FEEDING BEHAVIOUR

any action of an animal that is directed toward the procurement of nutrients. The variety of means of food procurement reflects the diversity of foods used and the myriad of animal types. Feeding behaviour has evolved as a result of the interaction between an animal's structure and its environment. Each species selects, by means of experience or adaptation, food for which it can successfully compete. Some species monopolize one type of food, whereas others use a variety of resources. An animal's food intake and metabolic expenditure must be balanced so as to ensure survival. Metabolism may be decreased to a level sustainable by intake capacity, such as during a famine, or intake may be limited by the capacity of the digestive system, as in some crustaceans. When food gathering and digestive capacity exceed normal metabolic use, food intake must be regulated by other mechanisms. In vertebrates, the activities of searching, obtaining, and ingesting food are in response to external stimuli, and the responsiveness varies with the hunger level. An increased appetite for salt in a sodium-deficient animal appears to be genetically determined, whereas increased intake of the correct foods in a thiamine-deficient animal results from a learning process. In caloric intake, each of the four critical regionsmouth, digestive tract, bloodstream, and storage sitescontains monitoring mechanisms. Depending on the type and amount of food consumed, various signals are sent to the brain that raise either the satiety level or the feeding motivation, and the consequent balance between caloric intake and energy expenditure results in a constant level of body weight. Feeding behaviour in vertebrates is largely controlled by the hypothalamus. Stimulation of the lateral hypothalamus will produce continued feeding in satiated animals, while its destruction results in aphagia, or noneating. Individuals with lesions in the lower central nucleus of the hypothalamus will keep eating until an abnormal level of obesity is reached. Some discrepancies between intake and expenditure are normal for particular periods in the life cycle, such as the storing of fat before hibernation or migration. The selection of particular foods from environments that offer a wide variety is almost entirely genetically determined among invertebrates but is learned in vertebrates, most of which will choose a varied diet when possible. The responsiveness of the young is initially wide in species that hunt almost from birth and narrow in those that depend upon parental feeding. Social responses often influence feeding; individual animals may start to feed or accept undesirable foods if they see others doing so. Feeding activities may be performed for the benefit of other individuals, as in the courtship feeding of many birds and insects or in the feeding of the young. In some species, such as ants and honeybees, individuals perform highly specialized roles in food gathering for the entire colony. any action of an animal that is directed toward the procurement of nutrients. The variety of means of procuring food reflects the diversity of foods used and the myriad of animal types. The living cell depends on a virtually uninterrupted supply of materials for its metabolism. In multicellular animals the body fluids surrounding each cell are the immediate source of nutrients. The contents of these fluids are kept at a relatively constant level in spite of tolls taken by the cells, primarily by mobilization of nutrients stored in the body; in vertebrates, for example, glucose is stored in the liver, fats in the fat tissues, calcium in the bones. These stores, however, will become exhausted unless the animal takes up nutrients from outside. Movements performed for this purpose are termed feeding behaviour. Additional reading W.C. Allee et al., Principles of Animal Ecology (1949), a classic survey of the entire field, is still useful as an introduction to the relation of animals to their food environment and gives many examples. The physiology of feeding behaviour is reviewed in Handbook of Physiology, section 6, vol. 1, Alimentary Canal: Control of Food and Water Intake, ed. by Charles F. Code (1967), which is largely though not entirely restricted to vertebrates. J.A. Colin Nicol, The Biology of Marine Animals, 2nd ed. (1967), gives a good introduction to the classification of feeding patterns with typical examples. A methodologically important systems analysis of the behaviour of vertebrate and invertebrate selective feeders may be found in C.S. Holling, The Functional Response of Predators to Prey Density and Its Role in Mimicry and Population Regulation (1965), and The Functional Response of Invertebrate Predators to Prey Density (1966). An introductory survey of the relations of insects to their food plants (and feeding behaviour of insects in general) is included in P.T. Haskell (ed.), Insect Behaviour (1966). More detailed and specialized material on this point is contained in J. de Wilde and L.M. Schoonhoven (eds.), Proceedings of the 2nd International Symposium Insect and Host Plant (1969).