Deserts: the arid lands

The principal desert areas of the world are found in the two subtropical belts between latitudes 10° and 35° north and south of the equator. Very little land in the Southern Hemisphere lies between these latitudes, with the exception of the southernmost tip of the African sub-continent arid the narrow tail of the South American island continent, and therefore the major deserts of the world lie in the Northern Hemisphere. Desert conditions are characterized by extreme dryness; annual precipitation is less than 25 centimetres, and the sun evaporates all water that falls as rain.
The lack of moisture reaching desert areas is due to a number of factors, The principal one is the descent of dry air from the upper atmosphere, a feature of the global circulation pattern that is typical of these latitudes. Air is drawn to the low-pressure zones in equatorial regions, where it is heated and forced to rise. In the upper reaches of the atmosphere the air spreads out from the equator, cools gradually and descends, reaching the ground in desert areas, where it has a very low water content. In some cases a desert may owe its existence to being situated in the heart of a continent, far from any marine influence or moist wind. Allied to this is the rain-shadow phenomenon, found in areas where air currents from the sea cross high mountains, dropping their rain on the seaward side as they rise. On the other side of the mountain the air descends, completely dry, giving rise to a desert area.
The intensity of solar radiation in the desert is very great in comparison with other areas. In moist regions up to 60 per cent may be reflected away by clouds, atmospheric dust, water and plant
surfaces, but in deserts only ten per cent of the total radiation is reflected in this way. On the other hand, due to the lack of insulating cloud cover, up to 90 per cent of the accumulated heat of the day is lost at night through radiation. The result is an extremely large temperature difference between day and night, which may be as great as 40°C.
The worldwide extent of deserts is now much smaller than was the case during the Age of Man. For one thing there is a smaller area of land lying within the desert belts than there was then - due largely to the movement of the Australian continent northwards out of the desert belt. Also in man's time, inefficient agricultural techniques and widespread grazing of domestic animals on poor land artificially enlarged the desert regions and was one of the factors that led to man's ultimate decline; with the desert becoming more extensive year by year, the area of the earth's surface suitable for cultivation decreased. After man's disappearance the earth's natural habitats re-established themselves and the deserts resumed their natural proportions.
Life in the desert has to cope with many hostile factors, such as lack of water and extremes of temperature. Even so a large number of animals and plants have evolved to cope with them successfully. Similar compensating adaptations - such as highly efficient kidneys producing particularly concentrated urine, large ears to dissipate heat and the ability to burrow to depths at which conditions are less severe - have been developed independently in many widely separated groups of animals in different areas of desert.

THE SAND DWELLERS

Survival in a waterless sea

The leaping devil is armed with long talons and a mouthful of sharp teeth.

By foaming, the spitting featherfoot both cools itself and gets rid of unwanted toxins.

When pouncing on prey, the leaping devil may travel up to 2 metres in single jump.

The flapjacks co-operate in moving stones to construct their condensation traps.

Although it is held out behind when running, the flapjack holds its tail aloft to catch the cooling breeze when standing still.

