Depletion of seed patches by Merriam's kangaroo rats: are GUD assumptions met?

Depletion of experimental seed patches by granivorous animals often is used as a qualitative assay of foraging activity. An optimal foraging model sug gests that seed amounts remaining when foragers leave patches ("giving-up-d ensity", GUD) also provide quantitative measures of foraging economics, die t strategies and foraging abilities. Such quantitative uses of GUDs rest on several largely untested assumptions. We tested two of these with Merriam' s kangaroo rats: that gain curves are smoothly decelerating, and that forag ers leave patches at a constant harvest rate. Harvest rates indeed declined with patch residence time, but in the piecewise linear fashion expected of systematic search. Animals also revisited areas within patches less freque ntly than expected with random search. In the field, they depleted patches in multiple visits and did not use a constant-rate leaving rule. These devi ations from model assumptions cast doubt on inferences about foraging ecolo gy that have been based on quantitative GUD theory.