They are a lot of insects that hibernate in a frozen or partially frozen state, but not many vertebrates, all of them terrestrial frogs (that I know of). Not only is freezing a seemingly death defying feat, but the adaptations employed to avoid permanent damage would kill most other animals.
There's a really excellent article written by Janet M. Storey at NatureNorth that explains the process in detail and in easy to understand language. There's a section on freeze tolerance in frogs in the book "Life in the Cold" by Peter J. Marchand (read it if you are at all interested in how animals and plants survive winter, you'll be more smarter if you do). So I'm not going to explain the process in detail, just outline some key points, and . . . an animated version of freezing in Grey Tree Frogs!
- Grey Tree Frogs hibernate under leaves, which offer very little protection from cold and ice.
- The changes in a Grey Tree Frog's body only begin when ice actually starts to form in the frogs body. Most animals anticipate the onset of freezing temperatures through environmental cues by experiencing changes in their physiology and/ or behavior.
- Freezing is limited to spaces outside of cells, leaving organs and tissues intact when the frog thaws in the spring.
- When ice starts to form, the liver starts producing glycerol; frogs use glucose, I'm not clear if Grey Tree Frogs use both or just glycerol. The glycerol aids in possibly three ways: by lowering the freezing point of cells, by strengthening cell membranes, and by reducing cellular dehydration. The first two help prevent the structural damage that can be caused by ice (think frost bite), the third helps with the fact that with all he water frozen outside of the cell, the water in the cell will flow "downhill", (osmotically speaking).
- The formation of ice also starts he heart beating faster, which seems counter intuitive; Grey Tree Frogs are cold-blooded so their metabolism should slow down with lowering temperatures. This would be true if the frog was just experiencing a temperature decrease, but it is also freezing. When a liquid turns into a solid, there is a release of heat, it's not a lot, but with a Grey Tree Frogs small body it is enough to get the heart pumping faster than normal. This allows the glycerin to be distributed quickly in the frog's body.
- When about 60% of the frog's body is frozen (up to 20 hours later), the heart stops pumping, and the little metabolism that occurs does so without oxygen. The Grey TreeFrog will remain this way until temperatures rise above freezing, at which point it will thaw out and shortly resume normal activities.
(note: the video my not appear on mobile devices. You can try this link if that's the case)