Though mammals share certain attributes, some of them do distinguish themselves from the crowd. All mammals have 4 legs except whales, dolphins, dugongs and manatees which have lost their hind limbs in the process of evolution. Mammals usually have 7 neck vertebrae. However the manatee and two-toed sloth have 5-7 neck vertebrae, while the three-toed sloth has 8-9 neck vertebrae.
Teeth are replaced once in the life cycle of a mammal and not continuously. The exceptions to this rule are pygmy rock-wallaby, silvery mole-rats and manatees, whose teeth are continuously replaced during their lifetime.
• Prototheria - These mammals lay eggs, incubate them for a certain period, after which the young ones are born. They are also called monotremes. The platypus, and the echidna are the only monotremes on Earth.
• Theria - These mammals carry their offspring in the womb for a certain period before it is born. This class is further divided into:
- Metatheria - it includes mammals which have abdominal pouches. These are also known as marsupials. They give birth to underdeveloped babies which are then carried in the pouches till they are fully developed. Kangaroos, Koalas are some well-known examples of marsupials.
- Eutheria - these mammals do not possess pouches and the baby is kept nourished within the body by a well-developed placenta. They are also known as placentals. About 95% of mammals belong to this group. Human beings, elephants, tigers, cows, bears are some examples of placentals.
► The largest mammal is the blue whale. It is also the biggest animal of any kind to have ever lived on Earth. It can grow to 30 meters in length and weigh around 30,000 pounds.
► The Virginian opossum, the rare water opossum or yapok of South America and the eastern native cat of Australia share the shortest gestation period of 12 days.
► The smallest mammal in the world is the Kitti's hog-nosed bat or bumblebee bat which grows to only 1.3 inches in length and weighs 2 grams.
► Bats are the only mammals that are capable of flying.
► The fastest mammal in water is the killer whale, which can swim at speeds of 34 mph.
► There are certain mammals that do not have any teeth such as platypus echidna, Baleen whales(which include 15 species), and anteaters.
► 12% of mammal species are found in Indonesia, which is the largest for any country.
► Monotremes are only found on the Australian continent.