Nutritional values 



Animals generally require proteins, carbohy- 

 drates, fats, vitamins, minerals, and water. Proteins 

 are used as the basic substance in the composition of 

 protoplasm; carbohydrates and fats are oxidized to 

 furnish energy for the body; vitamins serve as cata- 

 lysts for specific metabolic processes ; minerals are 

 needed to regulate osmotic pressure and as constitu- 

 ent elements of various body organs ; and water is 

 used as a general solvent, lubricant, and circulatory 

 medium. Species differ, however, in their needs for 

 particular substances. The beetles, Triboliuin, Lasio- 

 derma, and Ptiniis. for instance, grow slowly but 

 nonetheless satisfactorily on diets lacking carbohy- 

 drates. Hence, they may be distributed more widely 

 than are species which require carbohydrates in their 

 diet (Fraenkel and Blewett 1943). 



Foods differ in composition. Foods staple to an 

 organism's diet are those easily digested, and of high 

 caloric and protein content. They are adequate to 

 sustenance of the weight and vigor of the animal, but 

 usually need to be supplemented with vitamins and 

 minerals. The bobwhite and ring-necked pheasant, 

 for example, eat certain cultivated grains and weed 

 seeds, such as corn, sorghum, barley, wheat, rye, soy 

 beans, pigeon grass, and lesser ragweed as staple 

 foods, at least on a mixed diet. 



Non-staple or emergency foods are not in them- 

 selves sustentative, and animals limited to them grad- 

 ually lose weight and die. Such foods are, however, 

 often abundant and easily procured in emergencies, 

 when staple foods are covered with snow or ice, and 

 furnish sufficient energy to tide the animal over the 

 critical period. In emergencies, the bobwhite and 

 ring-necked pheasant eat black locust beans, fruits of 

 the bittersweet and sumac, rose hips, dried wild 

 grapes, and sweet clover seeds (Errington 1937). 



During good acorn years, squirrels, deer, and rac- 

 coons feed extensively on the acorns of white and 

 black oak, but almost completely ignore northern red 

 oak. Experiments with fox squirrels show that the 

 animals gain weight on an exclusive diet of white 

 oak acorns, scarcely maintain weight on acorns of the 

 black oak, and lose weight rapidly on acorns of the 

 red oak. The percentage of tannin in red oak acorns 

 is twice that in white oak acorns, and animals are 

 probably able to distinguish red oak acorns by a bitter 

 taste (Baumgras 1944). 



Vitamins are necessary for the maintenance of 

 good health in wild animals, just as in domestic ani- 

 mals or man. The symptoms of vitamin deficiency, 

 induced experimentally, are similar. Evidence has 

 been difficult to secure, however, that animals suffer 

 from vitamin deficiencies in their natural environ- 

 ments (Nestler 1949, House and Barlow 1958). 



Animals obtain most of their required minerals 



from their food and water. Additional salt must 

 sometimes be given caged animals to prevent canni- 

 balism. The gnawing of castoff deer antlers by ro- 

 dents is apparently for additional salts. The use of 

 certain soil deposits and springs as natural "licking 

 sites" by deer and other ruminants is apparently for 

 sodium salts lacking in their general diet (Stockstad 

 et al. 1953). Some birds, such as the evening gros- 

 beak, are also attracted to sources of salt supply. 

 There is some disagreement as to the need for grit in 

 the stomach as an aid for the grinding of seeds and 

 hard vegetable matter in gallinaceous and other birds 

 (Nestler 1949) ; this grit may be instead a source of 

 minerals. 



Animals appear to become aware of nutritional 

 deficiencies in their diet through physiological and 

 neurological mechanisms. Experiments with rats 

 show that when the body lacks some necessary ele- 

 ment such as sugar, salt, or a vitamin, the animal 

 consumes more of that particular substance than 

 usual. Discrimination and selection are apparently 

 made by taste, and a special need for a particular sub- 

 stance sharpens the taste for that substance so that 

 it can be detected even when present in food or water 

 in but very small quantities (Richter 1942). Nutri- 

 tional needs are neither the sole nor necessarily the 

 most important factor involved when animals show 

 preference for one type of food over another. Many 

 other factors condition the choice (Dethier 1954). 



Palatability 



Different species of animals vary considerably 

 in efficiency of digestion and utilization of particular 

 food substances. Thus, clothes-moth larvae can digest 

 cloth and bird lice can digest feathers, because among 

 other things they have an exceptionally high hydro- 

 gen ion concentration in their intestines. 



Digestive enzymes occur generally throughout the 

 animal kingdom although less is known about them 

 in the Protozoa. In the lower phyla the enzymes are 

 generalized in respect to the kinds of foods on which 

 they act ; in the higher phyla, they become highly spe- 

 cialized (Prosser et al. 1950). A specific enzyme, 

 however, does not differ greatly from one animal 

 group to another. Carnivores have strong proteases 

 and weak carbohydrases, correlated with their meat 

 diet. Herbivores, on the other hand, have weak pro- 

 tein, but active carbohydrate, enzymes. Herbivorous 

 mammals and birds possess a bacterial flora in their 

 digestive tracts that makes possible digestion of cellu- 

 lose. Omnivores have a full complement of enzymes 

 and can utilize a wide variety of foods. 



Practically all food contains some indigestible 

 matter ; ordinarily, that is passed through the di- 

 gestive tract and eliminated in the feces. If the indi- 



190 Ecological processes and dynamics 



