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Mammals are warm-blooded vertebrates that nurse their young
with milk. They evolved from reptiles. Their body is well insulated,
usually with hair or fur. The right and left sides
of the 4-chambered heart are completely separate.
Class Mammalia: Vertebrates covered with insulation, usually hair or fur (occasionally blubber). Metabolic rates and body temperatures are kept high (homeothermy). Glands in skin secrete sweat and oily secretions (sebum). Young mammals are nursed by their mothers; milk is secreted by mammary glands, derived from sweat glands. Frequent parental care generally follows.
Three tiny ossicles (malleus, incus, stapes) transmit sounds in middle ear. Four-chambered heart has complete separation of oxygen-rich and oxygen-poor blood. Only one bone, the dentary, makes up the lower jaw on each side. A muscular diaphragm is responsible for most breathing movements. A bony hard palate separates nasal cavity from oral cavity, allowing breathing and chewing at the same time.
Brain and braincase larger than in reptiles. Homeothermy is key to most of the other traits: Insulation is needed to maintain a body temperature different (usually higher) than that of the surroundings. Sweat glands prevent overheating because the evaporation of sweat cools the body surface. Sebum slows down (and thus controls) the evaporation of sweat. Mammary glands are derived from sweat glands, showing that sweat glands must have evolved first. In monotremes, milk is a sweaty fluid that exudes from the mother's ventral surface (without any well-defined nipples), and the infants lick it off this surface. In other mammals, the milk comes out through welll-defined nipples from which the infants feed; in marsupials, these nipples are located inside the pouch. The elevated standing posture requires more energy to maintain and also allows quicker escape from danger. The reprilian articular bone becomes the malleus, and the quadrate bone becomes the incus. More efficient breathing is required for the higher metabolic rate that homeothermy requires. Breathing cannot be interrupted for chewing, and mammals also eat more and chew their food more because of the higher energy needs. The bony palate separates the air passages above from the mouth below, allowing chewing and swallowing to occur without interrupting the breathing process. Stronger and more complex teeth are required for better chewing of food. Larger brain indicates higher intelligence and more complex behavior, which requires more parental care. Parental care includes feeding of milk (and often of other foods, especially in species that hunt). Young mammals learn by imitating the behaviors of their parents. Origin of mammals: Mammals evolved during Triassic times from mammal-like reptiles (Synapsida). The transition involved changes in the teeth and tooth replacement, the replacement of one jaw hinge (between articular and quadrate bones) with another (between dentary and squamosal bones), and the conversion of the articular and quadrate bones into the malleus and incus. Monotremata: Egg-laying mammals. Example: platypus. Marsupials: Pouched mammals. Examples: kangaroo, opossum, koala. Placental mammals (Eutheria): Mammals in which the fetus is nourished in utero by a placenta. Includes the vast majority of mammals, arranged in over 30 orders, about half extinct and half with living members. Examples: shrews, mice, bats, rats, cows, deer, pigs, dogs, cats, monkeys, humans, whales, horses, elephants, rabbits. Among the many orders of placental mammals are these:
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