Antlers
With the exception of the Chinese Water Deer, all male deer have antlers that are shed and regrown every year from a structure called a pedicle. Sometimes a female will have a small stub. The only female deer with antlers are Reindeer. Antlers grow as highly vascular spongy tissue covered in a skin called velvet. Before the beginning of a species' mating season, the antlers calcify under the velvet and become hard bone. The velvet is then rubbed off leaving dead bone which forms the hard antlers. After the mating season, the pedicle and the antler base are separated by a layer of softer tissue, and the antler falls off.
One way that many hunters are able to track main paths that the deer travel on is because of their "rubs". A rub is used to deposit scent from glands near the eye and forehead and physically mark territory.
During the mating season, bucks use their antlers to fight one another for the opportunity to attract mates in a given herd. The two bucks circle each other, bend back their legs, lower their heads, and charge.
Each species has its own characteristic antler structure – for example white-tailed deer antlers include a series of tines sprouting upward from a forward-curving main beam, while Fallow Deer and Moose antlers are palmate, with a broad central portion. Mule deer species within the same genus as the white-tailed deer, instead have bifurcated (or branched) antlers—that is, the main beam splits into two, each of which may split into two more. Young males of many deer, and the adults of some species, such as brocket deer and pudus, have antlers which are single spikes.
Most species of deer in the "True Deer" subfamily have large, impressive antlers with several tines that are highly prized by game hunters and collectors. Four Members of the Odocoleinae subfamily whose antlers are also popular and sought after are the moose, caribou, White-tailed deer, and mule deer. The most impressive White-tailed deer antlers come from populations in Texas, the Northern Great Plains Region,and the Great Lakes/Midwest Agricultural Region. The most impressive mule deer antlers come from populations in the Rocky Mountains and the deserts of the Southwestern United States and Northern Mexico. The most impressive moose and caribou antlers come from populations living in Siberia, Canada, and Alaska. For Elk and Red Deer, a stag having 14 points is an "imperial", and a stag having 12 points is a "royal". Occasional individual red deer males may have no antlers: these are known as hummels, and they may grow significantly larger than normal males.

Color

Piebald Deer
A piebald deer is a deer with a brown and white spotting pattern which is not caused by parasites or diseases. They can appear to be almost entirely white. In addition to the non-standard coloration, other differences have been observed: bowing or Roman nose, overly arched spine long tails, short legs, and underbites.

White Deer
Seneca County, New York State maintains the largest herd of white deer. White pigmented White-tailed Deer began populating the deer population in the area now known as the Conservation Area of the former Seneca Army Depot. The U.S. Army gave the white deer protection while managing the normal colored deer through hunting. The white deer coloration is the result of a recessive genes.

Evolution
The earliest fossil deer including Heteroprox date from the Oligocene of Europe, and resembled the modern muntjacs. Later species were often larger, with more impressive antlers. They rapidly spread to the other continents, even for a time occupying much of northern Africa, where they are now almost wholly absent. Some extinct deer had huge antlers, larger than those of any living species. Examples include Eucladoceros, and the giant deer Megaloceros, whose antlers stretched to 3.5 metres across.

Economic significance
Deer have long had economic significance to humans. Deer meat, for which they are hunted and farmed, is called venison. Deer organ meat is called umble.
The Sami of Scandinavia and the Kola Peninsula of Russia and other nomadic peoples of northern Asia use reindeer for food, clothing, and transport.
The caribou in North America is not domesticated or herded as is the case of reindeer (the same species) in Europe, but is important as a quarry animal to the Inuit. Most commercial venison in the United States is imported from New Zealand.
Deer were originally brought to New Zealand by European settlers, and the deer population rose rapidly. This caused great environmental damage and was controlled by hunting and poisoning until the concept of deer farming developed in the 1960s. Deer farms in New Zealand number more than 3,500, with more than 400,000 deer in all.
Automobile collisions with deer can impose a significant cost on the economy. In the U.S., about 1.5 million deer-vehicle collisions occur each year, according to the National Highway Traffic Safety Administration. Those accidents cause about 150 human deaths and $1.1 billion in property damage annually.

Taxonomy
Note that the terms indicate the origin of the groups, not their modern distribution: the water deer, for example, is a New World species but is found only in China and Korea.
It is thought that the new world group evolved about 5 million years ago in the forests of North America and Siberia, the old world deer in Asia.

Hybrid deer
In On the Origin of Species (1859), Charles Darwin wrote "Although I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, I have some reason to believe that the hybrids from Cervulus vaginalis and Reevesii are perfectly fertile." These two varieties of muntjac are currently considered the same species.
A number of deer hybrids are bred to improve meat yield in farmed deer. American Elk (or Wapiti) and Red Deer from the Old World can produce fertile offspring in captivity, and were once considered one species. Hybrid offspring, however, must be able to escape and defend themselves against predators, and these hybrid offspring are unable to do so in the wild state. Recent DNA, animal behavior studies, and morphology and antler characteristics have shown there are not one but three species of Red Deer: European Red Deer, Central Asian Red Deer, and American Elk or Wapiti. The European Elk is a different species and is known as moose in North America. The hybrids are about 30% more efficient in producing antlers by comparing velvet to body weight. Wapiti have been introduced into some European Red Deer herds to improve the Red Deer type, but not always with the intended improvement.
In New Zealand, where deer are introduced species, there are hybrid zones between Red Deer and North American Wapiti populations and also between Red Deer and Sika Deer populations. In New Zealand, Red Deer have been artificially hybridized with Pere David Deer in order to create a farmed deer which gives birth in spring. The initial hybrids were created by artificial insemination and back-crossed to Red Deer. However, such hybrid offspring can only survive in captivity free of predators.
In Canada, the farming of European Red Deer and Red Deer hybrids is considered a threat to native Wapiti. In Britain, the introduced Sika Deer is considered a threat to native Red Deer. Initial Sika Deer/Red Deer hybrids occur when young Sika stags expand their range into established red deer areas and have no Sika hinds to mate with. They mate instead with young Red hinds and produce fertile hybrids. These hybrids mate with either Sika or Red Deer (depending which species is prevalent in the area), resulting in mongrelization. Many of the Sika Deer which escaped from British parks were probably already hybrids for this reason. These hybrids do not properly inherit survival strategies and can only survive in either a captive state or when there are no predators.
In captivity, Mule Deer have been mated to White-tail Deer. Both male Mule Deer/female White-tailed Deer and male White-tailed Deer/female Mule Deer matings have produced hybrids. Less than 50% of the hybrid fawns survived their first few months. Hybrids have been reported in the wild but are disadvantaged because they don't properly inherit survival strategies. Mule Deer move with bounding leaps (all 4 hooves hit the ground at once, also called "stotting") to escape predators. Stotting is so specialized that only 100% genetically pure Mule Deer seem able to do it. In captive hybrids, even a one-eighth White-tail/seven-eighths Mule Deer hybrid has an erratic escape behaviour and would be unlikely to survive to breeding age. Hybrids do survive on game ranches where both species are kept and where predators are controlled by man.