Giraffe - biology.

Giraffe - biology. I INTRODUCTION Giraffe, the tallest living animal, instantly recognizable by its exceptionally long neck. Male giraffes average about 5.3 m (17 ft) high, but some grow to a height of almost 6 m (20 ft)--tall enough to span two floors of a typical office building. Giraffes live in tree-scattered terrain in Africa. The giraffe is the better known of two species in the giraffe family. The other species is the okapi, an animal that was not even discovered by scientists until about 1900. Although much shorter than the giraffe, the okapi also has a long neck and eats leaves, and both animals have long tongues and skin-covered horns. The giraffe's ancestors first appeared in central Asia about 15 million years ago, but the earliest fossil records of the giraffe itself, from Israel and Africa, date back about 1.5 million years. II RANGE AND HABITAT Most giraffes live either in East Africa or in Angola and Zambia in southwestern Africa. Until the middle of the 20th century giraffes were also commonly found in West Africa, south of the Sahara. But populations there have fallen sharply and become increasingly fragmented. Giraffes live in savanna and open woodland, habitats where the available food varies throughout the year. During the dry season, the animals eat evergreen leaves, but once the rainy season begins, they switch to new leaves and stems that sprout on deciduous trees. When there is a choice, male and female giraffes feed in different ways. Males concentrate on leaves from the highest branches, while the females arch their necks to eat closer to the ground. This behavior is so characteristic that a giraffe's sex can be identified from a long distance away simply by its stance while eating. Male giraffes are also more inclined to wander into dense woodland, a habitat that females generally avoid. Giraffes are not great travelers, despite their long legs. They cannot walk over swampy ground because their hooves quickly sink, and they very rarely wade across rivers. Giraffes on opposite banks of a river may never come into contact, unless the water level subsides. III PHYSICAL DESCRIPTION In addition to its great height, the giraffe is also one of the heaviest land animals. Exceptionally large males may weigh up to 1,900 kg (about 4,200 lb). Females are smaller, rarely reaching half that weight. Compared to other hoofed mammals the giraffe has a relatively short body, but its legs are disproportionately long. The front legs are marginally longer than the hind legs, a feature that contributes to the animal's steeply sloping back. Mature giraffes have large hooves about the size of dinner plates. Giraffes have two gaits--a loping walk and a gallop. When they walk, the animals move both feet on one side of their body in unison, followed by both feet on the other side. When they run, giraffes move the front feet together, then the back feet, swinging the hind feet up and planting them in front of the forefeet. While running, the neck of a giraffe moves backward and forward to keep the animal balanced. Giraffes have a top speed of about 56 km/h (35 mph), but because its legs are so long a galloping giraffe does not appear to be going very fast. A giraffe's neck can be over 1.5 m (5 ft) in length, but it contains only seven vertebrae--the same number as in most other mammals, including humans. These vertebrae are enormously elongated and are separated by very flexible joints. At the base of the neck, the thoracic vertebrae have spines that project upward, forming a hump over the shoulders. These bones anchor muscles that hold the neck upright, in the same way that cables hold up a construction crane. The giraffe's long neck and immense height help it eat leaves that are beyond the reach of other animals. A giraffe can extend its tongue for as much as 45 cm (18 in), wrapping it around leaves like a grasping hand, and the animal's canine teeth have deep grooves for stripping foliage from twigs. The tongue and lips of giraffes are covered by hard growths called papillae--a vital adaptation for feeding on trees with vicious thorns. Compared to other mammals, giraffes have an unexceptional sense of smell, but their eyes are large and their vision acute. Combined with their lofty vantage point, this gives the animals a panoramic view of their surroundings. For giraffes, bending down is an anatomical challenge. To reach ground level--for example, when drinking--a giraffe has to splay its front legs at an angle of almost 45 degrees. A giraffe's circulatory system is also specially modified, because the high pressure needed to pump blood up to its head could cause brain damage when the head is lowered. To deal with this problem, giraffes have elastic blood vessels that relieve some of the excess pressure. They also have a series of valves in their neck veins that ensure that blood always flows from the head back toward the heart, even when this means going against gravity. Giraffes have short, dense fur with an intricate pattern of dark patches that range from tannish-yellow to chestnut brown. This coloration helps giraffes blend in among trees and leaves, making them harder for predators to spot. Although no two giraffe skin patterns are exactly the same, some regional patterns are so common and distinct that zoologists have identified a number of giraffe subspecies based on the patterns. One of the most striking of these subspecies is the reticulated giraffe, which has large brown patches separated by thin contrasting lines, making a pattern that looks like broken paving. Another variety, the Masai giraffe, has patches with complex jagged outlines. In all giraffes, the coat color tends to become darker with age. In captivity giraffes have lived into their mid-30s, but their maximum lifespan in the wild is about 25 years. The heads of both male and female giraffes have a pair of