Harvest rates and foraging strategies in Negev Desert gerbils

We examined the foraging strategy and quantified the foraging traits of two nocturnal rodent species, Allenby's gerbil (Gerbillus allenbyl) and the gr eater Egyptian sand gerbil (Gerbillus pyramidum). In the laboratory, both s pecies used two distinct foraging strategics: either they immediately consu med seeds found in a patch (seed tray); or they collected and delivered die seeds to their nest box fur later consumption. Moreover, we found a transi tion in foraging strategy among individual G. allenbyi under laboratory con ditions; they all began by consuming the seeds on the tray and, after 7 day s on average, switched to the collecting strategy. By contrast, in the fiel d both species used only one foraging strategy; they collected and delivere d the seeds to their burrow or to surface caches for later consumption. Fur thermore, G. allenbyi and G. pyramidum collected seeds at significantly hig her rates in the field than in the laboratory because the seed encounter ra tes for both species were higher in the field. This suggests that in natura l conditions, probably involving predation risk and competitive pressure, g erbils must respond in two ways: (1) they must choose a foraging strategy t hat reduces predation risk by minimizing time spent feeding outside their b urrows; and (2) they must forage more efficiently. In the field, seed handl ing time of the larger species, G. pyramidum, was shorter than that of the smaller one, G. allenbyi. This difference mat give G. pyramidum an advantag e when resource levels are high and when most of a forager's time is spent handling seeds rather than searching for more seeds. Additionally, our fiel d study showed that the seed encounter rate of G. allenbyi was higher than that of C. pyramidum. This difference may give G. allenbyi an advantage whe n resource levels are low and when searching occupies most of the forager's time. The different advantages that each species has over the other, under different conditions, may well be factors promoting their coexistence over a wide range of resource densities.