The impact of long-term continuous risk of predation on two species of gerbils

Individuals of Gerbillus allenbyi Thomas, 1918 were subjected to artificial illumination in large field enclosures (2-ha sandy-substrate plots in the Negev Desert, Israel). The illumination was similar to that provided by a full moon and was used to mimic the elevated risk of avian predation that accompanies a full moon. We artificially illuminated the enclosures during all hours of darkness for 3 consecutive months. In some cases, we also added individuals of Gerbillus pyramidum Geoffroy, 1825 to provide a competitive challenge for the G. allenbyi. In the presence of the light source, individuals of G. allenbyi shifted their foraging activity to favor experimental areas of darkness. They also foraged less in the open and more beneath bushes. In the absence of the light source, G. allenbyi shifted its activity from the subplot with G. pyramidum to the subplot without the competitor. However, the competitive effect of the G. pyramidum disappeared in plots that were artificially illuminated. These results closely resemble those of earlier experiments during which we increased the apparent risk of predation in brief pulses lasting only 2 h/night.

New observations on urine contents in water-deprived Negev Desert rodents

In past studies, several rodent species of the murid subfamilies Gerbillinae and Cricetomyinae from the Namib Desert, when deprived of water, excreted allantoin precipitate in their urine. Shifting nitrogen excretion from urea to allantoin allows them to save much water. This phenomenon has not been reported in other rodents, and whether it is a trait that is common among desert rodents, but undocumented, or is unique to these Namib Desert species, is not known. We found no allantoin precipitate in the urine of any of five additional species of water-deprived murid rodents of two subfamilies from the Negev Desert. There was no discernible precipitate in the urine of Gerbillus andersoni allenbyi (Gerbillinae), Gerbillus pyramidum (Gerbillinae), or Acomys cahirinus (Murinae). Sodium oxalate was found in both the precipitate and the liquid urine of Psammomys obesus (Gerbillinae), and the as yet unidentified precipitate in the urine of Gerbillus dasyurus (Gerbillinae) was not allantoin. This preliminary study suggests that not all gerbilline rodents have the capacity to switch from urea to allantoin excretion. The Namib Desert gerbilline and cricetomyine rodents may be examples of closely related mammalian taxa that have evolved a new metabolic pathway to produce a nitrogenous product that results in sizable water savings, i.e., by switching from urea to allantoin.

Photoperiodic responses of four wild-trapped desert rodent species

The purpose of this study was to examine the effect of the photoperiod on reproductive status and body and lipid masses in four Egyptian desert rodent species ( Dipodillus dasyurus, Acomys cahirinus, Gerbillus andersoni, and Gerbillus pyramidum). Adult males and females were housed in long days for 11 wk. At that time, one-half of the animals were killed and the remaining animals were moved to short days (SDs) for 11 wk. Some individuals of Gerbillus andersoni and Gerbillus pyramidum had access to running wheels. Testes index and spermatogenesis, but not testis mass, were decreased in all species in SDs. In contrast, SDs did not affect female reproductive status in all species. Exercise stimulated spermatogenesis but did not affect female reproductive status. SDs increased body and lipid masses in male Acomys cahirinus, but not in other species. Collectively, these desert rodent species were responsive to day length changes, but these changes alone did not induce robust alterations in reproductive status and body and lipid masses.

Foraging Under Predation - a Comparison of Energetic and Predation Costs in Rodent Communities of the Negev and Sonoran Deserts

We used patch-use theory, giving-up densities in experimental food patches, and harvest-rate measurements within these patches to determine the relative contributions of predation risk and energy to foraging costs in four species of rodents from communities in the Sonoran and Negev deserts. To partition costs into components of energy and predation, we converted field measurements of giving-up densities into harvest rates (J min(-1)), used these harvest rates as an estimate of total foraging costs, estimated energetic foraging costs from published physiological measurements of activity and thermoregulatory costs, and assumed that missed opportunity costs were either zero or negative. Our results showed that predation costs predominate. Energetic costs represented only 24%, 19%, 16% and 13% of the foraging costs for Merriam's kangaroo rat (Dipodornys merriami; Sonoran), the round-tailed ground squirrel (Spermophilus tereticaudus; Sonoran), the greater Egyptian sand gerbil (Gerbillus pyramidum; Negev), and Allenby's gerbil (G. allenbyi; Negev), respectively. Equally important were predation-risk differences between bush and open microhabitats; the microhabitat differences in predation cost were often 2-4 times larger than the animals' energetic costs. Seasonal patterns in foraging costs also were predominantly influenced by predation rather than energetic costs. Predation costs appear to be greater in the Negev Desert, but rodents of the Sonoran desert experience greater seasonal and microhabitat variability in predation costs. As a result, predation risk may contribute more towards species coexistence in the community of the Sonoran Desert than that of the Negev Desert.