en zh es ja ko pt

Volume 21, Number 1January/February 1970

In This Issue

Back to Table of Contents

Fit to Survive

Evolutionary adaptations of both form and habit help make Arabia's desert mammals.

Written and illustrated by David L. Harrison

Photographs of the bleak surface of the moon and Mars, so commonplace last year, emphasize an important fact that is often forgotten: how fertile is our planet Earth. Even in its most inhospitable reaches —the poles, the mountain peaks, the depths of the ocean, and the deserts—life, in some form, has somehow established itself and survived. In parts of the Arabian deserts, for instance, conditions for survival are as demanding as any on—or maybe even off—Earth: intense heat, nearly unrelieved aridity, sudden cold; yet the mammalian fauna there is surprisingly varied. Despite the great and very special problems of survival, many wild mammals manage to survive, and some even to flourish.

The Arabian Peninsula has a total area of about a million square miles, the greater part of which is arid steppe and desert terrain, part of the great Palaeartic Desert tract stretching from the Sahara to Sind, in West Pakistan. About one fifth is the sandy desert of film and fiction: like the Rub' al-Khali and the Nafud, in which the wind has built up great ridges and dunes of sand. Other vast tracts are composed of undulating, featureless wastes with stony or dusty surfaces, occasional water holes and permanent coarse vegetation in hollows. Most of the peninsula is incomparatively dry; most parts receive less than 10 inches, annual rainfall, some as little as one inch. In the hearts of the deserts rain may not occur at all in a whole year. In summer the heat is scorching, July average temperatures—the figure midway between the highest and lowest readings each day—in places exceed 95° F. Shade temperatures exceeding 120° F. are by no means unusual—and in winter bitterly cold days are not rare.

For mammals those conditions mean trouble. Because of them vegetation is extremely sparse, which creates a food problem, and makes concealment from predators difficult. (Conversely, of course, predatory animals have greater difficulty in approaching and capturing their prey undetected.) This lack of cover is one reason why nearly all desert mammals are nocturnal, only leaving the security of their burrows or lairs under cover of darkness. Almost all desert mammals are pallid. It has long been debated whether or not these pale hues—"desert coloration"—provide a camouflage, but recent discoveries among Arabian rodents tend to confirm that they do.

Two species of Spiny Mouse are found in Arabia, living in arid rocky terrain. One exception to the nocturnal rule is the Golden Spiny Mouse (Acomys russatus) . The Golden Spiny is often seen darting about among the rocks in the intense heat of midday. Its normal coloration is a light reddish brown, but recently a new form was discovered, living on fields of black lava in eastern Jordan, which has evolved an entirely blackish pelage. The related but nocturnal species, Acomys dimidiatus, is also affected by soil color and on darker soils and rocks a darker, greyish race is found. Such instances are widespread among the desert mammals, and exceptions such as the strikingly obvious black and white pattern of the ratel, or Honey Badger, are generally easy to explain. The ratel, for example, with its vicious bite, tough, leathery hide and skunk-like ability to eject an evil-smelling secretion from the anal glands, is clearly a case of warning coloration. It advertises to all the fact that its possessor is best left in peace. Protective coloration is effective only when animals are motionless; it is highly developed among the desert hares, which often blend invisibly with the predominant soil color.

The problem of escape from predators in such bare terrain is an important factor in the lives of small desert mammals. Many small rodents seldom wander far from the security of their burrows, but the jerboas, a form of rodent, have evolved a special solution to this problem. By progressive elongation of the hind foot and elevation of the stance onto the tips of the three longest toes, followed by total loss of the outer toes, a method of moving by rapid and irregular jumps has been evolved. This makes the jerboa extremely difficult to capture and allows it to wander far afield. Two species are found in the Arabian deserts: the more primitive Five-toed Jerboa (Allactaga euphratica), which lives on the stony steppe-deserts of the north (with its two outer toes still present on its foot, but functionless) and the Three-toed Jerboa (Jaculus jaculus), a ubiquitous creature that is, without doubt, one of the most highly adapted desert mammals. This engaging little rodent is able to live in true sand desert, thanks in part to its ability to obtain a sure foothold when moving on soft sand by using tufts of long, rather rigid hair on the three remaining toes. A similar adaptation is found on the feet of that unique desert predator, the Sand Cat (Felis margarita). The pads which are normally visible on the soles of cats' feet are wholly concealed in this species by long tufts of wavy hair. The desert hare shows a similar tendency.

The sparsity of vegetation in the desert, which means that food supply is scarce for herbivores, leads in turn to a low population density and wide dispersal of individuals. Predators, in these conditions, tend to be even more widely dispersed than usual. Thus, for all desert mammals the problem of locating other individuals of their own species becomes important and almost all have exceptionally large ears. There is scarcely an exception to this rule; from the little Fennec Fox to the Sand Cat to the tiny Arabian Hare with its almost ludicrously large ears. Even the handsome black- and white-faced Ethiopian Desert Hedgehog (Paraechinus aethiopicus), which wanders in the most arid desert, has very large ears indeed. One reason for this is probably physiological (facilitating greater heat loss from the body) but there is no doubt that a greater acuity of sound localization also results because of an enlarged bony chamber surrounding the middle ear. This chamber, the tympanic bulla, is often greatly enlarged in desert mammals, attaining extreme degrees in rodents such as the Sand Rats (Meriones), where it may form almost the whole back of the skull. Recent research has shown that such enlarged bullae form a resonating chamber that sharpens the hearing for a sound frequency range corresponding to the rodent's own cry.

