Tooth Whales, called Odontocetes, (which include Dolphins and Sperm Whales) use sound
rather than sight to see their environment. In the illustration a dolphin is sending out sound
waves which are produced in the fatty cushion of the melon, while incoming echos bouncing off the fish, seem to be received and conveyed through the lower jaw.
CLICK HERE to view a FLASH animation demonstrating how a dolphin may echolocate to find food.
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Illustration: Genevieve Johnson
December 7, 2001
Seeing With Sound - Echolocation
This is Genevieve Johnson speaking to you from the Odyssey. While surveying the waters of Northern Australia, we have encountered the Indo-pacific Humpback dolphin, an inshore species we have not seen so far during the Voyage. Interestingly, we observe these animals in shallow, muddy water, yet they appear to navigate and hunt with perfect precision.
Although most mammals typically have good eyesight, cetaceans (whales) are an exception. Water is dense therefore light does not penetrate far beneath the surface.
Often, as is the case for the Indo-Pacific Humpback dolphin, the water is too cloudy for good visibility. So how do they navigate through their murky realm, communicate with one another as well as detect and capture prey?
To try and understand the perceptual world of cetaceans, it is necessary to imagine changing your primary sense from sight to sound. In this case, 'sound' images rather than 'visual' images are stored in the brain. Your sense of those around you, where you are and who you are with, are all determined by the sounds of others, or the sounds that you make.
All toothed whales, called Odontocetes, which include dolphins and Sperm Whales have developed the ability to 'see' with sound, by utilizing it as a kind of underwater 'acoustic flashlight'. Various species send out extremely powerful bursts of sounds like 'clicks', 'squeaks', 'whistles' and 'squeals', the type of sound emitted depends on whether they animals are communicating or foraging for food. Because sound propagates through water five times faster than on land, the waves will move through the water until they encounter an object that is of greater density than water. Once the waves hit an object, the sound will bounce back in the form of an echo.
Sound appears to be produced in the fatty cushion of the melon, while incoming echos seem to be received and conveyed through the lower jaw. It is then processed by the brain into information that allows the animal to 'see' it's surroundings including direction, speed, distance, shape, texture and density of objects.
If a human is in the water with an odontocete and that animal echolocates on them, scientists believe that the animal is capable of seeing inside the body. Returning echoes will differ depending on the density of the object they hit. To the animal, it is possible that the human body may look something like an x-ray.
Odontocetes have the ability to place outgoing clicks between the spaces of the returning echo, interpreting the unaltered sound waves. Likewise, the animal is capable of placing the echo so that it interferes with the outgoing clicks. Some scientists believe this is a way to judge location, direction and speed in a more accurate way.
Low sonar frequencies produced by odontocetes are designed to travel long distances, while high frequencies travel shorter distances, giving a more detailed picture of the surrounding environment. When toothed whales are travelling in the open ocean, they are able to utilize directional sound to focus on both near and far objects, by using multiple frequencies simultaneously. Dolphins are capable of switching frequencies in less than one-thousandth of a second, clicking at a rate of 300 per second, while sperm whales average 2 clicks a second while foraging.
The Indo-Pacific Humpack Dolphin is a species that lives close to shore.
As a result, they are often vulnerable to many human pressures.
Photo: Rebecca Clark
Cetaceans such as sperm whales that inhabit the clear waters of the open ocean probably use echolocation quite differently when compared to an inshore species such as the Indo-pacific humpback dolphin that generally inhabits tidal flats and muddy harbours. The echolocation clicks of sperm whales are intense and mainly low pitched for long range, deepwater exploration. While cetaceans such as the Amazon River dolphin and the Indo-Pacific Humpback dolphin, use faint, rapid, high-pitched clicks suited to the muddy bottom of their respective habitats.
In many parts of the world, inshore dolphin species such as the Indo-Pacific humpback and other inshore and river dolphins are suffering. Their preferred habitat which often includes crowded ports, means these animals are fighting to survive amongst filth and toxic waste, noise pollution, entanglements in fishing gear and boat traffic, while competing with humans for declining fish stocks. Whales did not evolve the ability to echolocate with such obstacles in mind, and many species around the world are suffering the consequences.
Dr. Peter Warshall discusses the marvels of echolocation in the book 'Mind in the Waters.'
"While using their sonar to hunt, dolphins and Orcas can simultaneously communicate their position in the hunt, their relation to their neighbor and their closeness to the fish. The criss-crossing sounds of a group of hunting cetaceans are like a multi-directional space-time map of the movements of the entire group. This map can be made without the necessity of extra signals. Further, since most sonar hunting sounds are above the hearing range of fish, the exchange of information among dolphins takes place without alarming the prey."
- "Mind in the Waters" - Joan McIntyre.
The Yolla Bolly Press, 1974
- "Cetacean Societies, Field Studies of Dolphins and Whales" - Mann, Connor, Tyack and Whitehead.
The University of Chicago Press, 2000.
Log by Genevieve Johnson