The class Mammalia is a relatively small one, containing about 4,500 species and 19 orders. Among the largest orders are the rodents, or Rodentia, which make up about half of all mammal species, and the bats, or Chiroptera, which account for about one-quarter of mammal species.
Mammals are one of the most diverse groups of animals, varying remarkably in structure, size, and habitat. The smallest living mammals are the shrews of the family Soricidae. The pygmy shrew, for example, weighs less than 1 ounce (28.35 grams). The largest mammal is the blue whale, with a weight of more than 220 short tons (200 metric tons).
Mammals are set apart from other animals by several characteristics. One of their most distinctive characteristics is their hair or fur, which in most mammals covers the entire body. Mammals are also endothermic and homeothermic that is, they produce heat internally and maintain a constant body temperature. Their coats of hair help regulate their body temperatures by acting as an insulator to prevent loss of body heat. Mammals maintain a fairly constant body temperature that normally lies between about 92° F. (34° C) and 102° F. (39° C). This heat is produced in a process called metabolism, which produces the energy and substances that an organism needs to live.
Mammalian metabolism produces heat. This heat production is controlled by a region of the brain called the hypothalamus. In most mammals, the hypothalamus works along with sweat glands in the skin that are distributed over much of the body. When a mammal’s body becomes too hot, the hypothalamus becomes active, causing blood vessels to dilate and release excess heat. The skin is cooled when sweat secreted onto the skin evaporates. When a mammal becomes too cold, the blood vessels contract, and the sweat glands dry up. In addition, reflex shivering occurs, which generates heat by muscle contractions. Such heat regulation allows mammals to remain active regardless of the temperature of their environment. In contrast, reptiles and other animals whose body temperature depends on the temperature of their environment cannot function in cold weather.
Other features of mammalian anatomy also set them apart from other classes of animals. The bones and teeth of mammals are designed for greater efficiency. For example, mammals have three small bones called ossicles in the inner ear. These bones help give them sensitive hearing. Their lower jaws are formed from a single bone, and a variety of teeth with different forms and functions and complex cusp patterns gives them the ability to chew their food more thoroughly. In addition, female mammals have mammary glands, or mammae, which produce milk to feed their young. Most mammals also have fleshy lips. When a mammal is young, it uses its lips to suckle, but as it grows, the mouth may become adapted to feed on specialized food.
Like fishes, birds, and crocodiles, mammals have a four-chambered heart and a diaphragm that separates the chest and abdominal cavities so that the lungs can work more efficiently in supplying increased amounts of oxygen to the blood.
Mammals have been able to exploit an amazing variety of habitats. Moles (family Talpidae) and many rodents make their homes in underground burrows or among leaf litter on the ground, as do shrews. Otters (family Mustel-idae) and beavers (family Castoridae) lead a semiaquatic life in fresh water. Whales and dolphins (order Cetaceae) spend all of their life in the sea. Monkeys and sloths (orders Primates and Edentata) inhabit tree canopies in forests. Bats (order Chiroptera) live in the air at least part of the time. Polar bears (order Carnivora) and musk oxen (order Artiodactyla) live in arctic environments, while camels (order Artiodactyla) and kangaroo rats (order Roden-tia) live in desert conditions.
Because mammals are warm-blooded, they consume more energy than cold-blooded animals. They therefore need to eat large amounts of food to fuel their metabolism.
Mammals’ complex teeth are specialized to cut, tear, and grind their food so that digestive processes can start earlier and are more thorough than if the food were processed only in the stomach.
Mammals’teeth closely reflect their diet. For example, cutting teeth in the front of the jaws, called incisors, are extremely well developed in the rodents, which need them for cutting through tough, woody materials. Carnivores have small incisors, but highly developed canines, which they use to tear and pierce flesh. They also have carnassials, which are modified forms of the last upper premolar and the first lower molar. Carnassials are bladelike to slice through flesh and cut meat into bite-sized pieces by their scissor action. In herbivores, which graze on grasses or nibble on leaves, the flat-topped premolars and molars, used for grinding non-woody vegetation, are the most important teeth.
Some mammals, such as pangolins and anteaters, which feed on termites and ants, and baleen whales, which feed on shrimplike crustaceans, have no teeth at all. These mammals use special mechanisms for collecting and controlling their prey. The giant anteater, for example, uses its long, sticky tongue to sweep up insects from the ground. The anteater’s digestive system includes a gizzard that grinds up the hard exoskeleton of ants and other invertebrates.
Most mammals are quadrupeds—that is, they move about on four feet. Some have plantigrade locomotion and walk by placing the whole foot on the ground. Plantigrade motion is usually found in species that hold or manipulate their food, such as primates and bears. Mammals with digitigrade locomotion walk on their toes, keeping their heels and palms off the ground.
