Bottlednosed Dolphins
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Bottlenose dolphins are among the most vocal of the nonhuman animals and exhibit remarkable development of the sound production and auditory mechanisms. This can be seen in audition, which is shown in the animals highly refined echolocation ability, and in tightly organized schools in which they live that are made up by sound communication. In testing the communication skills of dolphins, extensive studies have been done on vocal mimicry, in which the animal imitates computer-generated sounds in order to test motor control in terms of cognitive ability. Language comprehension on the other hand has been tested through labeling of objects, which has proven to be successful regarding the association of sound and object stimulus. The biggest question in dolphin communication is whether or not the species is capable of intentional communicative acts. Though results from studies have been debatable, the key to understanding the extent to this ÐŽ§languageÐŽÐ is to determine whether they have a repertoire of grammatical rules that generate organized sequences. In determining this, the greatest accomplishment for both the scientist and all of humanity, would be to accomplish interspecies communication, creating a bridge between humans and animals which could open up a new understanding of the unknown world of wildlife. Most importantly, it is necessary to understand the incredible aptitude of dolphin communicative skills, and the impressive intelligence the animal possesses which allows for a great deal of intraspecies and interspecies communication (Schusterman, Thomas, & Wood, 1986). The acoustical reception and processing abilities of the bottlenosed dolphins have generally been shown to be among the most sophisticated of any animal so far examined (Popper, 1980 as cited by Schusterman et al. 1986).
In order to understand the complexity of these highly mechanized acoustic systems, it is necessary to learn the process for which the dolphin hears. In most water-adapted cetaceans, tissue conduction is the primary route of sound conduction to the middle ear. The isolation of the bullae shows an adaptation for tissue-conducted sound. The lower jaw contains fat that is closely associated with the impedance of seawater. The lower jawbone of most odontocetes becomes broadened and quite thin posteriorly, and the fat forms an oval shape that closely corresponds to the area of minimum thickness of the jaw. This fat body leads directly to the bulla, producing a sound path to the ear structures located deep within the head. Paired and single air sacs are scattered throughout the skull, which serve to channel these tissue-conducted sounds (Popov & Supin, 1991). Other than this description, there are still more studies needed to determine the function of the middle ear and the type of bone conduction that occurs within the bulla. Due to detailed audiograms, dolphins have been shown to have the ability to detect high-frequency sounds. In an experiment by Johnson (1966) as cited in Schusterman et al. (1986), sine-wave sounds ranging in frequency from 75 Hz to 150 Hz were presented to a bottle-nosed dolphin. The animal was trained to swim in a stationary area within a stall and to watch for a light to come on. Following the light presentation a sound was sometimes presented. If the dolphin heard the sound, its task was to leave the area and push a lever. Sound intensity levels were varied by a staircase method of 1, 2, or 3 dB steps. The resulting audiogram, compared to the human aerial audiogram, showed that at regions of best sensitivity for each, thresholds for human and dolphin are quite similar, but separated by about 50 kHz in frequency, showing that the animals inner ear function is very similar to a human. The experiments done on dolphin auditory functions have generally shown a finely adapted sound reception system. This would be expected due to the highly adapted echolocation ability of the bottlenosed dolphin and other cetaceans. Results of work on absolute thresholds, critical bandwidths, frequency discrimination, and sound localization all indicate that the dolphin auditory system is at least as good as or better than the human system. This is in spite of the fact that sounds travels five times as fast under water as it does in air (Popov et al. 1991). The bottlenosed dolphin in captivity produces two categories of vocalizations: (a) narrow-band, frequency-varying, continuous tonal sounds referred to as ÐŽ§whistlesÐŽÐ and (b) broad-band pulsed sounds expressed as trains of very short duration clicks of varying rates (Evans, 1967, as cited in Schusterman et al. 1986). The pulsed sounds are used for both communication and echolocation, and the whistles are found to be used primarily for communication (Herman & Tavolga, 1980, as cited in Schusterman et al. 1986). Descriptions in literature emphasizing either the whistles or the pulsed sounds have led to contradictory hypotheses concerning the communication system of the dolphin. It has been reported that individually specific whistles often make up over 90% of the whistle repertoire of captive bottlenosed dolphins (Popov et al. 1991). A number of observations of apparent vocal mimicry have been made, though with no systematic investigation of the degree of vocal flexibility. The observed variability in the whistles, combined with the difficulty of identifying individual vocalizing dolphins in a group, has led to speculation that the whistles might be a complex, shared system, in which specific meanings could be assigned to specific whistles. Consideration of vocal mimicry has been taken to understand its relation to cognitive complexity, and to the potential use of vocal response for communication in an artificial language. In one study done by McCowan, Hanser, & Doyle, (1999), the dolphin was able to learn to mimic a number of computer-generated model sounds with high fidelity and reliability. The dolphin using its whistle mode of vocalization imitated all of the sounds, and all were distinct from the unreinforced whistles produced prior to training. The large majority of each dolphins whistle vocalizations were individually specific acoustic patterns, described as a ÐŽ§signature whistleÐŽÐ; the rest of the whistles were short chirps. The results of the mimicry training have shown that dolphins can mimic tonal sounds with frequencies between 4 and 20 Hz. Due to this research, scientists can now learn from these mimicry skills how to understand and develop natural communication based on a stronger emphasis on the animals cognitive abilities (Brecht, 1993). In object labeling, the dolphins seemed to understand the task of associating model sounds with displayed objects. Progress was most rapid when the model sound was always presented at full intensity, but the probability of its being presented on any given trial was systematically decreased over successive trials.