Behavioural adaptations

To the physiological, anatomical, and morphological adaptations of plants, animals can add adaptive behaviour. Many birds and most large mammals, like pronghorn antelopes or wild sheep, can evade critical spells by migrating along the desert plains or up into the mountains. Smaller animals cannot migrate such long distances, but regulate their environment by seeking out cool or shady places. In addition to flying to other habitats during the dry season, birds can reduce heat loads by soaring. Many rodents, invertebrates, and snakes avoid heat by spending the day in caves and burrows, and procuring food during the night. Even diurnal animals may reduce their activities by resting in the shade during the hotter hours of the day. Fossoriality, a lifestyle based in burrows, is the norm for small animals in all deserts, as it allows them to stay away from the gruelling heat during the hotter part of the day and it also provides them a warm refuge during the cold desert nights. Additionally, humidity inside burrows (ca. 30-50 per cent ) allows desert animals to preserve water. When the normal mechanisms to keep body temperature within acceptable limits fail, many small rodents and some desert tortoises (Testudo) salivate to wet the chin and throat and allow evaporative cooling. Such mechanisms have a high cost in water and are used only as emergency measures to prevent death.

At dawn, the dry desert ground may approach freezing temperatures and at midday it may heat up into an 80°C inferno. A few inches above the ground, variations in air temperature are much less pronounced, and, just a few inches below the surface, underground temperatures are almost constant between day and night. For this reason, thermoregulation is a particularly challenging problem for small surface-dwellers and especially for reptiles, which cannot regulate their body temperature metabolically. Most desert reptiles have developed peculiar ways of travelling over hot sandy surfaces. Side-winding, a form of lateral movement in which only a small part of the body is in contact with the surface, is employed by many sand snakes (Figure 1.13). Many lizards and some ground birds avoid overheating by running rapidly over the hot desert surface while maintaining their bodies well separated from the ground (Safriel 1990). Some lizards assume an erect, bipedal position when running, while others regulate their contact with the hot desert pavement by doing "push-ups" with their forelegs.

Many large mammals that cannot avoid being in the sun during a large part of the day orient their bodies so as to reduce the incidence of the sun's rays. By standing upright, ground squirrels reduce solar incidence upon their bodies. The African ground squirrel Xerus inauris even orients towards the sun and shades itself with its tail when foraging (Figure 1.14). The jackrabbit Lepus californicus warms its body in the early morning by exposing its large, highly vascularised ears perpendicular to the sun's rays, using them as a form of solar collector. Similarly, it cools at midday by keeping in the shade and putting the ears parallel to the incoming solar radiation, thus minimizing exposure while keeping the same radiative surface.

Morphological and anatomical adaptations

In mammals, desert fur coats are short, hard and compact, but at the same time well-ventilated, to allow sweat to evaporate directly from the skin. Birds, in addition, can fluff or compact their feathers to regulate heat exchange. In the ostrich, a desert dweller, the uncovered head, throat, legs and abdomen allow for radiation and convection cooling, while the feathers on the back protect the larger part of the body from direct solar radiation (Figure 1.15). Bipedalism, a common trait in small desert mammals such as kangaroo rats, allows for fast travel in open spaces and also keeps the body separated from the extreme temperatures of the ground surface. Indeed, bipedal desert rodents use open microhabitats much more frequently than their quadrupedal relatives, who restrict their activities to sheltered habitats.

Sand-dwellers have evolved several traits that allow them to survive in dunes, including fleshy footpads in camels, scaly fingers in certain lizards and digital membranes in some geckoes. Additionally, camels have long dense lashes that protect their eyes and they can close their nostrils to protect them from wind-blown dust. Many snakes have upwardly-turned nostrils that allow them to burrow rapidly in loose sand; others are flat and can bury laterally. Many other reptiles also show adaptations that protect their eyes, nose, and ears from sand and dust, and many insects have specially-adapted legs that allow them to bury themselves rapidly and to walk efficiently on hot sand.

Physiological adaptations

The most basic physiological problem of desert animals is to maintain their water balance by maximizing water intake and/or minimizing water loss. In deserts, free-standing water is scarce, found only in isolated oases and reservoirs. Camels and wild asses, for example, can drink large quantities of water in a very short time causing a dramatic dilution of the bloodstream, sufficient to cause death in other animals. In coastal deserts, animals obtain water by licking fog-drenched rocks. Desert amphibians can absorb water through the skin from humid underground dens by accumulating urea in their blood and raising its osmotic pressure. Most herbivores, like eland and oryx, obtain water from the foliage of the shrubs that compose their diet, often feeding at night when the plants are turgent. Some succulent plants have high salt contents, toxic compounds, or spines that deter their consumption. Herbivores, however, have found their way around these obstacles: some reptiles and birds have developed efficient salt-excreting glands, and many mammals have kidneys that can cope with salty water. Whitethroated packrats (Neotoma albigula), which feed almost exclusively on juicy cacti, have metabolic adaptations to prevent poisoning from the oxalates contained in these plants. Animals lose water through urine, faeces, respiration, and transpiration. Desert rodents have kidneys that are capable of producing highly concentrated urine, with an electrolyte concentration many times higher than that of blood plasma. Reptiles, birds and insects excrete uric acid, which requires less water, and sometimes complement the excretion process with specialized excretion from salt glands. Amphibians produce little urine, and can store large amounts of urea within their bodies, drastically reducing water loss. In droughts, some rodents can produce dry faeces, efficiently reabsorbing liquids in the rectum. Metabolism produces CO and water as byproducts of respiration. In most animals, this metabolic water is exhaled through the lungs, but many desert animals, including invertebrates, reptiles, and mammals, possess physiological and anatomical adaptations to reduce respiratory water loss, including modifications in the morphology of the nasal passages and the capacity to reabsorb water along the respiratory tract. One of the most extreme examples of this is given by the kangaroo rat (Dipodomys), which can survive on a diet of perfectly dry seeds.

In addition to the mechanisms that reduce water loss, many desert animals are extremely tolerant to dehydration, a condition that causes a fatal increase in blood viscosity in non-desert dwellers. In order to achieve this, camels, for example, are able to loose water selectively from tissues other than blood. In contrast, desert amphibians are tolerant to increased fluid viscosity, and some reptiles can excrete excess electrolytes through urine and salt glands, avoiding the thickening of the blood as they dehydrate. A problem related to dehydration is that of temperature regulation. In smaller animals the high surface area-tobody volume ratio makes sweating a dangerous enterprise and panting is the most common method of cooling. Even larger animals that usually sweat, like the oryx, begin to pant when their body temperature exceeds 41ºC.

Nocturnal hypothermia, exhibited by some large mammals like the eland, allows them to reduce their metabolic rate and to exhale air with less humidity during the night. Diurnal hyperthermia allows animals to reach body temperatures that would be normally lethal for non-desert vertebrates and to save on water needed to prevent overheating. Camels and elands, for example, can reach body temperatures of 44ºC with no harmful consequences, and save as much as 5-10 litres of water during extremely hot days. Hyperthermal species have a special disposition of veins and arteries that allows their brains to remain at a temperature lower than that of the overheated body. Like ephemeral plants, many smaller desert animals can also evade drought by entering into a dormant phase: desert butterflies and grasshoppers thrive in huge numbers when conditions are good and survive dry spells in the form of eggs or pupae. Spade-foot toads (Scaphiopus) spend most of their lifetime buried in dry mud and become active only after rains refill their ephemeral pools. Many other organisms go into some form of torpor during dry periods.