Don Larson, a PhD student at the University of Alaska, Fairbanks, studied wood frogs to find out how many frogs actually survive the harsh northern winters. Elizabeth Preston over at Discover Magazine breaks it down:
Frogs in the wild stayed frozen for an average of 193 days. During this time, sensors showed that the temperature in their habitats averaged –6.3°C (21°F). At some points, temperatures dropped as low as –18.1°C, or just below 0 Fahrenheit. Despite the conditions, which were worse than anything recorded for wood frogs before, every frog survived.
The research notes that frogs use the glucose in their tissues as a kind of natural antifreeze -- it protects their cells from the cold, so they can wake up and hop away every spring.