November 1997

High-Tech for Ancient Turtles

Continued from page 1

By Peter Tyson

The Leathery Turtle

Dermochelys coriacea, "the turtle covered in leathery skin," is unique among sea turtles, which also include the loggerheads, greens, hawksbills, and ridleys. It is the most ancient living reptile, around in its current form for at least 20 million years and possibly over 100 million years. It is also the biggest, having watched its rivals for size, the dinosaurs, go extinct some 65 million years ago. The female before me this tranquil night checks in at about five feet long and 550 pounds, the average for nesters on Playa Grande. But a male leatherback once caught in fishing nets off the coast of Wales stretched fully nine feet from head to tail and weighed a ton.

Despite their bulk, leatherbacks migrate farther and dive deeper than any other reptile-indeed, than almost any other animal. After nesting in the tropics, the giant turtles typically swim thousands of miles to reach their favorite feeding grounds in subpolar waters. Along the way, they regularly dive more than 3,000 feet straight down, in search of food or to escape the rare predator such as a killer whale. Their streamlined black body, with its smooth, leathery skin and ridged, highly tapered carapace-which lends the species the popular names of "keelback" and "trunkback"-eases long-distance swimming. A collapsible body frame that includes a high cartilage-to-bone ratio and few fused ribs (even their shell is as flexible as an eraser) allows leatherbacks to deal with the pressure, which at 3,000 feet approaches 1,500 pounds per square inch. Astonishingly, they manage all these feats on a diet of nothing more than jellyfish.

Despite its uniqueness, the leatherback is on the road to extinction. Frank Paladino, a biologist at Indiana-Purdue University at Fort Wayne who has run the project at Playa Grande since 1988 with James Spotila of Drexel University, recently estimated that the global population has dropped by two-thirds since 1980 alone, from 115,000 to 34,500 nesting females. (Censusing males is impossible, as they never come ashore and are rarely observed in the open ocean.)

Though the leatherback spends most of its life far out to sea, its chief threats, ironically, lie ashore. Laying its eggs on land is the one trait it has retained from its earliest ancestors, the land-dwelling tortoises, and that trait has now come back to haunt it. On leatherback nesting beaches throughout the tropical world, people raid the turtle's nests for their delectable eggs, build over their habitat with houses and hotels, and occasionally kill nesting females for their meat. Increasingly, though, leatherbacks are also losing their lives on the high seas, where fishermen harpoon them for food or for the thick, yellow oil contained in their flesh. (The oil is used in the Caribbean as an aphrodisiac or as a chest rub to relieve congestion and by fishermen in Arabia and India as a treatment for boat timbers.) The turtles also perish at sea when longline fishing gear unintentionally snags and drowns them, and floating plastic garbage chokes them when they mistake it for jellyfish.

To slow or even reverse this precipitous decline, biologists are rushing to better understand the species. "We desperately need to know where these animals go in the open ocean and what they do out there," says Scott Eckert of the Hubbs-Sea World Research Institute in San Diego, Calif.

Biologists have no idea where the turtles, which depart their natal beaches as two-ounce hatchlings, spend their developmental years. Nor do they know how fast they grow, at what age they reach sexual maturity, or how long they live. They're not sure how females find their way to feeding grounds, or how they find their way back to their favorite nesting beaches. No one has ever reported seeing leather-backs mate. "When you realize those very basic pieces of information are missing, you realize why we're having such a struggle trying to save these creatures," says Eckert, who says he has begun to fear, for the first time in 15 years in the field, that their extinction may occur in his lifetime.