Mammals

Mammals are a group of warm-blooded animals that have hair or fur and secrete milk to feed their young Mammals are the smallest group of vertebrates, with just roughly 5,000 species have been described now known to science. All mammals are categorized in the eukaryote domain since their cells include nuclei, placing them in the animal kingdom's class Mammalia. Even whales and other aquatic creatures can grow hair or fur. Mammals use their hair as camouflage, to assist regulate their body temperature, to sense their environment, and for communication.

All female mammals have what is known as a mammary gland, a specialized sweat gland that produces milk through a process known as lactation. This milk is rich in fat and protein for feeding and also gives their young immunity from infections and viruses. Mammals have warm blood, or are endothermic, which means they generate their own heat to maintain a relatively constant body temperature.

The mammals' robust four chambered hearts, skin-surface sweat glands, and lungs with an effective gas exchange mechanism are all necessary to maintain this steady body temperature. Although most animals are placental, there are several exceptions. Many mammals carry their offspring inside a uterus, where the growing young receive nourishment from the mother through a unique organ called the placenta and are delivered alive. Monotremes, such as the duck-billed platypus, lay shell eggs, while marsupials, such as opossums, kangaroos, and koalas, feed and shelter their tiny young in pouches.

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