The effects of water deprivation and salt load on water conservation efficiency in two Indian desert gerbils

The effects of water deprivation and intraperitoneal salt loading on urine volume and on various urinary constituents have been examined in two gerbil species of the Rajasthan desert, the Indian desert gerbil (Meriones hurrianae Jerdon) and the Indian gerbil (Tatera indica indica Hardwicke). During summer, hydrated T. indica excreted 0.782 ml urine.100 g-1.d-1 which was about 60.5% higher than the volume of urine excreted by hydrated M. hurrianae (0.487 ml.100 g-1.d-1). During winter, both species excreted around 1.5 ml urine.100 g-1.d-1. The experimental treatments caused reductions in urine volume in M. hurrianae from 40 to 76% during summer and from 35 to 71% in winter. Similar treatments in T. indica caused reductions in urine volume of 50-82% in summer and 5-60% in winter. The mean increase in urine osmolarity following various salt loading treatments in T. indica ranged from 3800 to 5761 mosmol.l-1 and from 4034 to 6255 mosmol.l-1 during summer and winter, respectively. The mean values of urine osmolarity for hydrated T. indica were 2831 and 3189 mosmol.l-1 during summer and winter, respectively. In M. hurrianae salt loading treatments caused increases of urine osmolarity between 3381 and 5646 mosmol.l-1 and between 4032 and 5434 mosmol.l-1, during summer and winter, respectively, over the values recorded for hydrated animals (summer = 3292; winter = 3294 mosmol.l-1). A maximum urine osmolarity of around 7000 mosmol.l-1 was found in both species when subjected to 2% salt-loading treatment. The treatments used in this study increased urinary urea level in both T. indica (3039-4056 mM) and in M. hurrianae (1900-2180 mM) compared to the level in their respective hydrated controls (T. indica = 1628 mM; M. hurrianae = 1372 mM). The results indicate that T. indica may be better adapted to produce more concentrated urine than M. hurrianae.