Deer are primarily herbivorous mammals that constitute Cervidae, the second most diverse family of artiodactyls after bovids (family Bovidae). There are 55 extant species of deer, most of which are characterized by long limbs adapted for running and jumping.
Almost all male deer possess bony outgrowths of the skull called antlers, which they use to attract females and assert their dominance over other males. In most species, these outgrowths are shed annually and are, in fact, one of the fastest-growing bony structures in the animal kingdom.
On average, deer measure between 2 and 4 ft (0.6 and 1.2 m) in shoulder height. However, the largest living species, the moose, is about 4 ft 7 in to 6 ft 11 in (1.4 to 2.1 m) tall and weighs up to 1,800 lb (800 kg), whereas the smallest, the northern pudu, is only about 13 to 14 in (32 to 35 cm) tall and weighs around 6.6 to 13.2 lb (3.3 to 6 kg).
In most species, males (bucks) tend to be larger than females (does).
A deer’s body is divided into a head, a flexible neck, a trunk, and a short tail. The head bears eyes, ears, nose, mouth, and jaws adapted for grazing, while the trunk houses vital organs, such as the heart, lungs, four-chambered stomach (since they are ruminants), and intestine.
The four limbs are long and slender, adapted for fast running and powerful jumping. Being artiodactyls (even-toed ungulates), deer walk on two main digits (digits 3 and 4), which are modified into cloven hooves. The other digits are either reduced or absent, making the limb easier to lift during jumps.
Males of all species except the water deer possess elaborately branched antlers. Although females typically lack them, reindeer females bear smaller, less branched antlers, which they use mainly for defensive purposes rather than sexual selection.
These structures develop from a permanent bony structure called the pedicel, which appears on the top of the skull by the time a deer is about one year old. In the next year, a single, unbranched spike antler forms from the pedicel, whereas by the third year, this spike is replaced by a larger, branched antler. Each year, the deer sheds its antlers and grows a new set that is usually larger and more complex than the previous one, a process that continues throughout life.
During growth, the antlers are covered with a soft, vascular tissue called velvet. With time, the blood supply to the tissue is cut off, and calcium and phosphorus get deposited in the tissue, rendering hard, fully mineralized antlers.
Depending on the species, antlers vary widely in both shape and size. For example, fallow deer possess the largest and heaviest antlers, distinguished by a broad, flattened central portion that resembles an open hand. In contrast, pudus have the lightest antlers, which are simple and unbranched, appearing as short spikes.
Most species have 32 teeth, with a dental formula of 0.0.3.3/3.1.3.3. Although most deer lack upper canines, elk and reindeer retain them. Others, like Chinese water deer, tufted deer, and muntjac, have enlarged upper canines that are modified into sharp tusks.
The color of their fur typically ranges from red to brown, with some species, such as the chital and sika deer, also featuring white spots.
The 55 extant species of deer are classified into 2 subfamilies, 5 tribes, and 19 genera.[1]
Deer are native to all continents except Antarctica and Australia, thereby occupying a broad range of habitats, ranging from Arctic tundra to tropical savannas. However, several species, including red deer, sambar, and chital, have been introduced to Australia and New Zealand, where they now have established wild populations.
The highest diversity of deer is found in Asia. The tropical seasonal forests and savannas of the Indian subcontinent are home to the shital, barasingha, sambar, and Indian muntjac, while in temperate East Asia, species such as the sika deer and Siberian roe deer occur in mixed deciduous and coniferous forests of northeastern China, Korea, and the Russian Far East.
Europe has fewer native species, including the red deer, roe deer, and fallow deer, which inhabit temperate deciduous forests, wetlands, and mountainous regions such as the Scottish Highlands, Alps, and Carpathians.
North America houses some of the largest deer species, such as white-tailed deer and mule deer, which dominate temperate forests and grasslands. On the other hand, the reindeer is adapted to extremely cold tundra regions. The white-tailed deer is most commonly found in ecotones, the transition zone between forests and grasslands.
In Central and South America, brocket deer inhabit dense lowland rainforests, where they typically lead a solitary life. In the Andes, taruca and Chilean huemul occupy montane to subalpine habitats, while wetland systems support the marsh deer.
Africa is unique in having only one native deer, the Barbary stag (a subspecies of red deer), which is restricted to the forests of the Atlas Mountains.
These animals are typically herbivorous, feeding mainly on forbs, shrubs, and the leaves of trees (browsing), as well as grasses and sedges (grazing) when available. Compared to other ruminants, deer have small, unspecialized stomachs and therefore feed on easily digestible plant parts, such as tender shoots, young leaves, and flowers, as well as fungi. However, during winter, reindeer rely heavily on high-carbohydrate lichens (also called reindeer moss), which they dig out from the snow.[2]
Moose feed underwater, especially during the peak of summer, when they consume mineral-rich aquatic plants like water lilies, pondweed, and horsetail. During winter, they switch to woody browse.[3]
Deer are crepuscular animals, meaning they are active either at dawn or dusk.
Most species typically run at speeds of 25 to 30 mph (40 to 48 km/h), particularly when escaping predators. Many species, such as the white-tailed deer, attain maximum speeds of 40 mph (60 km/h).[4] In fact, this species can also cover 8.5 ft vertically and 30 ft horizontally at full speed in a single jump.[5]
On average, male deer live about 3 years in the wild, while females survive around 6 years.[6]
Deer mate during the rut, their breeding season, typically beginning in fall (between late August and December). The onset of breeding is regulated mainly by day length (photoperiod) rather than temperature, with shortening days triggering hormonal changes. During rut, the males undergo physiological and behavioral changes driven by rising testosterone levels. They actively compete with other males for access to females through posturing, vocalizations such as grunts or roars, and antler fights.
The females enter estrus for a short period, usually lasting 24 to 48 hours, during which ovulation occurs, and they are sexually receptive. They tend to prefer dominant, healthy males that exhibit posturing and vocalizations such as grunts or roars, and emerge triumphant from antler fights.
After a brief period of mating, females typically become solitary, while males experience a rapid decline in testosterone levels and eventually shed their antlers. After about 6.5 to 7 months, the female gives birth to 1 to 2 relatively developed (precocial) fawns in late spring to early summer. Each fawn is born with a reddish-brown coat and white spots that usually fade before the first winter, leaving the coat uniformly brown. In the first 20 minutes or so of a fawn’s life, it learns to take its first steps.
As the fawns grow, they steadily increase in size and strength, with male fawns beginning to develop their first simple antlers. Females typically reach sexual maturity within their first year, whereas males mature more slowly, usually attaining sexual maturity at around 1.5 years of age.[7]
Adult deer are primarily preyed upon by wolves and large cats such as tigers and cougars, while fawns are more vulnerable to smaller predators, including coyotes, foxes, bears, and bobcats.[8] In some regions, reptiles such as crocodiles and alligators attack deer when they come to rivers to drink. Moreover, humans exert the greatest predatory pressure on deer populations worldwide through hunting and poaching.
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