Energy and water balance during torpor and hydropenia in the pigmy gerbil,Gerbillus pusillus

Abstract

Under ad lib. food conditions,G. pusillus used 107% of the predicted daily energy expenditure. This increased significantly with water deprivation to 116% of the allometric expected value, thereby reflecting an increase in activity in search of food with high moisture content, a shift to metabolic water for meeting the animal's water requirements and a change in the state of hydration of the animal. Economic water expenditure through efficient kidney concentrating ability, reduced pulmocutaneous evaporation, reduced faecal water loss, and tolerance to haemoconcentration, was sufficient to impart virtual independence of exogenous water. Gerbillus pusillus responded to food shortage by abandoning homeothermy. In doing so, it was able to maintain a fairly stable body mass by monitoring energy intake and ensuring that energy expenditure did not exceed intake. There was no significant difference in energy expenditure between gerbils on a restricted energy intake and ad lib. water and those gerbils on a restricted energy intake and no water. Both these experimental groups were able to maintain a positive water balance, for whilst less metabolic water was available, water loss primarily through evaporation decreased concomittantly with reduced body temperature. In addition, the volume of urine excreted when torpid accounted for approximately 5% of the volume excreted by coenothermic rodents deprived of free water. Urine, although less concentrated than that of non-torpid gerbils deprived of water, was ten times more concentrated than the plasma. Daily torpor, although uniquely suited to energy conservation, is not without cost. A reduced body temperature resulted in a decline in ‘apparent assimilation efficiency’ (AE). It seems therefore that the cost of maintaining an elevated body temperature for the full duration of digestion is impractical. The cost of employing torpor as calculated from the reduced AE of food (0.5 kJ·day−1), was however insignificant in comparison to the savings accured (32.25 kJ·day−1) by the use of torpor.