Mammals are found in a variety of habitats: polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some have adapted to fly or live in water. The most unique mammalian characteristic is the presence of milk-producing glands, the mammary glands, by which the young ones are nourished. They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. The skin is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. The heart is four-chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate and fertilisation is internal. They are viviparous with few exceptions, and development is direct. Examples include the oviparous Ornithorhynchus (platypus) and viviparous animals such as Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Rattus (rat), Canis (dog), Felis (cat), Elephas (elephant), Equus (horse), Delphinus (common dolphin), Balaenoptera (blue whale), Panthera tigris (tiger) and Panthera leo (lion). The mammary glands and hair are the decisive identification features, while the platypus is an important exception because it lays eggs, and the flying fox and blue whale show mammalian adaptation to flight and aquatic life, a point often asked in NEET.