Page created: 960624 Updated: 960624
Page maintainer: Wesley R. Elsberry.
Dolphins and whales are known to produce a wide variety of sounds. The sounds that an individual produces are determined in large part by the species of the individual. It is generally assumed that the range of hearing is a limit on the usual range of vocalizations produced. In the toothed whales, many species have been shown to be sensitive to ultrasonic sounds, with some sensitive to sound above 200KHz. The baleen whales are relatively more difficult to test, but the projections from models of inner ear anatomy indicate hearing from the infrasonic through most of the human audio range. In the bottlenose dolphin (Tursiops truncatus), vocalizations have been noted as "clicking, moans, whistles, trills, and squeaks". In the bioacoustics community, the large categories are whistles and pulse-based vocalizations. The names for the various sounds map into these two categories nicely. Different ranges of frequencies make whistle-based vocalizations sound like moans, whistles, trills, or squeaks. The clickings are pulse-based vocalizations.
The vocalizations of dolphins are assumed to have communication, navigation, and food location functions. Some researchers have advanced a "signature whistle" hypothesis, that states that certain vocalizations that are emitted by preference by individuals in a pod represent an acoustic "signature" for that individual.
Many people have asked, "What's the purpose of the dolphin sonar?"
The short answer is, "We don't know." There is a lot of conjecture about dolphin bisonar in the wild, but very little actual research on the subject. There is a lot of research on discrimination capabilities in experimental tests, but that does not tell us about the purpose of biosonar. Some people assert that biosonar is mostly used for food location and assessment, others that it is used mostly for navigation. Neither group can claim that they have the data in hand to support their assertions.
SONAR is an acronym for "Sound Navigation And Ranging". In animals, the terms that are used for active acoustic sensing are "echolocation" and "biosonar". Toothed whales in general are assumed to have biosonar capabilities, since all species tested so far do. In particular, the capabilities for discrimination of objects by dolphins has been extensively tested. While controversy continues over the functional anatomy of biosonar production and reception, it is generally believed that the biosonar sounds are produced by the nasal sacs, which are located behind the melon (the rounded region of a dolphin's forehead). The melon in various species is thought to act as an acoustic lens, to distribute or focus the sound in a particular manner. Theoretically, it would make sense for dolphins using biosonar to use lower frequencies for longer range navigation and higher frequencies for close range discrimination, because lower frequency sounds attenuate more slowly in water than do higher frequency sounds. However, it is difficult to measure biosonar activity in the wild due to several practical constraints: many biosonar pulses have energy in the ultrasonic range, to which common recording equipment is not sensitive; biosonar in dolphins is highly directional, requiring that the recording transducer be on-axis with the dolphin or very close to it; biosonar is a function, not a characteristic, so behavioral monitoring must accompany acoustic monitoring to establish that biosonar is the purpose of the recorded vocalization; and the usual problems involved with getting sophisticated electronics anywhere near large bodies of water. Not all pulse-based vocalization is biosonar, as some people have erroneously asserted. Au has recorded bottlenose dolphin biosonar in Kaneohe Bay in Hawaii as having a mean sound pressure level around 220 dB re 1 micropascal at 1 meter, and peak frequency around 130KHz. The human auditory range is canonically 20 Hz to 20KHz by comparison. The sound that a dolphin produces propagates outward and encounters objects in the water. The produced sound is high energy (as noted in bottlenose dolphins, about 220 dB re 1 micropascal at 1m), but the portion of sound that bounces off or reradiates from an object is very low energy. The time that the sound takes to propagate from the dolphin to the object and back gives the dolphin information about how far away the object is. Other information in the return echo tells about characteristics of the target. One controversial topic is whether dolphins form an "acoustic image" from the pattern of sounds returning from an object. Dolphins have been shown to be able to acoustically discriminate metal cylinders that differ by 1/3000th of an inch in diameter. What this means in terms of how they use echolocation in the wild is not yet known. There are differences between species in the characteristics of biosonar pulses. Bottlenose dolphins use broadband pulses (the energy is distributed over a wide frequncy range), but porpoises and the bouto use very narrowband pulses. What these differences mean for the purpose of biosonar are also unknown.
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