Harvest rates and foraging strategies in Negev Desert gerbils

Abstract

We examined the foraging strategy and quantified the foraging traits of two nocturnal rodent species, Allenby's gerbil (Gerbillus allenbyi) and the greater Egyptian sand gerbil (Gerbillus pyramidum). In the laboratory, both species used two distinct foraging strategies: either they immediately consumed seeds found in a patch (seed tray); or they collected and delivered the seeds to their nest box for later consumption. Moreover, we found a transition in foraging strategy among individual G. allenbyi under laboratory conditions; they all began by consuming the seeds on the tray and, after 7 days on average, switched to the collecting strategy. By contrast, in the field both species used only one foraging strategy; they collected and delivered the seeds to their burrow or to surface caches for later consumption. Furthermore, G. allenbyi and G. pyramidum collected seeds at significantly higher rates in the field than in the laboratory because the seed encounter rates for both species were higher in the field. This suggests that in natural conditions, probably involving predation risk and competitive pressure, gerbils must respond in two ways: (1) they must choose a foraging strategy that reduces predation risk by minimizing time spent feeding outside their burrows; and (2) they must forage more efficiently. In the field, seed handling time of the larger species, G. pyramidum, was shorter than that of the smaller one, G. allenbyi. This difference may give G. pyramidum an advantage 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 field study showed that the seed encounter rate of G. allenbyi was higher than that of G. pyramidum. This difference may give G. allenbyi an advantage when 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.