Posted 5 Mar 1998
IN THE MURKY undersea world, light waves don't travel far. Sound carries far more information. Dolphins and whales navigate and communicate with a complicated system of sonar and signals. Nuclear submarines have elaborate listening systems. But how do sea turtles use hearing to sense their world?

Nobody knows, but a team at the New England Aquarium in Boston is trying to find out. They're not trying to crack turtle language -- if such a thing exists, it must be extremely subtle. Instead, they're working at the most basic level, trying to learn exactly which frequencies a turtle can hear.

The results could help in the conservation of the six sea turtle species, all of which are classified as endangered or threatened.

For the past year or so, the aquarians have been trying to train Myrtle, a fifty-something green sea turtle, to respond reliably to sounds in her environment. Green sea turtles live in the tropics worldwide.

The effort to learn about Myrtle's hearing has started with trial and error. "Nobody's done it -- it's quite an adventure," says Kathy Streeter, who has 24 years experience training marine mammals. "She's very curious, easy to work with. She's more tuned in to things than we ever anticipated."

Anybody got the recipe?
The program uses positive reinforcement that would be familiar to anybody who has trained a dog: Myrtle gets fed every time she responds to the training sound by pressing a button. But the trainers don't wheedle, "Come here, Myrtle, you pretty turtle, it's time for supper." Instead, they clang a pair of pipes in her tank.

If she gives the right signal in return, she's fed.

How to reward a turtle? Myrtle's favorite reward is squid, but Streeter says she'll also gulp herring, shrimp, smelt and romaine lettuce. Lettuce? You read that right. Since Myrtle is fat, hunkering in at 550-plus pounds, her keepers reward her with low-fat treats. Sometimes she also gets a cube of lettuce soaked in squid ink.

Only one word for that: Yum.

But understanding Myrtle's palate proved easier than devising a system to learn when she's hearing a sound. About all that's known about sea turtle hearing, says Streeter, comes from brain studies showing that they can hear sounds between 200 and 800 hertz.

Working in the aquarium's 23-foot tall main tank, amidst dozens of species of fish and the occasional shark, Streeter has already trained Myrtle to approach when the pipes clank.

But the next step in training was a belly-flop. The researchers taught Myrtle to climb on an underwater platform and poke a blue circle with her beak after the test sound. Since green sea turtles have less neck extension than ballerinas, the switch had to be, well, in her face. Since that arrangement set off too many false alarms, the circle was moved further away, forcing Myrtle to paddle forward to touch the circle. Yet because turtles don't have much of a reverse gear, Myrtle couldn't return to her starting position.

Now the trainers are training Myrtle to swim among several speakers, and to touch whichever speaker was making the test sound. Using that technique, the researchers can answer the important questions: What is the private life of shrimp? Do corals sing carols -- or choral music?

Seriously
The work could help wild turtles. Marine scientists are concerned that the growing level of artificial noise in the oceans may affect animals that depend on sound for navigation or communication.

The populations of all sea turtles, including the green sea turtle, are thought to be declining. One problem is getting caught and drowning in fishing and shrimping nets. Some shrimpers solve the problem with "turtle excluder devices"-- screens that prevent large turtles from being trapped in their traps.

Streeter notes that noisemakers are already used to warn dolphins away from nets, and suggests that sound might work with turtles as well. Myrtle, she observes, doesn't like loud noise. "At some level of sound, Myrtle will come to the speaker and veer off, but at a lower level, she won't veer off," Streeter says. "It seems like a sound deterrent would work if we play it right."

-- David Tenenbaum