Mammals are animals that have hair or fur on their bodies and feed milk to their young. Adult female mammals produce milk from mammary glands. Mammals have well-developed lungs for breathing and four-chambered hearts that pump blood through a closed circulatory system. Mammals are endotherms, which means they can maintain a constant internal body temperature. This helps them adapt to a wide range of biomes, from deserts to polar regions and even oceans. It should come as no surprise that mammals are found on every continent on Earth.