Mammalia

Mammalia is a class of vertebrate animals characterized by the presence of mammary glands which in females produce milk for the nourishment of young; the presence of hair or fur; and specialized teeth. The class Mammalia includes around 5,400 species, distributed in about 1,200 genera, 153 families, and 29 orders, though this varies by classification scheme.

Characteristics

Mammals are distinguished by several unique features. The most distinctive of these is the presence of mammary glands used to nurse their young. Other defining characteristics of mammals include the presence of hair or fur, a high metabolic rate, a four-chambered heart, and a complex brain relative to other animals.

Classification

Mammals belong to the class Mammalia, which is divided into two primary subclasses: the Prototheria, representing the monotremes, and the Theria, which includes the infraclasses Metatheria and Eutheria. The Metatheria comprises the marsupials, while the Eutheria includes the placentals.

Evolution

The first mammals appeared during the Mesozoic Era, around 225 million years ago. They evolved from the synapsids, a group of reptiles which themselves evolved from the amphibians.

Diversity

Mammals inhabit every continent and most oceanic islands. Some species, such as the house mouse and the brown rat, have been transported around the world by humans. Mammals range in size from the tiny bumblebee bat to the massive blue whale.

Reproduction

Most mammals give birth to live young, but a few, such as the platypus and the echidna, lay eggs. The young of live-bearing mammals are typically born relatively helpless, but grow rapidly.

Conservation

Many species of mammals are threatened by habitat destruction caused by human activity. Some, such as the tiger and the giant panda, are endangered and protected by international agreements.

See also

• List of mammals
• Mammal classification
• Evolution of mammals