The biggest living member of the deer family is the moose (weighing about 800 kg); the smallest is the Andean Pudu (weighing about 10 kg).

The earliest deer appeared during the late Oligocene Epoch, roughly 35 million years ago. The "Irish elk" Megaloceros was a large, prehistoric deer that had antlers that were 11 feet (3.3 m) wide.

Habitat: Deer are found in many biomes around the world, including forests, rainforests, grasslands, and tundras.

Anatomy: Deer are long-legged animals with two-toed, hoofed feet (they are even-toed ungulates, belonging to the order Artiodactyla). They have short hair and a camouflaged coat (it helps hide them from their predators). The deer's snout is elongated and the tail is short. Males are usually larger than females The males of most species of deer have bony antlers; each year, the old antlers are shed and new antlers emerge the following spring.

Deer have a four-chambered stomach. When they eat food, it goes into the first chamber (called the rumen), where acids and bacteria help break down the tough plant fibers. Later the deer regurgitates the partly-digested food (called the bolus) and chews the "cud" again, and later re-swallows the food. The food then passes through the rumen into the second chamber of the stomach, and on to the third and fourth chambers of the stomach, and eventually, through the intestines where the digestion process is completed.

Diet: Deer are herbivores (plant eaters); most are browsers (eating leaves, shoots, soft vegetation, twigs, etc.), but some are also grazers (eating mostly grass).

Deer are ruminants; they store partly-chewed food, and later regurgitate this cud and thoroughly chew it (this is called "chewing the cud"). This process lets deer process a large amount of low nutrient food.

Predators: Deer have many predators; their main defenses are running away and hiding, although hooves, antlers, and teeth are used as a last resort. Their predators include large animals like grizzly bears, mountain lions, coyotes, and man. Small deer (like the pudu) are eaten by large birds, foxes, and small wild cats.

Classification: Kingdom Animalia (animals), Phylum Chordata, Class Mammalia, Order Artiodactyla (even-toed ungulates), Suborder Ruminantia (ruminants), Family Cervidae (deer, elk, moose, etc.), Subfamilies: Cervinae (elk, fallow deer, etc.), Hydropotinae (Chinese Water Deer), Muntiacinae (Barking Deer, Muntjac), Odocoileinae (moose, caribou, roe deer, Andean deer, mule deer, white-tailed deer, reindeer, pudu, marsh deer, etc.).

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