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Mammals (Click to select text)
Mammal, common name applied to any warm-blooded animal belonging to the class that includes humans and all other animals that nourish their young with milk, that are covered with varying amounts of hair, and that possess a muscular diaphragm. Mammals have the most highly developed nervous systems of all animals. Most members of the group have four appendages, usually legs. These may be adapted for use as swimming appendages, as in seals, or as wings, as in bats. Some Types,, however, have two limbs that have been reduced to small vestiges beneath the skin, as in whales, or have been lost altogether, as in sea cows. All mammals, except the egg-laying monotremes, produce live young that undergo the early stages of development within the body cavity of the mother. Some mammals are helpless at birth, others are able to walk and even run immediately, and may be born fully furred and with their eyes and earsopen. The largest mammal, the blue whale, Often exceeds 30 m (100 ft) in length, and the smallest shrews, mice, and bats are often less than 5 cm (less than 2 in) in length. In many mammals the color of the skin or fur blends with the animals natural surroundings. In others there is great contrast with the natural surroundings to favorvisual signals that provide information about theidentity of a species, and about the gender, age, orsocial status of an individual. The skin also functions as a sensory and excretory organ and contains specialized glands. Mammary glands, which are present in fully developed form in all adult female mammals. Aquatic mammals, such as whales, dolphins, and sea cows, have no sweatglands. Sweat glands are usually located at the base of hairs, except those in regions of skin bordering mucous membranes, such as the sweat glands surrounding the edges of the lips and covering the genitalia. Many mammals, however, have few functional sweat glands; in dogs and cats, for example, only the glands on the soles of the feet are functional. The eyes, ears, and nose of mammals also have their external endings in the integument. All mammals have two eyes, but the eyes of several burrow-dwelling mammals, such as moles, have lost their function partially or completely or have become covered with skin. All mammals reproduce sexually, and two types of reproductive acts are used to bring about sexual conjugation. In the primitive egg-laying mammals, excretory and genital organs open into a common orifice, called the cloaca. Transfer of sex cells from the male to the female is accomplished by bringing the cloacae into apposition. In all other mammals, however, the male sex cells are transmitted by copulation. After fertilization, development of offspring takes place entirely within the body of the mother in all mammals except the monotremes, which produce leathery-shelled eggs with large yolks, and in many marsupials, in which the gestation period only lasts about 10 to 15 days, with most of the development taking place in the mother's pouch afterbirth. Mammalian young are not prepared to pursue an independent existence immediately following birth but must be nursed during infancy. Mammals probably appeared on the earth during the early Mesozoic era. Most zoologists believe that mammals evolved from a group of extinct mammal-like reptiles, which existed during the Triassic period. The earliest animal fossils that have definitely been identified as mammals were found in rocks from the Jurassic period. During the Jurassic period, five distinct orders of mammals existed. One order was made up of small, rodent tike mammals, having gnawing front teeth and grinding teeth with several cusps. A second order consisted of small, carnivorous mammals, having molar teeth equipped with three simple, cone like cusps, that became extinct before the end of the Eocene epoch. A third group of small insectivorous mammals are the probable ancestors of present-day mammals. Of the mammalian subclasses that still exist, themonotremes are unrepresented by fossil remains.
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