148-540: The Richtersveld is a desert landscape characterised by rugged kloofs and high mountains, situated in the north-western corner of South Africa ’s Northern Cape province. It is full of changing scenery from flat, sandy, coastal plains, to craggy sharp mountains of volcanic rock and the lushness of the Orange River , which forms the border with neighboring Namibia . The area ranges in altitude from sea level, to 1,377 m (4,518 ft) at Cornellberg. Located in
296-530: A mammal adapted to desert life. It minimizes its water loss by producing concentrated urine and dry dung , and is able to lose 40% of its body weight through water loss without dying of dehydration. Carnivores can obtain much of their water needs from the body fluids of their prey. Many other hot desert animals are nocturnal , seeking out shade during the day or dwelling underground in burrows. At depths of more than 50 cm (20 in), these remain at between 30 and 32 °C (86 and 90 °F) regardless of
444-456: A sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes. Many people think of deserts as consisting of extensive areas of billowing sand dunes because that is the way they are often depicted on TV and in films, but deserts do not always look like this. Across
592-428: A semi-arid climate and are less extreme than regular deserts. Like arid deserts, temperatures can vary greatly in semi deserts. They share some characteristics of a true desert and are usually located at the edge of deserts and continental dry areas. They usually receive precipitation from 250 to 500 mm (9.8 to 19.7 in) but this can vary due to evapotranspiration and soil nutrition. Semi-deserts can be found in
740-489: A branch and stays perfectly still, with only its eyes moving. When an insect lands, the chameleon focuses its eyes on the target and slowly moves toward it before projecting its long sticky tongue which, when hauled back, brings the attached prey with it. Geckos feed on crickets , beetles, termites and moths . Termites are an important part of the diets of some species of Autarchoglossa, since, as social insects , they can be found in large numbers in one spot. Ants may form
888-438: A desert conglomerate . In time, bacteria that live on the surface of the stones accumulate a film of minerals and clay particles, forming a shiny brown coating known as desert varnish . Other non-sandy deserts consist of exposed outcrops of bedrock , dry soils or aridisols , and a variety of landforms affected by flowing water , such as alluvial fans , sinks or playas , temporary or permanent lakes , and oases. A hamada
1036-505: A destructive effect on the exposed rocky surfaces. The repeated fluctuations put a strain on exposed rock and the flanks of mountains crack and shatter. Fragmented strata slide down into the valleys where they continue to break into pieces due to the relentless sun by day and chill by night. Successive strata are exposed to further weathering. The relief of the internal pressure that has built up in rocks that have been underground for aeons can cause them to shatter. Exfoliation also occurs when
1184-733: A diverse range of mammal , reptile , and bird species. These include the Grey rhebok , Duiker , Steenbok , Klipspringer , Hartmann's mountain zebra , Chacma baboon , Vervet monkey , Caracal and the African leopard . Reptiles include snakes such as the Puff adder , Black Spitting Cobra , and the Nama Tiger Snake, and lizards which mainly comprise species of Agama . Species of weaver birds, doves , and Guineafowl are often found inland, although bird species become more diverse during
1332-422: A doubled clitoris. The majority of species are oviparous (egg laying). The female deposits the eggs in a protective structure like a nest or crevice or simply on the ground. Depending on the species, clutch size can vary from 4–5 percent of the females body weight to 40–50 percent and clutches range from one or a few large eggs to dozens of small ones. In most lizards, the eggs have leathery shells to allow for
1480-885: A fragile succulent ecosystem. The community conservancy is bordered to the north by the Richtersveld National Park (managed by the Richtersveld community and the South African National Parks) the Nababiep Provincial Nature Reserve and designated community grazing areas that allow the Nama people to continue with their lifestyle. Richtersveld National Park is the South-African part of the ǀAi-ǀAis/Richtersveld Transfrontier Park . In June 2007,
1628-560: A limited ability to regulate their body temperature, and must seek out and bask in sunlight to gain enough heat to become fully active. Thermoregulation behavior can be beneficial in the short term for lizards as it allows the ability to buffer environmental variation and endure climate warming. In high altitudes, the Podarcis hispaniscus responds to higher temperature with a darker dorsal coloration to prevent UV-radiation and background matching. Their thermoregulatory mechanisms also allow
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#17327658477241776-415: A low basal metabolic rate and by remaining underground during the heat of the day, reducing loss of water through its skin and respiratory system when at rest. Herbivorous mammals obtain moisture from the plants they eat. Species such as the addax antelope , dik-dik , Grant's gazelle and oryx are so efficient at doing this that they apparently never need to drink. The camel is a superb example of
1924-463: A movable quadrate bone , distinguishing them from the rhynchocephalians , which have more rigid diapsid skulls . Some lizards such as chameleons have prehensile tails, assisting them in climbing among vegetation. As in other reptiles, the skin of lizards is covered in overlapping scales made of keratin . This provides protection from the environment and reduces water loss through evaporation. This adaptation enables lizards to thrive in some of
2072-484: A number of ways, generally combining total precipitation, number of days on which this falls, temperature, and humidity, and sometimes additional factors. For example, Phoenix, Arizona , receives less than 250 mm (9.8 in) of precipitation per year, and is immediately recognized as being located in a desert because of its aridity-adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and
2220-571: A pouch beneath its eyes, to a distance of about two metres (6.6 feet); the blood tastes foul to these attackers. The closest living relatives of lizards are rhynchocephalians , a once diverse order of reptiles, of which is there is now only one living species, the tuatara of New Zealand. Some reptiles from the Early and Middle Triassic , like Sophineta and Megachirella , are suggested to be stem-group squamates, more closely related to modern lizards than rhynchocephalians, however, their position
2368-463: A predator that has caught them; attempt to outrun the rattlesnake , which does not pursue prey; but stay still, relying on their cryptic coloration, for Masticophis whip snakes which can catch even swift prey. If caught, some species such as the greater short-horned lizard puff themselves up, making their bodies hard for a narrow-mouthed predator like a whip snake to swallow. Finally, horned lizards can squirt blood at cat and dog predators from
2516-430: A prominent part of the diet of some lizards, particularly among the lacertas. Horned lizards are also well known for specializing on ants. Due to their small size and indigestible chitin , ants must be consumed in large amounts, and ant-eating lizards have larger stomachs than even herbivorous ones. Species of skink and alligator lizards eat snails and their power jaws and molar-like teeth are adapted for breaking
