Popular Science Monthly/Volume 38/December 1890/Animal Life in the Great Desert
By WILLIAM MARSHALL.
THE surface of the earth, with its division of land and water its diversities of climate, and its various elevations, offers to the world of plants as well as to animals a complexity of life-conditions to which their organisms are compelled to adapt themselves if they would even exist.
Few regions exhibit to so large an extent such even, uniform, and original character, as that vast desert expanse which stretches through southern Arabia and northern Africa from the Persian Gulf to the Atlantic Ocean. This uniformity is the result of the correspondence of the desert tract with the same degrees of latitude, and of its never departing from the subtropical regions. Since, also, the elevation of the land seldom greatly exceeds 3,000 feet, the temperature conditions, however much they may vary in single places in the course of a day, are as a whole more uniform than they would be in a similar tract running north and south, and marked by important elevations. The midday heat in the desert rises to over 120° Fahr., while at night the cold, in consequence of the rapid radiation, sometimes makes itself very unpleasantly felt, and in winter descends below the freezing-point. More unfavorable to the development of animal life than the temperature is the want of water, both running and standing, as well as the absence of rain and dew. Sufficient water and a thin surface soil are found only in the oases, which exercise an influence over the distribution of life like that of the presence of the numerous islands in the great ocean. Even including the oases, vegetation is very scanty; the immense territory of the Sahara, with an area of upward of 2,500,000 square miles, harbors only 560 species of plants; while the Japanese Islands, having only one seventeenth the area, 150,000 square miles, support not less than 2,745 species. Most of the desert vegetation is deficient in quality as well as quantity; the plants are sparse, generally small, with inconspicuous gray leaves, and often covered with sand. Many plants that are usually annual develop, under the influence of life in the desert, long roots reaching down to the ground water, and become perennial. Monocotyledonous plants are represented only by dry, tough grasses, like the esparto, and by a few palms in the oases. Woods, the chief resorts of animal life, are wanting.
Most of the scanty fauna is concentrated in the oases. The oasis of Bachariel, according to the French entomologist Lefevre, swarms with insects at certain seasons, which would yield a rich harvest to the collector if he would stay there long enough to secure the varieties. On the borders of the deserts, where the cultivated land cuts into them, especially in the region of the Nile Valley and the Red Sea, organic life is fairly well developed. The broad valleys in those regions are changed after rains into green meadows, and in January the perennial plants in every mountain-clove and ravine are covered with foliage and flowers; and annuals spring up, affording a luxuriant flora from February till April. Day moths sport themselves, in few species indeed, but in multitudes of individuals. Along with them buzz numerous wasps and flower-visiting beetles, and in the oases the troublesome ants are associated with a series of insects whose larvæ are bred in the water. Dragon-flies appear in multitudes, often swarming like locusts, and miles from the water, and myriads of stinging flies for short periods make the sojourn of Europeans intolerable. The pests of the home are here too, and vermin that make life a burden even to camels.
Scorpions are plenty, both in the oases and the desert proper, and spiders abound at the opening of the rainy season. Especially is this the case with a little purple spider of a velvety sheen, of which, according to Nachtigall, the people of Bournou believe the red velvet of the Western countries is made. Little crustaceans are numerous in the springs, and one species (Artemia oudenyi) occurs so frequently in some of the salt lakes of Fezzan as to serve, with the larvæ of certain flies, as food for the people. Fish are found in the ponds and underground springs; but the last are individuals which have, as Carl Vogt has shown, only casually reached the springs through underground channels from surface waters; for they betray no sign, either in coloring or the structure of their eyes, that they were ever accustomed to constant darkness. Of double interest is a fish living in the hot springs of Tofra and Lafra, in Tunis; first, because it can bear a temperature of 167° Fahr. without injury, and also because it belongs to a genus of which the other species live only in the sea. A few small fresh-water mollusks are found here and there, and land shells of a class which are capable of enduring protracted drought in a passive condition, and reviving when it begins to rain, and thus afford a remarkable example of adaptation to life in the desert. Frogs and salamanders, which do not easily adapt themselves to an arid environment, can not exist under the conditions of life that prevail in the Sahara, not even in the oases. Some reptiles, birds, and mammals fare better there. These vertebrates, in fact, with insects, are the only animal inhabitants of the desert.
Nearly all these animals, from lions and gazelles to locusts, wear the yellow color of the desert sand, verifying the phrase of the Latin poet, "Flavæ, leænece arida nutrix" ("Dry nurse of the tawny lioness"). The weakling is thus protected by a coat that withdraws him from the lurking view of hidden enemies, while the strong beast of prey may conceal himself behind a rock, by the aid of the color of which he can the more easily steal unobserved upon his prey. Only such animals as fear no enemies display so conspicuous a color as black. "What strikes the traveler," says Carl Vogt, "when he comes to the desert from the coast, where the greenness of vegetation predominates, is the absence of all lively colors—of red, green, and blue, in the animals." The fullgrown ostrich is white and black; it is so large and swift that it has nothing to be afraid of but mounted men, and its food is not of such a kind that it needs a protective coloring in order to approach it without observation. The great desert crow (Corvus umbrinus), in which the negro of the Soudan perceives and worships his "uncle," is strong enough to keep off all its enemies, and agile enough to seize its prey when it has once had its eye upon it. The beetles, too, of the desert are black; not the "black beetles" of the Mediterranean region, but other kinds such as often have bright colors or a metallic luster. Carl Vogt asserts that these beetles are defended by an offensive odor or taste, that they have highly arched wing-covers and a depressed corselet or a withdrawn head, and can feign death when they believe they are threatened. When driven into close quarters, they become motionless, assume the likeness of the excrement of gazelles or goats, and thus avoid pursuit.
