Dolphin by Alan Rauch

Author:Alan Rauch
Language: eng
Format: epub
Publisher: Reaktion Books

An early publicity drawing from Marineland.

Human systems of echolocation date back to the early twentieth century, but the term SONAR (SOund NAvigation and Ranging) was coined during the Second World War as the military perfected it to target submarines. Echolocation in dolphins is a system by which animals emit sound waves that rebound off a ‘target’ object (such as a fish) in a pattern that is recognizable and decipherable to the animal. Most dolphins are capable of generating very high-frequency waves (at least 120,000 Hz), which because of their narrow wavelength can generate what we must assume are fairly precise images. Humans can hear up to 16,000 Hz (or cycles per second), which is less than one tenth of some of the sounds produced by dolphins. To employ the term ‘images’, however, is misleading, because it is not clear how the dolphins actually process the returning echoes. In medical sonograms, such as for pregnancy, the echoes reflecting off the foetus (or an organ) are translated into an image better suited to visually oriented humans. Typically, lower frequencies are used to generate these images, since high frequencies (with intense, rapid pulses per each millisecond) might damage the foetus. In fact, dolphins can actually employ their high-frequency sonar to stun schools of fish, which makes for less frantic chases and more convenient meals.

Although humans have very good hearing and rely heavily on sound to identify and even locate objects, it is still difficult to comprehend the refined use of sound in dolphins. A dolphin, whether swimming during the day or the evening, must always contend with limited light, water turbidity and murky conditions. Under all of these conditions, echolocation is used to penetrate and to ‘read’ through the water, observing (as it were) what is ahead of the animal and to determine, if necessary, the topography of the sea floor. Recent work by Whitlow Au has demonstrated that dolphins can detect, via echolocation, very small objects from a great distance and distinguish the density of objects presented to them. It is not surprising, then, that dolphins are excellent foragers of prey species that seek refuge under the sand (fish and crabs); although they disappear from sight, their form, shape and density are readily apparent to an echolocating dolphin. Even squid, whose bodies are roughly the density of water, betray themselves by having a dense beak at their core that, we must assume, can be detected from a substantial distance.

Dolphins may also have the ability to retain sonic images of each other, given that they have the capability to ‘read’ muscle from bone, an empty stomach from a full one and a pregnant uterus from a ‘normal’ or vacant one. All of this is enhanced by the fact that dolphins swim in water, which because of its density is an ideal medium for the transfer of sound waves. Bats, which also echolocate, face the additional obstacle of emitting sound in the air, which is a far less conductive medium for the transmission of sound.

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