The Impact of Physiological Demands on Foraging Decisions under Predation Risk: A Test with the Whelk Acanthina monodon Rube´n E. Soto, Juan C. Castilla & Francisco Bozinovic Center for Advances Studies in Ecology and Biodiversity and Departamento de Ecologı´a, Facultad de Ciencias Biolo ´gicas, Pontificia Universidad Cato ´lica de Chile, Santiago, Chile Abstract We study whether and how physiological demands affect foraging decisions under predation risk, by evaluating the effect of starvation on the rate of food consumption and prey-size preferences and the potential trade-off between starvation and predation risk on foraging behavior in the whelk Acanthina monodon, a gastropod inhabiting the intertidal rocky shores of central Chile. These whelks appear to adjust their foraging strategy to physiological (nutritional) demand and predation risk. Starvation reduced the effect of predation risk on the rate of food consumption by A. monodon. Thus, in the absence of the predator sea star, the rate of food consumption by starved and satiated whelks was similar. When a predator was present, starved whelks fed faster than satiated whelks. Our results indicate that foraging behaviour represents an integrated and hierarchical response to environmental conditions and the physiological conditions of the forager. Correspondence: Francisco Bozinovic, Center for Advances Studies in Ecology and Biodiversity and Departamento de Ecologı´a, Facultad de Ciencias Biolo´gicas, Pontificia Universidad Cato´lica de Chile, Santiago 6513677, Chile. E-mail: fbozinov@bio.puc.cl Introduction Classic frameworks dealing with foraging ecology state that behaviour depends on the ecological context in which foraging takes place, and is based on the premise that nutrient intake is related to the survival and reproductive success of the forager (Holling 1959; Stephens & Krebs 1986). Nevertheless, the physiological condition of animals, in combination with cognition skill, the availability and properties of food resources, and predation risk may also have an important, albeit relatively little-studied effect on foraging choice. A clear example is the trade-off between starvation and predation risk, which represents a key element in modern behavioral ecology (Houston et al. 1993; Bozinovic & Ethology 111, 1044—1049 (2005) Ó 2005 Blackwell Verlag, Berlin