The Desert Adaptations of Birds & Mammals
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.
The primary method for cooling down a hot bird or mammal is through evaporative cooling. As water evaporates from a surface, it cools that surface. When a coyote pants, it rhythmically moves air over the moist surfaces of the mouth, throat, and tongue. Water is evaporated and these surfaces are cooled. Abundant dilated blood vessels are near these surfaces and are cooled by them. The resulting cooled blood is then circulated throughout the body. A hot owl will flap the loose skin under its throat to move air over its mouth cavity. This is called gular fluttering and achieves the same result as panting. Panting and gular fluttering are energy efficient movements that produce very little heat themselves.
Brains are very sensitive to heat. In sheep, and in members or the dog and cat families, evaporative cooling of the nasal passages results in the cooling of a special network of blood vessels to the brain. The brain of an exercising dog, for instance, is cooler than the rest of its body.
Vultures use evaporative cooling in an interesting way. A vulture urinates on its legs if the daytime temperatures are over 70°F (21°C). The urine will evaporate, cooling the legs and drawing more heat from the body of the hot vulture. This is why, when the daytime temperatures are consistently about 70°F, a vulture's legs are white, but when the temperatures are consistently lower than 70°F, a black vulture's legs are gray and a turkey vulture's legs are red.
Water Income and Water Expense
Birds and mammals have a great need for water. Water serves as the basic transport medium for nutrients, and it is the medium for dilution and removal of body wastes. Water functions in most chemical reactions of the living process, and as we have seen, water is the body's primary coolant.
The water-budget balancing act of desert animals has been compared to balancing a bank account: there is water income and water expense. Not surprisingly, it is always a tight budget for desert animals. Stored water is generally limited to what can be placed in the gut or crop. Debts are not tolerated. A 10 to 15 percent loss in body weight due to water loss can impair an animal's ability to recover; a 20 percent loss often means death. Water loss can happen quickly on a hot day in the desert, one to two liters per hour in humans.
What are the sources of water income and water expense?
Water income can come from three sources:
- Free water (for example, a bighorn drinks at a water hole)
- Water in food for example, a Phainopepla eats a juicy mistletoe berry)
- Oxidation water (the water produced by all animals when they metabolize food)
Water expenses can come from:
- Evaporative cooling
- Dilution and excretion of toxic body wastes
- Eggs or milk
Some rodents, such as pocket mice and kangaroo rats, are independent of any free water -or even of moist food. The kangaroo rat is probably the best known of these. It eats primarily dry, high carbohydrate seeds; one gram of grass seed produces one-half gram of oxidation water. Seeds with much fat or high protein content are avoided: the former produce too much heat that may have to be lost through evaporative cooling; the latter require too much water for diluting waste products. A kangaroo rat can live on water produced when food is metabolized, but that is only part of its arsenal of strategies for desert survival. Additional water is available from dry seeds which, when stored in its burrow, absorb as much as 30 percent of their weight in water from the higher humidity in the burrow. The evaporative loss from a kangaroo rat is low, as the animal has no sweat glands and little water is passively lost through its skin. Respiratory water loss is reduced by a nasal cooling system that extracts water from air as it passes through the nasal chambers as it is exhaled - a cooling system now known to be shared with other rodents and most other mammals. A kangaroo rat can produce urine twice as concentrated as sea water and feces five times drier than a lab rat's droppings. It conserves moisture further by being nocturnal. Finally, a kangaroo rat typically breeds only when green vegetation or insects are available to supplement its water balance.
Other rodents that do not have regular access to free water consume juicy animals and succulent plants and their fruits. Pack rats and cactus mice are good examples this feeding strategy. During June, the driest month of the year, pack rats can survive on cholla and prickly pear; cactus mice can survive on cactus fruit and insects. There are many other animals besides rodents that get most of their water from food. Elf owls survive on katydids and scorpions. Pronghorns can survive on the water in cholla fruits. Kit foxes can satisfy their water needs with the water in their diet of kangaroo rats, mice, and rabbits, along with small amounts of vegetable material. Other desert dwellers, such as coyotes, mule deer and bighorn sheep, require periodic free water. In fact their home ranges revolve around water holes. Such animals, including we humans, are found only where free water exists, or where it can be transported.
Humans In a Hot, Arid Environment
Humans are physiologically very good at keeping cool, but rather poor at conserving water. Sweating is the primary method of cooling the body; the evaporation of this sweat from all over the body cools the naked skin. During a really hot day in the desert, however, a human will lose as much as 12 liters (a little over 3 gallons) of water through sweat.
Humans have a special mechanism for cooling their big brain: The blood cooled by evaporation of sweat on the face and head penetrate the skull through tiny emissary veins, thus delivering freshly-cooled blood to the brain. This cranial radiator is unique among primates.
Humans' upright, two-legged stance also confers some advantages for keeping cool. When the sun is directly overhead, only the head and shoulders are in full sunlight-a four-legged animal has its entire back, shoulders and head exposed to the sun. Humans therefore gain much less radiant heat than four-legged animals. Also, by standing upright, most of the body is raised above the hot desert floor; this means that humans' rate of heat gain from the desert surface is much less than that of quadrupeds. Being upright also exposes more of the body to cool air currents, and thus body heat can be lost by convection.
Nakedness is also an advantage. Without insulating fur, heat can be lost more easily through convection, and sweat can be more easily evaporated. And that patch of thick hair on top of our heads is more than mere decoration-it shades the head and its heat-sensitive brain from the sun.
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?