bony horns, permanently covered by skin and ending in a tuft of black hair. The horns of males are larger than those of females, growing up to 25 cm (about 10 in) long. The skulls of males continue to deposit layers of new bone throughout life. As a result, a male giraffe's head becomes progressively heavier and more angular, with bony lumps that look like extra horns. This characteristic--found in no other mammal--is linked to the fights that male giraffes stage to establish dominance for mating. IV BEHAVIOR Giraffes spend up to half their time feeding, and most of the remainder is taken up either by searching for food or slowly digesting what they have eaten. Giraffes are ruminants (animals that regurgitate partially digested food and chew it again) like sheep and cows. Giraffes are mostly diurnal (active during the day). Sometimes they doze during the daytime, often while standing. They normally lie down only at night, tucking their feet under the body and usually keeping the head upright. However, when a giraffe is sleeping--something it does for just a few minutes at a time--it curves its neck around and rests its head on or near its rump. Although giraffes are social animals, their herds are less structured than those of most other mammals. A typical herd of giraffes contains up to ten members, and animals can leave or join it at any time. The giraffes are often so widely scattered that they seem out of contact with one another, but the animal's keen eyesight can keep neighbors in view from great distances away. Giraffe herds do not have a leader, and individual giraffes show no particular preferences for others in the herd. Notable exceptions to this rule are young males, which often form bachelor herds, and females with calves, which often feed together. One of the most striking elements of giraffe behavior is the duel between males fighting for mating privileges. Giraffe duels are among the most extraordinary in the animal kingdom. They start when two males approach each other and begin to rub and intertwine their necks. This behavior--known as necking--allows the opponents to assess each other's size and strength. Often, necking alone is enough to establish seniority. If not, the rivals begin to exchange blows with their heads. Each giraffe braces its front legs and swings its head upward and over its shoulder. If a blow lands solidly--and many do not--the recipient may stagger under the impact, and in rare cases may even collapse onto the ground. More often the contest breaks off after a few minutes, and the loser simply walks away. Unlike many of Africa's plant-eating mammals, giraffes can get all the moisture they need from their food--particularly when it is covered in dew. As a result, they do not need to migrate in search of water. The average home range of a giraffe is about 150 sq km (44 sq mi), although giraffes can spend their whole lives in an area as small as 5 sq km (1.5 sq mi) in regions where food is abundant throughout the year. Immature male giraffes wander the farthest, but as they become fully grown their home range becomes dramatically smaller. Adult giraffes have no enemies other than lions and humans, as their huge hooves are very effective in defending against predators. They are more vulnerable when they are lying down or drinking, because this gives lions the opportunity to leap up and seize them by the nose or throat. Newly born calves are at much greater risk. Despite their mothers' best efforts to protect them, over 50 percent of all giraffe newborns are killed by hyenas and big cats such as lions and leopards during the first month of life. V REPRODUCTION Female giraffes start to breed when they are about four years old, and they have a gestation period of almost 15 months--one of the longest in the animal world. Males start breeding at about age five, when they begin the ritual combat over mates. Giraffes are only weakly territorial, and a successful male will mate with receptive females whenever and wherever it finds them. Over a year later, when a pregnant female is ready to give birth, she makes her way to a calving area that she will use throughout her life. The moment of birth is dramatic, with the mother standing on all fours and the calf tumbling onto the ground. Remarkably, the calf is rarely injured by its fall. Newborn giraffes are often on their feet within 20 minutes and are soon feeding on their mothers' milk. They are about 2 m (6 ft) tall at birth, and double their height in their first year. They are weaned at one year and become fully independent by 15 months of age. Females are fully grown by age five and males by age seven. VI STATUS Like many of Africa's large mammals, giraffes have declined in numbers and in range over the last century. At one time, herds of over 100 animals were common in savanna regions across the continent, but today concentrations like these exist only in East Africa--particularly Tanzania's Serengeti National Park. The decline of giraffe populations has largely been due to hunting. In Africa, the giraffe is a traditional source of hide and hair and also of tough but nutritious meat. Hunting of giraffes has not yet had a catastrophic effect, as it has on some of Africa's big-game animals, but it is a cause for concern. The natural habitat of the giraffe is also being impacted more and more by human activities, reducing the animal's range. The giraffe is currently a protected species throughout most of its range and is classed as conservation-dependent by the World Conservation Union (IUCN). The giraffe's prospects for survival are good for those living in national parks and game reserves, but for animals living outside these areas the future is less secure. Scientific classification: The giraffe belongs to the family Giraffidae. It is classified as Giraffa camelopardalis. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.