Antelope - biology.

Antelope - biology. I INTRODUCTION Antelope, common name applied to a diverse group of hollow-horned mammals that belong to the same family as cattle, goats, and sheep. About 100 species of antelopes live in Africa and Asia, including some of the world's fastest and most elegant hoofed mammals, as well as some of the most endangered. In size, antelopes range from the tiny royal antelope, which stands about 25 cm (10 in) high at the shoulder, to the massively built common eland, which can weigh as much as 900 kg (nearly 2,000 lb)--more than an average car. Some better-known antelopes include impalas, gazelles, and gnus. Antelopes vary widely in their physical appearance and the way they live. Some antelopes, including many of the smallest species, stay close to vegetative cover and disappear into the undergrowth if disturbed, but most live in more open habitats where they rely on speed and alertness to escape attack. The species that flee also use their superb jumping skills to escape from predators: impalas, for example, can leap over fences 3 m (10 ft) high, and cover 10 m (33 ft) in a single bound. Unlike deer, which have branched antlers that they shed annually, antelopes have pointed horns that they keep throughout life. Antelope horns can grow up to 1.5 m (about 5 ft) long; they look formidable but their value as weapons is limited. There are records of antelopes impaling (spearing) and even killing predators as large as lions, but when faced with danger antelopes are far more likely to run away. Some antelopes are solitary but most live in herds. In the late 1800s, herds of springbok in southern Africa sometimes included over 10 million animals that spread over a distance of 160 km (100 mi). Although herds no longer reach this astounding size today, antelopes still dominate life in African plains. They help grass thrive by nibbling away competing plants, and they provide food for predators and also for people. II RANGE AND HABITAT About 5 million years ago at the beginning of the Pliocene Epoch, antelopes were widespread throughout Europe, Asia, and Africa. Today they live only in Africa and in central and southern Asia, with Africa having the greatest number of species. Antelopes have never lived in the Americas, although one North American mammal, the pronghorn, looks very much like an antelope and shares a similar way of life. Although antelopes all eat plants, some are grazers, preferring to feed on grasses on the ground, while others browse vegetation from low-lying branches on trees and bushes. These feeding preferences determine their habitat. Some species, such as Thomson's gazelles, live almost entirely on grass and rarely turn to any other kind of plant food. Thomson's gazelles feed where the grass is short, a preference that restricts them to dry, open plains. By contrast, the common reedbuck feeds in swampy ground. It specializes in eating coarse grasses that other grazing antelopes find difficult to digest. Browsing antelopes, unlike grazers, live in a wider variety of habitats, from tree-studded savannas to dense rain forests. Some of them feed exclusively on leaves, but most eat other kinds of food as well. This additional food usually includes flowers and fruit, but in the case of small forest antelopes called duikers, it also includes birds and animal remains. Water also plays an important part in determining where antelopes live. For example, gnus need to drink every day, so they cannot wander more than about 15 km (9 mi) from the nearest river or waterhole. Other species can get all the water they need from their food. The addax, a critically endangered antelope that was once widespread along the southern fringes of the Sahara Desert, needs at least 3 litres (5 pints) of water a day. However, the addax can absorb all the water it needs from roots, bulbs, and fruit, enabling it to survive indefinitely without drinking from a waterhole. III PHYSICAL DESCRIPTION Antelopes differ from one another almost as much as they differ from other members of the cattle, goat, and sheep family. Size is one of the most striking and variable features of antelopes: The common eland towers over most breeds of domestic cattle and can be 300 times heavier than the royal antelope. In many species, including the eland, males are considerably larger than females. In a few species, such as the Asian black buck, males and females also differ in color. Antelopes have a dense coat with short fur. Most antelopes have fawn or brown fur that helps camouflage them as they feed. But there are some exceptions to this rule. The rare zebra duiker has dark vertical stripes, while the gemsbok has gray and black fur and a vivid black-and-white face. A common feature of gazelles is a white rump, which flashes a warning signal to others when they run from danger. One gazelle, the springbok, also has a pouch of white brushlike hairs running along its back. When a springbok is alarmed, its pouch opens