Foraging choice in laboratory rats: Constant vs. variable delay.

Deprived animals choosing between a variable delay (with mean t ) and a constant delay of t s prior to availability of food usually prefer the variable delay. Models of discounted future rewards predict such preference. For comparison we write a model assuming that a forager minimizes the probability that its total food intake falls short of a fixed requirement. This model predicts preference for the constant delay at sufficiently high average feeding rates. In a test of the models, laboratory rats (Rattus norvegicus ) chose between a constant t s delay and a variable option with equiprobable delays of 1 and (2t -1)s. Each subject was presented with the same sequence of mean delays. Each delay was experienced by the subjects for seven consecutive test days. Between the first and the fourth test day, a subject's body weight was decreased from 85 to 75 percent of free- feeding weight. Between the fourth and the seventh test day, a subject's body weight was increased to 85 percent of free-feeding weight. As t increased from 5 to 50 s, subjects first preferred the constant delay and then came to prefer the variable delay. Thereafter, as t was decreased to 5 s, subjects retained preference for the variable delay, but the strength of that preference declined at t decreased. Short-term variation in body weight, at a given value of t , did not influence preference significantly. Despite the rats' initial preference for constant delays, we tentatively conclude that our results appear more consistent with the discounting model than with the energy budget model.