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.

With few exceptions, laboratory studies of auditory masking in marine mammals have been limited to examining detection thresholds for simple tonal signals embedded in broadband noise. However, detection of a sound has little adaptive advantage without the knowledge of what produced the sound (recognition) and where the sound originated (localization). In the current study, a bottlenose dolphin's masked detection thresholds (energetic masking) and masked recognition thresholds (informational masking) were estimated for a variety of complex signals including dolphin vocalizations, frequency modulated signals, and a 10 kHz pure tone. Broadband noise types included recordings of natural sounds and computer generated sounds. Detection thresholds were estimated using a standard go, no-go adaptive staircase procedure. The same dolphin learned to associate whistle-like FM sounds with specific arbitrary objects using a three alternative, matching-to-sample (MTS) procedure. The dolphin's performance in the MTS task was then tested in the presence of the same masking noise types used in the detection task. Recognition thresholds were, on average, about 4 dB higher than detection thresholds for similar signal-noise conditions. The 4 dB difference is likely due to additional cognitive demands of recognition, including attention and pattern recognition.

Bottlenose dolphins performing echolocation tasks at long ranges may utilize a transmission mode where bursts, or "packets," of echolocation clicks are emitted rather than single clicks. The clicks within each packet are separated by time intervals well below the two-way travel time, while the packets themselves are emitted at intervals greater than the two-way travel time. Packet use has been shown to increase with range; however, the exact function of packets and the advantages gained by their utilization remain unknown. In this study, the capability for dolphins to utilize multi-echo processing within packets of echoes was investigated by manipulating the number of available echoes within each packet as a dolphin performed a long-range echolocation task. The results showed an improvement in detectability with an increase in the number of echoes in each packet and suggest that packet use is an adaptation to allow multi-echo processing at long ranges without introducing range ambiguity.

Bottlenose dolphins (Tursiops truncatus) use the frequency contour of whistles produced by conspecifics for individual recognition. Here we tested a bottlenose dolphin’s (Tursiops truncatus) ability to recognize frequency modulated whistle-like sounds using a three alternative matching-to-sample paradigm. The dolphin was first trained to select a specific object (object A) in response to a specific sound (sound A) for a total of three object-sound associations. The sounds were then transformed by amplitude, duration, or frequency transposition while still preserving the frequency contour of each sound. For comparison purposes, 30 human participants completed an identical task with the same sounds, objects, and training procedure. The dolphin’s ability to correctly match objects to sounds was robust to changes in amplitude with only a minor decrement in performance for short durations. The dolphin failed to recognize sounds that were frequency transposed by plus or minus ½ octaves. Human participants demonstrated robust recognition with all acoustic transformations. The results indicate that this dolphin’s acoustic recognition of whistle-like sounds was constrained by absolute pitch. Unlike human speech, which varies considerably in average frequency, signature whistles are relatively stable in frequency, which may have selected for a whistle recognition system invariant to frequency transposition.

Dolphins are hypothesized to deduce the swimming direction of group members by attending to the spectral pattern of whistle harmonics. This is known as the direction of movement cue hypothesis and may facilitate coordination of complex group behavior when visibility is poor. The direction of movement cue hypothesis hinges on the assumption that dolphins can discriminate between whistles with different harmonic patterns that are associated with signaler orientation. This assumption was tested with a bottlenose dolphin. Whistles were recorded from a dolphin at different azimuth positions (0° to 180° in 45° increments). Noise-free, synthetic whistles were created to mimic the direction-dependant spectral profiles of the recorded whistles. A dolphin was then tested in its ability to discriminate between the synthetic whistles using fixed level and roving level conditions. The dolphin's discrimination performance in both the fixed and roving level conditions was near 100% for whistles separated by angles greater than 45°, and near chance for 45° separations. Computer simulations of the task, along with the dolphin's performance, suggest that the dolphin's discrimination was level invariant and based on the spectral pattern of the whistles.

In matched filter processing, a stored template of the emitted sonar pulse is compared to echoes to locate individual replicas of the emitted pulse embedded in the echo stream. A number of experiments with bats have suggested that bats utilize matched filter processing for target ranging, but not for target detection. For dolphins, the few available data suggest that dolphins do not utilize matched filter processing. In this study, the effect of time-reversing a dolphin's emitted click was investigated. If the dolphin relied upon matched filter processing, time-reversal of the click would be expected to reduce the correlation between the (unaltered) click and the echoes and therefore lower detection performance. Two bottlenose dolphins were trained to perform a phantom echo detection task. On a small percentage of trials ("probe trials"), a dolphin's emitted click was time-reversed before interacting with the phantom echo system. Data from the normal and time-reversed trials were then analyzed and compared. There were no significant differences in detection performance or click emissions between the normal and time-reversed conditions for either subject, suggesting that the dolphins did not utilize matched filter processing for this echo detection task.

Dolphins are social animals that rely heavily on passive and active acoustics for communication, navigation, foraging, and detecting predators. Auditory masking, from both natural and anthropogenic noise sources, may adversely affect these fitness-related capabilities. The dolphin's ability to detect a variety of complex signals (both dolphin phonations and tonal signals) masked by Gaussian, comodulated, snapping shrimp, and ice squeaks noise was tested. Detection thresholds were measured using a go/no-go adaptive staircase procedure. Masking patterns were similar for all signals (whistles, burst-pulse, and pure tones) except for click signals. Masking from ice squeaks resulted in the largest masked thresholds, while snapping shrimp and comodulated noise resulted in a release from masking relative to thresholds from Gaussian noise. Click signals were most difficult to detect when masked by snapping shrimp. Recognition thresholds were estimated for whistle-like signals using a cross-modal, matching-to-sample procedure. Recognition thresholds were on average 4 dB greater than detection thresholds for all noise types. The auditory mechanisms governing the results are discussed. [Work supported by the ONR.]

Whistle use by bottlenose dolphins serves several functions including individual identification, maintaining group cohesion, long-range communication, recruitment during feeding, and advertising emotional state. Whistles can vary considerably in amplitude, duration, and frequency. Despite this variability, dolphins must learn to associate specific whistle features (e.g., frequency contour) with meaningful events or objects, requiring plasticity in the dolphin’s recognition abilities. To test robust recognition, a bottlenose dolphin learned to associate whistle-like frequency contours with arbitrary objects. For example, whistle-A would cue the dolphin to touch object-A, and whistle-B would cue her to touch object-B. The whistles were then altered by varying amplitude, duration, and transposing frequency. Changes in amplitude and duration had little effect on recognition, however frequency transposition as small as 1/3 octave resulted in poor performance. The results are discussed from a comparative cognition perspective.