50 Coniparative Animal Physiology 



vaporizes and drinks less, and Microtus, the least. High water exchange is 

 associated with the greatest water supply and least evaporative forces in the 

 subterranean habitat, while the greatest adaptation for water conservation 

 permits life in the prairie grassland. The evaporative loss of water from the 

 lungs of desert rats is half that from most rodents. ^^^ 



The water relations of mammals, including man, in the 'desert have been 

 examined.^' ^^ The loss of water by skin evaporation and from the lungs 

 parallels resting metabolism. The sweat glands are under control of the autono- 

 mic nervous system, and serve a cooling function. Active sweating in man 

 normally begins when air temperature reaches about 86 °F. If evaporation is 

 prevented at high temperature, as by high humidity or by rubber or oilskin 

 clothing, the heart rate goes up and exhaustion occurs. Loss of water by the 

 skin is necessary in temperature control, but sweating diverts blood to the 

 skin from other areas, and by sweating the body loses salt. Both the salt and 

 water lost must be replaced. A dog suffers more in the desert than does man. 

 The dog loses only water, whereas man loses salt also, hence the dog's blood 

 concentration rises. The dog loses water mostly from its mouth and respiratory 

 passages, hence cooling and oxygenation are associated, and a panting dog may 

 thus develop an alkalosis, whereas in man cooling and respiration are separate. 

 Finally, the dog's fur-coated skin absorbs heat. A burro, on the other hand, 

 has many sweat glands, but its sweat contains little salt, hence the blood 

 chloride rises during work in heat. Buxton"'^^ gives an extensive list of desert 

 animals which may dispense entirely with drinking and get their water from 

 food and possibly from dew. Camels apparently can live without water if there 

 is plenty of good grazing, but at hard work Buxton claims that about 8 days 

 is their limit without water. Fat produces more metabolic water per gram 

 oxidized and yields more energy for a given amount of water than does carbo- 

 hydrate; fat is often stored by desert mammals. 



A man walking in the desert at 110°F. loses one quart of water per hour; 

 sweating increases by 20 gm. per hour for each 1°F. rise in air temperature. ^ 

 During such exposure an exercising man does not voluntarily drink enough 

 water to restore his water balance. Signs of serious dehydration are diminished 

 blood volume, high concentration of blood cells, nausea, numbness, and in- 

 adequate circulation. If water loss exceeds 12 per cent of the body weight in 

 dry heat a heat stroke is suffered. Water can be conserved if man ceases 

 activity, avoids the sun, remains clothed and reduces urine output by reducing 

 intake of protein and salt. In the tropical jungle where humidity is high, 

 cooling cannot be effected by sweating, and dehydration is not significant; 

 water intake is much less in the tropical jungle. ^^^ 



There are striking species differences in the water drunk and the water 

 excreted as a function of water deficit. Figure 22 shows the course of relative 

 water load when drinking was permitted after periods of privation. The dog 

 and burro rapidly drink more than they have lost, whereas man drinks less. "^ 

 Recovery from excessive hydration follows a different pattern; the rat recovers 

 more rapidly than the dog. Desert animals (kangaroo rat and pocket mouse) 

 on a dry diet excrete urine with chloride as concentrated as 900 mM, compared 

 with maximum chloride concentration of 370 mM in man and of 330 mM in 

 the dog. ^^^ 



Marine mammals have blood only a little more concentrated than that of 



