Atlantic bottlenose dolphin (Tursiops truncatus) hearing threshold for brief broadband signals

The hearing sensitivity of an Atlantic bottlenose dolphin (Tursiops truncatus) to both pure tones and broadband signals simulating echoes from a 7.62-cm water-filled sphere was measured. Pure tones with frequencies between 40 and 140 kHz in increments of 20 kHz were measured along with broadband thresholds using a stimulus with a center frequency of 97.3 kHz and 88.2 kHz. The pure-tone thresholds were compared with the broadband thresholds by converting the pure-tone threshold intensity to energy flux density. The results indicated that dolphins can detect broadband signals slightly better than a pure-tone signal. The broadband results suggest that an echolocating bottlenose dolphin should be able to detect a 7.62-cm diameter water-filled sphere out to a range of 178 m in a quiet environment.     

Cardiac responses to acoustic playback experiments in the captive bottlenose dolphin (Tursiops truncatus)

Acoustic recordings were used to investigate the cardiac responses of a captive dolphin (Tursiops truncatus) to sound playback stimuli. A suction-cup hydrophone placed on the ventral midline of the dolphin produced a continuous heartbeat signal while the dolphin was submerged. Heartbeats were timed by applying a matched-filter to the phonocardiogram. Significant heart rate accelerations were observed in response to playback stimuli involving conspecific vocalizations compared with baseline rates or tank noise playbacks. This method documents that objective psychophysiological measures can be obtained for physically unrestrained cetaceans. In addition, the results are the 1st to show cardiac responses to acoustic stimuli from a cetacean at depth. Preliminary evidence suggests that the cardiac response patterns of dolphins are consistent with the physiological defense and startle responses in terrestrial mammals and birds.     

Prosociality and reciprocity in bottlenose dolphins (Tursiops truncatus)

Animal Cognition - Some moral behaviours, often regarded as reflecting high cognitive abilities (such as empathy, cooperation, targeted helping) are known to only be present in very few species,...     

Spontaneous pointing by bottlenose dolphins (Tursiops truncatus)

Two bottlenose dolphins (Tursiops truncatus) participating in a symbolic communication project spontaneously developed behaviors that resembled pointing and gaze alternation. The dolphins' behavior demonstrated several features reminiscent of referential communicative behavior. It was triadic, involving a signaler, receiver, and referent. It was also indicative, specifying a focus of attention. The dolphins' points were distinct from the act of attending to or acting on objects. Spontaneous dolphin pointing was influenced by the presence of a potential receiver, and the distance between that receiver and the dolphin. These findings suggest that dolphins are capable of producing referential gestures. Accepted after revision: 14 August 2001 Electronic Publication     

Matching-to-sample by an echolocating dolphin (Tursiops truncatus)

An adult male dolphin was trained to perform a three-alternative delayed matching-to-sample task while wearing eyecups to occlude its vision. Sample and comparison stimuli consisted of a small and a large PVC plastic tube, a water-filled stainless steel sphere, and a solid aluminum cone. Stimuli were presented under water and the dolphin was allowed to identify the stimuli through echolocation. The echolocation clicks emitted by the dolphin to each sample and each comparison stimulus were recorded and analyzed. Over 48 sessions of testing, choice accuracy averaged 94.5% correct. This high level of accuracy was apparently achieved by varying the number of echolocation clicks emitted to various stimuli. Performance appeared to reflect a preexperimental stereotyped search pattern that dictated the order in which comparison items were examined and a complex sequential-sampling decision process. A model for the dolphin's decision-making processes is described.     

Cooperation increases bottlenose dolphins’ (Tursiops truncatus) social affiliation

Animal Cognition - Dolphins live in a fission–fusion society, where strong social bonds and alliances can last for decades. However, the mechanism that allows dolphins to form such strong...     

The ability to recognize objects from bottlenose dolphin (Tursiops truncatus) echoes generalizes across multiple orientations in humans and neural networks

Object constancy, the ability to recognize objects despite changes in orientation, has not been well studied in the auditory modality. Dolphins use echolocation for object recognition, and objects ensonified by dolphins produce echoes that can vary significantly as a function of orientation. In this experiment, human listeners had to classify echoes from objects varying in material, shape, and size that were ensonified with dolphin signals. Participants were trained to discriminate among the objects using an 18-echo stimulus from a 10° range of aspect angles, then tested with novel aspect angles across a 60° range. Participants were typically successful recognizing the objects at all angles (M = 78 %). Artificial neural networks were trained and tested with the same stimuli with the purpose of identifying acoustic cues that enable object recognition. A multilayer perceptron performed similarly to the humans and revealed that recognition was enabled by both the amplitude and frequency of echoes, as well as the temporal dynamics of these features over the course of echo trains. These results provide insight into representational processes underlying echoic recognition in dolphins and suggest that object constancy perceived through the auditory modality is likely to parallel what has been found in the visual domain in studies with both humans and animals.     

Bubble ring play of bottlenose dolphins (Tursiops truncatus): implications for cognition

Research on the cognitive capacities of dolphins and other cetaceans (whales and porpoises) has importance for the study of comparative cognition, particularly with other large-brained social mammals, such as primates. One of the areas in which cetaceans can be compared with primates is that of object manipulation and physical causality, for which there is an abundant body of literature in primates. The authors supplemented qualitative observations with statistical methods to examine playful bouts of underwater bubble ring production and manipulation in 4 juvenile male captive bottlenose dolphins (Tursiops truncatus). The results are consistent with the hypothesis that dolphins monitor the quality of their bubble rings and anticipate their actions during bubble ring play.     

