question: What do mammals have under the skin to help insulate the body?
distractor3: cartilage
distractor1: bone
distractor2: nerve cells
correct_answer: a layer of fat
support: Conserving heat is also important, especially in small mammals. A small body has a relatively large surface area compared to its overall size. Because heat is lost from the surface of the body, small mammals lose a greater proportion of their body heat than large mammals. Mammals conserve body heat with their hair or fur. It traps a layer of warm air next to the skin. Most mammals can make their hair stand up from the skin, so it becomes an even better insulator (see Figure below ). Mammals also have a layer of fat under the skin to help insulate the body. This fatty layer is not found in other vertebrates.