up, and the hairs stand on end. All antelopes have long slender legs. Powerful muscles where the upper legs meet the antelope's body provide leverage, increasing leg stride and speed. Although antelopes are good jumpers they are not great climbers, particularly when compared with sheep and goats. But a few do exhibit good balance--most notably the klipspringer, which stands on the tips of its hooves. Another African species, the gerenuk, is one of the few antelopes that habitually stands on its back legs. It does this to reach leaves in trees: As it stretches upward, the gerenuk uses its front legs to hold itself steady against the tree. Antelopes are ruminants, animals that regurgitate partially digested food, called cud, and chew it again. Like other ruminants, including cattle and sheep, antelopes have well-developed cheek teeth or molars, which grind cud into a pulp. They have no upper incisors, and in order to tear off grass stems or leaves, their lower incisors press against an upper hard gum pad when they bite. Whether they are eating or resting, antelopes rely on their keen senses to avoid danger. Their eyes face sideways, and their pupils are elongated horizontally, giving them a good view of danger from behind as well as in front. Their hearing and sense of smell are also acute--valuable features for life in the open where many predators are on the prowl after dark. Whereas in deer only males for the most part grow antlers, in antelopes, both sexes usually have horns, although the horns of males are normally larger. In species where males have only one mate, such as the dik-dik and klipspringer, horns are little more than spikes. In species where the males compete to mate with several females, antelope horns may grow as long as 1.5 m (5 ft). Despite their large size, antelope horns are hollow and lightweight. Antelope horns are always slightly curved, and in some species, such as the black buck, they are shaped like a pair of corkscrews, spiraling in opposite directions. Antelope life spans in the wild are difficult to determine, and most known figures relate only to captive animals. Captive gnus, for example, have lived to be over 20, while impalas have lived into their late teens. But in the wild, where predators weed out all but the fastest and fittest animals, few antelopes reach their teen years. IV ANTELOPE BEHAVIOR Compared to some mammals, such as carnivores (meat-eaters) and primates (apes, monkeys, and humans), antelopes are not noted for high intelligence. This trait is partly explained by their plant-eating lifestyle: Because their food cannot run away, they do not need to be quick-witted or resourceful to track it down. However, antelopes still display complex patterns of behavior, although much of it is instinctive rather than learned. In open habitats, antelopes run a high risk of predation (being preyed upon). To survive they use several kinds of defensive strategy, including living in herds. Herd living ensures that many pairs of eyes and ears are on the alert for danger. Herd living also gives individuals a better chance of avoiding attack, because predators can choose from many potential targets. When danger threatens, antelopes behave in characteristic ways. Paradoxically, many species, particularly gazelles, walk toward potential enemies, such as lions or cheetahs, when they first come into view. This behavior is not as reckless as it sounds, because it alerts the herd and allows the antelopes to assess the threat that they face. If the approaching animals do turn out to be predators, gazelles keep them under constant surveillance, always at the ready to run. The decision to start running is based on the type of predator and its distance. Gazelles will permit lions to come within 200 m (650 ft), because they instinctively know that a hunting lion prefers to stay hidden while it stalks its victim and a visible lion is unlikely to launch an attack. Cheetahs, which are superb sprinters, pose more of a threat--gazelles will often start to run when cheetahs are still over 800 m (0.5 mi) away. Antelopes communicate with one another using a variety of sounds. Dik-diks, for example, whistle when alarmed, a habit that alerts other animals to danger and makes dik-diks unpopular with hunters. But for antelopes generally, sight is a much more important form of communication. They indicate their mood by their posture, and also by the way they move. When they are excited or alarmed, many medium-sized antelopes bounce up and down on all four legs, keeping their legs stretched out straight. This behavior, known as pronking or stotting, acts as an alarm display. Some biologists theorize that stotting also communicates a message to predators, showing that individual antelopes are fit and alert and therefore not worth pursuing. In addition to visual displays, antelopes use scent signals to communicate. Scent signals have the advantage that they can linger for many days. Antelopes that