Land animals, lacking constant contact with the 

 water medium, iue faced with the prohlems of obtain- 

 ing water and preventing excessive water losses from 

 the IkkIv. 



Water lieconies available to animals in varying 

 amounts in tlie forms of rain, snow, hail, frost, and 

 fog. Whatever the form it arrives in, the significant 

 things are the amount of free, liquid water added to 

 the substratum, accessible to plants and animals, and 

 the humidity of the air. Considerable amounts of 

 moisture are lost to organisms as run-ofT water flow- 

 ing into streams, by evaporation back into the air. 

 and as water bound in snow and ice. 



Kvaporation of water from the earth's surface or 

 from the bodies of organisms increases as tempera- 

 ture rises, air movement (wind) accelerates, and the 

 amount of moisture already in the air decreases. 

 When measured as grains per cubic foot or as milli- 

 meters mercury pressure, the actual amount of mois- 

 ture vapor in the air is known as absolute humidity. 

 This measurement is of less ecological importance 

 than is relative humidity, the ratio of amount of 

 water vapor actually in it to the quantity required to 

 saturate the air at existing temperature and baro- 

 metric pressure. Relative humidity is easily deter- 

 mined by means of sling or cog psychrometers, and 

 may be continuously recorded with temperature by 

 bygrothermographs. 



The evaporation rate of water is more closely re- 

 lated to saturation deficit than it is to relative hu- 

 midity. Saturation deficit is a quantity which cannot 

 he directly measured, it must be calculated. It is that 

 additional amount of moisture required to saturate air 

 under prevailing temperature, relative humidity, and 

 barometric pressure conditions, commonly expressed 

 as grains per cubic foot or as millimeters of mercury 

 pressure. 



The most exact and desirable measurement of 

 water evaporation is the vapor pressure gradient ob- 

 taining between the organism and the surrounding 

 air. The gradient is positive if water molecules leave 

 the organism at a rate faster than the rate at which 

 the organism is absorbing them from the air, and 

 negative if the reverse is true (Table 8-1). The de- 

 termination of gradient magnitude involves the meas- 

 urements of body temperature of the organism and 

 air, permeability of body membranes, and rate of air 

 movement over the body surface (Thornthwaite 

 1940). 



Water is obtained by a land animal by various 

 devices. There may be some direct absorption 

 through body surfaces such as occurs in the toad in 

 moist soil and in some beetle larvae in moist air : 

 this device is important in only a few species. Large 

 mammals frequently travel several miles each day to 

 water holes to imbibe drinking water. Many, but not 

 all, birds require drinking water ; some species, for 



c 



^ ir;^ 



FIG. 8-1 Apparatus for measur- 

 ing weather factors, (a) rain 

 gauge (courtesy Friez Instrument 

 Division); (b) cog psyehrometer; 

 (c) hygrothermograph; (courtesy 

 Friei Instrument Division); (d) 

 Livingston spherical atmometer. 



instances quail and partridge, get it as morning dew 

 on vegetation. Butterflies may frequently be observed 

 drinking water from small pools. An important 

 source of water is the jree water in food, particularly 

 in succulent vegetation and in the blood and body 

 fluids of animals. Desert animals depend almost en- 

 tirely for water on that contained in their food and 

 on metabolic ivater, liberated when fats and carbo- 

 hydrates, and to a lesser extent, proteins, are oxidized 

 in their bodies. 



Water is lost from the body through the skin and 

 lungs as insensible moisture and perspiration. Rapid 

 and largely uncontrolled loss of moisture through the 

 skin of amphibians, snails, annelids, and insect larvae 

 is a limiting factor confining these animals to moist 



Rock, sand, and clay 97 



