The Noisy World of Dolphins
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In the seawater world of the dolphin, sound is the very best way to communicate or to learn about the surroundings —obstacles or prey or predators.
Scientists have studied two kinds of sounds that are a big part of dolphin life. One kind is a whistle, usually a few seconds long and in many different patterns. Among its many whistles, each dolphin has a special pattern—like a signature—that it uses to tell others where it is.
A very different dolphin sound is the click. That’s a sharp burst less than one thousandth of a second long. It is mostly ultrasonic (with a pitch too high for human ears) and used for sonar. By making that loud click and listening to the echoes, a dolphin can find out a lot about what’s out there. That works especially well in water, where sound travels about five times faster than it does through air.
An echo may contain a lot of information. The direction of the echo tells the direction of an object that reflected the sound. The time delay tells about the distance the click traveled plus the distance for the echo to travel back. And the details of the echo may tell about what kind of object reflected it.
Dolphins are so good at using their sonar that lots of studies have been designed to find out how they do it.
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| Is an object there? The blindfolded dolphin wins this game (and receives a fish to eat) if it presses the correct paddle—“yes” or “no.” Scientists are using the game to learn about dolphins’ sonar. |
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Sonar Game
As in most studies of animal abilities, scientists first
teach the animal a game that it can play over and over.
For dolphins, the game usually is designed to tell about
how they use sonar. The drawing at right shows a dolphin
in a floating playpen specially designed for the game.
The best way to understand the game is to imagine that you are playing it yourself. You be the dolphin, and I’ll be the trainer helping with the game.
You are blindfolded by rubber eyecups so that you can tell what’s around you only through your ears. You have been trained to start in a spec
ial position with your head in a hoop, as shown in the diagram. A game trial starts when I pull a string and lower a sound-blocking screen out of the way. That’s your cue to start making clicks and listening to their echoes.
Detecting
the Target
Today’s target is a four-inch steel ball, but sometimes
it may be much smaller. It may be hung at some measured
distance in front of you. (In the diagram it is about 20
feet away.) My job is to control the target. It is suspended
by fishing line so I can pull it up out of the way. The
target is either there or not there. Your job is to use
your sonar to tell which.
If you hear an echo from the target, you swim up and push the right paddle to say “It’s there.” If you can’t hear any echo telling about the target, you swim up and push the left paddle to say “No, it’s not there.”
If your choice was correct, I press a buzzer that tells you to come up for a snack. (For a dolphin, that’s a tasty fish.) If you have made the wrong choice, there is no buzzer and no reward. You just swim back to the hoop and get ready for the next trial.
At
first the game seems too easy. You can always hear echoes
when the steel ball is there, and you never make a mistake
when the target is not there.
The game gets harder as I move the target farther away and
the echoes become weaker. Then you will begin making mistakes.
I move the steel ball way out to 230 feet—about three-fourths the length of a football field. At that distance, you can detect it only nine times in every ten trials. Now, every extra small distance makes the echoes harder to hear. At 240 feet, you are correct in your echo detection only about five times in every ten trials.
Mysteries
of Sonar
Our game was taken from a book by Dr. Whitlow Au. He played
the game for real with a dolphin named Sven in Kaneohe Bay,
Hawaii. The diagram I used was taken from one of his experiments.
From the results of the game, you can see that a dolphin
can easily find a table-tennis ball in a big, Olympic-size
swimming pool.
Dr. Au has gone on to do a whole book full of experiments
on dolphin sonar. He trained dolphins to listen to echoes
from a standard target and then tell when some other target
was used instead. He has used targets of different shapes
and different materials.
He is still searching for the dolphins’ secret: How does a dolphin use echoes to learn if an object is round or flat, rough or smooth, and hard or soft?
Dr. Au has said: “The dolphin’s ability to discriminate and recognize features of targets with its sonar is a characteristic that man-made sonar systems do not possess.” By studying dolphins he hopes to make man-made sonar as good as theirs.
