Deer - Mating strategies
Deer are members of the order Artiodactyla, the even-toed ungulates. This order also includes the antelopes, bovines, and giraffes. Deer are generally slender and long-legged, and their most striking characteristic is the presence of antlers, which are often used to differentiate species.
The deer family, Cervidae, includes about 45 species, which are divided among 17 genera and five subfamilies: the Hydropotinae, the Chinese water deer; the Muntiacinae, the muntjacs of Asia; the Cervinae, the Eurasian deer; the Odocoleinae, the New World deer, moose, and caribou; and the Moschinae, the musk deer of China, Southeast Asia, and the Himalayas. Some taxonomists argue that the Moschinae should not be considered a subfamily of the Cervidae, but an entirely separate family (Moschidae), based on the relatively pronounced differences between Moschus and other deer. Unlike other deer, Moschus has a gall bladder, and where the females of other species have two pairs of teats, Moschus has only one pair.
Deer have short hair ranging in color from yellowish to dark brown. The underbelly and throat are lighter colored, and many species have a distinctive rump patch, an area of light hair fringed with darker hair. (A startled deer will lift its tail and flash the white of its rump patch as an alarm to other deer nearby.) The head of deer is angular, with the eyes set well on the side. The ears are oblong and the nose is usually covered with soft hair. The senses of hearing and smell are excellent. Vision is less so, as far as giving the animal an accurate picture of the world around it. Although a deer cannot accurately perceive form at distances greater than about 200 ft (60 m), it can detect slight movements up to 1,000 ft (300 m) away.
Besides the flash of the rump patch, deer communicate through sound and smell, and they produce a variety of vocalizations, from the roar of the red deer to the bark of the muntjac. Deer also have scent glands near their eyes, which they use to mark their territory on branches and twigs. Dung is also used as a territorial marker. Males will sniff a female's urine to learn if she is in estrus.
The legs of deer are long and slender, well-suited for fast running to escape their many predators. During evolutionary time, the leg bones of deer became longer and the weight of the animal became supported entirely on the third and fourth toes, eventually resulting in the evolution of cloven hooves. The second and fifth toes are short and positioned up, as dewclaws. The first digit has vanished and the bones of the palm (metacarpals and metatarsals) have been forged into a single bone, the cannon bone. Similar evolutionary changes have occurred in other herbivores that run to escape predators, such as horses.
Deer range in size from the Pudu (two species, standing 10-17 in [25-43 cm] at the shoulder and weighing 13-29 lb [6-13 kg]) to Alces, the moose, which stands 56-94 in (140-235 cm) at the shoulder and weighs 440-1,900 lb (200-850 kg). Most species of deer have antlers, which are usually found only on the males. However, in Rangifer, the caribou, both sexes have antlers. Other species, such as the Chinese water deer and the tufted deer, have tusks. Tusked deer are considered to be more evolutionarily ancient than those with antlers, because tusks are characteristic of the primitive chevrotains, or mouse deer. More recent species of deer are generally considered to have larger bodies, larger and more complex antlers, and a more gregarious social system.
Deer originated in Eurasia in the Oligocene, and were present in North America in the Miocene, and in South America in the Pleistocene. Perhaps the most well-known fossil species is the Irish elk (Megaloceros gigantus). Although not as large as the modern moose, Megaloceros carried a rack of antlers that had a spread of 6 ft (1.8 m) and weighed more than the rest of the animal's skeleton. Analysis of fossil specimens suggests that the Irish elk was well-suited for life in the open and able to run quickly for long distances. The common name is misleading, for Megaloceros was neither an elk nor exclusively Irish, although the first specimens were found in Irish peat bogs. (Long before Megaloceros came to the attention of science, the Irish were using its great antlers as gateposts and, in County Tyrone, even as a temporary bridge.)
