What do you think of this fact?
Picture this: you’re lying under the leaves in a forest, your heart stops, you stop breathing, and a big chunk of your body turns to ice. For almost any animal, that would be game over. For the North American wood frog (Lithobates sylvaticus), it’s just winter.
An ice age inside its body
Wood frogs don’t burrow deep into the ground to hibernate. They stay just beneath the leaf litter, and when it freezes, they freeze too. Up to 65 to 70 percent of their body water can turn to ice. At that point there’s no detectable heartbeat, no breathing, no blood flow and no brain activity.
In the wild, some frogs stay frozen for months at a stretch. In one study of frogs in natural hibernation, it was about 193 days in a row on average.
Sugar as antifreeze
The trick happens in the liver. As soon as ice starts forming in the body, the liver turns stored glycogen into glucose and floods the whole body with it. Together with urea, that sugar works as antifreeze: it keeps enough water inside the cells so they don’t dry out or get shredded by ice crystals.
The water freezes mostly outside the cells, while the sugar solution protects what’s inside them. Depending on the region, wood frogs can survive temperatures down to about -16 degrees Celsius (around 3 degrees Fahrenheit). Frogs from Alaska carry more glycogen in their livers and more urea in their blood than their relatives farther south.
The wood frog is the only North American frog that lives north of the Arctic Circle.
Reboot in spring
When it warms up, the frog thaws from the inside out. Its heart starts beating again, and the other organs slowly come back online. Soon after, it hops off to the nearest pond to mate as if nothing had happened.
No wonder scientists are fascinated by it: the wood frog shows that a vertebrate can handle ice inside its own body, as long as the chemistry is right.
A frog popsicle, just one that comes back to life every spring.