2664-423: A series of long, linear dunes known as seif dunes may form. These also occur parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the prevailing wind direction. Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point. They tend to grow vertically; they can reach a height of 500 m (1,600 ft), making them
2812-478: A single clade , the Toxicofera ). However, most of these putative venom genes were "housekeeping genes" found in all cells and tissues, including skin and cloacal scent glands. The genes in question may thus be evolutionary precursors of venom genes. Recent studies (2013 and 2014) on the lung anatomy of the savannah monitor and green iguana found them to have a unidirectional airflow system, which involves
2960-461: A sparse trophic network . Plant growth is limited by rainfall , temperature extremes and desiccating winds. Deserts have strong temporal variability in the availability of resources due to the total amount of annual rainfall and the size of individual rainfall events. Resources are often ephemeral or episodic, and this triggers sporadic animal movements and 'pulse and reserve' or 'boom-bust' ecosystem dynamics. Erosion and sedimentation are high due to
3108-700: A species affects the structure of territories, for example, rock lizards have territories atop rocky outcrops. Some species may aggregate in groups, enhancing vigilance and lessening the risk of predation for individuals, particularly for juveniles. Agonistic behaviour typically occurs between sexually mature males over territory or mates and may involve displays, posturing, chasing, grappling and biting. Lizards signal both to attract mates and to intimidate rivals. Visual displays include body postures and inflation, push-ups, bright colours, mouth gapings and tail waggings. Male anoles and iguanas have dewlaps or skin flaps which come in various sizes, colours and patterns and
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#17327658477243256-439: A steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate. At greater wind speeds, some particles are lifted into the air stream. When they land, they strike other particles which may be jerked into the air in their turn, starting a chain reaction . Once ejected, these particles move in one of three possible ways, depending on their size, shape and density; suspension , saltation or creep. Suspension
3404-584: A storm that may be many kilometers away. Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders. The River Nile , the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby. There may also be underground sources of water in deserts in
3552-485: A tolerable level. In many ways, birds are the ablest to do this of the higher animals. They can move to areas of greater food availability as the desert blooms after local rainfall and can fly to faraway waterholes. In hot deserts, gliding birds can remove themselves from the over-heated desert floor by using thermals to soar in the cooler air at great heights. In order to conserve energy, other desert birds run rather than fly. The cream-colored courser flits gracefully across
3700-632: A type of cold desert . While they do not lack water, having a persistent cover of snow and ice, this is merely due to marginal evaporation rates and low precipitation. The McMurdo dry valleys of Antarctica, which lack water (whether rain, ice, or snow) much like a non-polar desert and even have such desert features as hypersaline lakes and intermittent streams that resemble (except for being frozen at their surfaces) hot or cold deserts for extreme aridity and lack of precipitation of any kind. Extreme winds and not seasonal heat desiccate these nearly-lifeless terrains. The concept of "biological desert" redefines
3848-423: A variety of different camouflage methods . Many lizards are disruptively patterned . In some species, such as Aegean wall lizards , individuals vary in colour, and select rocks which best match their own colour to minimise the risk of being detected by predators. The Moorish gecko is able to change colour for camouflage: when a light-coloured gecko is placed on a dark surface, it darkens within an hour to match
3996-400: A year; they have no annual seasonal cycle of precipitation and experience twelve-month periods with no rainfall at all. Arid deserts receive between 25 and 200 mm (1 and 8 in) in a year and semiarid deserts between 200 and 500 mm (8 and 20 in). However, such factors as the temperature, humidity, rate of evaporation and evapotranspiration, and the moisture storage capacity of
4144-422: Is paraphyletic as some lizards are more closely related to snakes than they are to other lizards. Lizards range in size from chameleons and geckos a few centimeters long to the 3-meter-long Komodo dragon . Most lizards are quadrupedal, running with a strong side-to-side motion. Some lineages (known as " legless lizards ") have secondarily lost their legs, and have long snake-like bodies. Some lizards, such as
4292-472: Is 150 km (93 mi) long and is the largest oasis in the Libyan Desert. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below. Lakes may form in basins where there
4440-512: Is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation . About one-third of the land surface of the Earth is arid or semi-arid . This includes much of the polar regions , where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by
4588-538: Is a jumble of eroded sandstone outcrops , canyons , blocks, pinnacles , fissures , slabs and ravines . In some places the wind has carved holes or arches, and in others, it has created mushroom-like pillars narrower at the base than the top. On the Colorado Plateau , it is water that has been the prevailing eroding force. Here, rivers, such as the Colorado , have cut their way over the millennia through
Richtersveld - Misplaced Pages Continue
4736-542: Is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes . Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind. These are called "reg" in the western Sahara, "serir" in the eastern Sahara, "gibber plains" in Australia and "saï" in central Asia. The Tassili Plateau in Algeria
4884-497: Is characterized by harsh summer temperatures which have been recorded to reach approximately 53 °C (127 °F) in mid-summer. Nights are cool and bring with them heavy dew. With water being scarce, life in the Richtersveld depends on moisture from the early morning fog. Locals call it 'Ihuries' or 'Malmokkies' and it makes survival possible for a range of small reptiles, birds and mammals. Temperatures drop between April and May to more temperate levels, however, through June - August
5032-633: Is continuous above 5,600 m (18,400 ft). Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific. When rain falls in deserts, as it occasionally does, it is often with great violence. The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. These can experience flash floods , becoming raging torrents with surprising rapidity after
5180-647: Is disputed, with some studies recovering them as less closely related to squamates than rhynchocephalians are. The oldest undisputed lizards date to the Middle Jurassic, from remains found In Europe, Asia and North Africa. Lizard morphological and ecological diversity substantially increased over the course of the Cretaceous . In the Palaeogene , lizard body sizes in North America peaked during
5328-479: Is higher than that of forests or the sea. A desert is a region of land that is very dry because it receives low amounts of precipitation (usually in the form of rain, but it may be snow, mist or fog), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside the area. Deserts generally receive less than 250 mm (10 in) of precipitation each year. The potential evapotranspiration may be large but (in
