By having large arteries and veins closely bundled within their leg muscles, leatherback turtles (Dermochelys coriacea) can keep their limbs warm even in frigid waters, a new study suggests. Despite having a low metabolic rate, leatherbacks—the sole living species of turtle in a once larger group—have a core body temperature that ranges between 25°C to 27°C (about 77°F to 81°F). Muscles must stay warm to remain efficient, but that’s a challenge for these beasts because they often swim in near-freezing waters, either in cold regions of the world or deep below the sun-warmed surface. To figure out how the animals maintain muscle warmth, researchers dissected six turtles that had unfortunately drowned in fishing nets. Besides the layers of insulating fat, which scientists knew about already, the team noted the unusual pattern of major blood vessels within the turtles’ leg muscles. The blood vessels are arranged such that veins carrying blood back to the body’s core give up heat generated within the ever-active muscles to the blood coming from the heart in arteries, the researchers report online today in Biology Letters. That keeps most of the heat within the muscles and outside the body core—which is a big help for nesting females, the researchers note, because the strenuous activity of hauling out onto the beach and digging nests (image) could easily cause them to overheat. The unusual arrangement of arteries and veins in turtle muscles runs counter to examples noted in several species of warm-blooded aquatic creatures such as seabirds and mammals exposed to the same frigid environments: In the limbs of those animals, blood vessels are arranged such that body heat, which is largely generated in the liver, remains in the body core rather than being lost to the surrounding seawater.