Xerocole

A xerocole (from Greek xēros 'dry' and Latin col(ere) 'to inhabit'), is a general term referring to any animal that is adapted to live in a desert. The main challenges xerocoles must overcome are lack of water and excessive heat. To conserve water they avoid evaporation and concentrate excretions (i.e. urine and feces). Some are so adept at conserving water or obtaining it from food that they do not need to drink at all. To escape the desert heat, xerocoles tend to be either nocturnal or crepuscular (most active at dawn and dusk).

Water conservation

Avoiding evaporation Xerocoles have developed a variety of mechanisms to reduce water loss via evaporation. Mammalian xerocoles sweat much less than their non-desert counterparts. For example, the camel can survive ambient temperatures as high as 49 °C (120 °F) without sweating, and the kangaroo rat lacks sweat glands entirely. Both birds and mammals in the desert have oils on the surface of their skin to "waterproof" it and inhibit evaporation. Desert insects use a similar method, as their cuticles are waxy to prevent water from escaping; however, at critical temperatures (ex. 30 °C (86 °F) for cockroaches), the wax molecules in the cuticle rearrange to become permeable and permit evaporative cooling. Amphibious xerocoles, such as species of the frog genus Phyllomedusa, have wax-like coatings on their skin to reduce water loss. The frogs secrete lipids from glands in their skin: when their skin begins to dry out, they move their limbs over the glands on their backs, and wipe the lipids over their bodies. Other desert amphibians, such as the frog genus Cyclorana, avoid desiccation by burrowing underground during dry periods and forming a cocoon from shed skin: rather than being sloughed off, the skin remains attached to create the cocoon. As skin layers amass, water impermeability increases.

During evaporation

Though desert birds lack sweat glands, they can still take advantage of evaporative cooling by panting, which cools the trachea and lungs, and gular flapping, which consists of rapidly fluttering the gular skin to move air over the inner mouth and throat. Kangaroo rats and other small mammals use evaporative cooling in a similar way. When air is respired, water evaporates from the nose, cooling the surface of the nasal passages to approximately 24 °C (75 °F). The low temperature causes moisture to condense, partially making up for the water that was lost. The process, called respiratory heat exchange, works best when the walls of the nasal passage have a large surface area. Some animals pour bodily fluids on themselves to take advantage of evaporative cooling. Xerocole birds such as storks, New World vultures, and ibis urinate on their legs, while desert tortoises sometimes salivate on their neck and front legs to keep cool. Similarly, many rodents and marsupials lick themselves to spread saliva, though this only remains effective for a short time, and requires the fur to become very damp.

Excretion

Urine

To excrete nitrogenous waste products, mammals (and most amphibians) excrete urea diluted in water. Such xerocoles have adapted to make their urine as concentrated as possible (i.e. use the least amount of water) to dissolve urea. Desert mammals have longer and more deeply inset nephrons, as well as smaller and fewer cortical and juxtamedullary glomeruli (glomeruli being capillary networks where both fluid and waste are extracted from blood). This in turn leads to a smaller glomerular filtration rate, and on the whole, less water is transferred from the blood to the kidney. The kidneys of desert mammals are also better adapted at reabsorbing water from the tubular fluid: though there are fewer glomeruli, the xerocole has larger juxtamedullary glomeruli than cortical glomeruli (the former playing an important role in concentrating urine), whereas the opposite is true for non-xerocoles. Desert mammals also have longer loops of Henle, structures whose efficiency in concentrating urine is directly proportional to their length. The efficiency of their loops of Henle is augmented by the increased antidiuretic hormone in their blood. Desert amphibians can store more nitrogen than aquatic ones, and do so when not enough water is available to excrete the nitrogen as urea. The African reed frog can store excess nitrogen in iridophore, pigmented granules in its skin, by converting the nitrogen to guanine, which makes up the majority of the iridophores' composition. Reptiles, birds, insects, and some amphibious species excrete nitrogenous waste as uric acid rather than urea. Uric acid is less toxic than urea and largely water insoluble.

Feces Most animal feces are over 75% water; xerocoles, however, reabsorb water in the gut and produce much drier feces. For example, the kangaroo rat's feces contain only 1⁄6 as much water as that of other, non-desert rodents. In insects, the rectal gland also absorbs water, and the insects excrete dry pellets. In birds, along with some other vertebrates, the ureter and rectum both lead to the cloaca, whose walls also absorb water.

Other methods

Camels can further conserve water by closing an orifice in their stomach to create two compartments: one for water and one for food. Seed-eating rodents maintain a low metabolic rate to reduce water lost to respiration (and to prevent their burrow from overheating). Rodent mothers produce concentrated milk for their young, and then eat their young's dilute urine and feces to regain some of the water that was lost. Desert canids and kangaroos eat their own young's excrement for the same reason. The Australian water-holding frog conserves water by retaining urine in the bladder, swelling up like a balloon; it then uses its bladder as a water reserve during the dry season.

Alternative water sources

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