live in herds have glands in their hooves that leave a scented record of their movements. Antelopes use these scented tracks to find their way back to the herd if they accidentally become separated from it. Antelopes that live in forests tend to stay in the same area all their lives, but species that live in open habitats often migrate to feed and breed. The most famous of these migrations are carried out by gnus, which live in the plains and open woodlands of eastern and southern Africa. In some places gnus are sedentary, but in others, such as Tanzania's Serengeti National Park, gnus journey between two home ranges--one used during the dry season and the other during the wet season, when rain produces a fresh crop of grass. Migration can be risky, particularly when it involves crossing crocodile-infested rivers, but it does guarantee that the gnus have the best supply of food at different times of the year. V REPRODUCTION Antelopes mature rapidly. The smallest species are ready to breed when they are just six months old, and the largest species take only three or four years to reach sexual maturity. Some species can breed at any time of the year, but most have breeding seasons that coincide with the changing seasons. This timing ensures that young are born when food is plentiful. Courtship and mating behavior in antelopes varies. Dik-diks pair up for life, but in most herd-forming species, courtship begins with a protracted contest between adult males as they compete to gather as many females as they can. In some species males claim a territory that overlaps the territories of several females. In other species, such as the black buck, males fight for control of a small courtship arena, known as a lek. Males who successfully hold their ground at the center of the lek mate with many females, while those on the outskirts of the lek are ignored by females. After mating the gestation (pregnancy) period ranges from five to eight months. Female antelopes give birth to a single calf or, more rarely, twins. A mother and her newborn calf are vulnerable to predators, and antelopes have evolved two quite different strategies for surviving this period. In most species, including all those that live in forests and woodlands, the female gives birth in dense cover and leaves the calf while she feeds. The calf comes to its mother when she calls it. After taking a meal of milk, the calf will hide away once more. Once in its hiding place, the calf remains completely still and will run away only if it is on the verge of being discovered. Gnus and their close relatives use a higher-risk strategy, giving birth out in the open. Unlike other young antelopes, the calves follow their mothers, and they have to do this in record time. Young gnus are usually on their feet within 15 minutes after birth, and within a few days they have little trouble keeping up with the rest of the herd. This system allows the herd to stay on the move, and the herd also provides some protection for the calf. But predators looking for an easy meal still single out young calves. A newborn calf depends almost entirely on its mother for survival. In antelopes that form permanent pairs, such as duikers and klipspringers, the male may defend the calf from predator attack, but in most species there is no permanent pair bond, and the female brings up the calf on her own. VI TYPES OF ANTELOPE Although antelopes all belong to the cattle family (family Bovidae), the term antelope is an informal one, and does not have any precise scientific meaning. Some mammalogists take it to mean all bovids except cattle, but many also exclude another group known as the goat antelopes. The goat antelopes include animals such as the serow, markhor, bharal and urial, and also the musk-ox, all of which often have a mixture of goat and antelope features. This exclusion leaves a collection of seven or eight subfamilies of true antelopes, the exact number depending on the classification system used. A Nilgai and Four-Horned Antelopes In evolutionary terms, among the most primitive true antelopes are the nilgai and four-horned antelopes (subfamily Boselaphinae). These two species are found primarily in India, although nilgai have been introduced into southern Texas in the United States as free-roaming game animals. The nilgai is a stocky animal with a horselike shape. The four-horned antelope, also known as the chousingha, is smaller, and it is the only antelope to have four horns--two on the crown of its head, and a smaller pair set just above its eyes. These antelopes browse on leaves, shoots, and fruits. The four-horned antelope is dependent on water and rarely strays far from a water source. B Spiral-Horned Antelopes The spiral-horned antelopes (subfamily Tragelaphinae) live in Africa and include the kudu, sitatunga, bushbuck, bongo, and eland. Some of these animals, particularly the elands, are similar to cattle, with heavily built bodies and a dewlap (loose fold of skin) hanging