5476-543: Is inhabited by Nama and other peoples. The local community, which owns the entire area, manages the National Park in conjunction with South African National Parks and is entirely responsible for management of the World Heritage Site. Both areas are used by traditional nomadic/transhumance herders to practice their ancient lifestyle and culture. It is the last place where the traditional way of life of
5624-666: Is known to occur in various species of whiptail lizards . Parthenogenesis was also recorded in species that normally reproduce sexually. A captive female Komodo dragon produced a clutch of eggs, despite being separated from males for over two years. Sex determination in lizards can be temperature-dependent . The temperature of the eggs' micro-environment can determine the sex of the hatched young: low temperature incubation produces more females while higher temperatures produce more males. However, some lizards have sex chromosomes and both male heterogamety (XY and XXY) and female heterogamety (ZW) occur. A significant component of aging in
5772-560: Is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Polar deserts cover much of the ice-free areas of the Arctic and Antarctic. A non-technical definition is that deserts are those parts of Earth's surface that have insufficient vegetation cover to support a human population. Potential evapotranspiration supplements
5920-434: Is often less than 40 mm or 1.5 in) to being very far from the nearest available sources of moisture and are often in the lee of mountain ranges. Montane deserts are normally cold, or may be scorchingly hot by day and very cold by night as is true of the northeastern slopes of Mount Kilimanjaro . Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from
6068-421: Is only possible for particles less than 0.1 mm (0.0039 in) in diameter. In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to 6 km (3.7 mi). They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds for distances of up to 6,000 km (3,700 mi). Denser clouds of dust can be formed in stronger winds, moving across
Richtersveld - Misplaced Pages Continue
6216-486: Is sufficient precipitation or meltwater from glaciers above. They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas. When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa . The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during
6364-682: Is the Atacama Desert . It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of Antofagasta is just 1 mm (0.039 in) per year. Some weather stations in
6512-503: Is the "Halfmensboom" ( Pachypodium namaquanum Welw.) meaning "half-person tree" , the name comes from the plant's resemblance to the human form. These trees are revered by the indigenous Nama people as the embodiment of their ancestors, half human, half plant, mourning for their ancient Namibian home. Also found here are three endemic species of the tree aloe Aloidendron : A. dichotomum , A. pillansii and A. ramosissimum , often referred to as "kokerbooms" or "quiver trees". The area
6660-450: Is the key to plant growth. It can only take place during the day as energy from the sun is required, but during the day, many deserts become very hot. Opening stomata to allow in the carbon dioxide necessary for the process causes evapotranspiration , and conservation of water is a top priority for desert vegetation. Some plants have resolved this problem by adopting crassulacean acid metabolism , allowing them to open their stomata during
6808-431: Is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia , kangaroo rats and jerboas , Phrynosoma and Moloch lizards. Deserts present a very challenging environment for animals. Not only do they require food and water but they also need to keep their body temperature at
6956-528: Is very cold and carries little moisture so little precipitation occurs and what does fall, usually snow, is carried along in the often strong wind and may form blizzards, drifts and dunes similar to those caused by dust and sand in other desert regions. In Antarctica , for example, the annual precipitation is about 50 mm (2 in) on the central plateau and some ten times that amount on some major peninsulas. Based on precipitation alone, hyperarid deserts receive less than 25 mm (1 in) of rainfall
7104-694: The Atacama Desert . Deserts of this type are often long and narrow and bounded to the east by mountain ranges. They occur in Namibia , Chile , southern California and Baja California . Other coastal deserts influenced by cold currents are found in Western Australia , the Arabian Peninsula and Horn of Africa , and the western fringes of the Sahara. In 1961, Peveril Meigs divided desert regions on Earth into three categories according to
7252-561: The Khoikhoi (of whom the Nama are the largest surviving clan) who once occupied the entire south-western part of Africa, survives to any great extent. The World Heritage Site is declared under the cultural criteria of the World Heritage Convention although it is recognised that the cultural values of the community and their continued existence are intrinsically connected to the environment. Desert A desert
7400-406: The common basilisk , can run across water. Lizards make use of their senses of sight , touch , olfaction and hearing like other vertebrates . The balance of these varies with the habitat of different species; for instance, skinks that live largely covered by loose soil rely heavily on olfaction and touch, while geckos depend largely on acute vision for their ability to hunt and to evaluate
7548-621: The horse latitudes at 30° to 35° North and South. These belts are associated with the subtropical anticyclone and the large-scale descent of dry air. The Sahara Desert is of this type. Mid-latitude deserts occur between 30° and 50° North and South. They are mostly in areas remote from the sea where most of the moisture has already precipitated from the prevailing winds. They include the Tengger and Sonoran Deserts . Monsoon deserts are similar. They occur in regions where large temperature differences occur between sea and land. Moist warm air rises over
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#17327658477247696-485: The microflora necessary for their transition to a plant-based diet. Perhaps the most herbivorous species is the marine iguana which dives 15 m (49 ft) to forage for algae , kelp and other marine plants. Some non-herbivorous species supplement their insect diet with fruit, which is easily digested. Lizards have a variety of antipredator adaptations , including running and climbing, venom , camouflage , tail autotomy , and reflex bleeding . Lizards exploit
7844-417: The windward slope of the mountain range . When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little precipitation occurs. The Taklamakan Desert is an example, lying in the rain shadow of the Himalayas and receiving less than 38 mm (1.5 in) precipitation annually. Other areas are arid by virtue of being a very long way from
7992-496: The "Richtersveld Cultural and Botanical Landscape", just to the south of the National Park and an area of equivalent size and beauty, was named a UNESCO World Heritage Site . Unlike the National Park, the Richtersveld Community Conservancy, which forms the core zone of the World Heritage Site, is not subject to diamond mining and is as a result the more pristine of the two areas. The Richtersveld
8140-492: The Atacama have never received rain. Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. It is so arid that mountains that reach as high as 6,885 m (22,589 ft) are completely free of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary , though permafrost extends down to an altitude of 4,400 m (14,400 ft) and
