For a long time, it was a puzzle as to how cold-blooded reptiles and amphibians such as turtles and frogs survived in northern climes. These animals’ body temperatures—unlike with warm-blooded animals such as dogs, deer, and people—plummet with the cooling temperatures. Many of them live underground near ponds and streams. As the temperatures outside cool, their body metabolism slows down, which makes it possible to survive long periods without much food, or even oxygen. Furthermore, many species of frogs and turtles have been found to use glucose compounds as a kind of antifreeze that allows their bodies to freeze without their blood and other body fluids forming harmful crystals that would otherwise destroy their tissues.