Creatures That Never Take a Drink
Any creatures living in a desert where eight months without significant rain is routine must have intricate adaptations to these conditions. For instance, the desert pocket mouse can go its whole life without a drink of water. The tiny, big-eyed rodents dig burrows up to five feet deep (1.52 m), usually with at least two entrances the size of a broom handle. Deep beneath the ground they are protected from the heat and cold and enjoy nearly constant conditions with three times the humidity of the desert floor above. That is essential, since they have such a large surface area relative to their weight that they would otherwise have a hard time keeping their body temperature constant. Moreover, they can slip in and out of a hibernation-like state, which dramatically slows their heart rate and metabolic process. They have such efficient kidneys and digestive systems that they can extract all the moisture they need from the seeds, plants, and insects on which they live.
On the whole, reptiles are better adapted to the desert than are mammals, which accounts for the much richer variety of lizards and snakes in most deserts. Most creatures store energy in the form of fats and carbohydrates or directly burn proteins. When proteins, carbohydrates, and fats are converted to energy, the chemical reaction produces waste products of carbon dioxide and water and nitrogen. Mammals get rid of the waste products in urea, which requires urine that consumes a lot of water. Lizards and snakes convert their nitrogen wastes to uric acid, which requires almost no water and gets rid of twice as much nitrogen per unit as the more soluble urea. As a result, reptiles all start off with a waste-disposal system that makes it easier for them to live in the desert.
Moreover, reptiles do not use energy to keep their body temperatures constant. Again, that gives them certain advantages in the desert, where they don't often have to cope with freezing conditions. The temperature of their environment determines reptiles' body temperatures, which is why most reptiles cannot live in areas with long winters. In the desert, they do have to avoid overheating, which they do by hiding in the shade or in burrows during the heat of the day in the warm season. Also, because they cannot cool themselves through sweating, they do not lose much water.
However, evolution can fashion solutions to heat and drought for even creatures that seem completely unsuited to desert life. For instance, the Rhabdotus land snail also manages to survive in the Chihuahuan Desert. The snails, with their soft, moist, easily dehydrated flesh, spend almost all their lives buried in the dry, desert soil sealed up in their coiled shells. Buried an inch below the surface, they seal the opening of their shells with a papery flap, relying on the trapped, insulating air to protect them from the heat and the cold. The shell itself reflects about 90 percent of the sunlight that strikes it. Sealed up in its shell, the snail can last for years, waiting for a good rain. When awakened by the drumming of such a rain, they emerge to feed and mate. When they are moving around, they use 45 times as much water as when they are holed up.
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