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An Ecological Theory on Foraging Time and Energetics and Choice of Optimal Food-Searching Method

R. Ake Norberg
Journal of Animal Ecology
Vol. 46, No. 2 (Jun., 1977), pp. 511-529
DOI: 10.2307/3827
Stable URL: http://www.jstor.org/stable/3827
Page Count: 19
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An Ecological Theory on Foraging Time and Energetics and Choice of Optimal Food-Searching Method
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Abstract

(1) A deductive, theoretical model is developed to allow calculation of a minimum energy budget and the corresponding time budget for foraging, both based on a diel (24 h) cycle. The model refers to animals with relatively high energetic costs of foraging, such as birds and mammals. (2) Various contributions to the daily time budget for foraging of a predator are identified and expressed as functions of prey density. (3) The prolongation of the daily search time, due to the energy expenditure for foraging, is expressed as a function of the total daily search time and the ratio between the energy consumption of the foraging activities and basal metabolism. (4) Different search methods are characterized by the search efficiency and energy expenditure linked to them. It is argued that, in general, the efficiency of a search method increases with its energy consumption. Optimization of foraging behaviour is taken to mean minimization of the necessary daily foraging time. Two constraints prevail: (i) the animal must gather a certain minimum amount of energy per day, and (ii) it has a certain maximum time available for foraging per day. (5) Equations are devised to allow comparison of the profitability of different search methods at various prey densities (or daily search times). From these equations, and from graphs based on them, predictions can be made about optimal choice of search methods. (6) The most energy-consuming, but also most efficient, search methods should be employed by a predator at the highest prey densities. When prey density decreases a predator should shift to progressively less energy-consuming search methods although these are connected with low search efficiency. At food bottle-neck periods due to low density of available prey, predators thus should resort to their least energy-consuming search methods of low efficiency. (7) The length of time available for foraging per day does not affect the choice of optimal food-searching method, nor do increases in energy-consumption, due for instance to reproduction or temperature regulation in cold weather. (8) The smaller the predator and the bigger the prey, the more energy-consuming (and efficient) search methods the predator should employ.(9) Different patch selection for foraging probably often require different locomotion types. When similar foods are eaten, small species, rather than big ones, should exploit the habitat patches demanding the most energy-consuming search methods, at least when food is scarce. When food is depressed due to exploitation, interspecific competition should lead to this apportionment of patches and cause adaptations accordingly.

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