8288-657: The Komodo dragon can kill prey as large as water buffalo . Dragons are prolific scavengers , and a single decaying carcass can attract several from 2 km (1.2 mi) away. A 50 kg (110 lb) dragon is capable of consuming a 31 kg (68 lb) carcass in 17 minutes. Around 2 percent of lizard species, including many iguanids, are herbivores. Adults of these species eat plant parts like flowers, leaves, stems and fruit, while juveniles eat more insects. Plant parts can be hard to digest, and, as they get closer to adulthood, juvenile iguanas eat faeces from adults to acquire
8436-576: The ability to sacrifice and regrow their tails . The adult length of species within the suborder ranges from a few centimeters for chameleons such as Brookesia micra and geckos such as Sphaerodactylus ariasae to nearly 3 m (10 ft) in the case of the largest living varanid lizard, the Komodo dragon . Most lizards are fairly small animals. Lizards typically have rounded torsos, elevated heads on short necks, four limbs and long tails, although some are legless. Lizards and snakes share
8584-532: The absence of available water) the actual evapotranspiration may be close to zero. Semi-deserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes . Most deserts on Earth such as the Sahara Desert , Grand Australian Desert and the Great Basin Desert , occur in low altitudes . One of the driest places on Earth
8732-401: The air but soon fall back to earth, ejecting other particles in the process. Their weight prevents them from being airborne for long and most only travel a distance of a few meters (yards). The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 cm (12 in). In a really severe steady blow, 2 m (6 ft 7 in) is about as high as
8880-439: The air moving in a loop through the lungs when breathing. This was previously thought to only exist in the archosaurs ( crocodilians and birds ). This may be evidence that unidirectional airflow is an ancestral trait in diapsids . As with all amniotes, lizards rely on internal fertilisation and copulation involves the male inserting one of his hemipenes into the female's cloaca . Female lizards also have hemiclitorises ,
9028-473: The amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location. Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks , which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter, and
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#17327658477249176-722: The amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least twelve consecutive months without precipitation, arid lands have less than 250 mm (9.8 in) of annual precipitation, and semiarid lands have a mean annual precipitation of between 250 and 500 mm (9.8 and 19.7 in). Both extremely arid and arid lands are considered to be deserts while semiarid lands are generally referred to as steppes when they are grasslands. Deserts are also classified, according to their geographical location and dominant weather pattern, as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts . Trade wind deserts occur either side of
9324-479: The area was proclaimed in 1991 after 18 years of negotiations between the National Parks Board and the local Nama people who continue to live and graze their livestock in the area. It has an area of 1,624.45 square kilometres (627.20 sq mi). This is a space for Nama people who live what is known as a transhumant lifestyle – where they migrate seasonally with their livestock and make use of
9472-411: The barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that even evaporate ice. Deserts, both hot and cold, play a part in moderating Earth's temperature, because they reflect more of the incoming light and their albedo
9620-457: The capture of large quantities of solar energy . English desert and its Romance cognates (including Italian and Portuguese deserto , French désert and Spanish desierto ) all come from the ecclesiastical Latin dēsertum (originally "an abandoned place"), a participle of dēserere , "to abandon". The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus
9768-523: The cellular structure of which has been modified to allow them to store water. When rain falls, the water is rapidly absorbed by the shallow roots and retained to allow them to survive until the next downpour, which may be months or years away. The giant saguaro cacti of the Sonoran Desert form "forests", providing shade for other plants and nesting places for desert birds. Saguaro grows slowly but may live for up to two hundred years. The surface of
9916-615: The coldest part of the year. Animals adapted to live in deserts are called xerocoles . There is no evidence that body temperature of mammals and birds is adaptive to the different climates, either of great heat or cold. In fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live. Many desert animals (and plants) show especially clear evolutionary adaptations for water conservation or heat tolerance and so are often studied in comparative physiology , ecophysiology , and evolutionary physiology . One well-studied example
10064-597: The collision between airborne particles and by the impacts of saltating sand grains landing on the ground. The mechanism is little understood but the particles usually have a negative charge when their diameter is under 250 μm and a positive one when they are over 500 μm. Deserts and semi-deserts are home to ecosystems with low or very low biomass and primary productivity in arid or semi-arid climates. They are mostly found in subtropical high-pressure belts and major continental rain shadows . Primary productivity depends on low densities of small photoautotrophs that sustain
10212-586: The color of the underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment. Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates. Oxidising a gram of carbohydrate produces 0.60 grams of water; a gram of protein produces 0.41 grams of water; and a gram of fat produces 1.07 grams of water, making it possible for xerocoles to live with little or no access to drinking water. The kangaroo rat for example makes use of this water of metabolism and conserves water both by having
10360-468: The concept of desert, without the characteristic of aridity, not lacking water, but instead lacking life. Such places can be so-called "ocean deserts", which are mostly at the centers of gyres , but also hypoxic or anoxic waters such as dead zones . Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures falling sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to 54 °F) and
10508-508: The desert floor to scarify the seed coat. The seed of the mesquite tree, which grows in deserts in the Americas, is hard and fails to sprout even when planted carefully. When it has passed through the gut of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect
10656-500: The desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles , and often spines to deter herbivory . Some annual plants germinate , bloom and die in the course of a few weeks after rainfall, while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Animals need to keep cool and find enough food and water to survive. Many are nocturnal , and stay in
10804-500: The distance to their prey before striking. Monitor lizards have acute vision, hearing, and olfactory senses. Some lizards make unusual use of their sense organs: chameleons can steer their eyes in different directions, sometimes providing non-overlapping fields of view, such as forwards and backwards at once. Lizards lack external ears, having instead a circular opening in which the tympanic membrane (eardrum) can be seen. Many species rely on hearing for early warning of predators, and flee at
10952-617: The driest deserts on earth. The skin is tough and leathery, and is shed (sloughed) as the animal grows. Unlike snakes which shed the skin in a single piece, lizards slough their skin in several pieces. The scales may be modified into spines for display or protection, and some species have bone osteoderms underneath the scales. The dentitions of lizards reflect their wide range of diets, including carnivorous, insectivorous, omnivorous, herbivorous, nectivorous, and molluscivorous. Species typically have uniform teeth suited to their diet, but several species have variable teeth, such as cutting teeth in
