292 Nutrients 



Influence of Nutrients on Animals 



As the great majority of animals display the holozoic mode of nutri- 

 tion, this type of heterotiophism will be considered first, together with 

 certain nutritional aspects of parasitism. The nutritional relations of 

 saprophytes— particularly the fungi and bacteria— will be included 

 in the discussion of decomposition in the succeeding section. 



The nutrients required by animals are proteins, carbohydrates, fats, 

 salts, and accessory substances such as vitamins. Limitation by the 

 nutrient factor may result from the shortage of necessary foods in 

 the environment or from the inability of the animal to obtain and 

 digest them. The deficiency of the food itself may occur in relation 

 to its abundance or its composition. In some situations the total 

 amount of nutriment is sufficient but the particles of food are so 

 scattered that they cannot be gathered fast enough to satisfy daily 

 nutritional needs. 



The main bulk of the animal diet consists of carbohydrates, fats, 

 and proteins. Considerable flexibility exists in respect to the use of 

 these materials as sources of energy, but at least some protein food is 

 required in the growth and repair of animal tissue. Insufficient 

 supply of food to meet energy requirements or deficiency in amount 

 of protein, minor mineral nutrients, or necessary trace elements may 

 act as a limitation on the growth, reproduction, or distribution of an 

 animal. 



Free-living animals and parasites obtain these nutrient materials 

 by feeding on the tissues of other organisms in their environment. 

 The bodies of the organisms preyed upon as a source of food all con- 

 tain carbohydrates, fats, and proteins, but in widely varying propor- 

 tions, and they may or may not furnish all the minor and trace ma- 

 terials that are needed. Furthermore, the possible food organisms 

 must be caught, killed, and dealt with mechanically, in the process 

 of ingestion, and chemically, in the process of digestion. 



Herbivorous animals may be either larger or smaller than the plant 

 food on which they feed, but carnivores must always be larger, 

 quicker, or stronger than their prey, or be able to overcome them by 

 the use of poison glands or some other adaptation. Parasites are 

 smaller than their hosts. Predators must also be equipped with ap- 

 pendages and mouthparts that are capable of breaking through the 

 bark of trees, the shells of invertebrates, the tough skin of verte- 

 brates, or other types of protective outer covering, and capable of 

 chewing up the food sufficiently for swallowing. Apparently soft 



