Seeing without eyes?

If you were a dolphin, and you lived in the murky waters of river deltas, or the blackness of the deep ocean, your eyes would be useless. You wouldn’t find food, you wouldn’t even find your fellow dolphins. Dolphin echo location has evolved to take over where eyes fail.

“Dolphins (and other toothed whales) can produce high pitched clicks. When these clicks hit an object, some of the sound will echo back to the ‘sender’. By listening to the echo and interpreting the time it took before the echo came back, the dolphin estimate the distance of the object.”*

With echo location, a dolphin can locate an object. “Depending on the material the object is made of, part of the sound may penetrate into the object and reflect off internal structure. If the object is a fish, some sound will reflect off the skin on the dolphins side, some of the bones, the internal organs and the skin on the other side. So one click can result in a number of weaker echoes. This will give dolphins some information about the structure and size of the fish. By moving its head (thereby aiming the clicks at other parts of the fish) the dolphin can get more information on other parts of the fish.”

“It is like a medical ultrasound probe, but the results are far less clear. A medical probe moves back and forth very rapidly, much faster than a dolphin can move its head. Also the frequency of the sounds of the medical probe is much higher than a dolphins sonar. Therefore the level of detail the echoes can provide is much higher in the medical probe.”

Can dolphins ‘see’ with their sonar?

“Just like people can visualize an object by just touching it, dolphins can get an idea of what an object looks like by scanning it with their sonar. They can also identify objects with their sonar that they have only been able to see.

“[Whether] they form a visual picture from the sonar information (visualization) or form an acoustical picture from visual information is still unresolved. This capability is called cross-modal transfer and it has been demonstrated in only a few animal species so far: bottlenose dolphins and the California sea lions. “

See the following references for more details on this subject.

R.J. Schusterman, D. Kastak and C. Reichmuth (1995) Equivalence class formation and cross-modal transfer: testing marine mammals.
R.A. Kastelein, J.A. Thomas and P.E. Nachtigall (eds): Sensory systems of Aquatic Mammals, pp. 579-584 De Spil Publishers, Woerden, the Netherlands ISBN 90-72743-05-9
A.A. Pack and L.M. Herman (1995) Sensory integration in the bottlenosed dolphin: Immediate recognition of complex shapes across the senses of echolocation and vision
J. Acoustical Society of America 98(2) Part 1: 722-733
For technical information on dolphinsonar, check out the following book: W.W.L.Au (1993) The sonar of dolphins. (Springer-Verlag New York).

*Quoted sections reprinted from “Dolphins FAQ” by Jaap van der Toorn (jaap@compuserve.com) at the alt.animals.dolphins newsgroup

For samples of these and other sounds, check out our LISTEN page.