FIELD METABOLIC RATES AND WATER INFLUXES OF 2 SYMPATRIC GERBILLIDAE - GERBILLUS-ALLENBYI AND G-PYRAMIDUM

Two primarily granivorous rodents of Old World deserts, Gerbillus allenbyi (mean adult body mass=26 g) and G. pyramidum (mean adult body mass=40 g), coexist in sandy habitats in the northwestern Negev desert. Both species are burrow dwellers and are nocturnal; however, in their overall distributions, G. pyramidum occurs in more extreme deserts than does G. allenbyi. In comparing field metabolic rate (FMR) and water influx of the two Gerbillus species, we considered two alternative hypotheses: (1) given the difference in their overall distributions, G. pyramidum has a lower FMR and water influx than G. allenbyi, and (2) given the similarity in their diets, and that we worked with sympatric populations, FMR and water influx are similar. The latter alternative proved to be correct. Field metabolic rates in summer were 7.29 kJ . g-0.51 . day-1 for G. allenbyi and 7.74 kJ . g-0.51 . day-1 for G. pyramidwn, values that were 69.3% and 74.5%, respectively, of those predicted for rodents of their body masses. Summer water influx of G. allenbyi was 0.167 ml . g-0.90 . day-1 and that of G. pyramidum was 0.144 ml . g-0.90 . day-1; these values were 79.4% and 68.6%, respectively, of water influxes predicted for rodents of their body masses. When compared allometrically, there were no interspecific differences in any of the measurements.