Classification of mammals

Biologists classify the 4,500 species of mammals alive today into 18 main orders on the basis of structural differences and development of the unborn. Members of these orders live throughout the world. Mammals differ in the way their young develop before birth. Mammals can be divided into three groups: egg-laying mammals, pouched mammals, and placental mammals.

Monotremesare mammals that lay eggs. Young monotremes develop within a protective shelled egg much as reptiles do. Pouched mammals are known as marsupials. Young marsupials develop within the mother’s body for only a short time. After birth, they complete their development inside a special pouch, or marsupium, located on the mother’s abdomen. Most modern mammals are placentalmammals. In placental mammals, the young remain within the mother’s body until they are able to maintain life independently. A special organ called theplacentaconnects the unborn young to the mother.



Only two types of monotremes are alive today, and they live only in a few isolated regions of the world. The duck-billed platypus lives only in Australia. The spiny anteater lives in some parts of Australia and New Guinea.

Monotremes are considered mammals because they have hair and produce milk for their young. In other ways, the monotremes are fundamentally different from all other mammals.

Monotremes show a curious mixture of traits. They have a cloaca, as do the lower vertebrates. Their limbs are attached to the sides of the body like the limbs of reptiles, and their feet have claws. The platypus has webbed feet and a flat tail that it uses for swimming. Unlike most other mammals, monotremes do not have true teeth. The platypus uses its flat bill to probe in the mud for worms, snails, and shellfish. The spiny anteater uses its beak to probe into anthills for food.

The major reptile-like feature of the monotremes is that they lay eggs.Monotremes produce two to three eggs at a time. After fertilization the eggs are kept inside the mother’s body and are nourished for a short period before they are laid in the nest. The female incubates her eggs by curling her body around them. The anteater has a special broodpouch on her lower abdomen in which she keeps the eggs until they hatch.

After the young monotremes hatch, they are fed with milk secreted by their mother. The milk is produced by more than 100 specialized sweat glands on the lower abdomen. Unlike other mammals, young monotremes do not suckle. Instead, they lap up the milk as it oozes onto the mother’s belly.



Kangaroos, koalas, and almost all other marsupials live in Australia. A few species of marsupials live in South America. North America has only one marsupial, the opossum. The one major difference between marsupials and other mammals is that young marsupials complete their development inside their mother’s pouch.

The fertilized egg develops into an embryo, which remainsin the female marsupial’s body for only a short time. The embryos feed from the yolk in the egg and do not attach to the uterine wall. In some species, internal development lasts only a matter of days. At birth, young marsupials are still in an incomplete stage of development. They are blind, helpless, and extremely small.

Once inside the pouch, the young marsupial fastens its mouth onto a nipple. The nipple swells, and the young becomes firmly attached and begins to suckle. Immature marsupials are fed and protected inside the pouch until they are able to feed and care for themselves. The hind legs of the newborn marsupial are poorly developed; actually, they are a little more than embryonic buds. The front legs, however, are more fully developed and tipped with claws. A young marsupial uses its front limbs to pull itself up the mother’s abdomen and into her pouch.


Placental mammals

More than 95 percent of all mammals are placentals. Early in the development of a placental mammal, the embryo becomes implanted in the wall of the mother’s reproductive organ, called the uterus. Then the placenta forms, connecting the young mammal directly to its mother’s uterus. The fluid-filled sac, the amnion, surrounds the embryo and supports it during development. Blood vessels from the amnion connect to the placenta as well. The circulatory systems of the mother and the embryo are not directly connected. Nutrients and oxygen from the mother’s blood pass across the tissues of the placenta into the blood of the developing embryo. Waste materials pass from the embryo to the mother’s blood.

Because the developing young mammals get their nourishment directly from the mother through the blood, development is not limited by the fixed amount of food found in an egg. The longer period of time for the young to develop permits the formation of a complex brain and nervous system. The period of time during which the young mammal develops within the uterus is known as gestation, or pregnancy. The length of gestation differs among mammals.

Placental mammals differ in size, shape, diet, and the way they move. Each order of placentals shows adaptations for a particular way of life. Some are adapted for walking; others are adapted for running, leaping, swimming, or flying. Some mammals so greatly differ in appearance that it is hard to believe they are closely related.


