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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2004.03.003 Byline: Stephen J. Simpson, Richard M. Sibly, Kwang Pum Lee, Spencer T. Behmer, David Raubenheimer Abstract: There is growing evidence that, rather than maximizing energy intake subject to constraints, many animals attempt to regulate intake of multiple nutrients independently. In the complex diets of animals such as herbivores, the consumption of nutritionally imbalanced foods is sometimes inevitable, forcing trade-offs between eating too much of nutrients present in the foods in relative excess against too little of those in deficit. Such situations are not adequately represented in existing formulations of foraging theory. Here we provide the necessary theory to fit this case, using an approach that combines state-space models of nutrition with Tilman's models of resource exploitation (Tilman 1982, Resource Competition and Community Structure, Princeton: Princeton University Press). Our approach was to construct a smooth fitness landscape over nutrient space, centred on a 'target' intake at which no fitness cost is incurred, and this leads to a natural classification of the simple possible fitness landscapes based on Taylor series approximations of landscape shape. We next examined how needs for multiple nutrients can be assessed experimentally using direct measures of animal performance as the common currency, so that the nutritional strategies of animals can be mapped on to the performance surface, including the position of regulated points of intake and points of nutrient balance when fed suboptimal foods. We surveyed published data and conducted an experiment to map out the performance landscape of a generalist leaf-feeding caterpillar, Spodoptera littoralis. Author Affiliation: (*) Department of Zoology and University Museum of Natural History, University of Oxford, U.K. (a ) School of Animal and Microbial Sciences, University of Reading, U.K. (a ) School of Biological Sciences, University of Auckland, New Zealand Article History: Received 26 August 2003; Revised 22 October 2003; Accepted 12 March 2004 Article Note: (miscellaneous) MS. number: 7831