The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or “warm-blooded” bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart.
Mammals encompass some 5500 species, distributed in about 1200 genera, 152 families, and up to 46 orders, though this varies with the classification scheme. The mammals include the largest animal on the planet, the whale, as well as some of the most intelligent, such as elephants, some primates, and some cetaceans.
The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placental, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30-40 m bumblebee bat to the 30-meter Convert blue whale.
With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents, bats, and Soricomorpha.
The next three biggest orders, depending on the biological classification scheme used, are the Primates(apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).
Some mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation.
Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.