Deer occur naturally throughout most of the world, with the exception of sub-Saharan Africa, Australia, and Antarctica. As an introduced species, deer have thrived in Australia, New Zealand, Cuba, New Guinea, and other places. For large herbivores, they are remarkably adaptable. Although most typically fond of wooded areas, some deer have adapted to semi-aquatic habitats (the Chinese water deer and moose), open grasslands (the Pampas deer of South America), and the arctic tundra (the caribou). Slowly, deer are returning to areas frequented by humans; in suburban America, white-tailed deer are becoming a common backyard sight, and throughout the mountains of New Hampshire and Maine roadsigns warn of moose crossing.
Deer are herbivores. Lacking upper incisors, they bite off forage by pressing their lower incisors against a callous pad on the upper gum. Their teeth have low crowns, well-suited to their diet that, depending on the species, includes twigs, leaves, aquatic plants, fruits, lichens, and grass. During hard winters, white-tailed deer may strip and eat the bark from trees. Furthermore, some temperate species, such as the white-tailed deer, actually alter their metabolism during the winter, lessening their need for food and therefore decreasing the likelihood of starvation. So strong is this natural adaptation, that even in captivity these species will eat less in the winter, even if the amount of food available remains constant.
Like other artiodactyls, deer are ruminants with a four-chambered stomach. When food is first eaten, it is stored in the first chamber, the rumen, where bacteria begin to break it down. It is later regurgitated into the mouth and, as a cud, is chewed again and mixed with saliva. When swallowed a second time, the food bypasses the rumen and goes into the second stomach chamber, the reticulum, and then passes into the third chamber, the omasum, and then into the fourth chamber, the abomasum. Food then moves into the small intestine, where nutrients are absorbed. Although this entire process takes about 80 hours, it converts about 60% of the cellulose in the food into usable sugars and thus is remarkably effective.
Deer vary their diet depending on the seasonal availability of forage and their nutritional needs. Fallow deer, for instance, eat a great deal of grass; it comprises about 60% of their diet during the summer. In the fall, there is less grass but more fruit is available, such as acorns. As the grass proportion of their diet declines, the deer turn to fruit, which at the height of fall makes up almost 40% of their food intake. The winter diet consists of browse, that is, woody stems of shrubs such as ivy and holly.
A three-year study of moose living on Isle Royale, Michigan, determined three major limiting factors on what moose could eat to satisfy their nutritional requirements. First was the capacity of the rumen, second was the time available for feeding, and third was the need for sodium, an important nutrient that is difficult to obtain on this glacier-scrubbed island in Lake Superior. Researchers calculated that to meet their needs, the moose would have to eat particular amounts of both terrestrial plants and higher-sodium aquatic plants each day. Remarkably, the observed diet of the moose in the study matched the scientists' predictions.
Like all herbivores, deer must spend a great deal of time eating in order to obtain sufficient nutrition from their food. Studies of wild red deer in Scotland found that females without calves spent 9.8 hours foraging each summer day, while the larger males spent 10.4 hours. Lactating females with calves spent 11.1 hours per day feeding. Spending large amounts of time feeding makes deer vulnerable to predators, but the tendency to herd, and the ability to eat fast and store food in the rumen, help make them less vulnerable.
In North America, predators of adult deer include the brown bear, bobcat, cougar, coyote, wolf, wolverine, and packs of roving domesticated dogs, while golden eagles sometimes take young deer. In South America, deer are taken by jaguars. Eurasian deer must deal with dholes (wild dogs), tigers, and wolves. One reptilian predator, the Komodo dragon of Indonesia, depends largely on the Timor hog deer (Cervus rusak timoensis). Deer have long been hunted by humans as well. Other causes of death include fighting between males, automobile and train accidents, falling through ice and drowning, becoming entangled in fences or stuck in the crotches of trees when reaching high for browse, being caught in forest fires, becoming stuck in swampy areas, and falling over snow-covered banks or cliffs. Many of the deer shot by hunters escape only to die of their wounds later. Particularly harsh winters also decimate deer populations.
