Seasonal Changes in Body Fluids in a Field Population of Spadefoot Toads

In both September and March, solute concentrations in the plasma and urine were typical of fully hydrated anurans. In June, however, levels of urea and electrolytes in the plasma were significantly higher, as were urinary urea levels. The moisture content of soil at the burrow sites was sufficient to permit passive water uptake by the toads in September and March. The low concentrations of urea in the body fluids of toads in March indicates that soil water was in fact utilized during the winter. By June the water potential of the soil was lower than that of the toads' body fluids, and the increase in plasma solute levels during the spring probably reflects net water loss coupled with the accumulation of metabolically formed urea.