The physiology of desert animals must tread a narrow path between water conservation and heat regulation. The lack of sweat glands - a water-saving measure - means that less conventional ways must be used to cool the animal in the heat of the day. Usually this is achieved by large ears or similar outgrowths which, criss-crossed by blood vessels, act as radiators to remove the animal's body heat.
A structure of this kind is found on the tail of the sand flapjack, Platycaudatus structor, a fairly large rodent found in sandy areas. Its excess body heat is carried away by the blood to the tail, where it is dissipated into the atmosphere. When pursued the animal can move at speed, running with its lengthy tail held well out behind as a counterbalance in the manner of its ancestor, the jerboa.
To conserve water the flapjack even constructs a condensate trap. As part of their courtship ritual each pair of flapjacks places a pile of stones over the site of the family burrow. These stones as well as protecting the burrow from the sun's direct rays during the day provide a large number of cold surfaces on which moisture can condense at night.
The spitting featherfoot, Pennapus saltans, is a rodent of a type that has existed in this environment ever since mammals first colonized the hot, dry areas of the earth's surface. It has small forelimbs and long hind legs, for jumping. The toes are fringed by short, stiff hairs. Its kidneys are highly efficient, recycling the animal's waste water to the point that its urine is more than twice as concentrated as that of a rodent of similar size living in a humid environment.
The featherfoot never drinks water but obtains all the moisture it needs from plants. It can even eat plants that are poisonous to other animals, and has the ability to excrete toxic substances without their having taken part in any metabolic process. It is a nocturnal animal, but if displaced from its deep burrow by a predator during the heat of the day, it can cool itself by producing copious quantities of saliva and coating the front of its body with foam. It also spits at its foe with deadly accuracy. As the saliva contains most of the excreted poisons from the plants it is an efficient weapon. Needless to say this defence mechanism dehydrates the animal very quickly and is therefore used only in dire emergency and even then only for short periods at any one time.
The leaping devil, Daemonops rotundus, an insectivore with carnivorous habits, is one of the featherfoot's chief predators and has a morphology and physiology similar to the featherfoot and to the other small desert rodents on which it preys.
A totally different predator, but one also descended from insectivore stock, is the desert shark, Psammonarus spp. It is sausage-shaped with a blunt, strong head and powerful shovel-like feet. It swims through the sand rather than burrowing, bursting into the rodents' nesting chambers, which it locates using the sensory pits at the end of its nose. It is almost completely hairless and avoids the extremes of temperature by remaining underground for most of the time. When it is at rest it lies just below the surface with only its eyes and nostrils protruding.

LARGE DESERT ANIMALS

The problem of size and its solution

The grobbit is purely herbivorous. Its long tail may grow up to over 100 centimetres.

The grobbit feeds mainly on the leaves and shoots of desert shrubs.

Its cloven hoofs and dew claws are used in conjunction to grasp branches.

The khilla's insectivore dentition has been adapted to suit its carnivorous habits.

It is largely nocturnal and is equipped with broad burrowing forefeet.

The kriskin, a common predatory bird in desert areas feeding principally on snakes, is, like many other predators of the region, mainly black in color.

The extinction of the camel at about the same time as man died out left a niche that was distinctly unattractive to any other animal. For a large animal to exist in desert conditions a quite remarkable physiology is required. The camel, for instance, was able to lose about 30 per cent of its body weight through dehydration without ill-effects, and it stored all the subcutaneous fat of its body in one lump, leaving the rest of the body free to radiate heat. It could tolerate fluctuations in its body temperature to some extent and had thick nostril covers and eyelids that effectively kept dust and sand out of its nose and eyes.
After some 50 million years of evolution these features have all developed again in one animal - the desert leaper, Aquator adepsicautus. The leaper is descended from the rodents, possibly one of the jerboas or sand rats, and has grown large - adult males may reach more than 3 metres from nose to tail. The tail is the most unusual feature of this animal; it is here that all its subcutaneous fat is stored. The fat is not a water store, but a store of food that enables the leaper to go for long periods without eating when food is unavailable. When the fat store is full the animal's body is well balanced and it can leap quickly along on its hind limbs. In this condition it can undertake journeys of 100 kilometres or more between waterholes and oases. It has broad, horny pads on the toes of its hind feet which prevent it from sinking into the sand and give it a good grip on naked rock.
The rocky areas of the desert are the habitat preferred by the grobbit, Ungulamys cerviforme. This rodent is about 60 centimetres long, excluding the tail, and has hooves developed on its third and fourth digits enabling it to run about the craggy landscape of the rocky desert. The second and fifth digits of its front feet have small claws that almost touch the hooves when the foot is bent, allowing the grobbit to grasp and pull down branches and feed on them. The grobbit lives in packs all over the rocky desert zone of the African and Asian sub-continents.
In the deserts, large predators are not common and very few meat-eating mammals of any description are found. The khilla, Carnosuncus pilopodus, however, descended from the insectivores, is one of the few. Standing about 60 cm high at the shoulder, it is largely nocturnal and spends most of the day hidden in a network of burrows excavated in soft sand. At night it hunts small mammals and obtains most of the water it needs from the moisture contained in their flesh.
Most desert animals are sandy yellow in colour to blend in with the surroundings and have white undersurfaces that counteract the effects of shade and give them a two-dimensional appearance. That this colour scheme is the result of evolutionary pressure is a belief supported by the darker appearance of animals found on black-grey lava areas and the almost white forms of the same animals found in salt-pan regions.
Animals that are not camouflaged are predominantly black. Predatory birds, reptiles and the most poisonous and unpalatable arthropods fall into this category. The colour resemblance may be due to a form of mimicry in which for some reason black is an advantageous colour for certain predators, and all others adopt the same colour to derive some similar benefit.