11100-587: The driest season, and others curl their leaves up to reduce transpiration. Others, such as aloes , store water in succulent leaves or stems or in fleshy tubers. Desert plants maximize water uptake by having shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water. The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. In common with cacti, many have developed spines to ward off browsing animals. Some desert plants produce seed which lies dormant in
11248-586: The environment. The chameleons in general use their ability to change their coloration for signalling rather than camouflage, but some species such as Smith's dwarf chameleon do use active colour change for camouflage purposes. The flat-tail horned lizard 's body is coloured like its desert background, and is flattened and fringed with white scales to minimise its shadow. Many lizards, including geckos and skinks , are capable of shedding their tails ( autotomy ). The detached tail, sometimes brilliantly coloured, continues to writhe after detaching, distracting
11396-454: The exchange of water, although more arid-living species have calcified shells to retain water. Inside the eggs, the embryos use nutrients from the yolk . Parental care is uncommon and the female usually abandons the eggs after laying them. Brooding and protection of eggs do occur in some species. The female prairie skink uses respiratory water loss to maintain the humidity of the eggs which facilitates embryonic development. In lace monitors ,
11544-658: The expansion of the dewlap as well as head-bobs and body movements add to the visual signals. Some species have deep blue dewlaps and communicate with ultraviolet signals. Blue-tongued skinks will flash their tongues as a threat display . Chameleons are known to change their complex colour patterns when communicating, particularly during agonistic encounters. They tend to show brighter colours when displaying aggression and darker colours when they submit or "give up". Several gecko species are brightly coloured; some species tilt their bodies to display their coloration. In certain species, brightly coloured males turn dull when not in
11692-537: The extant non-avian reptiles. Archosauromorpha [REDACTED] [REDACTED] Pantestudines [REDACTED] † Kuehneosauridae [REDACTED] Rhynchocephalia [REDACTED] Squamata [REDACTED] Both the snakes and the Amphisbaenia (worm lizards) are clades deep within the Squamata (the smallest clade that contains all the lizards), so "lizard" is paraphyletic . The cladogram
11840-455: The external temperature. Jerboas , desert rats , kangaroo rats and other small rodents emerge from their burrows at night and so do the foxes, coyotes, jackals and snakes that prey on them. Kangaroos keep cool by increasing their respiration rate, panting, sweating and moistening the skin of their forelegs with saliva . Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath
11988-521: The fine material has been blown away and the surface consists of a mosaic of smooth stones, often forming desert pavements , and little further erosion takes place. Other desert features include rock outcrops , exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water, in the form of springs and seepages from aquifers . Where these are found, oases can occur. Plants and animals living in
12136-463: The first two groups. The pads are composed of millions of tiny setae (hair-like structures) which fit closely to the substrate to adhere using van der Waals forces ; no liquid adhesive is needed. In addition, the toes of chameleons are divided into two opposed groups on each foot ( zygodactyly ), enabling them to perch on branches as birds do. Aside from legless lizards , most lizards are quadrupedal and move using gaits with alternating movement of
12284-480: The forest-dwelling Draco , are able to glide. They are often territorial , the males fighting off other males and signalling, often with bright colours, to attract mates and to intimidate rivals. Lizards are mainly carnivorous, often being sit-and-wait predators ; many smaller species eat insects, while the Komodo eats mammals as big as water buffalo . Lizards make use of a variety of antipredator adaptations , including venom , camouflage , reflex bleeding , and
12432-572: The form of springs , aquifers , underground rivers or lakes. Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water . The Great Man-Made River is a scheme launched by Libya's Muammar Gaddafi to tap this aquifer and supply water to coastal cities. Kharga Oasis in Egypt
12580-458: The fragmentation of harder granites , limestone and sandstone . There is a certain critical size (about 0.5 mm) below which further temperature-induced weathering of rocks does not occur and this provides a minimum size for sand grains. As the mountains are eroded, more and more sand is created. At high wind speeds, sand grains are picked up off the surface and blown along, a process known as saltation . The whirling airborne grains act as
12728-417: The front of the jaws and crushing teeth in the rear. Most species are pleurodont , though agamids and chameleons are acrodont . The tongue can be extended outside the mouth, and is often long. In the beaded lizards, whiptails and monitor lizards, the tongue is forked and used mainly or exclusively to sense the environment, continually flicking out to sample the environment, and back to transfer molecules to
12876-439: The ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called cryptobiotic soil ) can be a vital link in preventing erosion and providing support for other living organisms. Cold deserts often have high concentrations of salt in the soil. Grasses and low shrubs are the dominant vegetation here and the ground may be covered with lichens . Most shrubs have spiny leaves and shed them in
13024-474: The ground have a marked effect on the degree of aridity and the plant and animal life that can be sustained. Rain falling in the cold season may be more effective at promoting plant growth, and defining the boundaries of deserts and the semiarid regions that surround them on the grounds of precipitation alone is problematic. A semi-arid desert or a steppe is a version of the arid desert with much more rainfall, vegetation and higher humidity. These regions feature
13172-432: The ground on its long legs, stopping periodically to snatch up insects. Like other desert birds, it is well-camouflaged by its coloring and can merge into the landscape when stationary. The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers (miles) away from the waterhole it needs to visit daily. Some small diurnal birds are found in very restricted localities where their plumage matches
13320-427: The ground rises quickly through a small pocket of cooler, low-pressure air above forming a whirling column of particles, a dust devil . Sandstorms occur with much less frequency than dust storms. They are often preceded by severe dust storms and occur when the wind velocity increases to a point where it can lift heavier particles. These grains of sand, up to about 0.5 mm (0.020 in) in diameter are jerked into
13468-429: The high desert floor, creating canyons that are over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion years old. Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation. Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. As
13616-783: The high elevations of the Tabernas Desert (and some parts of the Spanish Plateu ), The Sahel , The Eurasian Steppe , most of Central Asia , the Western US , most of Northern Mexico , portions of South America (especially in Argentina ) and the Australian Outback . They usually feature BSh (hot steppe) or BSk (temperate steppe) in the Köppen climate classification . Coastal deserts are mostly found on
