Depending on their size and species, male deer are called bucks (smallest), stags, or bulls (largest) and females are called does, hinds or cows. Young deer are called kids, fawns or calves. A group of deer is commonly called a herd. Hart, from Old English heorot "deer", is an alternative term for a stag, particularly a Red Deer stag past its fifth year (compare with the modern Dutch word hert meaning deer). The county Hertfordshire is named after a place where deer ford a watercourse. The word hart is not commonly used, but Shakespeare makes several references, punning on the sound-alike "hart" and "heart", for example in Twelfth Night. "The White Hart" and "The Red Hart" are common English pub names. Whinfell Forest once contained a landmark tree called Harthorn. The word "Hart" also exists in Saterfrisian, being a synonym for the Word "Hirsk" which is more similar to the German Word "Hirsch".
Deer live in a variety of biomes ranging from tundra to the tropical rainforest. While often associated with forests, many deer are ecotone species that live in transitional areas between forests and thickets (for cover) and prairie and savanna (open space). The majority of large deer species inhabit temperate mixed deciduous forest, mountain mixed coniferous forest, tropical seasonal/dry forest, and savanna habitats around the world. Clearing open areas within forests to some extent may actually benefit deer populations by exposing the understory and allowing the types of grasses, weeds, and herbs to grow that deer like to eat. Additionally, access to adjacent croplands may also benefit deer. However, adequate forest or brush cover must still be provided for populations to grow and thrive.
Small species such as the brocket deer and pudus of Central and South America, and the muntjacs of Asia generally occupy dense forests and are less often seen in open spaces, with the possible exception of the Red Brocket and the Indian Muntjac. There are also several species of deer that are highly specialized, and live almost exclusively in mountains, grasslands, swamps and "wet" savannas, or riparian corridors surrounded by deserts. Some deer have a circumpolar distribution in both North America and Eurasia. Examples include the reindeer (caribou) that live in Arctic tundra and taiga (boreal forests) and moose that inhabit taiga and adjacent areas. Huemul Deer (taruca and Chilean Huemul) of South America's Andes Mountains fill an ecological niche of the ibex or wild goat, with the fawns behave more like goat kids.
The highest concentration of large deer species in temperate North America lies in the Canadian Rocky Mountain and Columbia Mountain Regions between Alberta and British Columbia where all five North American deer species (White-tailed Deer, Mule deer, Caribou, Elk, and Moose) can be found. This is a region has several clusters of national parks including Mount Revelstoke National Park, Glacier National Park (Canada), Yoho National Park, and Kootenay National Park on the British Columbia side, and Banff National Park, Jasper National Park, and Glacier National Park (U.S.) on the Alberta and Montana sides. Mountain slope habitats vary from moist coniferous/mixed forested habitats to dry subalpine/pine forests with alpine meadows higher up. The foothills and river valleys between the mountain ranges provide a mosaic of cropland and deciduous parklands. The rare woodland caribou have the most restricted range living at higher altitudes in the subalpine meadows and alpine tundra areas of some of the mountain ranges. Elk and Mule Deer both migrate between the alpine meadows and lower coniferous forests and tend to be most common in this region. Elk also inhabit river valley bottomlands, which they share with white-tailed deer. The White-tailed Deer have recently expanded their range within the foothills and river valley bottoms of the Canadian Rockies owing to conversion of land to cropland and the clearing of coniferous forests allowing more deciduous vegetation to grow up the mountain slopes. They also live in the aspen parklands north of Calgary and Edmonton, where they share habitat with the moose. The adjacent Great Plains grassland habitats are left to herds of Elk, American bison, and pronghorn antelope.
The Eurasian Continent (including the Indian Subcontinent) boasts the most species of deer in the world, with most species being found in Asia. Europe, in comparison, has lower diversity in plant and animal species. However, many national parks and protected reserves in Europe do have populations of Red Deer, Roe Deer, and Fallow Deer. These species have long been associated with the Continent of Europe, but also inhabit Asia Minor, the Caucasus Mountains, and Northwestern Iran. "European" Fallow Deer historically occurred over much of Europe during the Ice Ages, but afterwards became restricted primarily to the Anatolian Peninsula, in present-day Turkey. Present-day Fallow deer populations in Europe are a result of historic man-made introductions of this species first to the Mediterranean regions of Europe, then eventually to the rest of Europe. They were initially park animals that later escaped and re-established themselves in the wild. Historically, Europe's deer species shared their deciduous forest habitat with other herbivores such as the extinct tarpan (forest horse), extinct aurochs (forest ox), and the endangered wisent (European bison). Good places to see deer in Europe include the Scottish Highlands, the Austrian Alps, the wetlands between Austria, Hungary, and Czech Republic. Some fine National Parks include Donana National Park in Spain, the Veluwe in Netherlands, the Ardennes in Belgium, and Bialowieza National Park of Poland. Spain, Eastern Europe, and the Caucasus Mountains still have virgin forest areas that are not only home to sizable deer populations but also for other animals that were once abundant such as the wisent, Eurasian lynx, Spanish lynx, wolves, and brown bears.
