Mammals are distinguished from the other vertebrates by the presence of hair or fur and milk-producing glands for the purposes of nourishing the young. Mammals are also endothermic, providing for the ability of continuous activity through day and night and alternating seasons. Mammals are also characterized by a placenta, an organ that provides nourishment and removes wastes from the internally-developing live embryo. Herbivorous mammals have modified digestive tracts, permitting them to make full use of cellulose as a diet. Other mammals have modified keratinous structures such as horns and hooves, providing important adaptation for climbing, fighting, and protection.
Monotremes are mammals such as the duck-billed platypus that lay eggs.
Marsupial mammals such as kangaroos give birth to immature young, which complete development in a special external pouch on the surface of the mother. Placental mammals complete the fetal development of the young completely within the mother, facilitated by the placenta. Placental animals are born alive.