Winterkill, Oxygen Relations, and Energy Metabolism of a Submerged Dormant Amphibian, Rana Muscosa

Both larval and metamorphosed Rana muscosa (mountain yellow—legged frog) over—winter for 6—9 mo in ice—covered lakes and streams at high elevation (≤3700 m) in California and Nevada. In some winters almost all the frogs in shallow lakes (<4 m deep) die. This mortality is associated with oxygen depletion, which occurs most rapidly in shallow lakes. However, tadpoles survive for months in nearly anoxic conditions when shallow lakes are frozen to the bottom. In the laboratory at low P 0 2 , oxygen metabolism of overwintering tadpoles at 4° differed from that of frogs. The critical oxygen tension (P c ) of tadpoles (1.98 kPa) was significantly lower than for frogs (4.04 kPa). Also, when P 0 2 was greater than P c , the resting oxygen consumption (V 0 2 ) of tadpoles partially conformed to P 0 2 , whereas the resting V 0 2 of frogs was constant. At the atmosphere P 0 2 of their natural environment (P 0 2 °13.3 kPa), however, the resting V 0 2 of tadpoles was not significantly different from that of frogs. Tadpoles apparently have two advantages over frogs during winter: (1) a greater tolerance of low P 0 2 , and (2) a reduced consumption of energy and oxygen at low P 0 2 . Unlike a number of anuran species which become dormant on land at temperatures above 4°, the resting V 0 2 of metamorphosed R. muscosa overwintering at 4° was nearly the same as for summer—acclimated individuals in air at 4°. During 7 mo dormancy, the standard metabolic rate of both tadpoles and frogs remained constant. Resulting V 0 2 of frogs in water at 4°, and in air at 15° was related to the 0.83—0.85 power of body mass.