The coloring of the other animals is often remarkably like that of the pebbly sand. Those creatures—beasts of prey, ruminants, and birds—which are not confined to the soil, but roam or fly around, are tawny, but sometimes striped with different tints. Fowls, larks, stone-chats, running and wading birds, do not form local races with clear or dark feathers, and have not the faculty of changing their color according to the background against which they may for the time find themselves. Another rule prevails with those animals which occur in districts of limited extent. The snakes and lizards of the desert, even when they are of the same species, wear different vestures according to their dwelling-places, while the colors of the same individual, of the lizards at least, are themselves changeable. The proverbial chameleon is not the only animal which is capable of unconsciously adapting its colors to those of its surroundings. Eminently accomplished in this respect are the plaice, while our brook-trout, frogs, and many lizards possess the useful faculty in a less degree. The spring-tailed lizard (Uromastix acanthinurus), which Carl Vogt observed in captivity, presented in darkness and the shade a dullgray slate color with indefinite blackish marblings, but when exposed to direct sunlight became brighter and brighter, and at last appeared of a dirty cream-color, with small, deep-black spots, resembling in its hues the fine desert sand mixed with black grain pebbles. Another lizard of the Sahara (Trapelus ægypticus) possesses the same peculiarity in a higher degree. The property of changing color depends on the presence of certain dark cells in the tissue of the skin, called chromatophores or color-bearers, which, contracting, under reflex influences of the nervous system, permit the full display of the ground-color of the animal, or, expanding to a certain extent, overlie it.
The power of changing color also exists in insects, but less commonly. We more frequently find among them varieties which are distinguished by constantly different but always protective colors. Lefevre observed in the Libyan Desert curious praying crickets of the same species as to other marks, which were brown on a brown soil, and a hundred paces away, on white fossil shells and fragments of limestone, were correspondingly white. They resembled the background against which they stood so much that the French naturalist could not detect them except when they moved. They had other peculiarities, among them wings so contracted that they could not fly; a phenomenon which is sometimes met among insects and birds inhabiting large territories and islands where they are but little exposed to pursuit. They have disused flight with advantage, for only a good flier can keep his ground under the conditions that prevail in such places. A weak flier would be taken by the wind and carried off helpless to destruction.
Sand-fowl (Pterocles) are represented by fourteen species. in the Old World, and are spread from the deserts and steppes of central Asia and India through all continental Africa. They visit southern Europe as breeding-birds, crossing the Strait of Gibraltar into the Iberian Peninsula. Their home is never in wooded regions; the more barren, stony, and arid the land, the less the extent of water and swamps, or contrast of mountain and valley, the more agreeable it is to them. In such regions live these modest birds, on the little which the land affords them, often on the sparse halfa grass; yet they can be found in coveys of hundreds, in places where it seems a puzzle how anything can live. Only ability to move speedily from place to place can make this possible. None but accomplished fliers can exist under such circumstances, and then when gathered in large groups. "It is easy for them," says Brehm, who has observed them more closely than any other naturalist, "to execute a flight, before going to sleep, which would appear to us equal to a day's journey or more." At breeding-time the coveys separate into pairs, and live in this state for a considerable period. When the brood is hatched they are still confined to their household duties, and, not being able to roam around, many suffer for want of the food which their narrow domain does not afford. Life in the desert is, therefore, one of the factors by which the sand-fowl is forbidden the polygamy affected by other members of the gallinaceous family. Scarcity of food also affects the life of these birds by adding to their hours of labor; for they require more time to find the quantity of food they need than other birds whose tables are more richly furnished, and may often be seen, when the moon is shining, active during a part of the night.
Their plumage is strikingly like the soil of their home, though I doubt if they are aware of the value of the feature, as Brehm believes. The squatting attitude and the stillness they assume when they believe themselves in danger are probably only instinctive. Bitterns in like manner resemble in plumage, and in the position they assume when they perceive anything suspicious, the old reeds and bushes on the shore. I have observed the same changes in captive birds when suddenly frightened, and when it can not be of any use to them. It is an involuntary reflex action, like the bristling of the hair and the exposure of the teeth in angry dogs.
With extraordinarily acute sight and hearing joined to a great power of flight, the sand-fowl is little exposed to danger, except when a desert fox or fennec succeeds in stealing upon a covey at their noon-rest, or at night, and snapping up one or two of the number.
This animal, which is a little larger than a cat, is a true child of the desert, and is represented by local varieties through all Africa. Its color is the characteristic yellow of the desert; it has a fine growth of hair on the paws, which prevents its sinking in the fine sand and muffles the sound of its footsteps. The most striking of the features that have adapted it to its abiding-place and its way of life is in a certain sense the complement of its soft foot—a very sharp organ of hearing, the sound-catching outer part of which is unusually large. Its eye is not adequate to perceive its favorite prey, so well protected by its color; and there is a limit to the development of the organ of sight in an animal which, while it does not shun the day, is eminently nocturnal; and, as is often the case, another sense, that of smell, comes in, besides the hearing, to take the place of sight. Hearing is the night-sense; and the fennec can hear the slightest movement of the sleepy khata (Pterocles alchata) at distances almost incredible to men, and slip upon its prey with noiseless steps. Then a leap, and one of the little sleepers, before it is aware of what has taken place, has breathed out its arduous but not unpoetic life; while its companions rush away affrighted, with loud cries of "khadda, khadda!"—Translated for the Popular Science Monthly from Daheim.