13764-454: The import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara , and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for
13912-523: The lack of heat to aid in the evaporation process. Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal". The characteristics of hot deserts include high temperatures in summer; greater evaporation than precipitation, usually exacerbated by high temperatures, strong winds and lack of cloud cover; considerable variation in the occurrence of precipitation, its intensity and distribution; and low humidity. Winter temperatures vary considerably between different deserts and are often related to
14060-483: The land with a billowing leading edge. The sunlight can be obliterated and it may become as dark as night at ground level. In a study of a dust storm in China in 2001, it was estimated that 6.5 million tons of dust were involved, covering an area of 134,000,000 km (52,000,000 sq mi). The mean particle size was 1.44 μm. A much smaller scale, short-lived phenomenon can occur in calm conditions when hot air near
14208-476: The land, deposits its water content and circulates back to sea. Further inland, areas receive very little precipitation. The Thar Desert near the India/Pakistan border is of this type. In some parts of the world, deserts are created by a rain shadow effect. Orographic lift occurs as air masses rise to pass over high ground. In the process they cool and lose much of their moisture by precipitation on
14356-723: The last ice age when the climate was colder and wetter. These include the Great Salt Lake , Utah Lake , Sevier Lake and many dry lake beds. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the Space Shuttle . Deserts have been defined and classified in
14504-510: The life processes of plants. Potential evapotranspiration , then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 mm (12 in) of rain per year, however about 2,500 mm (98 in) of water could evaporate over the course of a year. In other words, about eight times more water could evaporate from the region than actually falls as rain. Rates of evapotranspiration in cold regions such as Alaska are much lower because of
14652-489: The lizard to maintain their ideal body temperature for optimal mobility. Most social interactions among lizards are between breeding individuals. Territoriality is common and is correlated with species that use sit-and-wait hunting strategies. Males establish and maintain territories that contain resources that attract females and which they defend from other males. Important resources include basking, feeding, and nesting sites as well as refuges from predators. The habitat of
14800-412: The location of the desert on the continental landmass and the latitude. Daily variations in temperature can be as great as 22 °C (40 °F) or more, with heat loss by radiation at night being increased by the clear skies. Cold deserts, sometimes known as temperate deserts, occur at higher latitudes than hot deserts, and the aridity is caused by the dryness of the air. Some cold deserts are far from
14948-553: The male tokay gecko is heard as "tokay-tokay!". Tactile communication involves individuals rubbing against each other, either in courtship or in aggression. Some chameleon species communicate with one another by vibrating the substrate that they are standing on, such as a tree branch or leaf. Lizards are found worldwide, excluding the far north and Antarctica, and some islands. They can be found in elevations from sea level to 5,000 m (16,000 ft). They prefer warmer, tropical climates but are adaptable and can live in all but
15096-404: The measurement of precipitation in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P − PE ± S , wherein P is precipitation, PE is potential evapotranspiration rates and S is the amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through
15244-503: The middle of the period. Mosasaurs likely evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous . Dolichosauridae is a family of Late Cretaceous aquatic varanoid lizards closely related to the mosasaurs. The position of the lizards and other Squamata among the reptiles was studied using fossil evidence by Rainer Schoch and Hans-Dieter Sues in 2015. Lizards form about 60% of
15392-666: The most extreme environments. Lizards also exploit a number of habitats; most primarily live on the ground, but others may live in rocks, on trees, underground and even in water. The marine iguana is adapted for life in the sea. The majority of lizard species are predatory and the most common prey items are small, terrestrial invertebrates, particularly insects . Many species are sit-and-wait predators though others may be more active foragers. Chameleons prey on numerous insect species, such as beetles , grasshoppers and winged termites as well as spiders . They rely on persistence and ambush to capture these prey. An individual perches on
15540-617: The nearest available sources of moisture. Montane deserts are arid places with a very high altitude ; the most prominent example is found north of the Himalayas, in the Kunlun Mountains and the Tibetan Plateau . Many locations within this category have elevations exceeding 3,000 m (9,800 ft) and the thermal regime can be hemiboreal . These places owe their profound aridity (the average annual precipitation
15688-409: The night to allow CO 2 to enter, and close them during the day, or by using C4 carbon fixation . Many desert plants have reduced the size of their leaves or abandoned them altogether. Cacti are present in both North and South America with a post-Gondwana origin. The genus is desert specialist, and in most species, the leaves have been dispensed with and the chlorophyll displaced into the trunks,
15836-497: The nights grow much colder. Strong gale-force winds often pick up in the winter and these cause sandstorms. The wind is often cold due to the influence from the Atlantic Ocean in the west. The Richtersfeld is a transitional zone between the coastal Succulent Karoo ecoregion in the west, and the drier Nama Karoo to the east. Rainfall in the Richtersveld varies from 5 mm per annum in the east to 200 mm per annum in
15984-595: The north-western side of the Northern Cape province in South Africa, the Richtersveld is regarded as the only arid biodiversity hotspot on earth and the majority of the area is inscribed on UNESCO's World Heritage List due to its cultural values. The Nama people of Richtersveld claimed title to their traditional land and set aside this conservancy for future research and tourism. The northern part of
16132-527: The ocean and others are separated by mountain ranges from the sea, and in both cases, there is insufficient moisture in the air to cause much precipitation. The largest of these deserts are found in Central Asia. Others occur on the eastern side of the Rocky Mountains , the eastern side of the southern Andes and in southern Australia. Polar deserts are a particular class of cold desert. The air
16280-441: The only venomous lizards. However, several species of monitor lizards, including the Komodo dragon , produce powerful venom in their oral glands . Lace monitor venom, for instance, causes swift loss of consciousness and extensive bleeding through its pharmacological effects, both lowering blood pressure and preventing blood clotting . Nine classes of toxin known from snakes are produced by lizards. The range of actions provides
16428-426: The outer surfaces of rocks split off in flat flakes. This is believed to be caused by the stresses put on the rock by repeated thermal expansions and contractions which induces fracturing parallel to the original surface. Chemical weathering processes probably play a more important role in deserts than was previously thought. The necessary moisture may be present in the form of dew or mist. Ground water may be drawn to
