Field Urine Concentration in Two Indian Desert Gerbils

Abstract

One of the most striking physiological features of many desert-dwelling rodents is the ability to produce concentrated urine. Several investigators (MacMillen, 1972; MacMillen and Christopher, 1975; MacMillen and Grubbs, 1976) reported urine concentrations of noncaptive desert heteromyid rodents, but comparable data generally are not available for rodents from other deserts. Herein, I provide information on urine concentrations of two Indian desert gerbil species, Meriones hurrianae and Tatera indica, in their natural habitats. Meriones hurrianae (body mass range, 56-70 g) and T. indica (body mass range, 80-120 g) are the most abundant and desert-specialized rodent species of the Indian desert (24-30.5*N, 70*-76.2*E). Animals were captured in Sherman live traps baited with peanut butter during several months. Immediately after capture, animals were brought to the animal house at the Central Arid Zone Research Institute at Jodhpur (26*10'N, 7301'E) and housed individually in wire-mesh cages for collecting urine over the next 24 h. For T. indica, traps were set late at night. Captured animals were moved to the animal house early in the morning and kept in cages for the collection of urine. The crepuscular M. hurrianae was removed from traps within 1 h of capture and placed in cages. During urine collection, animals were provided with pearled millet (Pennisetum typhoides) ad lib. but no drinking water. Simultaneously with studies on freshly captured animals, laboratory-acclimatized animals of both species were provided with pearled millet and water ad lib., and urine samples were collected over a 24-h period. Urine samples were collected by placing a large petri dish with a layer of mineral oil under each cage. Urine was pipetted into plastic vials and stored frozen in capped vials under a layer of mineral oil. Urine osmolarity was measured using a Precision Osmette 'A' milliosmometer. Necessary dilutions were made with ,l pipettes. Each urine sample was measured at least twice and the mean value used for comparisons. Meriones hurrianae usually had lower urine osmolarity than T. indica (Fig. 1), although values differed significantly (P < 0.01) only during July. During the monsoon period (July), urine osmolarity was reduced from respective summer (May) values by 52% in M. hurrianae and by 29% in T. indica, presumably reflecting reduced need for water conservation during the monsoon. During summer, a mean urine osmolarity of 3,180 mOsm/l in M. hurrianae (maximum value, 4,645 mOsm/1) and a mean of 5,128 mOsm/l in T. indica (maximum value 7,547 mOsm/1) were recorded. Mean urine osmolarities in summer did not differ significantly between species. During November and December, urine osmolarity values for freshly captured animals of both species were similar to values for laboratory-acclimatized, hydrated conspecifics. Freshly captured T. indica excreted more concentrated urine in most months than laboratory-acclimatized, hydrated conspecifics. This suggests that the natural milieu of this species is more water-stressful than laboratory conditions. In their natural environment, these animals encounter the need to conserve body water, and likely do so through their powerful kidneys. Purohit and Ghosh (1963) reported that Tatera indica possesses longer kidney papillae than M. hurrianae. T. indica consumes large quantities of insects (Prakash, 1975), and the qualitative pattern of their urine osmotic concentration in the field is similar to that reported by MacMillen (1972) for the carnivorous-insectivorous Onychomys torridus that also exhibits fairly high and seasonally uniform urine concentration. Urine of freshly captured M. hurrianae had either similar or lower concentration than that from laboratoryacclimatized, hydrated conspecifics. This pattern suggests that these crepuscular rodents are not water stressed in the field, even in summer. This state of hydration is attributed to the high water content of their diet of roots and rhizomes. In the view of MacMillen and co-workers (MacMillen and Grubbs, 1976; MacMillen and Hinds, 1983), the relationship between metabolic-water production, evaporative-water loss, and ambient temperature ultimately determines the extent of urine concentration; at low temperatures, metabolic-water production is high relative to evaporative-water loss, water balance is accordingly favorable, and there is reduced need to produce highly concentrated urine. These workers reported that during nocturnal surface activity in the warmer summer months, desert heteromyid rodents frequently encounter temperatures exceeding that at