Future value and patch choice in least chipmunks

Animals subsisting on non-renewing patches confront a unique valuation problem, in which the objective value of a patch is inversely related to the amount already harvested. We investigated whether the subjective patch valuations of captive least chipmunks, Tamias minimus, reflected this environmental constraint as they foraged for buried patches of seeds. On the day following partial depletion of two patches, chipmunks spontaneously selected the least depleted patch, despite having no direct information about its former or present value. These findings held across experiments in which the order and extent of patch depletion were under the forager's control or experimental control. The animals consistently chose sites from which they had taken the fewest items, despite experiencing no difference in relative capture rate, suggesting that they discounted patch value on the basis of amount harvested or harvest time. Furthermore, the animals refused to return to patches they had completely depleted, and when induced to re-dig depleted patches by placing seeds above them, the animals chose randomly despite a three-fold difference in the number of items previously taken from each, suggesting that they disregarded rewards taken, attending only to the amount remaining. Subsequent experiments showed that chipmunks can take past reward history into account. When we refilled previously depleted patches and induced chipmunks to re-dig and deplete them again, they treated the patches as renewable types and represented their value in terms of the amount removed, not the amount left. Thus, the assessment mechanism of least chipmunks is facultative: they expect that patches will not replenish, but are prepared for exceptions. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.