Water Exchanges of the California Vole, Microtus californicus

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Abstract
Daily ad libitum water consumption was approximately 25%\of body weight. Progressive restriction of water intake was invariably followed by proportional weight loss; weight stabilized if water privation was not too severe. Voles restricted to 2/3 ad libitum intake declined in weight to 78.8 [plus or minus] 1.1% of initial body weight in 13 + 1.4 days post-restriction. Those restricted to 1/3 intake declined to 69.8 + 1.4% of initial weight in 36 [plus or minus] 2.1 days. Some individuals lost half of their body weight and lived for more than 100 days while subsisting on subfatal water regimens. Total water deprivation killed the voles in 5.6 [plus or minus] 0.5 days; their weight at death was 71.1 [plus or minus] 3.4% of initial body weight. Rate of exchange of body water, as estimated by radioisotope (H3) dilution, was inversely related to water intake, both on an individual basis and when grouped according to water regimen. Half-times in days of labelled body water were for animals living on water ad libitum, 1.62 [plus or minus] 0.15; 2/3,2.20 [plus or minus] 0.14; 1/3,2.58 + 0.15; less than 1/3, 3.68 [plus or minus] 0.32. Evaporative (pulmocutaneous) water loss at 15-16 C in dry air flowing at 1-1/2 1/min. through a 1/2-liter chamber was in terms of ml H2O lost/g O2 consumed: for animals living on water ad libitum, 1.06 [plus or minus] 0.04; 2/3,0.976 [plus or minus] 0.034; 1/3, 0.916 [plus or minus] 0.041; less than 1/3,0.941 [plus or minus] 0.058. The differences are not statistically significant. The percentage of water in the fat-free carcasses was: for animals''living on water ad libitum, 71.1 + 0.6; 2/3, 68.9 [plus or minus] 1.0; 1/3,70.4 [plus or minus] 0.6; less than 1/3, 69.4+. 0.6. The voles maintained a constant body water: solids ratio during water privation, thus indicating that they did not tolerate general dehydration. The populations of M. californicus studied require free water and can only partially compensate for drinking water insufficiency. They respond positively to water privation by reducing their rate of urinary-fecal water loss through increased recycling of their exchangeable body water, but not reducing pulmocutaneous water loss. Apparently, these voles possess no special resistance to exsiccation or significant water conserving adaptations. It seems reasonable that M. californicus'' responses to experimentally imposed water privation are paralleled in nature during drought.

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