Two of the challenges of life in the desert faced my animals are thermoregulation and osmoregulation: maintaining an optimal body temperature and retaining water. Many of the prominent desert animals, such as reptiles and insects, are ectothermic; their body temperatures fluctuate with the temperature of the surrounding environment. The metabolic rate of these animals increases as their body temperature increases, but their low metabolic rates during periods of inactivity helps them to conserve energy. Birds and mammals, on the other hand, are endothermic, and they maintain relatively high metabolic rates that sustain their relatively constant body temperatures. Within this general context, desert animals use a variety of morphological, physiological, and behavioral attributes to maintain optimal body temperatures and minimize water loss, particularly through their respiratory and excretory systems. Many of these adaptations are not unique to desert animals, but they are certainly beneficial in this environment.DEALING WITH HEAT: Thermoregulation
Animals avoid overheating spatially through the use of various microclimates. For example, they can move to shade, underground, or into a plant to avoid the heat on the surface of the soil or rock. Some flying animals, such as birds, can reach cooler temperatures at higher altitudes or higher elevations. Animals also maintain optimal body temperature temporally by altering the timing of their activities. Some animals are nocturnal, some are diurnal, and some are crepuscular.Color is important for a variety of reasons, such as cryptic coloration that helps animals hide from predators and display coloration that signals members of the same species. Color also can be used to help animals, such as some species of tenebrionid beetles and desert iguanas of the Sonoran Desert, maintain optimal body temperatures. Dark colors help these ectotherms increase their body temperature and their capacity for activity during the cool morning; their color becomes lighter during the afternoon to help them avoid hyperthermia.
In addition to color, large extremities and small body size potentially help some animals avoid overheating by increasing the surface-area-to-volume ratio, which allows them to dissipate more heat. Vasodilatation of skin blood vessels helps an animal dissipate heat; conversely, vasoconstriction of skin blood vessels helps an animal conserve heat. Vasodilatation of skin blood vessels also can help a lizard, for example, absorb heat that is transferred to deeper parts of its body, raising it core body temperature. In addition to dissipating heat from blood flowing near the external surface of their body, mammals and reptiles use evaporative cooling from their respiratory systems (e.g., panting, mouth gaping) and birds use gular flutter (throat movement to draw air into moist mouth), which allows them to dissipate heat from blood flowing near these surfaces.
Some animals also regulate blood flow to their brains to prevent it from overheating. To keep the brain cool, reptiles can shunt blood returning from their head through the internal jugular vein to their external jugular vein to keep it from warming blood moving toward the brain through the artery adjacent to the internal jugular vein. Some mammals (e.g., canids, felids, and ruminants) have a carotid rete below the brain, where heat moves from warm arterial blood to venous blood cooled in the nasal passages (similarly, birds have an ophthalmic rete).
DEALING WITH ARIDITY: Osmoregulation
Birds and larger mammals are "obligatory drinkers", but they have the ability to travel great distances to find water. Most desert animals are "facultative drinkers" (they drink when water is available), and some animals do not drink at all. These animals obtain some of their water directly from the water in their food. The diet of desert animals that obtain water from their food often changes during the course of the year because the content of water in their food changes during the year. For example, they might eat hydrated plant material during one season and switch to animals during other parts of the year. Some animals, notably kangaroo rats, obtain all of their water from their food, either directly or as a by-product of their metabolism. During cellular respiration (specifically the electron transport system in mitochondria), O2 serves as an electron acceptor and combines with H+ to form water. In this regard, it is more efficient for the animal to obtain its water through the metabolism of carbohydrates rather than fats or proteins. To breakdown fats and proteins more O2 is required, which means more water is lost through respiration. Metabolism of proteins also means more water loss through urine to excrete the nitrogenous wastes created.To limit water loss through the excretion of nitrogenous wastes, terrestrial animals use energy to convert ammonia into less toxic molecules that can be more concentrated (i.e., require less water for dilution). Mammals convert ammonia to urea. Arthropods, reptiles, and birds use additional energy to produce uric acid or guanine, which can be excreted as moist solids. To help retain water, arthropods, amphibians, reptiles, and birds pass their urine through their hindgut or cloaca, which absorbs water from the urine in addition to absorbing water from material passing through their digestive tract.
The exchange of O2 and CO2 occurs on the surface of wet membranes, which means that much water can be lost during respiration. This loss is reduced somewhat when dry air drawn into the nasal passages causing evaporative cooling. After the air picks up moisture in the lungs, it passes back through the somewhat cooler nasal passages and moisture condenses before it is exhaled. Burrowing animals also can create a more humid microclimate in their cool, underground chamber, which also reduces water loss through respiration.
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