August 9, 2001
How Do Sperm Whales Produce Sound?
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This is Genevieve Johnson speaking to you from the Odyssey in Papua New Guinea.
Sperm whales are highly acoustic animals that emit powerful, regular clicks almost continuously while they are underwater. In large males, up to one third of the entire body length is made up of the huge nose, the world's largest biological sound generator.
The incredibly successful evolution of sperm whales as acoustic specialists is due to the fact that sound propagates through water more effectively than any other form of energy. Indeed, sound travels through water five times faster than it does in air. We know that the sperm whale spends its life immersed in sound. It is believed that they use sound to navigate through the depths, to find prey and to communicate with one another.
So how does the sperm whale use its enormous nose to produce the sounds that appear to be paramount to its success as the world's largest predator?
When the sperm whale dives, the air remains locked in a closed system made up of the nasal passages and air sacs, where it is assumed it can be recycled. There are two nasal passages inside the nose; the left nasal passage is mainly used in respiration, while the right nasal passage is mainly used in the production of sound. Air is forced through the right nasal passage under pressure and out through two black lips, half a meter in diameter, called the 'monkey lips'. It is believed that the sound is produced when the pressurized air is forced between these lips. The main part of the sound energy produced by the lips is reflected off the distal air sac, it then propagates back through the spermaceti organ, to the frontal air sac attached to the bony crest of the skull. The sound is then reflected off the frontal air sac, where it is propagated forward through the junk and out into the water, presumably producing directional sound in the form of clicks. It is proposed that the oil-filled bodies in the junk may have a collimating function, i.e: making a directional beam of the highest sound pressure in the animal kingdom. The spermaceti organ and the spermaceti bodies of the junk are sacs filled with wax esters that conduct sound at approximately the same speed as water.
The spermaceti organ is a unique feature of the sperm whale, were as the junk is homologous with the melon found in all odontocetes or toothed whales, where it is used as an adapter between the sound producing tissues and the water.
A Sperm Whale
Photo: Chris Johnson
So why has this sophisticated system of sound production evolved?
It is believed that a directional sound beam and thereby a much higher sound pressure can be achieved when the sound is processed through the acoustic mechanisms of the nose, resulting in a finite amount of sound energy being produced directionally, the animal is able to use this sound beam as a kind of 'acoustic flashlight' which is very effective for sonar purposes.
This theory of sound production in the nasal complex of the sperm whale was originally proposed by Norris and Harvey in 1972. The theory presented here
is the modified version of the Norris and Harvey theory, proposed by Bertel
Moehl in 1978.
Recent research in 1998 off Northern Norway led by Dr. Bertel Moehl with the collaboration of Magnus Wahlberg and Peter T. Madsen from the department of Zoophysiology, at the University of Aarhus, has shown that sperm whales produce the highest sound pressure levels in the animal kingdom. They have also shown that the clicks are directional, thereby justifying the idea of the nasal complex as the sound producing organ involved in biosonar, as well as meriting the enormous size of it.
Log by Peter Teglberg Madsen & Genevieve Johnson