The legs of seabirds in polar regions extend into frigid waters that are often below standard freezing temperatures. These creatures use their circulatory systems to retain heat inside the body. The arteries that run from the legs to the feet are bundled in parallel to the veins that return the blood to the body. The heat in the arteries is picked up by the colder veins that return blood to the animal’s core. This means that the warmth of the blood in the arteries stays in the body and does not dissipate into the environment. With this neat trick, little heat is lost.
