CONTRASTING PATTERNS OF ENERGY SUBSTRATE USE IN 2 SPECIES OF FROGS THAT BREED IN COLD WEATHER

We investigated nightly and seasonal variation in trunk-muscle and liv er glycogen stores in males of two species of frogs that breed in cold weather, the wood frog (Rana sylvatica) and the spring peeper (Pseuda cris crucifer). Wood frogs have an explosive breeding period that begi ns soon after ice clears from their breeding ponds and lasts only 3-5 days. Males call at low rates (500-600 call notes/h) and move about th e pond in search of females. They began the breeding season with large glycogen reserves in their trunk muscles (14-22 mg/g), but these were significantly depleted by the end of the breeding period. Males colle cted late in the evening consistently had lower trunk muscle glycogen reserves than those collected early in the evening. Liver glycogen sto res were unchanged on either a nightly or seasonal basis. Wood frogs d o not feed during the breeding season and cannot replenish depleted gl ycogen reserves. They also lack substantial lipid reserves and appear to depend entirely on carbohydrates to support call production and oth er reproductive activities. Spring peepers begin breeding at about the same time as wood frogs, but they continue for up to two months. Male s call at very high rates (3000-4000 call notes/h) for several hours e ach night. They began the season with very little glycogen in the trun k muscles (2 mg/g), but the muscles have enormous stores of lipids. Gl ycogen contents of both trunk muscles and liver increased late in the season when the frogs began to feed, but also were depleted on some ni ghts when males were calling. Spring peepers derive approximately 90% of their energy for call production from lipid reserves and only 10% f rom glycogen. This pattern is similar to that seen in other frogs with very high calling rates.