Axerocole (from Greek xēros/ˈzɪroʊs/ 'dry' and Latin col(ere) 'to inhabit'),[2][3][4] is a general term referring to any animal that is adapted to live in adesert. 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).[1] 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 eithernocturnal orcrepuscular (most active at dawn and dusk).
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Xerocoles have developed a variety of mechanisms to reduce water loss via evaporation. Mammalian xerocolessweat much less than their non-desert counterparts. For example, thecamel can survive ambient temperatures as high as 49 °C (120 °F) without sweating,[6] and thekangaroo rat lacks sweat glands entirely.[7] Both birds and mammals in the desert haveoils on the surface of their skin to "waterproof" it and inhibit evaporation.[8]
Desert insects use a similar method, as theircuticles arewaxy 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.[5]
Amphibious xerocoles, such as species of the frog genusPhyllomedusa, have wax-like coatings on their skin to reduce water loss. The frogs secretelipids 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.[9] Other desert amphibians, such as the frog genusCyclorana, 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.[9][10]
Though desert birds lacksweat glands, they can still take advantage of evaporative cooling by panting, which cools thetrachea andlungs, and gular flapping, which consists of rapidly fluttering thegular skin to move air over the inner mouth and throat.[11] Kangaroo rats and other small mammals use evaporative cooling in a similar way. When air isrespired, water evaporates from the nose, cooling the surface of thenasal passages to approximately 24 °C (75 °F). The low temperature causes moisture to condense, partially making up for the water that was lost.[9][12] The process, called respiratory heat exchange, works best when the walls of the nasal passage have a large surface area.[13]
Some animals pour bodily fluids on themselves to take advantage of evaporative cooling. Xerocole birds such asstorks,New World vultures, andibis urinate on their legs,[11][14] while desert tortoises sometimes salivate on their neck and front legs to keep cool.[5] 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.[13]
To excretenitrogenous waste products, mammals (and most amphibians) excreteurea diluted in water.[15][16] 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 insetnephrons,[17] as well as smaller and fewercortical andjuxtamedullary glomeruli (glomeruli being capillary networks where both fluid and waste are extracted from blood). This in turn leads to a smallerglomerular filtration rate, and on the whole, less water is transferred from the blood to the kidney.[5][17][18] The kidneys of desert mammals are also better adapted at reabsorbing water from thetubular fluid: though there are fewer glomeruli, the xerocole has larger juxtamedullary glomeruli than cortical glomeruli (the former playing an important role in concentrating urine),[19] whereas the opposite is true for non-xerocoles. Desert mammals also have longerloops of Henle, structures whose efficiency in concentrating urine is directly proportional to their length.[5][17][20] The efficiency of their loops of Henle is augmented by the increasedantidiuretic hormone in their blood.[5]
Desert amphibians can store more nitrogen than aquatic ones, and do so when not enough water is available to excrete the nitrogen as urea.[10] TheAfrican reed frog can store excess nitrogen iniridophore, pigmentedgranules in its skin, by converting the nitrogen toguanine, which makes up the majority of the iridophores' composition.[9]
Reptiles, birds, insects, and some amphibious species excrete nitrogenous waste asuric acid rather than urea. Because uric acid is less toxic than urea, it does not need to be dissolved in water to be excreted (as such, it is largely insoluble).[10][15][16][20]
Most animal feces are over 75% water; xerocoles, however, reabsorb water in the gut and produce much drier feces.[21] For example, the kangaroo rat's feces contain only1⁄6 as much water as that of other, non-desert rodents.[22] In insects, the rectal gland also absorbs water, and the insects excrete dry pellets.[21] In birds, along with some other vertebrates, theureter andrectum both lead to thecloaca, whose walls also absorb water.[5][8]
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Camels can further conserve water by closing an orifice in their stomach to create two compartments: one for water and one for food.[23]
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. Desertcanids and kangaroos eat their own young's excrement for the same reason.[13]
The Australianwater-holding frog conserves water by retaining urine in thebladder, swelling up like a balloon; it then uses its bladder as a water reserve during the dry season.[8][10]
Xerocoles get a substantial amount ofhygroscopic water from their food. Many feed on moisture-filled plants: theaardvark obtains water from wild cucumbers (Cucumis humifructus)[25] and the camel eatssucculents and bushes in the winter, getting enough water to go two months without drinking.[12] Theoryx eatsAcacia leaves late at night, when water content is highest: in the hot, arid daytime the leaves are only 1% water; but in cooler, more humid night the leaves are 40% water. Some xerocoles are able to obtain water fromhalophytic (saltwater) plants, as they can metabolize high amounts ofoxalic acid and produce very concentrated urine. Thechisel-toothed kangaroo rat also mitigates the saltiness of the halophyte it eats (theshadscale) by using its broad, sharp lower incisors to scrape off the leaves' salty outerlayer to reach the less-salty center.[13]
Carnivores derive water from their prey's meat and blood.[7][13] Insectivores, such as theaardwolf (a type of hyena) and thesouthern grasshopper mouse, are thus largely independent from free water.[13][26]