Deer antlers are found primarily in the males and are a social and sexual symbol as well as a weapon. The huge antlers of the Irish elk were the long-term result of sexual selection, whereby females consistently bred with males that had the largest antlers. This can explain how the gene for larger and larger antlers was passed down through generations until the tremendous 6 ft (1.8 m) span was reached.
Antlers differ from horns. Horns are a permanent outgrowth of the skull and are covered by a layer of keratin. Antlers, on the other hand, are grown and shed annually. They consist of a bare bony core supported on bony disks, called pedicles, that are part of the skull. There is a tremendous investment of energy in the regrowth of antlers, which regrow to be more elaborate with each year as the deer ages, adding more prongs, or "points." Antlers are often damaged during mating-season fights, which seriously curtail a male's reproductive success. A study of red deer males with damaged antlers showed that they had less mating success than did males with intact racks. However, the experimental removal of the antlers of a dominant male showed it to be only a temporary setback; despite the loss, the male retained his status and his females.
The antlers of temperate-region species of deer begin to grow during early summer. The growing antlers are covered by a thin layer of skin covered by short, fine hairs. Aptly called velvet, this skin nourishes the antlers with a plentiful supply of blood until they reach full growth in late summer. The blood supply then ceases, and the velvet dries up. The deer rubs off the velvet to reveal fresh new and, for a short time after shedding the velvet, gory antlers.
Those species of deer considered to be more evolutionarily recent are generally more gregarious, but the diet of these species may also be related to social organization. Those deer that primarily browse, such as roe deer, live in small groups or alone, for their food is generally found only in small patches. On the other hand, caribou, which graze on lichens and sedges over extensive open areas, may occur in large herds of several thousand animals. Such grazers may find an extra benefit in their herding behavior, with extra eyes and ears alert for predators.
During the mating season of temperate species, males use one of three strategies to obtain access to receptive females. They may defend a territory that overlaps the ranges of females, as does the muntjac. They may defend a single doe against all suitors, as does the white-tailed deer. Or they may attempt to assemble and hold a harem of females, as does the red deer (Cervus elaphus). The males and females of this gregarious species spend most of the year in single-sex herds, which generally have particular ranges. Come September, the females gather in rutting areas, and are soon joined by the males, which compete for the females through displays of roaring, spraying urine, and fighting.
Fighting begins when the challenger appears, and he and the holder of the harem roar at each other. After several minutes of vocalizing, they walk parallel to each other, tense and alert, until one of them turns toward the other and lowers his antlers. They lock antlers and begin shoving each other. When one succeeds in pushing the other backwards, the loser runs off.
The fights are dangerous. Almost a quarter of the males in a Scottish study were injured during the rut, 6% permanently. A male between ages of seven to 10 has the best chance of winning such an encounter, which a harem holder must face about five times during the mating season. There is another danger besides injury; young males often lurk at the fringes of a harem, waiting until the harem holder is distracted and then spiriting away a female or two. The apparent benefits of holding a harem are deceiving; although there may be as many as 20 females in the harem, the male will father only about four or five calves.
Females of tropical species of deer come into estrus several times a year. Gestation lasts from 176 days in the Chinese water deer to 294 days in the roe deer. The female deer delivers from one to six young (six in Hydropotes), but one or two is the norm. The young of most deer are born spotted.
The males of the Cervinae (such as the red deer) are called stags, the females, hinds, and the young, calves. Among the Odocoileinae, the male deer are called bucks, the females does, and the young fawns. Exceptions are Alces (moose) and Rangifer (caribou), in which the males are bulls, the females, cows, and the young, calves.
Besides the moose, other North American species include the white-tailed deer (Odocoileus virginianus) found from southern Canada, throughout most of the United States, Mexico, and down to Bolivia and northeastern Brazil. The white-tailed deer may be the most abundant species of wild large mammal, with a population of about 60 million individuals. The mule deer (O. hemionus), named for its large ears, ranges from the southern Yukon and Manitoba to northern Mexico. The tiny Key deer (O. v. clavium) is an endangered sub-species of the white-tailed deer; only about 250 remain in the western Florida Keys.
F. C. Nicholson