THE NORTH AMERICAN
DESERTS

Living in the shadow of the mountains

To retain as much water, as possible, the rootsucker lies on the desert surface with its headshield drawn in thight against its body shell.

The desert spickle sucks nectar from cactus flowers through its long snout. For such an ungainly little animal it can run surprisingly swiftly over the desert.

Only the male long-legged quail has a head plume.

The females are otherwise identical although a little smaller.

The eggs of long-legged quail are laid in sheltered hollows in the desert sand.

The North American deserts are rain-shadow deserts. The wet westerly winds that blow towards the continent across the Pacific Ocean first meet the great western mountain barrier and are forced to rise, dropping their rain on the seaward side. Past the peaks the winds are dry and desert conditions prevail on the extensive plains beyond.
The deserts are not completely barren, but contain an intermittent vegetation consisting of cacti and other succulent plants, normally growing as single specimens, each widely separated from one another. The barren soil surface between the plants conceals a vast network of roots spreading out to collect enough water for each plant to survive.
Among the roots lives the rootsucker, Palatops spp., an animal heavily armoured to protect it from desiccation rather than to defend it from attack. Its head is shielded by a broad spade-like plate and its back is covered by a shiny nut-like shell composed of compacted hair. Its tail and feet are also armoured, but with articulated plates that permit total mobility. The rootsucker moves through the sand using its broad feet like paddles and its head shield as a shovel to reach the roots of succulents on which it feeds, gnawing them with the edge of its head shield and lower incisors.
Among the thorns found in the vertical grooves of cactus stems lives the little desert spickle, Fistulostium setosum, its narrow body covered by spines that are partly for defence and partly for camouflage among the cactus thorns. It has no teeth and subsists entirely on the nectar of cactus flowers which it drinks through its long snout. When collecting nectar it often picks up pollen on its head. The pollen is eventually deposited on the stigmas of other flowers, thus effecting the cross-pollination of the cacti. Living almost solely on nectar, the spickle's digestive system is a very primitive affair, since nectar is very easily broken down.
Lizards and other reptiles do not have the sophisticated mechanisms that mammals and birds have for regulating body temperature. Their temperature is entirely dependent on the surroundings. Several desert reptiles have, however, developed rudimentary devices for keeping themselves cool. The fin lizard, Velusarus bipod, for instance, a small bipedal reptile, has a system of erectile fins and dewlaps on its neck and tail which it raises into the wind when its body becomes too hot. The heat is transferred through the fins via the blood stream into the air. When cooling itself, the lizard typically balances on one leg while keeping the other off the hot desert surface to get maximum benefit from the system.
Small mammals of the desert, like the desert spickle and the fin lizard, are preyed on by ground-dwelling birds such as the long-legged quail, Deserta catholica. Its eggs, which are laid in sand scrapes in sheltered spots beneath bushes or overhanging rocks, are sat on continuously to protect them from the extremes of heat and cold that are typical of the desert climate's daily temperature range.
The breeding cycle of this and many other desert birds is dependent on the rainy season, the birds nesting as soon as the first spring rains appear and continuing as long as the wet season lasts. In unusually dry years no breeding takes place.