Xerocole

Source: Wikipedia, the free encyclopedia.
A fennec fox standing around in tall grass.
The fennec fox's large ears help keep it cool: when the blood vessels dilate, blood from the body cycles in and dissipates over the expanded surface area.[1]

A xerocole (from

crepuscular
(most active at dawn and dusk).

Water conservation

A kangaroo rat can live its entire life without ever having to drink.[5]

Avoiding evaporation

Xerocoles have developed a variety of mechanisms to reduce water loss via evaporation. Mammalian xerocoles

oils on the surface of their skin to "waterproof" it and inhibit evaporation.[8]

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.[5]

Amphibious xerocoles, such as species of 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.[9][10]

During evaporation

Further information:
Heat of vaporization

Though desert birds lack

nasal 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 as storks, New World vultures, and ibis 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]

Excretion

Urine

Further information: Renal physiology
Camels carrying baggage in the desert
Arabian camels can survive several days and travel up to 160 kilometres (100 mi) without water.[5][6] One way they save water is by excreting very concentrated urine.[7]

To excrete

antidiuretic 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] 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.[9]

Reptiles, birds, insects, and some amphibious species excrete nitrogenous waste as uric 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]

Feces

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 only 1⁄6 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, the ureter and rectum both lead to the cloaca, whose walls also absorb water.[5][8]

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.[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. Desert

canids and kangaroos eat their own young's excrement for the same reason.[13]

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.[8][10]

Alternative water sources

An addax's face – he's looking into the camera and raising his upper lip, making it look as though he's laughing.
Some antelope, such as the addax (pictured) and the oryx, are so efficient at getting water from plants that they never need to drink.[13][24]
See also:
Fat catabolism

Xerocoles get a substantial amount of

shadscale) 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 the aardwolf (a type of hyena) and the southern grasshopper mouse, are thus largely independent from free water.[13][26]

Xerocoles obtain a large percentage of their water from the metabolic 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]

Thermal regulation

Morphology

See also: Allen's rule and Bergmann's rule

Xerocoles such as the

hare 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), the cape hare near Abu 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 the

conduction.[13]

Similarly, desert birds have fewer feathers on the underwing and

dorsal 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]

A reddish-gold lizard basking on a log
Ectotherms, such as this Cunningham's spiny-tailed skink, often bask in the sun to regulate body temperature.

Burrows

Most small xerocoles live in

microenvironments: 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]

Circadian rhythms

All desert rodents except ground squirrels and chipmunks are nocturnal.[13] Amphibians are usually nocturnal as well, while many other xerocoles are diurnal, 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] Unlike hibernation, which leads to a state of torpor, estivation induces lethargy, and can go unnoticed in some animals if their body temperature is not measured.[13]

Protection from the sun

Further information: Thermal radiation § Surface effects
Three cape ground squirrels emerging from a burrow in the Namib Desert
When the Cape ground squirrel scurries from one shady spot to the next, it holds its wide, flat tail over its back to provide shade.[13]

Xerocoles are usually light and sandy in color as a means to reflect solar radiation and reduce heat absorption.

Iguanid lizards can change color on a much smaller time scale by varying melanin concentration. They become darker when burrowing and lighter when basking – both the desert iguana and the zebra-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 black

UV 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]

Protection from sand

Desert animals such as the camel, addax, and kangaroo rat have large feet to prevent them from sinking in the sand.[6][29] The fennec 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 a nictitating 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 the sand cat have long hairs protruding from them.[39][40] The camel and the saiga 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]

Speed

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]

Known xerocoles

The following animals are known xerocoles:

See also

References

Citations

  1. ^ a b Nakate, Shashank (20 September 2011). "Desert Animals List". Buzzle. Archived from the original on 5 November 2012. Retrieved 24 November 2012.
  2. ^ "xero-". The New Oxford American Dictionary (2nd ed.). Oxford University Press, Inc. 2005.
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  6. ^ a b c "Arabian (Dromedary) Camel". National Geographic. National Geographic Society. Retrieved 25 November 2012.
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  9. ^
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  10. ^
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  12. ^
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  13. ^
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  16. ^ a b Ophardt, Charles E. "Urea Cycle". Virtual Chembook. Elmhurst College. Archived from the original on November 15, 2012. Retrieved 26 November 2012.
  17. ^
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  18. ^ "glomeruli". Gale Encyclopedia of Medicine. The Gale Group, Inc. 2008. Retrieved 27 November 2012.
  19. ^ "Regulation of Urine Concentration". Anatomy & Physiology. CliffsNotes. Archived from the original on October 25, 2012. Retrieved 27 November 2012.
  20. ^
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  21. ^
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  22. ISBN 9780520008663
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  23. ISSN 0031-9333
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  27. ^ "Large ears used to cool off: jackrabbit". Ask Nature. The Biomimicry 3.8 Institute. 2012-06-23. Retrieved 2012-12-03.
  28. ^ "Desert hares". The National. United Arab Emirates: Abu Dhabi Media. Retrieved 2012-12-03.
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  32. ^
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  33. ISBN 9780806131467
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  34. ISBN 9780806131467
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  35. ^ a b "Animal Adaptations" (PDF). Classroom Activities. SeaWorld/Busch Gardens. December 2002. Archived from the original (PDF) on September 15, 2012.
  36. ^
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  37. ^ a b c d Lull, Richard Swann (1920). "Desert Adaptations". Organic evolution. Macmillan. pp. 393–408.
  38. ^
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  39. ^ Bronx Zoo. "Camel Adaptations". Wildlife Conservation Society. Archived from the original (Flash) on June 26, 2012. Retrieved 29 November 2012.
  40. ^ Lincoln Park Zoo (23 September 2010). "Sand cat". Retrieved 6 December 2012.
  41. ^ "Camels". Traveling the Silk Road. American Museum of Natural History. Retrieved 8 December 2012.

Sources

  • Mares, Michael A.; Oklahoma Museum of Natural History, eds. (1999). Deserts. University of Oklahoma Press.
    ISBN 9780806131467
    .

External links
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