16576-460: The painted dragon lizard Ctenophorus pictus is fading breeding colors. By manipulating superoxide levels (using a superoxide dismutase mimetic) it was shown that this fading coloration is likely due to gradual loss with lizard age of an innate capacity for antioxidation due to increasing DNA damage . The majority of lizard species are active during the day , though some are active at night , notably geckos. As ectotherms , lizards have
16724-466: The pavement is formed. It may be that after the sand and dust is blown away by the wind the stones jiggle themselves into place; alternatively, stones previously below ground may in some way work themselves to the surface. Very little further erosion takes place after the formation of a pavement, and the ground becomes stable. Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form
16872-401: The potential for new medicinal drugs based on lizard venom proteins . Genes associated with venom toxins have been found in the salivary glands of a wide range of lizards, including species traditionally thought of as non-venomous, such as iguanas and bearded dragons. This suggests that these genes evolved in the common ancestor of lizards and snakes , some 200 million years ago (forming
17020-515: The predator's attention from the fleeing prey. Lizards partially regenerate their tails over a period of weeks. Some 326 genes are involved in regenerating lizard tails. The fish-scale gecko Geckolepis megalepis sheds patches of skin and scales if grabbed. Many lizards attempt to escape from danger by running to a place of safety; for example, wall lizards can run up walls and hide in holes or cracks. Horned lizards adopt differing defences for specific predators. They may play dead to deceive
17168-478: The presence of rivals or females. While it is usually males that display, in some species females also use such communication. In the bronze anole , head-bobs are a common form of communication among females, the speed and frequency varying with age and territorial status. Chemical cues or pheromones are also important in communication. Males typically direct signals at rivals, while females direct them at potential mates. Lizards may be able to recognise individuals of
17316-471: The rainier winter season. The threatened Richtersveld katydid ( Africariola longicauda ) is endemic to the area. In addition there are several species of spiders and scorpions . Approximately 4,849 plant species, 40 percent of which are endemic , have been documented in the Richtersveld and comprise mainly succulent and aloe species. The area is home to a number of rather unusual plants, many of which are found nowhere else on earth. Chief among these
17464-546: The resulting fragments and rubble strewn over the desert floor are further eroded by the wind. This picks up particles of sand and dust, which can remain airborne for extended periods – sometimes causing the formation of sand storms or dust storms . Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes . Other deserts are flat, stony plains where all
17612-592: The right and left limbs with substantial body bending. This body bending prevents significant respiration during movement, limiting their endurance, in a mechanism called Carrier's constraint . Several species can run bipedally, and a few can prop themselves up on their hindlimbs and tail while stationary. Several small species such as those in the genus Draco can glide: some can attain a distance of 60 metres (200 feet), losing 10 metres (33 feet) in height. Some species, like geckos and chameleons, adhere to vertical surfaces including glass and ceilings. Some species, like
17760-438: The rock surface experiences even greater temperature differentials. During the day the sky is usually clear and most of the sun 's radiation reaches the ground, but as soon as the sun sets, the desert cools quickly by radiating heat into space. In hot deserts, the temperature during daytime can exceed 45 °C (113 °F) in summer and plunge below freezing point at night during winter. Such large temperature variations have
17908-527: The same species by their scent. Acoustic communication is less common in lizards. Hissing , a typical reptilian sound, is mostly produced by larger species as part of a threat display, accompanying gaping jaws. Some groups, particularly geckos, snake-lizards, and some iguanids, can produce more complex sounds and vocal apparatuses have independently evolved in different groups. These sounds are used for courtship, territorial defense and in distress, and include clicks, squeaks, barks and growls. The mating call of
18056-503: The sand stream can rise as the largest sand grains do not become airborne at all. They are transported by creep, being rolled along the desert floor or performing short jumps. During a sandstorm, the wind-blown sand particles become electrically charged . Such electric fields , which range in size up to 80 kV/m, can produce sparks and cause interference with telecommunications equipment. They are also unpleasant for humans and can cause headaches and nausea. The electric fields are caused by
18204-435: The shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine . Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy. People have struggled to live in deserts and
18352-441: The sheet consists of thin horizontal layers of coarse silt and very fine to medium grain sand, separated by layers of coarse sand and pea-gravel which are a single grain thick. These larger particles anchor the other particles in place and may also be packed together on the surface so as to form a miniature desert pavement. Small ripples form on the sand sheet when the wind exceeds 24 km/h (15 mph). They form perpendicular to
18500-482: The shells. Larger species, such as monitor lizards, can feed on larger prey including fish, frogs, birds, mammals and other reptiles. Prey may be swallowed whole and torn into smaller pieces. Both bird and reptile eggs may also be consumed as well. Gila monsters and beaded lizards climb trees to reach both the eggs and young of birds. Despite being venomous, these species rely on their strong jaws to kill prey. Mammalian prey typically consists of rodents and leporids ;
18648-446: The skin and various thermoregulatory strategies to maintain its body temperature in one of the harshest environments on Earth. Lizard Sauria Macartney , 1802 Lizard is the common name used for all squamate reptiles other than snakes (and to a lesser extent amphisbaenians ), encompassing over 7,000 species , ranging across all continents except Antarctica , as well as most oceanic island chains . The grouping
18796-412: The skin. The arctic weasel has a metabolic rate that is two or three times as high as would be expected for an animal of its size. Birds have avoided the problem of losing heat through their feet by not attempting to maintain them at the same temperature as the rest of their bodies, a form of adaptive insulation. The emperor penguin has dense plumage, a downy under layer, an air insulation layer next to
18944-415: The slightest sound. As in snakes and many mammals, all lizards have a specialised olfactory system, the vomeronasal organ , used to detect pheromones . Monitor lizards transfer scent from the tip of their tongue to the organ; the tongue is used only for this information-gathering purpose, and is not involved in manipulating food. Some lizards, particularly iguanas, have retained a photosensory organ on
