Water and Osmotic Balance in Overwintering Tadpoles and Frogs, Rana muscosa

Abstract

Water content, extracellular osmolality, and total solute content were measured in tadpoles and frogs of Rana muscosa submerged in the laboratory for 0, 1, 7, and 12 mo at 4 C. Edema was maintained at a constant level in both tadpoles and frogs during most of the natural overwintering period (7 mo), whereas extracellular osmolality and total solute content varied. During the first month of submergence, water content (excluding fat bodies, ovaries plus eggs, and tadpole digestive tracts) increased by 5.7% in tadpoles and 14.0% in frogs. Concomitantly, extracellular osmolality declined by 13.6% and 9.6%, respectively. The decline in osmolality was due solely to dilution in frogs, but it was due to both solute loss and dilution in tadpoles. Between 1 and 7 mo, water content in both tadpoles and frogs remained constant, whereas solutes were accumulated and extracellular osmolality increased. Between 7 and 12 mo, water content increased further, whereas solutes were lost and extracellular osmolality diminished. These findings are consistent with those for adult anurans of other species at low temperature, but the initial solute loss in tadpoles is not. The changes in water content, extracellular osmolality, and total solute content during overwintering were independent of body mass and skin surface area. Mass-specific water content was negatively related to dry body mass in both tadpoles and frogs, but tadpoles were more hydrated than frogs of the same size.