Xerocoles obtain a large percentage of their water from themetabolic processes used to break down their food. The water gained from fat is nearly twice the amount gained from carbohydrates, as the former contains more hydrogen (which determines the amount of water produced). The water gained from metabolism is more than enough to offset the water lost from evaporation in the lungs (which increases due to the need for oxygen to break down food).[5][7][12]
Xerocoles such as thehare have large ears that help them keep cool: when the ears stand up, blood flow increases to the numerous vessels there and heat is dissipated.[7][27] However, at 48 °C (118 °F), thecape hare nearAbu Dhabi, UAE sits in the shade and drapes its ears over itself, as erecting them in such weather would absorb more heat.[28]
Desert animals have less fat than their non-desert counterparts, as fat would act as insulation, so retaining heat. What fat they do have is localized, such as in the camel's hump or thebison's neck.[5] In terms of fur, however, desert animals have thick insulating coats that impede the conduction of heat towards the body.[7] The coats are not uniformly distributed, but rather leave sparsely covered patches called "thermal windows" at theaxilla,groin,scrotum, andmammary glands. Heat can be dissipated from thermal windows viaconvection andconduction.[13]
Similarly, desert birds have fewer feathers on the underwing andflank – heat stress induces some birds to raise their wings, increasing the surface area of exposed skin. Birds adjust their feathers to create or dissipate an insulating layer, as typified by theostrich. At high temperatures, the ostrich elevates its longdorsal feathers to create a barrier against solar radiation while allowing air to move across the skin's surface. In the cool nights, the feathers lower and interlock, trapping an insulating layer above the skin.[11]
Most small xerocoles live inburrows to avoid the desert heat.[29] The burrows act asmicroenvironments: when they are deeper than 50–60 cm (20–24 in) below the surface, they maintain humidity and temperatures between 30 and 32 °C (86 and 90 °F), regardless of external weather.[13][30] Some animals seal their burrows to keep them moist.[7][31]
Ectotherms also use burrows as a means to keep warm in the cold desert nights.[5] As ectotherms are usually small and unable to store their own body heat, they quickly take on the external temperature of the environment, which necessitates controlled microenvironments. For example, while reptiles are able to operate at temperatures exceeding optima, they become sluggish when cold. As such, they spend their nights in burrows or crevices, where they create warm environments by quickly generating metabolic heat.[5][32] Desert lizards usually use other animals' burrows to meet their purposes.[9]
All desert rodents exceptground squirrels andchipmunks are nocturnal.[13] Amphibians are usually nocturnal as well, while many other xerocoles arediurnal, but reduce activity at midday and increase in the mornings and evenings.[9] Some xerocoles change their activity patterns depending on the season: nocturnal ants, for example, become diurnal during colder periods.[33]
Many xerocoles, especially rodents,estivate in the summer, becoming more dormant.[5] Some desert amphibians estivate underground for over a year at a time.[10] Unlikehibernation, which leads to a state oftorpor, estivation induceslethargy, and can go unnoticed in some animals if their body temperature is not measured.[13]
Xerocoles are usually light and sandy in color as a means to reflect solar radiation and reduce heat absorption.[32] Some change color with the seasons to reflect more sunlight in the summer: addaxes change from gray-brown to nearly white.[24][34]Iguanid lizards can change color on a much smaller time scale by varyingmelanin concentration. They become darker when burrowing and lighter when basking – both thedesert iguana and thezebra-tailed lizard become so pale that they appear to shine due to the amount of light they reflect.[32]
Most desert lizards also have a blackperitoneal lining in theirabdominal cavity to absorbUV radiation and prevent it from damaging internal organs.[9]
Shade under shrubbery provides resting spots for diurnal lizards, nesting sites for birds, as well as temporary oases for diurnal rodents, who skirt among shady spots.[13] Large animals such as camels and carnivores also spend the hottest parts of the day under shade.[29][32]
Desert animals such as the camel, addax, and kangaroo rat have large feet to prevent them from sinking in the sand.[6][29] Thefennec fox has extra fur on the soles of its feet to give it traction and protect it from the hot sand.[35] Most animals in arid environments are slender with long legs, giving them the speed as they travel long distances for food and water.[36]
The three main vulnerabilities against the sand are through the eyes, ears, and nose.[37] To keep sand out of their eyes, xerocoles including reptiles and birds, and some amphibians and mammals[38] have anictitating membrane in their eyes: a third, transparent eyelid that protects the cornea from blowing sand and can dislodge it from the eye.[35][38] Reptiles also have eyes the size of pinholes or protected by valves.[37] To keep sand out of their ears, mammals such as the camel and thesand cat have long hairs protruding from them.[39][40] The camel and thesaiga antelope also have adaptations to protect their noses from sand: the former has narrow nostrils it can close, and the latter has a large nose with its nostrils set wide apart and far back to prevent sand from entering when grazing.[29][36][41] Reptile diggers have nostrils that face upwards instead of forwards for the same reason.[37]
Xerocoles, having to travel long distances for food and water, are often adapted for speed, and have long limbs, feet that prevent them from sinking in the sand, and are overall slender in form.[36] As there is little cover to protect them from predators, desert animals also use speed as a defense mechanism. For example, a desert jackrabbit can run much faster than a coyote; as such, "an ordinary wolf or coyote will not attempt to chase him, for they realize the hopelessness of it."[37]
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The following animals are known xerocoles:
