November 1997

High-Tech for Ancient Turtles

Continued from page 2

By Peter Tyson

Turning to Technology

When we finish with the ultrasound, we all step back and watch the turtle cover over her nest. Her giant front flippers send shovelfuls of sand flying behind her, and within minutes I can only guess where the nest actually lies. Leatherbacks must take special care to conceal their eggs, as many non-human predators-from mongooses to dogs-also dig them up. Soon she finishes her work and begins dragging herself down the berm toward the breaking waves.

But we are not through with her yet. Rostal wants to take a blood sample, to see if there is a correlation between her testosterone levels and the presence of those preovulatory follicles. The goal is to discover what triggers a female to lay her eggs and, when she's done, to leave the region for the season. By better understanding the egg-laying cycle, conservationists can know not only where but more precisely when to extend protection to nesting females.

No longer in her trance, the leatherback is now hellbent on reaching the water and will not sit still while we try to draw her blood. So we lay down a tarp of thick rubber webbing in her path. When she crawls onto it, we take up the four corners and, after several attempts, succeed in pinning her powerful flippers at her side. Rostal wastes no time in getting the sample, which he extracts from her neck with a long hypodermic needle. Along with samples from other turtles, the blood later reveals that females bearing mature ovaries with multiple large follicles, such as our turtle that night, do show high testosterone levels, while those with depleted ovaries have correspondingly low levels of the hormone. Such clues to the timing and mechanics of reproduction will help scientists better monitor discrete leatherback populations across the tropics, from Mexico to Malaysia.

As sophisticated as the blood and ultrasound testing are, they represent just the tip of the proverbial iceberg for Paladino and other leatherback researchers. In their efforts to gather as much information as they can about each turtle, nest, egg, and hatchling on Playa Grande and elsewhere, they rely on a wide range of techniques. Certain methods remain low-tech, such as recording each mother's carapace length and width with a measuring tape (to determine average sizes of nesters there) and marking the exact location of each nest with a wooden stake (to learn what makes an ideal nesting site). But other methods rely on some of the most advanced technologies available.

Arguably the most valuable technology Paladino employs at Playa Grande is also the smallest. About the size of a grain of rice, the Passive Integrated Transponder, or PIT, is a glass-encapsulated microchip identification tag that his team injects into the shoulder muscle of every nester that crawls onto Playa Grande. PIT tags are more reliable than metal flipper tags, which Paladino uses as well, even though they often fall off during the animal's peregrinations at sea. As each turtle comes ashore, project staff members pass a hand-held scanner, like those used in supermarket checkout lines, across the reptile's shoulder to read the I.D. code.

By identifying individuals, researchers can answer a bevy of questions. Over the years, Paladino and his colleagues have proven, for instance, that Playa Grande-which at its height in the late 1980s saw about 1,600 nesters a season-is one of the largest leatherback nesting colonies in the Pacific Ocean. They have also shown that females tend to lay eggs, on average, five times during the October-to-February nesting season. This finding may have conservation implications. For example, if females hang around relatively close to the beach, boating and fishing near nesting beaches may need to be restricted during these months.

Paladino now hopes to begin implanting PIT tags into newborns. Though he fears that as hatchlings grow, the tags may become so buried in dense tissue that they become unreadable, he hopes enough will remain viable sufficiently long to help clear up such enigmas as how fast leatherbacks mature. For instance, if a tagged newborn female returns as an adult to its natal beach to lay eggs-as many researchers believe leatherbacks do-then, with a quick read of the tag, the scientists can know just how old that turtle is and how long it took to reach sexual maturity.

Other technologies that come into play on shore shed light on leatherback physiology. As each turtle on Playa Grande lays her eggs, Paladino's crews place the wires of a thermocouple, which measures temperature, into the nest during the 60-day incubation period. Nest temperatures, it turns out, largely determine the sex of hatchlings. Warmer temperatures mean more females, colder temperatures more males. The temperature at which an equal proportion of the two sexes results reportedly lies between 84 and 86 degrees F. Therefore, knowing the nest temperature on Playa Grande and other beaches (which may vary slightly) is crucial to conservation efforts, which often include relocating nests that are prone to egg-poaching or seawater erosion.

One of the most tantalizing mysteries surrounding the leatherback is how it manages to survive in icy waters. Leatherbacks have been caught in 44 degrees F waters with internal body temperatures of over 77 degrees F. Some biologists have suggested they must have high metabolisms, like mammals or birds. But biologists Frank Paladino and James Spotila believe otherwise.

To analyze the turtle's resting metabolism, the two researchers placed sealed masks over the heads of nesting leatherbacks and collected their respiratory gases in large meteorological balloons. They later analyzed the gases for total volume and percentages of oxygen and carbon dioxide to develop an estimate of the turtle's resting metabolic rate-the amount of oxygen it burns per kilogram of body weight-after sitting immobile on shore for two hours. They found that leatherbacks have a metabolism less than half that of a similar-sized mammal, such as a cow.

Paladino and Spotila theorize that leatherbacks are relying on a unique metabolic system they have termed gigantothermy. Gigantotherms-which may have included the dinosaurs, they say-have large body sizes and low metabolisms, and they use peripheral tissues (in the case of leatherbacks, their blubbery skin) as insulation. In leatherbacks, for instance, arteries and veins lie side by side, so that heated blood pumped out from the heart can warm chilled blood coming in from the extremities. This helps to keep the turtle's core body temperature high even in waters that would kill a human in minutes.