SUMMARY
The physiological responses supporting freeze tolerance in anurans are well known, but the evolution of this trait remains little studied. This is the first common-garden study of geographic variation in cryoprotective responses to freezing and the degree of freeze tolerance. We studied the gray treefrogs(Hyla versicolor and H. chrysoscelis) from sympatric sites in Minnesota, Indiana and Missouri. Patterns in the literature suggest that northern frogs produce more cryoprotectants upon freezing, but we found no geographic variation in cryoprotective responses or degree of freeze tolerance. The concentration of glucose produced upon freezing was higher than previously reported for this species (liver: 475 μmol g–1dry mass). Unfrozen frogs had high levels of glycerol (liver: approx. 150μmol g–1 dry mass), and did not produce more upon freezing. Liver glycogen content (concentration multiplied by liver mass) was highest in frogs from Minnesota and Missouri, and was stored in preference to lipids in Minnesota frogs, possibly to provide energy for the longer northern winters. Minnesota frogs accumulated more ice (53.4±1.8%) after freezing to–2.5°C than Indiana frogs (45.5±3.3%). The two species differed in body size but not in any of the physiological parameters measured. We conclude that these populations show no adaptive variation in freeze tolerance and that comparing published studies may be misleading because of different acclimation and feeding regimes.
