Hot and Dry Lifestyles
from the book, "Our Fascinating Earth"
Hot and Dry Lifestyles
Although the word desert means, literally, "deserted" or "uninhabited," plants and animals that live in this harsh, relentless environment have evolved excellent adaptations. The basic scarcity of water produces a hostile setting for species as water—dependent as most of earth's life forms; in addition, desert landscapes are often accompanied by a savage sun and cutting wind. For both plants and animals the secret of successful desert dwelling lies in adjustments in physiology and lifestyle. The adaptations are innovative, ingenious, often bizarre, but amazingly effective.
The giant saguaro of the Sonoran Desert is a prime example of efficient living within the constraints of a home in an arid environment. In its over—200—year life span the saguaro can reach a height of up fifty feet. Its shallow roots extend radially as far as sixty—five feet in all directions around the plant, forming an effective plumbing system. When it rains, the network of roots sponges up water at a rapid rate, pumping it into the plant, where it is distributed throughout. So efficient is the root system that after a typical summer storm it can send enough water into the saguaro for the plant to get along without another drink for as long as four years.
The saguaro's accordion—pleated stem permits it to expand and contract, a critical factor in its adaptation to the desert. After a summer cloudburst the saguaro may absorb as much as a ton of water; this is made possible by the expansion properties of the pleated stem. A mature saguaro may weigh over ten tons, four—fifths of which is water. The cactus is then well prepared for the dry months that will follow.
The mesquite, a common tree in the deserts of the American South—west, has made its own compromises to achieve a good life in a dry domain. Its extensive root system reaches as far as 175 feet below the surface in the constant search for zones of underground water. The mesquite's crowning achievement occurs just after birth, for its growth above ground will be delayed until its roots have located water! Only then will stem and leaves begin to sprout, for now the tree is adequately equipped to face the harsh desert environment.
Cacti have been the most successful desert plants because of their efficiency in collecting, storing, and restraining their need for moisture. Their typical waxy surface discourages evaporation, and the white hairs that adorn many species reflect heat away from vulnerable tissue. Spines replace leaves, for they give off heat, expose a minimal surface to drying winds, and point downward to direct runoff of moisture to their thirsty roots. Additionally, spines discourage animals from eating the plant. Other successful desert plants produce leaves only after rain, hug the ground protectively, or manufacture toxins that protect their territory from competitors.
Food is the most important source of water for animals that have adapted to an arid environment. Because nighttime desert temperatures dip sufficiently for dew to form, this is significant moisture for both plants and animals. The water content of most desert plants, as high as 90 percent, makes them a critical source of water for plant—eating animals. An herbivorous animal such as the desert wood rat obtains almost all of its water by eating cactus. The wood rat itself is also made up mostly of water, so predators such as the gopher snake can obtain most of the moisture they need from a diet of rodents.
Perhaps the most successful desert mammal is the kangaroo rat of the American Southwest. It never drinks water! Subsisting on dry plants and seeds, it get its fluids almost entirely from the water of oxidation, which is produced during the digestive process. An average kangaroo rat can produce fifty—four grams of water from about one hundred grams of plant seeds.
Other animals have developed specific adaptations to meet the demands of the desert's stark domain. The strategy of spadefoot toads is to dig their way backward into the sand and remain dormant for eight to nine months. When they detect a summer downpour, the toads emerge, mate frantically, and lay eggs in puddles of rainwater. In about thirty—six hours the eggs hatch and tadpoles emerge and race desperately to become adults before the puddle evaporates. Those that succeed can join their elders underground and wait for the rains of the next year.
The unique specialties of various animals include the transparent eye—lids of certain lizards that permit them some vision during violent sandstorms. And the absorbent plumage of the sand grouse that allows it to become drenched at a distant water hole and bring a supply of water home to its young. There is also the ground squirrel of the Kalahari Desert, which seldom finds a rock or plant large enough to provide shade; therefore, its plumed tail draped over its back creates needed shade.
Of all desert dwellers humans are the least able to survive in an arid environment. The human body has evolved no water—storing organs or similar aids to endure desert conditions. Actually the reverse is true, for a person caught in the summer heat of a desert could expend a gallon of water in sweat. To survive comfortably on a hot day in the Sahara, those who are not accustomed to the desert should drink about six quarts of water. This is not always feasible, and every year a number of tourists die in the great Sahara Desert.
A few years ago in the Egyptian desert five tourists tried to reach an oasis 125 miles away in three vehicles, all unfit for desert travel. To add insult to their rashness, they completely ignored the recommendation of six quarts of water per person and carried only one quart each.
They did manage to drive ninety—five miles before the cars, choked with dust and/or stuck in the sand, could travel no farther. The men then set out on foot, retracing their tracks to reach the point of origin rather than following the trackless sands to their destination just thirty miles away.
A few days later a desert patrol found their abandoned automobiles and followed the tire tracks. Not one of the tourists had survived. The first body was discovered only ten miles from the abandoned vehicles. The toughest of the group was a little poodle whose mummified body was found fifty miles from the cars; it had obviously been headed back in the right direction.
The prescribed six quarts of water is based merely on an average, since individuals vary greatly in their water requirements. Those whose body hydration of water is extremely low should never venture out into the desert, because getting stranded could be fatal if they became dehydrated before becoming aware of it.
An incident that occurred in the Sahara in August 1973 is a tragic example. A couple traveling from one oasis to another developed engine trouble, and the man left the woman sitting in the car shade while he went for help. Securing assistance, he returned by car only five hours later. He found her sitting where he had left her, but she had perished from the effects of unrelenting heat and thirst!