Freeze tolerance and cryoprotectant mobilization in the gray treefrog (Hyla versicolor).

Abstract

Interstudy differences have been reported in the cryobiology of Hyla versicolor, especially for southern vs. northern populations, although laboratory conditioning likely was a contributing factor. This study measured freeze tolerance and cryoprotectant levels in H. versicolor from a southern population (Illinois). Frogs fully tolerated freezing at -1.5 and -3.5 degrees C for 24-48 hr but their survival rate declined at -5.5 degrees C (< 50%). Calorimetry revealed that 46% of the body water froze after 24 hr at -1.5 degrees C. Levels of plasma glucose and glycerol were substantially elevated, 14x and 5x respectively, in recently thawed frogs vs. unfrozen frogs. Plasma osmolality correspondingly rose from 242 to 304 mOsmol/L. Three unfrozen frogs had levels of plasma glycerol ranging between 17.1-36.8 mmol/L, suggesting an anticipatory response to freezing, but another three unfrozen frogs had a glycerol level of 1.1 mmol/L. A direct relationship existed between glycerol content and plasma osmolality and an inverse relationship was observed between plasma osmolality and ice content. Glycerol clearly was a major component of cryo-protectant production in these frogs, which was likely essential to their freeze tolerance. The cryobiology of Illinois frogs was only marginally less developed than seen in northern populations of this species.