Humans and all other warm-blooded, hair-bearing vertebrates (vertebrates have backbones) are classified as mammals. Compared to other animal species, they have better-developed brains and nurse their young with milk.
Apart from these defining milk glands, animals possess several additional distinctive characteristics. Although hair is a characteristic trait of mammals, it has vanished in many whales, except during the prenatal period. Unlike all other vertebrates, which hinge the lower jaw through a separate bone called the quadrate, mammals hinge the lower jaw directly to the skull. Sound waves are transmitted through the middle ear by a chain of three little bones. The lungs and heart are kept apart from the abdominal cavity by a muscular diaphragm.
There is just the left aortic arch. (In fish, amphibians, and reptiles, both aortic arches are maintained; in birds, only the right arch remains.) Every mammal has mature red blood cells (erythrocytes) without a nucleus, whereas every other vertebrate has nucleated red blood cells.
The Eulipotyphla (which includes hedgehogs, moles, and shrews) and rodents are the biggest orders of mammals in terms of number of species. The next three are the even-toed ungulates (pigs, camels, and whales), the Carnivora (cats, dogs, and seals), and the Primates (humans, monkeys, and lemurs).
There are between 5,000 to 5,500 different species of mammals, making them a varied collection of creatures with adaptations for living in a range of settings. While certain mammal species—like otters, seals, and dolphins—live in freshwater and saltwater environments, the great majority of mammal species are terrestrial. Mammals with the ability to fly are called bats.