from the neck. Males are larger than females, and in some species only males have horns. Primarily browsers, these antelopes use their long tongues to strip leaves from low branches and shrubs. They live in woodlands and forests, and rely on fur markings that blend into their surroundings and unobtrusive movements to avoid predators. Elands are among the few antelopes that have been raised on farms, although with limited success. Elands produce good meat in climate conditions that are too hot for domestic cattle, and they have been raised for their milk, but their nomadic lifestyle makes it difficult to keep them under control. The sitatunga inhabits swampy beds of sedges, reeds, or grasses throughout Central Africa. It is a good swimmer and diver and if pursued it may take refuge in deep water, submerging itself with only its nostrils above the surface. The sitatunga has physical characteristics specialized for a semiaquatic life. Its hooves are long and splayed to provide support on soft, muddy ground. This hoof modification makes sitatungas awkward on dry ground, and they rarely venture out into the open. C Duikers Duikers (subfamily Cephalophinae) form a group of small antelopes that live mainly in forests and scrub. Their name comes from an Afrikaans word meaning 'diver,' a reference to their habit of diving into vegetation when disturbed. There are about 18 species of duikers and the largest stands little more than waist-high. Duikers live exclusively in Africa, and although they are widespread, their secretive habits often make them go unnoticed. Both sexes have horns, which are often completely hidden by a characteristic tuft of hair on top of the head. Unusually for antelopes, females have slightly larger horns than males. Duikers browse on leaves and some also eat small birds or the remains of dead animals. D Reedbucks, Waterbucks, and Their Relatives Reedbucks, waterbucks and their relatives (subfamily Reduncinae) include ten species of antelopes that live in wetlands and tussock grasslands in southern Africa. Their need for water means they rarely stray far from a river or waterhole. The horns of these antelopes typically curve backward from the base and then forward toward the tips, giving them an S-shape. Their horns have a series of ridges or rings known as annuli that give the horns a knobbly appearance. The antelopes in this group have a characteristic shaggy coat. Some species have long fur around the neck, and the two species of waterbuck are shaggy all over with oily fur that gives off a powerful smell. Mammalian hunters, such as lions and leopards, prey upon reedbucks and waterbucks, and crocodiles have been known to attack these antelopes as they drink from rivers. E Rheboks The rhebok or reebok (subfamily Peleinae) is so named because it reminded early white settlers of the roebuck, a type of deer in Europe. Found only in South Africa, the rhebok's preferred habitat is grassy hills and mountains. The rhebok reaches about 75 cm (30 in) in height at the shoulder. Males possess horns that may extend 28 cm (11 in) in length. Its woolly, curly coat is brownish gray in color. F Horse Antelopes Seven species known as horse antelopes (subfamily Hippotraginae) include the sable antelope, roan antelope, oryx, and addax--large-bodied animals that have a horselike build. They live in Africa and the Arabian Peninsula in a variety of habitats from woodlands to deserts. Despite their collective name, these antelopes are impossible to confuse with real horses because they have exceptionally long horns. The horns of the gemsbok, a type of oryx, are almost straight, and measure up to 1.5 m (nearly 5 ft) long, while the horns of the scimitar-horned oryx curve backward in a graceful arch. Trophy hunters prize horse antelope horns, a fact that has brought some of these animals to the verge of extinction. G Gnus, Hartebeests, Blesboks, and Their Relatives The gnus, hartebeests, blesboks, and their relatives (subfamily Alcelaphinae) make up a group of seven species, or eight if the impala is included (some authorities class the impala in a subfamily of its own called Aepycerotinae). Unlike horse antelopes, these African antelopes have an ungainly build, with sloping backs, narrow heads, and short, sharply bent horns. Some of the most numerous antelopes alive today, these antelopes have a marked tendency for living in large herds. They live in open woodlands or grassy plains, often migrating long distances to find food. Together with gazelles, these antelopes form an important source of food for many of Africa's largest predators, including lions and hyenas. H Other Antelopes The remaining true antelopes are often classified into a single group (subfamily Antilopinae), although some authorities prefer to divide them into three subdivisions. The smallest of these subdivisions contains just two species, the saiga and the chiru (also known as the Tibetan antelope). Both animals are unusual. The saiga