The highest concentration of large deer species in temperate Asia occurs in the mixed deciduous forests, mountain coniferous forests, and taiga bordering North Korea, Manchuria (Northeastern China), and the Ussuri Region (Russia). These are among some of the richest deciduous and coniferous forests in the world where one can find Siberian Roe Deer, Sika Deer, Elk, and Moose. Asian Caribou occupy the northern fringes of this region along the Sino-Russian border.
Deer such as the Sika Deer, Thorold's Deer, Central Asian Red Deer, and Elk have historically been farmed for their antlers by Han Chinese, Turkic peoples, Tungusic peoples, Mongolians, and Koreans. Like the Sami people of Finland and Scandinavia, the Tungusic peoples, Mongolians, and Turkic peoples of Southern Siberia, Northern Mongolia, and the Ussuri Region have also taken to raising semi-domesticated herds of Asian caribou.
The highest concentration of large deer species in the tropics occurs in Southern Asia in Northern India's Indo-Gangetic Plain Region and Nepal's Terai Region. These fertile plains consist of tropical seasonal moist deciduous, dry deciduous forests, and both dry and wet savannas that are home to Chital, Hog Deer, Barasingha, Indian Sambar, and Indian Muntjac. Grazing species such as the endangered Barasingha and very common Chital are gregarious and live in large herds. Indian Sambar can be gregarious but are usually solitary or live in smaller herds. Hog Deer are solitary and have lower densities than Indian Muntjac. Deer can be seen in several national parks in India, Nepal, and Sri Lanka of which Kanha National Park, Dudhwa National Park, and Chitwan National Park are most famous. Sri Lanka's Wilpattu National Park and Yala National Park have large herds of Indian sambar and chital deer. The Indian sambar are more gregarious in Sri Lanka than other parts of their range and tend to form larger herds than elsewhere.
The Chao Praya River Valley of Thailand was once primarily tropical seasonal moist deciduous forest and wet savanna that hosted populations of Hog Deer, the now-extinct Schomburgk's Deer, the Eld's Deer, Indian Sambar, and Indian Muntjac. Both the Hog Deer and Eld's Deer are rare, whereas Indian Sambar and Indian Muntjac thrive in protected national parks such as Khao Yai.
Many of these South Asian and Southeast Asian deer species also share their habitat with various herbivores such as Asian elephants, various Asian rhinoceros species, various antelope species (such as nilgai, four-horned antelope, blackbuck, and Indian gazelle in India), and wild oxen (such as Wild Asian Water Buffalo, gaur, banteng, and kouprey).
Australia has six introduced species of deer that have established sustainable wild populations from Acclimatisation Society releases in the 19th Century. These are Fallow Deer, Red Deer, Sambar Deer, Hog Deer, Rusa deer, and Chital Deer. Red Deer introduced into New Zealand in 1851 from English and Scottish stock were domesticated in deer farms by the late 1960s and are common farm animals there now. Seven other species of deer were introduced into New Zealand but none are as widespread as Red Deer.
Nearly all deer have a facial gland in front of each eye. The gland contains a strongly scented pheromone, used to mark its home range. Bucks of a wide range of species open these glands wide when angry or excited. All deer have a liver without a gallbladder. Deer also have a Tapetum lucidum which gives them sufficiently good night vision.
A doe generally has one or two fawns at a time (triplets, while not unknown, are uncommon). The gestation period is anywhere up to ten months for the European roe deer. Most fawns are born with their fur covered with white spots, though they lose their spots once they get older (excluding the Fallow Deer who keeps its spots for life). In the first twenty minutes of a fawn's life, the fawn begins to take its first steps. Its mother licks it clean until it is almost free of scent, so predators will not find it. Its mother leaves often, and the fawn does not like to be left behind. Sometimes its mother must gently push it down with her foot. The fawn stays hidden in the grass for one week until it is strong enough to walk with its mother. The fawn and its mother stay together for about one year. A male usually never sees his mother again, but females sometimes come back with their own fawns and form small herds.
Deer are selective feeders. They are usually browsers, and primarily feed on leaves. They have small, unspecialized stomachs by ruminant standards, and high nutrition requirements. Rather than attempt to digest vast quantities of low-grade, fibrous food as, for example, sheep and cattle do, deer select easily digestible shoots, young leaves, fresh grasses, soft twigs, fruit, fungi, and lichens.
With the exception of the Chinese water deer, all male deer have antlers that are shed and regrown each year from a structure called a pedicle. Sometimes a female will have a small stub. The only female deer with antlers are Reindeer (Caribou). 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 torn away leaving hard antlers. After the mating season, the pedicle and the antler base are separated by a layer of tissue, and the antler falls off.
The 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, e.g. each white-tailed deer antler includes a series of tines sprouting upward from a forward-curving main beam. Mule deer (and Black-tailed 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.
Most species of deer in the "True Deer" subfamily (Cervinae) have large, impressive antlers with several tines 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, Northern Great Plains Region, Great Lakes Region, and the Piedmont (United States). The most impressive mule deer antlers come from populations in the Rocky Mountains. 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". If the antlers deviate from the species' normal antler structure, the deer is considered a non-typical deer.
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 deaths and $1.1 billion in property damage annually.
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 antler 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.