Unlike other desert mammals, humans have come up with many cultural and technological adaptations to the desert heat and aridity. Picture yourself on a typical summer day in the Sonoran Desert. What techniques and devices are you using to keep cool and hydrated?
Basic depiction of desert animals
Have you ever wondered how animals can live in a hostile desert environment? Water, so necessary for life processes, is often scarce. Temperatures, which range from freezing to well over 100�F (38�C), make maintaining a safe body temperature a constant challenge. Add to this the catch-22 of desert survival: an organism’s need for water increases as temperature rises-available water usually decreases the hotter it gets. This might sound like an impossible situation, yet, as we’ll see, desert birds and mammals have developed many adaptive strategies for coping with temperature extremes and limited water.
The primary strategy for dealing with high desert temperatures is avoidance-many mammals simply avoid the high daytime temperatures by being nocturnal or crepuscular (dusk- or dawn-active). A bobcat, for instance, is typically most active at dusk and dawn; a javelina is never active during the day in summer, but it may be in winter. Even day-active birds are most active at the cooler dawn. Many mammals, such as ringtails or kangaroo rats, are never active during the day.
Microclimates and Burrows
Another avoidance strategy is to seek out a cool microclimate. A cactus wren may simply rest quietly in the shade of a jojoba; a prairie falcon will nest on a ledge of a cool north-facing cliff and avoid the hot south face. A cool, deep crevice in the cliff face may be the daytime refuge of a pallid bat, while a ringtail is sleeping away the day in a jumble of rocks at the base of the cliff.
Some mammals create their own microclimates. A white-throated wood rat (or pack rat) builds a den made of desert litter-cholla joints, prickly pear pads, sticks, and stones-within a clump of prickly pear cactus. It looks a little like a trash heap and may be three feet high and eight feet across. At the bottom of this pile is a series of tunnels leading to a nest of soft plant fibers. The pack rat spends its day in the soft nest, somewhat insulated from an exterior air temperature that may be 110� F (43�C), with a ground surface temperature of 160�F (71�C).
Any small mammals dig burrows in the desert soil. The burrow environment is much more moderate than is the surface temperature, which may have an annual fluctuation of between 15�F (9.5�C) and 160�F (71�C). Many desert rodents spend the entire day within the mild environment of a burrow. (A Merriam’s kangaroo rat, for instance, will venture to the desert surface for less than one hour each night!) White-tailed antelope squirrels are diurnal rodents that forage for brief periods on the hot daytime desert surface. As they look for seeds, fruits, and insects, their chipmunk-sized bodies heat up, even though their bushy tails hang like parasols over their backs. Above ground, the squirrels may often be observed pressing their bellies, with legs spread, against the cool soil-or even tile of suburban patios-in shady spots, allowing, it is presumed, their body heat to be conducted to the cool earth or tile. It has been speculated that the squirrels use the cooler earth in their burrows in a similar fashion when they retreat to them in on hot days.
Speculations made decades ago regarding the behavior of desert rodents in their burrows, and the temperature fluctuations of the rodents and their burrows during the heat of desert summers, have taken on a life of their own as “facts.”So have generalizations about temperatures in burrows and pack rat nests that were based on very limited measurements at elevations and conditions far different from those of our desert extremes. The truth is, we have much to learn about these animals’ temperature tolerances and their strategies to avoid overheating. Ongoing and future research assisted by modern technology will, it is hoped, provide us with more complete answers.
Large mammals do not burrow to escape the desert heat. The kit fox, however, is the exception. Unlike any other North American canid, the kit fox uses burrows year round. Burrows help it thrive in hot, dry desert valleys-an environment that is too challenging for other canids. Other large mammals, such as bighorn sheep and mule deer, seek shady spots during the day and remain inactive. Large body size actually has its advantages in the hot desert environment: a large body heats up more slowly than a small body. This phenomenon is called thermal inertia. It may buy enough time to get through a blistering summer day.
Heat Conduction and Radiation
Birds or mammals can conduct heat from their bodies to the environment by decreasing the insulating value of feathers or fur. On a hot day, a curve-billed thrasher sleeks its feathers which creates a thinner insulating layer. Coyotes lose their thick winter coats in late spring; their early summer coats are relatively thin. A bighorn sheep also sheds its winter coat in the spring-but it sheds it in stages. During the heat of June, the belly and shaded parts of the legs are shed first, providing an area from which to lose body heat; the back, however, remains covered with thick woolly fur that insulates and shades the bighorn sheep from the hot overhead sun.
Birds have some advantages over mammals in dealing with heat. The normal body temperature of birds is generally higher than that of mammals. This higher body temperature means that a Gambel’s quail, for instance, with a body temperature of 107�F (42�C), can continue to conduct heat to the air until the ambient temperature reaches 107�F. (A coyote, by comparison, has a body temperature of 102�F.) Also, by dilating the blood vessels going to its bare scaly legs, a bird can dump excess body heat to the environment. A bird’s leg temperature may increase 15�F (9.5�C) after its blood vessels dilate. Thus, a hot bird sleeks its feathers and stands tall to expose its legs to the air. Mammals too have “radiators.” The long ears of a jackrabbit can transfer excess heat to the air through dilation of the blood vessels to the ear. This works best when the air temperature is below the jackrabbit’s normal body temperature (104�F/40�C), or after the jackrabbit has been active.
Shivering in the Desert
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Many deserts were formed 8,000 to 10,000 years ago. Some are superhot in the day. In fact the highest temperature ever recorded on Earth was 134°F (56.6°C) in California and Nevada’s Death Valley in 1913. Even though many deserts can reach temperatures of well over 100°F (37.8°C) during the day in summer, they can get cold at night. Why? In most places, clouds and water vapor hold in heat, sort of like a blanket. But deserts don’t have enough clouds and water vapor to do this.
Some deserts are always cold—in fact the biggest desert in the world is Antarctica! Even though it’s covered in snow and ice, it rarely rains or snows in Antarctica, which makes it a desert.
Famous Deserts
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Deserts can be huge spaces. For instance, the entire continental United States could almost fit inside Africa’s Sahara desert. The giant Gobi desert in Asia stretches across parts of China and Mongolia. North America has large deserts, too, including the Mojave in California and parts of Nevada, Arizona, and Utah. The Sonoran is a large desert located in Mexico and parts of the Southwestern United States. More than one-fifth of the continent of Australia is covered in desert. The Great Victoria Desert is the largest on the continent.
Look Closely
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It may seem like nothing can live in a desert because it’s so dry. But most deserts are full of life, with plants and animals that have adapted to survive without much water. Some plants, like cacti, store enough water in their stems to last until the next rain. Other plants, like mesquite grass, have very small leaves that curl up in the daytime to conserve the water they have. Some desert plants sprout and bloom only when it rains.
Desert animals also have adaptations that help them survive without much water. Kangaroo rats in the Sonoran Desert get water from the seeds they eat. Some carnivores, such as desert foxes, get enough liquid from their prey.
Another trick? Most desert animals stay underground or beneath shady rocks during the day. Many of them come out to hunt for food at night, when it’s cool.
If you plan to explore a desert, be sure to pack water, sunscreen, and protective clothing. After all, you’re not a kangaroo rat!