Digitigrade locomotion enables such animals as cats and dogs to run at high speeds. Such animals also have a limited ability to manipulate food or other objects. Animals with unguligrade locomotion, called ungulates, or hoofed animals, walk only on the tips of the toes, which are protected by large nails or hoofs. Unguligrade locomotion is associated with fast, long-distance travel, and the animals that use it typically run to avoid their predators. Ungulates have almost no ability to use their hands or feet to manipulate their food or environment.
Humans are the only animals to walk on two feet, or bipedally. But many rodents and marsupials, such as kangaroos, hop bipedally.
The limbs of aquatic mammals have evolved into paddlelike flippers or fins. Some, such as the otter and the platypus, have a membrane between the digits on the forelimbs, which aids them in swimming. In seals, all the flippers are used for moving through water. In whales, the hind limbs have completely disappeared and locomotion is powered by movements of the back and tail, whereas the function of the foreflippers is only to steer and balance.
Gibbons, orangutans, and other apes have strong arms and elongated hands that allow them to swing from branch to branch in a type of locomotion known as brachiation. Some South American monkeys also have prehensile tails that operate as a fifth limb. Some squirrels and marsupials that live in the tree canopy have folds of skin that extend from the forelimb to the hind limb and tail, on which they glide from tree to tree. The only mammals that are true fliers are the bats. Their forearm is the major support of a wing membrane. The hind limb and the tail often also support the wing.
Classification of mammals
The class Mammalia is divided into two subclasses: the Prototheria and the Theria. The prototherians are a group of primitive mammals. Today, only one order of prototherians exist the Monotremata. The order has two families: the Ornithorhynchidae, or bird-noses, and the Tachyglossidae, or echidnas.
Monotremes have characteristics of both reptiles and mammals. Like reptiles, they are oviparous, or egg-laying. They are also toothless, though young platypuses produce three tiny teeth soon after birth, which they lose and replace with horny plates. Also, like reptiles, monotremes have only one opening at the hind end of the body. This opening, called the cloaca, serves both the processes of elimination and reproduction. Female monotremes have mammary glands but no teats, or nipples, as do all other mammals. Young monotremes therefore do not suckle, but lap up milk as it is secreted by the mammae. Female platypuses also differ from other mammals in that they have only one ovary, located on the left side of the body, whereas other mammals have two ovaries, one on each side.
The subclass Theria contains all other living mammals that give birth to live young. It is divided into two infraclasses: the Metatheria, which has only one order the Marsupialia, or pouched mammals and the Eutheria, or placental mammals. Female marsupials have two uteri (singular, uterus), the muscular organs in which young develop. Marsupials give birth to live young, but they are born blind and greatly undeveloped. As soon as they are born, they crawl through the mother’s fur to her teats, where they clamp on with their jaws and remain attached for several months. Most marsupials have pouches, in which the young are protected as they develop. However, many mouse-sized species lack pouches, and the voung dangle exposed during their period of attachment. Kangaroos and other marsupials leave the pouch before they are weaned.
In contrast to marsupials, eutherian development takes place mostly within the mother and most young are born at an advanced stage of development. Eutherians have only one uterus, although in some it is divided into two halves. As with all mammals, fertilization is internal. Once the female’s egg, or ovum, has been fertilized by the male’s sperm, it becomes fixed to the wall of the uterus. As in birds and reptiles, special structures then develop around the embryo. These are the amniotic sac, or amnion, which contains amniotic fluid, and the allantois, or respiratory membrane. In addition, a placenta is produced from tissues that arise from both mother and embryo. The placenta is usually attached to the uterus wall and connected to the embryo by the umbilical cord. The purpose of the placenta is to allow the passage of oxygen and nutrients from mother to embryo and waste products from embryo to mother. These substances are carried in the blood and pass through a thin partition separating the two blood systems. This form of reproduction is highly efficient and allows the fetus to remain inside the mother until it is very well developed. Even so, in most mammals the newborn are not fully mobile and are often helpless.
The testes, or sperm-producing organs, of most male mammals lie in two sacs called scrota (singular, scrotum) that protrude on the underside of the body just in front of the pelvis and fuse behind the penis. In marsupials, however, the scrotal sacs lie in front of the penis. The testes lie outside the body because high body temperatures hinder the process of sperm maturation. In many mammals, such as some primates, the sacs descend during breeding season and are retracted when the breeding season is over.
The length of time young mammals remain physically, or at least nutritionally, dependent on the mother varies from group to group. Some newborns are able to fend for themselves when they are only a few days old. Others may not become physically independent for many months. In many species, such as elephants, chimpanzees, and wolves, which live in family groups, the young may remain with the mother for many years. Elephants and other community animals cooperate in finding food, avoiding danger, and caring for the young. This life style necessitates some sort of code of behavior for herd members, which is learned from the parents and other members of the group.