cetacean hearing, with a 12 particular focus on odontocetes and recent advancements. While this broad topic has been 13 studied for several centuries, new technologies in the last two decades have been leveraged to 14 improve our understanding of a wide range of taxa, including some of the most elusive species. 15 This paper addresses topics including how sounds are received, what sounds are detected, 16 hearing mechanisms for complex acoustic scenes, recent anatomy and physiology studies, the 17 potential impacts of noise, and mysticete hearing. We conclude by identifying emerging 18 research topics and areas which require greater focus. 19 20 3

The directional properties of bottlenose dolphin clicks, burst-pulse, and whistle signals were measured using a five element array, at horizontal angles of 0°, 45°, 90°, 135°, and 180° relative to a dolphin stationed on an underwater biteplate. Clicks and burst-pulse signals were highly directional with directivity indices of ~11 dB for both signal types. Higher frequencies and higher amplitudes dominated the forward, on-axis sound field. A similar result was found with whistles, where higher frequency harmonics had greater directivity indices than lower frequency harmonics. The results suggest the directional properties of these signals not only provide enhanced information to the sound producer (as in echolocation) but can provide valuable information to conspecific listeners during group coordination and socialization.

Previous measurements of toothed whale echolocation transmission beam patterns have utilized few hydrophones and have therefore been limited to fine angular resolution only near the principal axis or poor resolution over larger azimuthal ranges. In this study, a circular, horizontal planar array of 35 hydrophones was used to measure a dolphin's transmission beam pattern with 5° to 10° resolution at azimuths from -150° to +150°. Beam patterns and directivity indices were calculated from both the peak-peak sound pressure and the energy flux density. The emitted pulse became smaller in amplitude and progressively distorted as it was recorded farther off the principal axis. Beyond ±30° to 40°, the off-axis signal consisted of two distinct pulses whose difference in time of arrival increased with the absolute value of the azimuthal angle. A simple model suggests that the second pulse is best explained as a reflection from internal structures in the dolphin's head, and does not implicate the use of a second sound source. Click energy was also more directional at the higher source levels utilized at longer ranges, where the center frequency was elevated compared to that of the lower amplitude clicks used at shorter range.

Auditory masking occurs when one sound (usually called noise) interferes with the detection, discrimination, or recognition of another sound (usually called the signal). This interference can lead to detriments in a listener's ability to communicate, forage, and navigate. Most studies of auditory masking in marine mammals have been limited to detection thresholds of pure tones in Gaussian noise. Environmental noise marine mammals encounter is often more complex. In the current study, detection thresholds were estimated for bottlenose dolphins with a 10 kHz signal masked by natural, anthropogenic, and synthesized noise. Using a band-widening paradigm, detection thresholds exhibited a pattern where signal thresholds increased proportionally to bandwidth for narrow band noise. However, when noise bandwidth was greater than a critical band, masking patterns diverged. Subsequent experiments demonstrated that the auditory mechanisms responsible for the divergent masking patterns were related to across-channel comparison and within-valley listening.