Mammal

Mammal, warm-blooded animal best distinguished by the possession of milk glands for feeding its young. Hair is a feature of mammals, although some, like the whales, have little or none. All mammals, except monotremes like the platypus, bear their young alive. Other shared characteristics are a lower jaw formed from 1 bone, 3 small bones in the middle ear, a neck of 7 vertebrae (even in giraffes), a diaphragm that forms a partition under the ribs, and a 4-chambered heart.

Mammals evolved from reptiles, but due to gaps in the fossil record, various stages in their development from reptiles are as yet undetermined. It is probable that the different groups of mammals arose independently from several kinds of intermediate mammal-like reptiles, so that there was no single ancestral mammal. The first mammals are believed to have been small and lived at the same time as the giant dinosaurs.

Mammals have evolved into many forms, and constitute some 3,200 species alive today. Mammals are divided into 3 main groups. The monotremes are the most primitive mammals and include the platypus of Australia. They lay eggs and feed their babies milk secreted from pores in the skin and not from milk glands with nipples. Marsupials are the pouched animals, including the kangaroo, opossum, wallaby, and Tasmanian devil. The young are born in an undeveloped state and complete their development in a pouch. Marsupials are found only in Australia and parts of America. Placental mammals, including humans, are the largest and most successful group. The young are born in varying states of development, from the relatively helpless human offspring to those like horses that are able to run within a few hours after their birth or, in the case of whales and dolphins, are able to swim as soon as they are born.