How wood frogs freeze solid and return to life

HostIt's always a bit of a shock to realize that right now, in the middle of a cold winter, there are thousands of tiny frogs just lying under the leaves in the woods, frozen as hard as a brick. They're not just huddling for warmth or sleeping deep in the dirt. They're actually turned into ice. I have always wondered how a living thing can just stop like that and then walk away when the sun comes out.

GuestIt really is one of the strangest things in nature. If you found a wood frog in January and picked it up, you could practically use it to hammer a nail. It doesn't breathe. Its heart doesn't beat. Its blood isn't moving. By almost any rule we have for what makes something alive, that frog looks like it's gone for good. But for these guys, being a frozen rock is just part of the yearly plan.

HostBut ice is sharp and dangerous to living things. If I put a bowl of strawberries in the freezer and then thaw them out, they turn into a pile of mush because the ice crystals poke holes in the fruit. I don't see how a frog avoids just turning into a puddle of goo once the ice melts.

GuestThat mushy strawberry is exactly what the frog has to avoid. When water freezes, it grows and forms jagged edges. In a normal body, those ice spikes would shred the tiny bags that hold everything together, which we call cells. To stop that, the wood frog uses a trick with its liver. As soon as it feels the first touch of frost on its skin, its liver starts pumping out a massive amount of sugar. It floods its blood and its body with it. We're talking about so much sugar that it would be deadly to almost any other animal on earth.

HostSo it turns its blood into a thick syrup? I have heard of people using salt to melt ice on the road, so is the frog just using sugar to keep the ice from forming at all?

GuestNot exactly. The frog actually lets itself freeze, but it controls where the ice goes. The sugar gets inside the cells and acts like a shield. It makes the liquid inside the cells so thick that it can't freeze into those sharp spikes. The ice still forms, but it stays in the gaps between the cells, like in the belly or the spaces under the skin. So the frog is mostly ice, but the actual living parts are packed in that sugary syrup that stays liquid even when it's way below freezing. It's like putting bubble wrap around all the important bits so the ice can't crush them.

HostI still find it hard to believe it's that simple. Even if the cells aren't shredded, the frog still isn't breathing. The brain needs a constant supply of air and fuel to stay alive. If the heart stops and the blood sits still for months, how does the brain not just rot or shut down forever?

GuestFor us, that would be the end, because our brains are always running hot and using up fuel. But the wood frog basically turns the lights off and locks the door. Everything inside slows down so much that it hardly needs any energy at all. Because it's so cold, the body doesn't spoil the way a warm body would. It's in a state of paused life. The sugar also helps protect the things that build a body, keeping them from falling apart while they're sitting there in the dark. It's not trying to live through the winter; it's just waiting it out.

HostBut what happens when the cold gets really bad? I mean, in places like Alaska, it can get down to forty or fifty below zero. A little bit of sugar syrup doesn't seem like enough to stop a deep freeze like that from turning a frog into a solid block that would just shatter.

GuestThere's a limit to what they can handle, but they're tougher than they look. They don't just sit out on the bare ground where the wind can hit them. They tuck themselves under a thick layer of dead leaves and wait for the snow to fall. Snow is a great blanket. Even if the air is forty below, the ground under that snow might stay much warmer. They're playing a game of hide and seek with the cold, trying to stay in a spot where their sugar shield can do its job.

HostIt's a huge risk to take every single year. When the ground finally warms up, how does it all start back up? It seems like if the heart doesn't start at the exact same time as the lungs or the brain, the whole system would just fail.

GuestThe wake-up is just as wild as the freeze. It starts from the inside out. The heart is actually the first thing to come back to life. As the ice melts and the blood thins out, the heart gets a tiny spark. It starts to twitch, then it starts to beat, pumping that warm sugar-water through the body to thaw out the rest. The brain is the last thing to wake up. Within a few hours, the frog is moving its legs, and by the end of the day, it's hopping toward a pond to find a mate.

HostSo the sugar does more than just save them; it makes sure they're the first ones at the party.

GuestThe wood frog is the only creature that treats a deep freeze like a nap that helps it get a head start on every other frog in the woods.

HostThe little brown frog under the leaves is just waiting for the first hint of spring to turn its heart back on.