TOPP Helps Reveal Leatherback Turtle At-Sea Physiology

A marine turtle on land is a ponderous yet awe-inspiring spectacle and none more so than the giant leatherback. While marine turtles make their living at sea, all are tied to the land, and a female leatherback must emerge from the surf every few years and deposit her eggs to ensure the continuation of her ancient and venerable lineage.

The task is not an easy one. And while this activity drives the entire life history of the species, according to TOPP sea turtle researcher (and Duke University post-doc research associate) Bryan Wallace, the amount of time the female spends nesting on land is roughly comparable to the amount of time most human males spend in the barber chair. (An adult female leatherback returns to her nesting ground every three plus years, with between six and eight land visits of an hour and a half each, or—roughly—between 2 and 6 hours a year).

More than just a curious factoid, Wallace's analogy is significant for the perspective it offers on a gap that has long existed in knowledge of this animal's energy needs. Because all previous sea turtle metabolism studies had been conducted on land during the animal's brief and strenuous terrestrial visits, the physical demands imposed by the animal's marine habitat and lifestyle were a mystery—up until now.

Wallace and colleagues have recently published an important study of leatherback metabolism that allow us to better understand the energy demands of leatherback behaviors at sea--foraging, diving and migrating --across a spectrum of temperatures and depths (all data points TOPP tags collect).

While turtle behavior can be studied by deploying tags on the animals, their metabolic rates can also be measured by "labeling" the hydrogen and oxygen atoms in water molecules with special stable isotopes (a technique called Doubly Labeled Water) and injecting them into the turtles' bloodstreams. By measuring the concentrations of these labeled isotopes in blood samples over time, researchers can determine how much energy the turtles are expending during the study period.

Wallace and his colleagues employed this technique to measure metabolic rates of five leatherback turtles during periods between nesting events (normally about a ten day period). Four of these turtles (and fourteen 'control' turtles) also carried archival data loggers during the study, helping Wallace record the turtles’ actual diving behavior and water temperatures they encountered while at sea. In this way, the researchers were able to capture information about marine turtles that has long eluded them and answer an important question: what's going on inside the animal while it is in the water?

Knowing how leatherbacks behave in the ocean during periods between a season's nesting events and how much energy the leatherbacks expend during these at-sea intervals allows researchers to make reasonable estimates of individual energy requirements across the spectrum of at-sea behaviors.

With eastern Pacific leatherback populations plummeting, better information about their needs and special vulnerabilities may offer these leatherbacks some much needed relief as resource managers begin to make better informed decisions.

To help understand why, Wallace offers a useful analogy. "Imagine that the turtles are cars. These measurements allow us to know their ratings for MPG on the road, not just idling in traffic." And with the help of habitat data the turtles are collecting, TOPP research may just help make traveling a bit safer for this highly endangered species.

 

by Diane Richards