18.3 Diversity of Mammals

Objectives Distinguish between egg-laying, pouched, and placental mammals. Infer why most egg-laying and pouched mammals live in one part of the world. Classify the major orders of placental mammals and give examples of each.

Life scientists begin to make sense of the great diversity of mammals by separating two kinds of mammals from all the rest. These two groups, the egg-laying mammals and the pouched mammals, reproduce very differently from most other mammals.

Egg-Laying Mammals As you probably know, most mammals give birth to live young. Living in Australia and New Guinea, however, are mammals that lay eggs. They are the duck-billed platypus and two species of spiny anteater.

These egg-laying mammals are called monotremes (MAHN oh TREEMZ). Monotremes have the two unique mammal characteristics: hair and mammary glands. Their eggs are soft-shelled and hatch after ten days.

When the young emerge, they are fed milk. The milk comes from glands on the mother's belly. Since the glands have no special opening, the milk seeps out and the baby sucks it off of the mother's fur. Spiny anteaters nurse their young for up to six months.

Pouched Mammals Other mammals with unusual reproduction are Called marsupials (mar SOO pee uhlz). Marsupials are mammals with pouches in which the young complete their development.

You have probably heard of two marsupials, the koala and the kangaroo. They both live in Australia. Another marsupial, the opossum, is common throughout North America. About 80 species of marsupials live in South America, including the rat opossum.

Marsupial eggs are fertilized inside the female's body and begin their development there. The embryos grow inside an organ called the uterus (YOOT ur uhs). They are nourished by a limited food supply that was part of the egg.

When that food supply is used up, the young are born while still embryos. The tiny, blind babies then crawl into the mother's pouch. Each finds a nipple, where milk comes out of a mammary gland, and begins to suckle. The babies stay in the pouch for one to two months, until their development is complete.

Placental Mammals Most mammals neither lay eggs nor have pouches. Their young complete their development inside the uterus. There, an embryo receives nutrients and oxygen from the mother's body through an organ called the placenta pluh SEHN tuh).

The placenta also removes wastes and carbon dioxide from the embryo. Thus the placenta provides for all the embryo's needs while it is developing. Mammals whose unborn young are nourished through placentas are called placental mammals.

The length of time an embryo develops inside its mother varies, depending on the kind of placental mammal. Mice have a short, 21-day period of development. Humans are born after 9 months of growth inside the uterus. Elephants take 22 months before they are born.

After they give birth, placental mammals spend more time caring for their young than do most other animals. Cnidarians, for example, release many eggs and sperm into the ocean.

Placental mammals care for their young until they can be on their own. However, the degree of parental care varies. An elephant invests more time and energy in raising each offspring than a rabbit does.

Diversity of Placental Mammals Scientists classify placental mammals into nearly 20 large groups called orders. The mammals in each order share certain important characteristics and adaptations. Here are some of the orders of placental mammals.

Carnivora As you might guess from the name, these mammals are carnivores, or meat eaters. They have teeth adapted for tearing flesh, clawed toes, and a well-developed brain. This group includes not only dogs, cats, and bears, but also seals, otters, and walruses.

Cetacea Whales and dolphins belong to this group of mammals that live in the ocean. They have evolved fishlike bodies with fins and paddlelike front limbs.

Insectivora Moles and shrews belong to this group of insect-eating mammals. Most have long skulls, narrow snouts, five-clawed feet, and are smaller than 46 cm. Their bodies, habits, and diet are similar to those of the first ancient mammals

Rodentia Rodents make up the largest order of mammals. They have chisel-like front teeth adapted for gnawing. Rodents include squirrels, beavers, rats, mice, porcupines, and gophers.

Proboscidea This order is named for the long, muscular trunks of its members, the elephants. Elephants have extra-long teeth called tusks.

Primates Mammals placed in this order have thumbs adapted for grasping objects. Their eyes face forward and they eat both plants and animals. Monkeys and apes are members of this group.

Chiroptera Bats, the mammals adapted for flight, make up this order. Their wings are flaps of skin stretched between their bodies and their long fingers. Bats are active mainly at night.

Perissodactyla Like sheep and goats, the mammals in this group have hooves and are plant eaters. But instead of having an even number of toes on each foot, they have an odd number . This order includes horses, zebras, and rhinoceroses. They have large, flat teeth used for grinding their food.

Artiodactyla The familiar sheep, goats, pigs, and deer belong to this order of mammals, with hooves and an even number of toes. The giraffe is also a member. They are all herbivores, or plant eaters, and range in size from the 3.5 kg mouse deer to the nearly 4.5 metric ton hippopotamus.

Lagomorpha The mammals grouped in this order have long rear limbs adapted for jumping. They include rabbits and hares.

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