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Water is essential for life. It is required to exchange gases (e.g., oxygen and carbon dioxide) in respiration, it transports nutrients through the body, it is involved in most metabolic processes, it dilutes and removes wastes, and it serves as a coolant. Water is acquired by animals through drinking (free water), through water in their food (preformed water), and through water produced as a by-product of metabolizing food (metabolic water, see respiration). Water is acquired by plants from the soil using their roots.

Water is lost by plants and animals by three avenues:

  • cutaneous water loss: leaking through outer surface (skin, exoskeleton, etc.).
  • excretory water loss: loss in urine and feces.
  • respiratory water loss: loss during gas exchange (breathing, and don't forget that plants must "breathe" for photosynthesis and respiration to occur.

Some of the ways to reduce water lost by the three avenues are given below. Torpor reduces water lost by all three avenues also.

  • cutaneous water loss:
    • stay in cool/moist microclimate; venture out when relative humidity is high.
    • use relatively impermeable materials on outer surface, such as scales, waxes, etc.
    • don't sweat
    • decrease surface area (e.g., small leaves)
    • reduce surface temperatures of body by shading (hairs/spines, etc.)
  • excretory water loss:
    • concentrate urine (e.g., the kangaroo rat is capable of producing urine twice as concentrated as seawater).
    • dry feces (e.g., kangaroo rat droppings are five times drier than lab rat feces).
    • produce uric acid rather than urea -- uric acid requires 10x less water than urea to rid the same amount of waste.
  • respiratory water loss:
    • stay in moist microclimate (e.g., burrow); venture out when relative humidity is high.
    • nasal water condensation: evaporative cooling in the nasal passageways cools exhaled air. Because cooler air holds less water than warmer air, water in the cooled, exhaled air condenses along the nasal passages. The longer, narrower, nasal passages found in many desert rodents cools air further and condenses more water.
    • have smaller leaves
    • be drought-deciduous, dropping leaves/twigs when soil dries up.
    • have only a few, small, sunken stomata on the bottomside of leaves.
    • use hairs/spines to increase shading and decrease wind across stomata.
    • have C4 and CAM photosynthesis