Seasonal water flux, urine and plasma osmotic concentrations in free-living fat sand rats feeding solely on saltbush

Fat sand rats (Psammomys obesus, Gerbillidae) are relatively large (200 g), diurnal desert rodents that may feed entirely on halophytic plants and, apparently, do not drink in the wild. We measured seasonal water fluxes, and plasma and urine osmolalities (Posm, Uosm) and concentrations of Na+, Cl− and K+ (PNa, PCl, PK, UNa, UCl, UK) of free-living fat sand rats that fed solely on A. halimus. Mass specific water influx of fat sand rats was significantly higher in spring than in winter and summer [0·43 vs. 0·27 and 0·18 ml H2O/(g·day), respectively] and was 1·5 to 3·5 times that predicted for a rodent of equivalent body mass. This was due to seasonal differences in water content of A. halimus and did not reflect the animal's water needs. Mean Posm and Uosm were 344 and 2119 mOsm/kg, respectively. Neither these measurements nor the mean PNa, UNa, PCl and UCl (145, 750, 118 and 775 mmol/1, respectively) varied during winter and spring; however, UK increased in spring (207 vs. 163 mmol/1 in winter). Na+, K+ and Cl− comprised 78 to 83% of total urine osmolytes. We concluded that fat sand rats were able, to some extent, to regulate their electrolyte intake by scraping off the salty epicuticular and cuticular layers of A. halimus leaves before eating them. More was scraped off drier plant material and, although fat sand rat water influx was 60% higher in spring than in winter, there was no seasonal difference in urine osmotic concentration.