When foragers discount the future: constraint or adaptation?

Foraging currencies based on average rates of energy gain often fail to predict observed behaviour when an animal must choose between a smaller, more immediate reward and a larger, more delayed reward. We review experiments demonstrating that a forager's choice may depend on the ratio of reward sizes, the difference in pursuit times, and the magnitude of the average pursuit time. The most telling result is that of Green et al. (1981), who showed that foragers prefer the smaller, more immediate reward when both pursuit times are relatively small, but they prefer the larger, more delayed reward when both pursuit times are relatively large. This paper proposes a new theoretical explanation by modifying a standard economic model for discounting future rewards. Our model of a variable time bias specification can explain the observed, temporally inconsistent preferences. Discounting future rewards might be an adaptive response to uncertainty in an animal's natural environment, but we cannot reject the possibility that preference for immediacy is a behavioural constraint on foraging efficiency.