Water Economy in Desert Organisms

The white-throated wood rat, Neotoma albigula, may go its whole life without ever taking a drink of water. It does not need to. The rat obtains all the water it needs from its food. The fact that the rat's diet includes dry seeds and cactus makes this even more surprising. It obtains about half of the water it needs from succulent plants. The white-throated wood rat is also able to chemically synthesize water from the molecules in the food it eats. It shares this adaptation with many other Neotoma species, also known as pack rats. The wood rat has many other adaptations that allow it to thrive with little or no water.

The small kit fox, Vulpes velox, rarely drinks water. It will if it is thirsty and has the opportunity, but it is generally a long way from the fox's den to any reliable source of water. Therefore, the fox consumes much more prey than it needs for energy requirements. The excess prey is consumed simply for the water it contains. Along with this behavior, the fox has evolved a digestive system that allows it to survive while rarely drinking water.

Both of these are examples of water economy as practiced by desert organisms. Many desert organisms, both plants and animals, have evolved strategies that allow then to survive on little or no water.

Plant Adaptations

Desert plants have many adaptations that reduce water loss, including extensive root systems to capture as much water as possible and thickened stems to store water. Plants also have waxy or resinous coatings on their leaves and a thick epidermis, and they keep their stomata closed much of the time. However, keeping the stomata closed causes another problem. Since the carbon dioxide molecule is even bigger than the water molecule, anything that reduces water loss also prevents carbon dioxide from entering the leaf. In these plants, the carbon dioxide levels can drop so low that normal photosynthesis cannot function.

To compensate, many desert plants have evolved a much more efficient form of photosynthesis that uses a molecule with four carbon atoms. Photosynthesis in non-desert plants uses a molecule with three carbon atoms. The two reactions are commonly known as C3 photosynthesis and C4 photosynthesis respectively. The corresponding plants are called C3 plants and C4 plants. Desert plants are frequently C4 plants.

C4 photosynthesis is more efficient at low levels of carbon dioxide. It is more costly in energy terms, but since deserts have plenty of sunlight, low light levels are not a problem.

Animal Adaptations

Kangaroo rats (Dipodomys sp.), small rodents closely related to pocket gophers that are not rats at all, exhibit many of the adaptations common to desert mammals. They have highly efficient kidneys that excrete almost solid urine, thus conserving water. To do this, they have evolved adaptations that allow salt concentration in their urine to be as high as 24 percent, compare to 6 percent in humans.

Also, kangaroo rats have complex passages in their nostrils that condense, collect, and recycle moisture. The temperature at the tips of their noses is 28°C (82°F) while their body temperature is 38°C (100°F). So, as they exhale, the air is cooled and water is condensed and recycled. Humans lose twenty times more water through respiration than do kangaroo rats.

Kangaroo rats also plug their burrows during the day to maintain a relative humidity in the burrow of 50 percent or higher while the humidity above ground drops to as low as from 10 to 15 percent. Kangaroo rats can also obtain water metabolically from the seeds they eat. Captive kangaroo rats may become stressed enough to drink water, but a wild kangaroo rat may go its entire life without drinking water.

The kangaroo rat is not unique. Many other animals have adaptations that allow them to live in the desert by practicing water economy. Several small rodents share some or all the adaptations of the kangaroo rat. These include the white-throated wood rat (also known as pack rat, Neotoma sp.), the canyon mouse (Peromyscus crinitus ) and many others. Birds also exhibit some adaptations, although most simply escape the dry conditions. The Phainopepla, Phainopepla nitens, breeds in the desert in the springtime and escapes to the mountains in summer.

The roadrunner, Geococcyx californianus, cannot easily escape the desert, nor does it try. It has several adaptations that allow it to thrive. It reabsorbs water from feces, it excretes excess salt through a nasal gland (other than the roadrunner, an adaptation found only in marine birds), and it is able to utilize effectively the water in its prey. Behavioral modifications include reducing its activity by 50 percent during the heat of midday.

Desert bighorn sheep (Ovis canadensis nelsoni ) do not drink water in the winter. They obtain all of the water they need from the vegetation they eat. In the summer they must drink about every three days. If water is not available, then the bighorn changes its behavior to rest during daylight hours and feed at night. Since they must rely on plants to supply all their water, they select succulent plants and the tender young pads of prickly pear (after scraping off the thorns with their hooves).

Here are three of the other physiological and behavioral adaptations for minimizing water loss:

• Many animals are active only at night. Others are crepuscular (active only at dusk and dawn);
• A few desert animals enter a period of dormancy during the heat of summer. These include the round-tailed ground squirrel and several species of desert toads; and
• Birds and reptiles excrete uric acid in the form of a semi-solid paste.

Of all the adaptations shared by desert-dwelling animals, the most amazing must be the kangaroo rat's ability to chemically synthesize water. Wild kangaroo rats get by on nothing but dry seeds. They do not drink water even if it is available. So the kangaroo rat is not practicing efficient water economy: it simply does not need to drink water. However, the kangaroo rat must have water in its blood and tissues just like other animals and will die from dehydration if the water in its tissues is deficient. The kangaroo rat gets the water it needs exclusively from digestion of carbohydrates. All animals produce some water, the water of metabolism, this way. But the kangaroo rat metabolizes all the water it ever needs.

see also Diurnal; Nocturnal.

Elliot Richmond

Bibliography

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Findley, James S., et al. Mammals of New Mexico. Albuquerque: University of New Mexico Press, 1975.

Findley, James S. The Natural History of New Mexican Mammals. Albuquerque: University of New Mexico Press, 1987.

Krutch, Joseph Wood. The Voice of the Desert: A Naturalist's Interpretation. New York: William Sloan Associates, 1962.

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