Insect-eating mammals

The oldest group of placental mammals is small, highly active animals called insectivores. As their name indicates, these animals eat mainly insects. Their diets vary, however, and may also include snakes, fruit, birds and other insectivores. Biologists think that insectivores are the ancestral stock that gave rise to the other placental mammals, even to the enormous elephants and whales. Compared with other mammals, insectivores have small brains. They also have enormous appetites. With their extremely high metabolic rates, insectivores would starve to death within a short time if they were deprived of food. Most insectivores live in burrows or trees and are active only at night.


Flying mammals

Bats are the only mammals that can truly fly. Without their large, folding wings, bats resemble insectivores in both habits and appearance. A bat’s wing is made of a flexible flap of skin stretched over extremely long arm and hand bones. The wing is supported by the bones of the last four fingers, which are exceptionally long and thin. The thumb is usually not attached to the wing and has a curved claw used for clinging or grasping.

Bats generally fly and live in groups. They are active only at night. Most bats eat insects. Some bats eat nectar, and others catch fish or frogs with their clawed hind feet.

Vampire bats live only in the tropics. They feed on the blood of other mammals, especially horses and cattle. With its front teeth, a vampire bat takes small cuts in a victim’s skin and laps up the blood with its tubelike tongue.

Hoofed mammals.Among land mammals, those with hoofs are called ungulates.Sheep, cattle, deer, pigs, camels, and other ungulates with an even number of toes make up the order Artiodactyla. Horses, tapirs, rhinoceroses, and other ungulates with an odd number of toes belong to the order Perissodactyla. Ungulates walk on tiptoe. The toes that touch the ground are broad, and the claw is enlarged, forming a hard, protective hoof. Ungulates’ main means of defense if their ability to run.

Hoofed mammals generally live together in herds. The young are well developed at birth and can move with their herd within a day or two after they are born. Hoofed mammals are herbivores. All animals in the order Artiodactyla except the pig and the hippopotamus have a four-chambered stomach. The first chamber is the rumen, which contains bacteria and other microorganisms that digest cellulose. Animals with this chamber are called ruminants.


Trunk-nosed mammals

Today only two living representatives of the order Proboscidea remain, the African elephant and the Asian elephant. These herbivores are noted for their enormous size and their long, grasping trunk. The trunk is really an elongated nose and upper lip. An elephant uses its trunk to take up water to drink, to spray water over its body, and to collect food and place it in its mouth.

Carnivorous mammals

Carnivores hunt other animals for food. This order includes land predators, such as tigers, lions, and wolves, as well as marine mammals, such as seals and walruses. Their long, sharp canine teeth are specialized for capturing prey and tearing flesh. Carnivores are intelligent and have keen senses of smell, vision, and hearing. On their cheeks are whiskers, called vibrissae that are sensitive to touch.

Scientists generally divide carnivores into three subgroups: the cat family, the dog family, and the sea lion family. The powerful limbs of land carnivores enable them to leap onto prey from trees or chase their prey across the ground. Their feet have thick pads that absorb the shock of landing or running. Most members of the cat family have sharp retractable claws for capturing their prey. Members of the dog family have lean, muscular bodies and slender legs. Some members of the order Carnivora are not strictly carnivores. Raccoons and bears, for example, are omnivores;they also eat plant materials.

Seals, sea lions, and walruses have streamlined bodies and a thick layer of insulating fat called blubber. Seals, sea lions, and walruses use their hind limbs as paddles to propel themselves through the water. Members of the seal family eat a wide variety of food, including mollusks, fish, and birds. Seals and walruses sometimes leave the water. When on land, seals and walruses move about very slowly. The reason, of course, is that these mammals do not have legs. Instead, their forelimbs are flippers. Their hind limbs form part of the rear flipper, or tail. The flippers allow the seals and walruses to move well in the water but not well on land.

Seals and walruses do not have a blowhole. Seals live in many parts of the world. Most seals live along the ocean coasts. A few live in freshwater lakes and island seas. They feed on fish, squids, and seabirds. Like toothed whales and dolphins, seals swallow their food without chewing it. Unlike all whales and dolphins, seals give birth to their young on land. Young seals are also nursed on land, even though they can swim as soon as they are born.

Though adapted to a life spent mainly in the water, marine carnivores mate, bear young, and rest on land.

Marine mammals

The largest mammals – whales – live in the sea. Like all mammals, sea mammals breathe with lungs, have milk glands to nurse their young, and have hair or fur for at least a part of their life. All sea mammals swim very well. They each have two flippers for swimming. They also have a streamlined shape somewhat like fish. This shape helps them move through the water easily.

