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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Freezing tolerance of the European water frogs: the good, the bad, and the ugly

¹Physiologie des régulations énergétiques, cellulaires, et moléculaires (UMR CNRS 5123), Université Claude Bernard Lyon1, Villeurbanne; ²Ecologie des hydrosystème fluviaux (UMR CNRS 5023), Université Claude Bernard Lyon1, Villeurbanne; and ³Laboratoire de RMN, Hôpital St-Louis, Paris, France

Submitted 20 October 2004 ; accepted in final form 20 January 2005

Survival and some physiological responses to freezing were investigated in three European water frogs (Rana lessonae, Rana ridibunda, and their hybridogen Rana esculenta). The three species exhibited different survival times during freezing (from 10 h for R. lessonae to 20 h for R. ridibunda). The time courses of percent water frozen were similar; however, because of the huge differences in body mass among species (from 10 g for Rana lessonae to nearly 100 g for Rana ridibunda), the ice mass accumulation rate varied markedly (from 0.75 ± 0.12 to 1.43 ± 0.11 g ice/h, respectively) and was lowest in the terrestrial hibernator Rana lessonae. The hybrid Rana esculenta exhibited an intermediate response between the two parental species; furthermore, within-species correlation existed between body mass and ice mass accumulation rates, suggesting the occurrence of subpopulations in this species (0.84 ± 0.08 g ice/h for small R. esculenta and 1.78 ± 0.09 g ice/h for large ones). Biochemical analyses showed accumulation of blood glucose and lactate, liver glucose (originating from glycogen), and liver alanine in Rana lessonae and Rana esculenta but not in Rana ridibunda in response to freezing. The variation of freeze tolerance between these three closely related species could bring understanding to the physiological processes involved in the evolution of freeze tolerance in vertebrates.

cold hardiness; ice content; osmolality; glucose