Leatherbacks: Adapted for the deep

By Brianna Forte

Editor’s Note: Three species of sea turtles – leatherback, loggerhead, and green – nest regularly on Brevard County beaches. Typically, leatherback sea turtles are the first to arrive in March as nesting season gets underway. They are followed by loggerhead sea turtles a few weeks later, and the green sea turtles are the last to the nesting party. This week, we will be taking a closer look at each of these species.

Leatherback sea turtles swim all around our oceans, and their habitat ranges from tropic and temperate waters along the equator to subarctic latitudes. In addition to occupying nearly freezing waters, leatherbacks are known to forage at depths up to 4,000 feet where the temperature plummets in comparison to the surface water that is warmed by the sun. So how do these gentle giants manage to keep cozy and warm?

Unfortunately, it is not as simple for leatherbacks as snuggling under a blanket with a mug of piping hot chocolate. Instead, they rely on the anatomical structure of their blood vessels to form a counter-current heat exchange system.

In mammals and birds that live in cold environments, the counter-current heat exchange system consists of arteries and veins arranged in a certain way to conserve the core temperature of the animal. The arteries, which contain warm oxygenated blood flowing to appendages such as the hands and feet, are alongside the veins, which contain cold deoxygenated blood flowing back to the heart. As the warm blood from the arteries flows past the cold blood in the veins, the heat is transferred to the veins going toward the center of the body and the core part of the body stays warm.

According to an article by Kara Feilich, for mammals and birds the counter-current heat exchange system functions to “keep heat in the core of the body while letting the limbs cool down to the temperature of the surroundings.” However, in a study published in the journal Biology Letters, it was hypothesized that the counter-current heat exchange system in leatherbacks functions differently.

The researchers in this study analyzed the bodies of six juvenile leatherbacks that were collected from accidental bycatch by NOAA fisheries observers. Through dissection and tissue manipulation, the researchers noticed an unusual network of blood vessels at the base of each of the turtle’s limbs that were not conducive to the previous counter-current exchange system that was believed to keep the core body of the turtle warm and the flippers cold.

Researchers concluded that the counter-counter current heat exchange system in leatherback sea turtles keeps the limbs warm while the core temperature of the body remains cooler. Through this unique counter-current heat exchange system, leatherbacks retain the heat generated by their muscles used for movement. This enables the leatherback’s flippers to be warm enough to function effectively in cold-water environments and helps them maintain abilities such as paddling and steering.

This unique counter-current heat exchange system also benefits female leatherbacks when they are nesting. Leatherbacks are at risk of overheating when they use their hind limbs to help them travel over the sand and dig a nest. The counter-current heat exchange system proposed by this study would prevent the females from overheating during the nesting process, as it would protect the core body from excessive heat generated by the limbs.

The research paper concluded by stating that leatherbacks rely on continuous exercise of the limbs to generate heat to keep their peripheral muscles warm, and this heat is mostly kept in the peripheral limbs instead of the core of the body through the counter-current heat exchange system. So although leatherbacks don’t have any cozy blankets to keep them warm, they do have a unique heating system to ensure that they will not be cold.

Sources:

https://royalsocietypublishing.org/doi/10.1098/rsbl.2015.0592

https://jeb.biologists.org/content/219/3/298

https://www.sciencemag.org/news/2015/10/how-sea-turtles-keep-their-legs-warm

https://oceana.org/marine-life/sea-turtles-reptiles/leatherback-turtle