When Old Man Winter comes along, it's natural to stay indoors. After all, it can be dangerous to your health when the weather outside gets frightful. But what about all the animals in the world?

You probably know that certain animals decide to hibernate for the winter. Large furry bears find a place where they can stay reasonably warm and simply go into a deep sleep-like state for several months. Other animals, such as birds, migrate to a warmer climate. Not all animals can migrate or hibernate, though.

Take frogs, for example. They don't have thick skin and fur like a bear. They also can't take to the skies to fly south for the winter. Yet, some frogs, such as the wood frog, live all over the United States and even as far north as Alaska and above the Arctic Circle. How do these little creatures survive the frigid temperatures in these areas?

Recently, scientists have studied wood frogs and discovered some amazing things about these unique animals. So what do wood frogs do when the temperature drops below the freezing mark? They freeze!

As the September air gets colder and colder, wood frogs that live in the woods take cover under layers of leaves and other ground cover. Then something miraculous occurs: they stop breathing, their hearts stop beating, and their blood stops flowing. Eventually, they freeze.

About two-thirds of the water in their bodies turns to ice. To the naked eye, they appear dead. They don't move. If you tried to bend one of their arms, it would break. Yet life still exists deep inside.

To study wood frogs in freezing environments, scientists attached tiny radio transmitters to a group of frogs. The transmitters allowed scientists to track their movements and study them in their frozen state when they made their homes for the winter.

Over the course of two years of study, not one of the frogs died. Scientists learned that the frogs could withstand temperatures as low as 0º F for as long as seven months. These findings surprised scientists, who had previously believed that these frogs could only survive for a few weeks in warmer temperatures.

Scientists discovered that the water inside individual cells does not freeze, but the water in between cells does freeze solid. Cells still function, but they can't communicate with each other.

Wood frogs have special nucleating proteins in their blood that cause the water in their blood to freeze first. Cells begin to shrivel as ice draws water out of nearby cells. This process could result in death if it weren't for a substance that acts like antifreeze within cells.

Scientists believe early freeze/thaw cycles help wood frogs to convert glycogen stored in their livers into glucose in their cells. They think high levels of glucose inside cells allow wood frogs to survive being frozen for so long, because the glucose keeps sufficient water inside cells to keep them alive.

Amazingly, when spring temperatures arrive, frozen wood frogs slowly thaw out. Their hearts start beating, their blood starts flowing, and they begin to breathe again. Glucose gets converted back to glycogen, and they're eventually able to hop away to the nearest pond or lake to start the mating process.