LEARN ABOUT MAMMALS

CLASS MAMMALIA

Class Mammalia consists of all mammals, an extremely diverse and very advanced group in the animal kingdom with certain distinguishing features. Mammals are endothermic and with a high rate of metabolism, just like class Aves. But unique to mammals are hair and mammary glands. The mammary glands produce milk that nourishes the young, and the hair helps insulate the body and maintain a warm, constant body temperature. Also serving a protective barrier is the skin, a layer of epidermis over a layer of fat. Mammals have efficient circulatory systems with a four chambered heart, a well-developed brain, and four limbs. Most mammals are terrestrial, but there are about 1,000 winged species, including bats, and 80 aquatic species, including whales and dolphins. There are three major groups of mammals, divided by their embryonic development.

Monotremes, with only three living species, are mammals that lay eggs. The eggs are incubated in a nest, and after the eggs hatch, the young feed on the milk secreted by the fur of the mother. One species of monotreme, the platypus, lives in Tasmania and Australia. Besides the few monotreme species, all mammals are characterized by giving birth to live young.

Marsupials are the second type of mammals. All marsupials live in Australia, New Zealand, and Central and South America. Marsupials give birth to tiny embryos that are attached to the mother's nipple. These embryos grow in an external pouch attached to the mother's abdomen, called a marsupium. One example of a marsupial is a kangaroo, with a pouch on the ventral side.

The most common kind of mammals are eutherians, or placentals. The placentas of these mammals make a long-lasting relationship between the mother and child. About 95% of mammals are eutherians, and natural selection favors a long term relationship between the mother and child because the originally weak and helpless child needs protection.

Mammals evolved during the age of reptiles from a lineage of reptiles about 225 million years ago. After the extinction of dinosaurs at the end of the Mesozoic Age, the species diversity and population of mammals boomed because there was no more food and fewer predators. There are currently 4,500 species of mammals. The order Primates contains our species, Homo sapiens

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