Leatherback sea turtles are the world’s only “warm-blooded” reptiles, maintaining a core body temperature of about 26 degrees Celsius (79 degrees Fahrenheit) even as they dive into near-freezing northern ocean waters. According to new findings published in Biology Letters this week, these turtles are able keep their legs warm thanks to bundles of arteries and veins in their hips.
Like us, leatherbacks (Dermochelys coriacea) are endothermic, and like seals and seabirds, they have what’s called vascular counter-current heat exchangers in their limbs: Heat is transferred between close bundles of veins and arteries. But does this system work in the same way in different animals? John Davenport of University College Cork and colleagues dissected six juvenile leatherbacks that were collected as bycatch on fishing vessels in the equatorial Pacific. (A necropsy revealed that they had drowned.) The team examined the vascular bundles in their hindquarters and studied their relationship with hindlimb musculature and anatomy.
The closely bundled arrangement of veins and arteries at the base of the legs, the researchers found, have a counter-current function that's the opposite as that of aquatic mammals and birds exposed to similarly cold conditions. Rather than transfer heat from (outgoing) arterial blood to (incoming) venous blood in order to maintain elevated core body temperatures while the limbs are kept cool, leatherback heat exchangers maintain higher temperatures within their limb muscles.
Their body core temperatures are typically lower than that of their muscles, and their endless amount of exercise – they’re always swimming – transfers some heat to the insulated core. This system keeps sea turtle muscles warm enough to work effectively in the cold. Leatherbacks are the sole living species of the family Dermochelyidae, which has a 50-million-year history of foraging in cool water.
Keeping heat in the muscles (and outside of the core) is especially important for nesting females, who use their legs for locomotion as well as nest digging; otherwise, they’d overheat.