Another striking feature of the jerboas is the prominent black and white tuft on the tail tip, which is very obvious, bobbing up and down as the animal hops about in the moonlight or in the fast-gathering gloom of the desert dusk. It is believed that-this also functions as a danger signal. The Sand Rats, too, have prominent black tail tips and one of the Arabian species which is active by day, Meriones lybicus, holds its tail erect as it flees to the burrow, apparently to warn other members of the colony of danger.

Other important problems which desert mammals must solve are the physiological restrictions imposed upon them by the climate. The problem of temperature regulation is probably the least difficult of these, however, because fortunately the intense heat of the desert sun does not penetrate far beneath the surface of the earth. Many desert rodents customarily close up the burrow entrance with a plug of soil during the daytime, thus preserving an equable microclimate in their home. Other mammals, like the unique little hyrax, seek shelter during the heat of the day in caves or rock crevices or, like the jackal, in dense thickets. For surface mammals, however, such as the Arabian Oryx, the gazelles and the hares, thermal insulation is a severe problem. Such species as these can rarely find more shade than that in shallow cavities scratched out in the sand or beneath some bush or overhanging rock.

By far the most exacting of all the physiological problems which must be solved by the desert's mammals, however, is that of water balance. Various methods have been evolved to overcome the virtual absence of available drinking water over long periods. Some desert rodents obtain necessary moisture by feeding on certain succulent plants which store water in their foliage. The Fat Jird (Psammomys obesus) feeds largely on the foliage of certain Salsolaceae, such as Traganum, the leaves of which contain almost 83 per cent water. The dwarf shrub Rhanteriunt eppaposum is an important food plant for the handsome Rhim, or White Gazelle (Gazella subgutturosa marica), of the Arabian sands, because its buds hold moisture in times of drought. Depending on plants for moisture, however, carries with it a great degree of ecological dependence and restricts the distribution of the animal species. A far greater degree of water independence has been achieved by those rodents that are able to exist for long periods on a diet of dry seeds. Excavation of the tunnel systems of some species of gerbil reveals numerous blind side-tunnels packed with dried seeds; these storage chambers provide food during the long, hot summer months. The Greater Egyptian Jerboa (Jaculus orientalis), which occurs in Sinai, is able to survive for three years on a diet of dry barley and wheat containing only 10 per cent water. An ordinary rat on this diet dies after three days, so that the jerboa has three hundred times the survival power of the rat.

Other desert animals can store water in special depots of fat, maintaining their hydration by obtaining metabolic water from the breakdown of the fat. The Arabian Camel is, of course, the classical example. The fat in a camel's hump weighs perhaps 20 to 30 pounds, and the breakdown of each pound provides 1.1 pounds of water (by combining released hydrogen with oxygen derived from respiration). This remarkable beast is able to tolerate a 25 per cent loss of body weight under conditions of prolonged water deprivation; it can endure a rise of body temperature of 9° above normal and when opportunity presents it will drink as much as 25 gallons at a time to restore its condition. Some animals, such as the beautiful Arabian Gazelle (Gazella gazella arabica), have been observed to drink sea water in situations when they are almost completely without fresh water and I have myself observed a marked partiality for salt in a captive gazelle. It habitually licked the sweat running from my arms and legs during the fiery heat of summer in Oman. It is interesting that desert bats such as the curious Trident Bat, with its three-. pronged noseleaf, are obliged to visit water holes, and often at dusk enormous numbers may be seen diving down to take sips of water from the surface. The reason for this water dependence is found in the nature of their diet. Most Arabian bats live on insects, which have a high protein content. And that means a considerable excretion of urea as waste product which in turn means a significant loss of water in urine.

Herbivorous mammals have another difficulty: the scarcity of pasture in periods of prolonged drought. Some of the larger species, such as the handsome Arabian Oryx (Oryx leucoryx), can cover great distances in search of those favored areas where recent showers have produced fresh pasture. The oryx has been known to cover almost 60 miles, nearly all at a walk, in less than 18 hours. In fact they don't have to have free water for months at a time, living on moisture from succulent plants and even, occasionally, from dew. The oryx, incidentally, is one of the mammals which can consume the bitter and cathartic gourd of the Desert Colocynth (Citrullus colocynthis). The smaller mammals are less fortunate in times of prolonged drought, however; rodents must retire deep into their burrows and live on stored food until better times return. Some species are probably capable of aestivation; that is to say they pass into a torpid condition similar to hibernation, thus greatly reducing their metabolic requirements. This has been observed in some of the gerbils and I suspect that the Three-toed Jerboa may also employ aestivation, since it seems to disappear during the hot summer months in Arabia.

Among the rodents that resort to food storage, surely the strange-looking Mole Rat (Spalax leucodon), which occurs in the fringes of the north-western deserts, must be the most prodigious worker. As much as 40 pounds of potatoes and sugar beets have been found stored in this rodent's burrows, which are marked by a series of mounds resembling molehills. This strange creature has become so totally modified for subterranean life that it has lost all external trace of eyes, ears and tail, using its enormous incisor teeth for digging and the flattened head for shoveling soil like a bulldozer, useful evolutionary adaptations for a moon-like desert environment where truly, only the fittest survive.

David L. Harrison, author of Footsteps in the Sand and the two-volume, standard reference work Mammals of Arabia (Ernest Benn Limited, London), would be pleased to receive information or specimens to further his research.

This article appeared on pages 30-32 of the January/February 1970 print edition of Saudi Aramco World.


Check the Public Affairs Digital Image Archive for January/February 1970 images.