Behavioural Processes 61 (2003) 109–121 Increased effort requirements and risk sensitivity: a comparison of delay and magnitude manipulations Ari P. Kirshenbaum * , Allen D. Szalda-Petree, Nabil F. Haddad Department of Psychology, Experimental, Animal Behavior Laboratory, The University of Montana, Missoula, MT, USA Accepted 12 November 2002 Abstract Reward magnitude and delay to reward were independently manipulated in two separate experiments examining risk-sensitive choice in rats. A dual-running wheel apparatus was used and the tangential force resistance required to displace both wheels was low (50 g) for half of the subjects, and high (120 g) for the remaining subjects. Concurrent FI30-s and FI60-s schedules delivered equivalent amounts of food reward per unit time (i.e. 5 and 10 pellets of food, respectively), and these conditions served as the baseline treatment for all subjects. Variability, either in reward magnitude or delay, was introduced on the long-delay (60s) schedule during the second phase. All subjects were returned to the baseline condition in the third phase, and variability was introduced on the short-delay (30 s) interval schedule during phase four. The subjects were again returned to the baseline condition in the fifth and final phase, ultimately yielding a five-phase ABACA design. Original baseline performance was characterized by a slight short-delay interval preference, and this pattern of performance was recovered with each subsequent presentation of the baseline condition. Overall, the data obtained from the reward magnitude and delay-to-reward manipulations were indistinguishable; subjects experiencing low-response effort requirement behaved in a risk-indifferent manner and subjects experiencing high-response effort requirement preferred the variable schedule. Implications for the daily energy budget rule on risk-sensitive foraging are discussed in light of these findings. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Concurrent schedules; Rats; Response effort; Risk-sensitive foraging theory 1. Introduction Natural resources may vary seasonally and daily. Therefore, it is reasonable to assume that foragers pos- sess the ability to alter their strategies in order to match changing environmental constraints. The capacity to evaluate changes in resource availability, and to alter strategy accordingly, has an evolutionary utility be- cause it may allow the forager to maximize caloric * Corresponding author. Present address: Human Behavioral Pharmacology Laboratory, Psychiatry Department, University of Vermont, 38 Fletcher Place, Burlington, VT 05401, USA. E-mail address: ari.kirshenbaum@uvm.edu (A.P. Kirshenbaum). income under stochastic conditions. Since fitness will ultimately be determined by foraging efficiency, it is imperative that organisms be sensitive to relative dif- ferences between concurrently available food sources. Resources in nature are patchily distributed, and al- though some patches yield better overall payoffs than others, the immediate energetic value associated with a patch is a function of the rate of caloric gain within that patch. Rate of gain is relative to encounter rate, and encounter rate is inevitably variable. Thus, forag- ing strategy may be governed by the degree of vari- ance in encounter rate. Understanding the organism’s response, or sensitivity, to resource variability is the primary focus of risk-sensitive foraging (RSF) theory. 0376-6357/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0376-6357(02)00165-1