has a distinctive trunklike muzzle, with downward-pointing nostrils. It lives in the low-lying steppes of Central Asia, and some experts believe that its strange nose is an adaptation for breathing cold air. Male saigas have short, slightly curved horns, up to 36 cm (14 in) long. The chiru lives in mountainous regions of Tibet. It has inflatable nasal sacs, which may be an adaptation to breathing at high elevations. Its horns reach 75 cm (30 in) in length, with sharply pointed tips. A second subdivision is made up by the dwarf antelopes, an entirely African collection of largely forest and woodland animals. They include the dik-diks, klipspringer, and steenbok, and also the royal antelope--the smallest antelope. Numbering about a dozen species, dwarf antelopes are physically very varied: the royal antelope, for example, has an arched back, long back legs, and a short neck, giving it a shape like a rabbit, while the dik-diks look more like typical antelopes that have been scaled down to a small size. Male dwarf antelopes have short straight horns, and the females have no horns. The third and largest subdivision in the subfamily Antilopinae consists of gazelle and their close relatives. These long-legged, slender, and graceful animals have fawncolored upperparts and pale undersides, and both males and females grow short S-shaped horns. Gazelles live in Africa and Asia, and their habitats vary from open woodlands and grassy plains to barren high-altitude steppes. There are 16 species, and they include some with large populations, such as Thomson's gazelle, and others, such as Przewalski's gazelle from northern China, that have become extremely rare. Gazelles are fast on their feet and some species have been credited with top speeds of nearly 100 km/h (60 mph). VII CONSERVATION STATUS To survive, antelopes rely on their ability to spot danger and to make a rapid escape. This defense system enables them to evade most wildlife predators, but it offers only partial protection from human hunters interested in antelope hide, horn, or meat, and no protection at all from changes to their habitat. In recorded history the only antelope to become extinct is the South African blue buck, which disappeared around 1800. Since then, however, most species of antelopes have suffered a fall in numbers, and some have come perilously close to disappearing. Foremost among the threatened species are the addax, scimitar-horned oryx, and Arabian oryx, all of which are hunted for their horns. These three species live in dry places and deserts devoid of large vegetation, where hunters in motor vehicles find them easy to chase. After several decades of motorized hunting, only a few hundred of the addax and the Arabian oryx remain in the wild. The Red List of Threatened Species compiled by the World Conservation Union (IUCN), a nongovernmental organization that collects global information on endangered species, now lists the scimitar-horned oryx as extinct in the wild. Small populations now survive only in protected parks and in some zoos. Many antelopes are hunted for food and for their hide, but another threatened species, the chiru, is hunted for its ultra-fine wool, which is known as shahtoosh. Chirus are extremely wary and their wool cannot be shorn like a sheep's wool. Instead, poachers kill chirus and then shear their wool. Although the sale of this wool is illegal under the Convention on International Trade in Endangered Species (CITES), the heavy demand for this wool on the black market has decimated the chiru's population. In recent decades conservation programs have had some positive impact with species that have been overhunted. Captive-breeding programs for the Arabian oryx, which began in the 1960s in zoos in the United States and Europe, have produced more than 2,000 animals. The species has been reintroduced into the wild in protected areas of Oman, and its future looks reasonably secure. Similar conservation programs involving zoos around the world exist for the addax and scimitarhorned oryx. Conservationists are using a different approach to help the chiru. By educating retailers and consumers about the full implications of buying shahtoosh, they hope to stem the demand for the wool and curtail its illegal trade. For antelopes in general, a more intractable problem is shortage of space. A century ago Africa was almost entirely unfenced, and grassland species such as gnus roamed freely across the continent. Throughout the 20th century much of their former habitat has been transformed into cattle ranches, bringing antelopes into direct competition with farmed animals. Large antelope herds still exist in some of Africa's national parks, but antelope populations are becoming smaller and more fragmented, a trend that is likely to create conservation problems in years to come. Scientific classification: Antelopes belong to the family Bovidae of the order Artiodactyla. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.