4
Communication and Acoustic
Behavior of Dolphins and Whales
PETER L. 'TYACK and CHRISTOPHER W. CLARK
1. Introduction
About 70 million years ago, the terrestrial ancestors of whales and dolphins
reentered the ocean where life originally began. Not only did this require
dramatic shifts in locomotion for swimming and in respiration for diving,
but the ocean also presented a very different sensory environment. The
explosive way in which cetaceans breathe reduced the usefulness of olfaction, which has limited utility underwater. Light propagates great distances
rapidly in air, which makes vision particularly useful for sensing distant
objects on land or in air, but light does not propagate well in water. Few
objects can be seen underwater at ranges of more than a few tens of meters.
By contrast, sound travels particularly well underwater. The potential for
the acoustic modality to sense distant sources of sound is highlighted by
recent discoveries that we can detect low-frequency calls of whales at
ranges of hundreds and sometimes thousands of kilometers (Costa 1993;
Clark 1994b, 1995).
Early human mariners relied upon their normal terrestrial senses, especially vision, to orient and communicate at sea. Humans are not particularly
good at sensing magnetic fields as weak as that of the earth, so the early
technological development of the compass greatly enhanced navigation
out of sight of land. During the past century, as the range of technological
options available to mariners has increased, seafaring humans have come
to appreciate and rely upon acoustic methods for orientation, communication, and location. We cannot see far below the sea surface, but we can use
ship-based sonars to find the sea floor, fish, and submerged objects such as
submarines or sunken ships. Humans took advantage of long-range sound
propagation in the sea as early as World War II. In order to find and rescue
the crew of a plane shot down at sea, aircraft were equipped with a special
explosive charge rigged to sink and explode if the plane sank. Listening
posts were manned round the clock, and the location of any downed plane
could be estimated by timing when the sound of the explosion reached
different posts hundreds or even thousands of miles away (Urick 1983).
156
Communication and Acoustic
Behavior of Dolphins and Whales
PETER L. 'TYACK and CHRISTOPHER W. CLARK
1. Introduction
About 70 million years ago, the terrestrial ancestors of whales and dolphins
reentered the ocean where life originally began. Not only did this require
dramatic shifts in locomotion for swimming and in respiration for diving,
but the ocean also presented a very different sensory environment. The
explosive way in which cetaceans breathe reduced the usefulness of olfaction, which has limited utility underwater. Light propagates great distances
rapidly in air, which makes vision particularly useful for sensing distant
objects on land or in air, but light does not propagate well in water. Few
objects can be seen underwater at ranges of more than a few tens of meters.
By contrast, sound travels particularly well underwater. The potential for
the acoustic modality to sense distant sources of sound is highlighted by
recent discoveries that we can detect low-frequency calls of whales at
ranges of hundreds and sometimes thousands of kilometers (Costa 1993;
Clark 1994b, 1995).
Early human mariners relied upon their normal terrestrial senses, especially vision, to orient and communicate at sea. Humans are not particularly
good at sensing magnetic fields as weak as that of the earth, so the early
technological development of the compass greatly enhanced navigation
out of sight of land. During the past century, as the range of technological
options available to mariners has increased, seafaring humans have come
to appreciate and rely upon acoustic methods for orientation, communication, and location. We cannot see far below the sea surface, but we can use
ship-based sonars to find the sea floor, fish, and submerged objects such as
submarines or sunken ships. Humans took advantage of long-range sound
propagation in the sea as early as World War II. In order to find and rescue
the crew of a plane shot down at sea, aircraft were equipped with a special
explosive charge rigged to sink and explode if the plane sank. Listening
posts were manned round the clock, and the location of any downed plane
could be estimated by timing when the sound of the explosion reached
different posts hundreds or even thousands of miles away (Urick 1983).
156