Chapter Outline   

During summer, much of the activity in the desert occurs at night when it is cooler and the relative humidity is higher.

Web Resources

The Excretory System summarizes how both invertebrates and vertebrates eliminate wastes. By M.J. Farabee of Estrella Mountain Community College.

The Kidney describes the human kidney but the principles of kidney function are well presented and apply to other organisms. By John W. Kimball.

Study Questions

1. Some insects have adopted unique ways of obtaining water including the absorption of moisture from the air. Describe how this can be and give specific examples of how insects do this.

2.  For a kangaroo rat, some 90% of their water gain may come from the metabolism of their dry food. a) Describe how the metabolism of food can result in water gain. b) Why might kangaroo rats favor a diet high in carbohydrates and low in fat and protein?

3.  An animal's integument can serve as an important barrier to water loss, but it also restricts necessary gas exchange. Compare and contrast how desert insects, large scorpions, and small mammals accomplish necessary gas exchange while minimizing water loss.

Desert tortoises can tolerate large fluctuations in solute concentrations within body tissues during cycles of hydration and dehydration.

5.  The kidney of a cactus wren is less efficient at concentrating urine than are the kidneys of a kangaroo rat, yet the cactus wren uses less water to rid the body of metabolic wastes, including nitrogenous wastes. Explain how this can be.

6.  Excess salts can be particularly troublesome for desert animals for it can exacerbate dehydration. a) How do excess salts exacerbate dehydration? b) Describe ways in which some desert animals can reduce salt intake and the unique ways that some rid themselves of excess salts.

7.  To survive periods of water deprivation, animals must store water which is gradually depleted until the body is again rehydrated. Indeed some desert animals are superb water hoarders. a) Describe how desert tortoises survive between drinks. b) Describe the observation that during drought, tortoise hatchlings consuming a potassium-rich diet grow more slowly than do hatchlings consuming a low-potassium diet.