Foraging under Multiple Costs: The Importance of Predation, Energetic, and Assessment Error Costs to a Desert Forager

Abstract

We investigated spatial and temporal variation in the use of resource patches by Chaetodipus penicillatus by documenting changes in foraging activity over several seasons, under two moon phases, and in two resource patch types. In particular, we examined the influence of nocturnal illumination (a surrogate for predation risk) and ambient temperature on: (1) foraging effort (which is inversely proportional to the giving-up density, GUD), and (2) GUD equality, i.e. the ability of foragers to accurately assess patch quality by equalizing heterogeneous resource patches to the same GUD. There were significant positive relationships between foraging effort and relative GUD equality, ambient temperature and foraging effort, and ambient temperature and GUD equality. In contrast, despite having a strong effect at the beginning of our study, moonlight did not have a consistent or significant impact on foraging effort and GUD equality when analyzed over several seasons. Our results indicate that as energetic or predation costs decline, foraging effort rises, which in turn, leads to an increase in GUD equality and a reduction in assessment error costs. This increased harvesting equality is due to both enhanced functional harvest rates and increased accuracy of patch assessment by foragers as more time is spent within a patch. In addition, predation risk and energetic costs appeared to switch in relative importance in C. penicillatus over the course of several seasons. These results emphasize the importance of considering multiple foraging costs and their additive effects in studies of foraging and patch use.