98 



lo. Sources and Role of Water used by 

 Prairie and Forest Anim,als 



The bodies of animals contain a very large proportion of water — 

 from 60 to 95 per cent. Growing animals in particular require water 

 in relatively large amounts. Practically all foods gain entrance into the 

 body in acpeous solutions, and are transported by water to all parts ; 

 and by the same means, the waste products, with the exception of the 

 excretion of carbonic acid, are removed. The methods by which aquatic 

 animals secure water are relatively simple, because they live in a 

 liquid medium; but the conditions upon land are quite different. Here 

 osmotic pressure does not operate as in water, and the air varies from 

 saturation to a very dry condition. This dryness tends to cause strong 

 evaporation from animals living in such a medium, and a proper bal- 

 ance between intake and water-loss is one of the most potent influences 

 in the life of land animals. In this relation lies the importance of the 

 sources of water available to them. These sources are as follows : 

 with the food, by drinking, from the atmosphere, and by metabolism. 

 The loss is by excretion and evaporation, the relative humidity and the 

 evaporating power of the air being, therefore, important considera- 

 tions. The loss of water is retarded in many ways. Some animals 

 possess a relatively impermeable skin, or a covering, as hair or feath- 

 ers, which retards air currents and evaporation through the skin, just 

 as a cover of vegetation retards soil evaporation. Other animals con- 

 serve their moisture by modes of behavior, being active mainly during 

 the cooler night, thus escaping the excessive evaporation of the heated 

 day; and still others live in burrows in the soil, where the humidity is 

 higher than in the air. Many animals can live only where the air is 

 humid. There is thus an almost endless series of conditions relating 

 animals to the supply and loss of water. 



On account of the herbivorous food habits of so many animals a 

 large number secure much water with the juicy vegetation eaten, and 

 others from nectar or from the sap drawn or escaping from plants. 

 The predaceous animals secure a large amount of water from the fluids 

 of the animals they devour or the juices sucked from their bodies, as 

 in the case of certain Hemiptera and some parasites. In addition to 

 the fluids derived from plants and animals, many animals also drink 

 water, some in small amounts and others in large quantities. Innu- 

 merable observations have been made by naturalists on the drinking 

 habits of animals, but I know of no general discussion of this subject, 

 and particularly of none from the standpoint of the variation of their 

 behavior in this resoect in different environments. But the sources 

 of water mentioned are not the only ones available to animals, although 



