Metabolic consequences of accelerated urea synthesis during seasonal dormancy of spadefoot toads, Scaphiopus couchi and Scaphiopus multiplicatus

Scaphiopus couchi newly emerged from natural dormancy produce urea at a greater rate than dormant toads kept in moist soil. Energy budgets during dormancy of Scaphiopus couchi and S. multiplicatus were calculated by measuring protein, carbohydrate, lipid, ash, and DNA content of newly emerged toads and toads near the end of the natural activity period. Scaphiopus couchi uses 17% of the protein, 36% of the lipid, and 28% of the total energy reserves available at the start of dormancy, and S. multiplicatus uses 32% of the protein, 62% of the lipid, and 51% of the total energy reserves. If all of the nitrogen derived from protein catabolism were accumulated as urea, the minimum difference in body fluid concentrations before and after dormancy would be 167 mM for S. couchi and 341 mM for S. multiplicatus . Because these predicted increases are greater than normally observed in the body fluids of spadefoots at the end of dormancy, the toads apparently are obtaining sufficient part of their nitrogen wastes. Simultaneous O 2 consumption, CO 2 production, and nitrogen waste production measurements provide evidence for a shift from lipid to protein metabolism in S. couchi when osmotically stressed. During osmotic stress, S. couchi does not simultaneously achieve the reduced levels of O 2 consumption associated with dormancy, and increased protein catabolism and urea synthesis. Feeding experiments show that protein, carbohydrate, and lipid and replenished in newly emerged. S. couchi in proportion to their representation in the diet.