Water and sodium requirements of field populations of house mice (Mus domesticus) and short-tailed mice (Leggadina lakedownensis) on Thevenard Island, in the arid Pilbara region of Western Australia

The coexistence of the Lakeland Downs short-tailed mouse Leggadina lakedown ensis and house mouse Mus domesticus on Thevenard Island, in the arid north of Western Australia, prompted a study to compare their seasonal water and sodium metabolism using tritiated water and sodium-22 as tracers. Fraction al water influx rates for M. domesticus (40.3 +/- 1.6% total body-water day (-1)) were significantly higher than those for L. lakedownensis (25.3 +/- 1 .2% total body-water day(-1)). Water effluxes were higher in both species o f mouse after the passage of a cyclonic storm near the study site. Water fl ux differences remained significant between species when turnover rates wer e scaled with body mass. A comparison of water influx rates of M. domesticu s with those predicted for field populations of other eutherian rodents sho wed that rates for M. domesticus on Thevenard Island were higher than expec ted. In contrast, water influx rates for L. lakedownensis did not differ si gnificantly from expected values for a desert rodent. Rates of sodium influ x for M. domesticus (41.7 +/- 3.6 mmol kg(-1) day(-1)) were over twice thos e of L. lakedownensis (19.7 +/- 4.8 mmol kg(-1) day(-1)), and were reflecte d in the significantly higher concentrations of sodium ingested in the diet , and excreted in the urine, of M. domesticus. Furthermore, the rate of wat er influx was positively correlated with the rate of sodium influx in M. do mesticus, suggesting that they were obtaining both water and sodium from th e one dietary source. There was no evidence to suggest that mice of either species were experiencing water or sodium stress, because water and sodium influxes and effluxes remained in balance. These results suggest that M. do mesticus on Thevenard Island had a higher-than-expected daily water require ment, and may represent a mesic deme of house mice that have yet to adapt t o the island environment.