Whales and dolphins live in the water all the time.. Whales and dolphins have a blowhole set high on their head. Air can be taken in and released through this hole. You may have heard about the spouts of whales and dolphins. A spout is formed by air being forced out of the blowhole.

Dolphins live in all the oceans of the world. They also live in some rivers, such as the Amazon River of South America. Like all whales, dolphins feed on fish and squids. Unlike whales, dolphins have teeth in both their upper and lower jaws. Dolphins are able to locate underwater objects by marking sounds and listening for the echoes. After making sounds, the echoes of the sounds reflecting from an object tell the dolphin about how far away the object is and about how large the object is.


Gnawing mammals

Rodents are distinguished from other mammals by their teeth, which are highly specialized for gnawing. All rodents have two pairs of large, curving incisors that grow constantly. Rodents is the largest order of mammals. Three factors account for the rodents’ remarkable worldwide distribution: their intelligence, their small size, and their rapid rate of reproduction.

Most rodents (mice, hamsters, guinea pigs, rats, and squirrels) are small animals. Larger rodents include prairie dogs, porcupines, and beavers.

Rodentlike mammals – Lagomorphs.Rabbits and hares resemble rodents in many ways, but are not closely related to rodents. Both rabbits and hares have long hind legs that are specialized for leaping and hopping. Their teeth are adapted for gnawing. Lagomorphs, unlike rodents, have an additional pair of teeth posterior to their upper incisors.

Both rodents and lagomorphs have a special intestinal pouch called the cecum that contains cellulose-digesting microorganisms. Like the rumen of ungulates, the cecum is an adaptation to a diet that consists mainly of grains and tough grasses.


Toothless mammals

Armadillos, anteaters, and tree sloths make up the order Edentata. But only the anteaters completely lack teeth. The other species have molars only. Most edentates have specialized features that are adaptations for an insect diet. The anteater uses its powerful clawed forefeet to rip open termite and ant nests. Then it uses its long, sticky tongue for capturing the insects.

Armadillos have a unique protective shield formed of small bony plates.



Lemurs, monkeys, apes, and human beings belong to the order Primates. Most primates are tree dwellers, and many of the features characteristic of primates were originally adaptations for life in the trees. Sensitive grasping hands with opposable thumbs and feet with big toes are aids in climbing and swinging through trees. Most primates have a flattened face with both eyes directed forward. The eyes of primates can focus and can discern color. Because the fields of vision slightly overlap, their eyes can also perceive depth. Generally, young primates are cared for by their parents for a longer period after birth than most other mammals.

The outstanding feature of primates is their highly developed brain. Primates are distinguished from other mammals mainly by their active life, their curiosity, and their exceptional ability to learn. Of the apes, only gibbons and orangutans live in trees. Gorillas spend their days on the ground but sleep in trees at night. Apes can stand upright and walk for short distances on their hind legs. More often, they lean forward and balance on the knuckles of their hands when they stand or move. Except for the orangutan, apes live in highly developed social groups. They communicate with each other through a large number of sounds.

So, the primates are characterized by possession of (1) a placenta, (2) three kinds of teeth (canines, incisors, molars), (3) opposable first digits (thumbs), (4) two pectoral mammae, (5) expanded celebral cortex, and (6) a tendency toward single births.

Man is distinguished from the other primates by his upright posture and his lack of body hair. Among all the mammals, he is one of the least specialized. He is an omnivore, eating a wide variety of fruits, vegetables, and other animals. His hands closely resemble those of a primitive insectivore, in contrast to the highly specialized forelimbs developed by, for example, the whales, bats, horses. His sensory organs are crude comparing with those of insects or of many other mammals. Man has, however, one area of extreme specialization: the brain. Because of his brain, man is unique among all the other animals in his capacity to reason, to speak, to plan, and to learn.

To the group of Placental Mammals also belong: Whales and Related Aquatic Mammals, Gnawing Mammals, Rodentlike Mammals, and Toothless Mammals. Because of the placenta, the embryo can stay inside the mother longer. When it is finally born, a baby placental mammal is more developed and active than other mammals. Still, some babies, such as mice, kittens, and humans, are born fairly helpless. Others, such as horses, cows, and whales, are very active when born.



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