19092-466: The soil until sparked into growth by rainfall. With annuals , such plants grow with great rapidity and may flower and set seed within weeks, aiming to complete their development before the last vestige of water dries up. For perennial plants, reproduction is more likely to be successful if the seed germinates in a shaded position, but not so close to the parent plant as to be in competition with it. Some seed will not germinate until it has been blown about on
19240-464: The sparse vegetation cover and the activities of large mammals and people. Plants and animals in deserts are mostly adapted to extreme and prolonged water deficits , but their reproductive phenology often responds to short episodes of surplus. Competitive interactions are weak. Plants face severe challenges in arid environments. Problems they need to solve include how to obtain enough water, how to avoid being eaten and how to reproduce. Photosynthesis
19388-440: The surface by evaporation and the formation of salt crystals may dislodge rock particles as sand or disintegrate rocks by exfoliation. Shallow caves are sometimes formed at the base of cliffs by this means. As the desert mountains decay, large areas of shattered rock and rubble occur. The process continues and the end products are either dust or sand. Dust is formed from solidified clay or volcanic deposits whereas sand results from
19536-464: The surface of the ground. Dunes are sometimes solitary, but they are more often grouped together in dune fields. When these are extensive, they are known as sand seas or ergs . The shape of the dune depends on the characteristics of the prevailing wind. Barchan dunes are produced by strong winds blowing across a level surface and are crescent-shaped with the concave side away from the wind. When there are two directions from which winds regularly blow,
19684-577: The surrounding mountains. Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska , are known as paleodeserts. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system , deserts would be classified as arid megathermal climates. Polar deserts are
19832-610: The surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available, and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert farming is possible with the aid of irrigation , and the Imperial Valley in California provides an example of how previously barren land can be made productive by
19980-399: The tallest type of dune. Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas. In deserts where large amounts of limestone mountains surround a closed basin , such as at White Sands National Park in south-central New Mexico , occasional storm runoff transports dissolved limestone and gypsum into a low-lying pan within the basin where
20128-508: The top of their heads called the parietal eye , a basal ("primitive") feature also present in the tuatara . This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps them detect predators stalking it from above. Until 2006 it was thought that the Gila monster and the Mexican beaded lizard were
20276-407: The trunk is folded like a concertina , allowing it to expand, and a large specimen can hold eight tons of water after a good downpour. Other xerophytic plants have developed similar strategies by a process known as convergent evolution . They limit water loss by reducing the size and number of stomata, by having waxy coatings and hairy or tiny leaves. Some are deciduous, shedding their leaves in
20424-496: The use of the word desert can cause confusion. In English before the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity ; but today the word is most often used in its climate-science sense (an area of low precipitation). Phrases such as " desert island " and " Great American Desert ", or Shakespeare 's "deserts of Bohemia " ( The Winter's Tale ) in previous centuries did not necessarily imply sand or aridity; their focus
20572-424: The vomeronasal organ responsible for chemosensation, analogous to but different from smell or taste. In geckos, the tongue is used to lick the eyes clean: they have no eyelids. Chameleons have very long sticky tongues which can be extended rapidly to catch their insect prey. Three lineages, the geckos , anoles , and chameleons , have modified the scales under their toes to form adhesive pads , highly prominent in
20720-416: The water evaporates, depositing the gypsum and forming crystals known as selenite . The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea. A large part of the surface area of
20868-574: The west. The western mountainous region receives mainly winter rainfall as well as life-giving mists from the ocean. The eastern area along the Orange River is drier and the rain more often arrives in summer, often as large thunderstorms. By late August through to early October spring arrives with a vast floral blooming of daisy species ( Osteospermum ) and "vygies" ( Lampranthus ), namely in Namaqualand . The Richtersveld offers habitats for
21016-476: The western edges of continental land masses in regions where cold currents approach the land or cold water upwellings rise from the ocean depths. The cool winds crossing this water pick up little moisture and the coastal regions have low temperatures and very low rainfall, the main precipitation being in the form of fog and dew. The range of temperatures on a daily and annual scale is relatively low, being 11 °C (20 °F) and 5 °C (9 °F) respectively in
21164-404: The wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill. When they reach the crest, they cascade down the far side. The upwind slope typically has a gradient of 10° to 20° while the lee slope is around 32°, the angle at which loose dry sand will slip. As this wind-induced movement of sand grains takes place, the dune moves slowly across
21312-511: The wind direction and gradually move across the surface as the wind continues to blow. The distance between their crests corresponds to the average length of jumps made by particles during saltation. The ripples are ephemeral and a change in wind direction causes them to reorganise. Sand dunes are accumulations of windblown sand piled up in mounds or ridges. They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle. As
21460-450: The world's deserts consists of flat, stone-covered plains dominated by wind erosion. In " eolian deflation ", the wind continually removes fine-grained material, which becomes wind-blown sand. This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles , leaving a desert pavement , an area of land overlaid by closely packed smooth stones forming a tessellated mosaic. Different theories exist as to how exactly
21608-414: The world, around 20% of desert is sand, varying from only 2% in North America to 30% in Australia and over 45% in Central Asia. Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes . A sand sheet is a near-level, firm expanse of partially consolidated particles in a layer that varies from a few centimeters to a few meters thick. The structure of
21756-629: The young hatch close to 300 days, and the female returns to help them escape the termite mound where the eggs were laid. Around 20 percent of lizard species reproduce via viviparity (live birth). This is particularly common in Anguimorphs. Viviparous species give birth to relatively developed young which look like miniature adults. Embryos are nourished via a placenta -like structure. A minority of lizards have parthenogenesis (reproduction from unfertilised eggs). These species consist of all females who reproduce asexually with no need for males. This
21904-459: Was the sparse population. Deserts occupy about one third of Earth's land surface. Bottomlands may be salt -covered flats. Eolian processes are major factors in shaping desert landscapes. Polar deserts (also seen as "cold deserts") have similar features, except the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Some of
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