Coordinated breathing in bottlenose dolphins (Tursiops truncatus) as cooperation: integrating proximate and ultimate explanations

In this study, coordinated breathing was studied in 13 common bottlenose dolphins because of its links with spontaneous coordinated behaviors (e.g., swimming, foraging, and playing). A strong link was shown between dyadic coordination levels and age/sex categories when both association patterns and spatial formation are considered. This is consistent with a significant influence of social relationships on cooperating and contrasts with an economic perspective based on immediate material outcomes alone. This cooperation bias is explained by linking proximate processes that evoke performance with ultimate evolutionary processes driven by long-term adaptive outcomes. Proximate processes can include 2 kinds of immediate outcomes: material reinforcements and affective states associated with acts of cooperating that can provide positive reinforcement regardless of immediate material benefits (e.g., when there is a time lag between cooperative acts and material outcomes). Affective states can then be adaptive by strengthening social relationships that lead to eventual gains in fitness.     

Whistle rates of wild bottlenose dolphins (Tursiops truncatus): influences of group size and behavior

In large social groups acoustic communication signals are prone to signal masking by conspecific sounds. Bottlenose dolphins (Tursiops truncatus) use highly distinctive signature whistles that counter masking effects. However, they can be found in very large groups where masking by conspecific sounds may become unavoidable. In this study we used passive acoustic localization to investigate how whistle rates of wild bottlenose dolphins change in relation to group size and behavioral context. We found that individual whistle rates decreased when group sizes got larger. Dolphins displayed higher whistle rates in contexts when group members were more dispersed as in socializing and in nonpolarized movement than during coordinated surface travel. Using acoustic localization showed that many whistles were produced by groups nearby and not by our focal group. Thus, previous studies based on single hydrophone recordings may have been overestimating whistle rates. Our results show that although bottlenose dolphins whistle more in social situations they also decrease vocal output in large groups where the potential for signal masking by other dolphin whistles increases.     

Understanding of the concept of numerically "less" by bottlenose dolphins (Tursiops truncatus)

In 2 experiments, bottlenose dolphins (Tursiops truncatus) judged the ordinal relationship between novel numerosities. The dolphins were first trained to choose the exemplar with the fewer number of items when presented with just a few specific comparisons (e.g., 2 vs. 6, 1 vs. 3, and 3 vs. 7). Generalization of this rule was then tested by presenting the dolphins with all possible pairwise comparisons between 1 and 8. The dolphins chose the exemplar with the fewer number of items at levels far above chance, showing that they could recognize and represent numerosities on an ordinal scale. Their pattern of errors was consistent with the idea of an underlying analog magnitude representation.     

Comprehension of signs by dolphins (Tursiops truncatus)

The authors assessed the ability of 6 captive dolphins (Tursiops truncatus) to comprehend without explicit training 3 human communicative signs (pointing, directed gaze, and replica). Pointing consisted of indicating the target item with the index finger and a fully extended arm. Directed gaze consisted of orienting the head and eyes toward the target item while the rest of the body remained stationary. The replica signal consisted of holding up an exact duplicate of the target item. On the initial series of 12 trials for each condition, 3 dolphins performed above chance on pointing, 2 on gaze, and none for replica. With additional trials, above chance performance increased to 4 dolphins for pointing, 6 for gazing, and 2 for replica. The replica sign seemed to be the most taxing for them (only 2 dolphins achieved results significantly above chance). Taken together, these results indicate that dolphins are able to interpret untrained communicative signs successfully.     

Bottlenose dolphin (Tursiops truncatus) calves appear to model their signature whistles on the signature whistles of community members

Bottlenose dolphins are unusual among non-human mammals in their ability to learn new sounds. This study investigates the importance of vocal learning in the development of dolphin signature whistles and the influence of social interactions on that process. We used focal animal behavioral follows to observe six calves in Sarasota Bay, Fla., recording their social associations during their first summer, and their signature whistles during their second. The signature whistles of five calves were determined. Using dynamic time warping (DTW) of frequency contours, the calves signature whistles were compared to the signature whistles of several sets of dolphins: their own associates, the other calves associates, Tampa Bay dolphins, and captive dolphins. Whistles were considered similar if their DTW similarity score was greater than those of 95% of the whistle comparisons. Association was defined primarily in terms of time within 50 m of the mother/calf pair. On average, there were six dolphins with signature whistles similar to the signature whistles of each of the calves. These were significantly more likely to be Sarasota Bay resident dolphins than non-Sarasota dolphins, and (though not significantly) more likely to be dolphins that were within 50 m of the mother and calf less than 5% of the time. These results suggest that calves may model their signature whistles on the signature whistles of members of their community, possibly community members with whom they associate only rarely.     

Inter-group alliance dynamics in Indo-Pacific bottlenose dolphins (Tursiops aduncus)

Animal Cognition - The social intelligence hypothesis holds that complex social relationships are the major selective force underlying the evolution of large brain size and intelligence. Complex...     

Acquired equivalences between auditory stimuli in dolphins (Tursiops truncatus)

This study investigated whether dolphins would show evidence of equivalence class formation between auditory stimuli. Bottlenose dolphins were trained to press one or other of two response levers depending on which one of four auditory stimuli had been previously presented. Once they had learned the initial discriminations, the stimulus-lever contingencies was repeatedly reversed. Within any given session, however, pressing of one lever always led to reward with one set of two tones and pressing the other lever led to non-reward with an alternative set of two tones. After sufficient experience with this response reversal procedure, the dolphins spontaneously chose the same levers they had first learned to be correct with one of the across-set stimulus pairs when later in the session they were presented with the other of the across-set stimulus pairs. They thus demonstrated that they had associated the tones belonging to the two sets within two separate functional classes. It is discussed why the dolphins succeeded with auditory stimuli when they had previously failed in a similar task with visual stimuli. Received: 7 December 1999 / Accepted after revision: 23 June 2000