Foraging under multiple costs: the importance of predation, energetic, andassessment error costs to a desert forager

We investigated spatial and temporal variation in the use of resource patch es by Chaetodipus penicillatus by documenting changes in foraging activity over several seasons, under two moon phases, and in two resource patch type s. In particular, we examined the influence of nocturnal illumination (a su rrogate for predation risk) and ambient temperature on: (1) foraging effort (which is inversely proportional to the giving-up density, GUD), and (2) G UD equality, i.e. the ability of foragers to accurately assess patch qualit y 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 begin ning of our study, moonlight did not have a consistent or significant impac t on foraging effort and GUD equality when analyzed over several seasons. O ur results indicate that as energetic or predation costs decline, foraging effort rises, which in turn, leads to an increase in GUD equality and a red uction 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, p redation risk and energetic costs appeared to switch in relative importance in C penicillatus over the course of several seasons. These results emphas ize the importance of considering multiple foraging costs and their additiv e effects in studies of foraging and patch use.