What if you heard there was a creature that could freeze solid like a Popsicle, stay frozen for months and then defrost and go on its way like nothing had happened?
You might think it is a creature from another planet with a superpower, but it’s not! This creature is just one of our local frogs — the wood frog.
Blending in with the others
Wood frogs are very much like other native frogs. Until it is time to activate their superpower and transform into their frozen state, they do regular frog things like hunting by day and calling at night.
These small amphibians are found throughout the forests of Alaska, Canada and the northeastern United States, including here in Will County. They are about 1½ inches to 3 inches long and are tan, gray or reddish-brown with a light-colored underside and moist skin. A dark stripe runs through the wood frog’s eyes, looking like a mask. This distinctive eye stripe is the easiest way to identify them from other native Illinois frogs.
Their long, sticky tongues are used to catch insects, worms, slugs and other small creatures. In turn, they are prey for snakes, turtles and many mammals. Their call sounds like a quacking duck.
Spring vernal pools
In the spring, female wood frogs lay eggs in vernal pools. Vernal pools are small, seasonal ponds or large puddles that form from heavy rains or melting snow. The lack of hungry fish in vernal pools makes them ideal places for frogs to lay their eggs.
Words to know
Distinctive: Characteristic of a person or thing to distinguish it from others.
Glucose: A sugar that is an important energy source in living things.
Hunker: To squat or crouch down low.
Plummet: A steep or rapid fall or drop.
Vernal: Related to spring.
The tadpoles that hatch from the eggs eventually grow legs, shed their tails and spend time out of the water. Eventually, the heat of summer causes the seasonal ponds to dry up. Wood frogs will then spend their time on the ground in moist woodlands and forested swamps.
When the weather begins to turn cool in fall, wood frogs leave the moist woods and head to higher, drier ground. Once there, they nestle under leaf litter and wait for their superpower to kick in!
The transformation
As winter moves in and temperatures plummet, the wood frog’s body gets colder and colder. The frog stops moving, her heart stops beating and she stops breathing. The lens of her eye freezes, and her eyes turn white. She becomes like a solid block of ice. She will stay frozen like this all winter. In some places, wood frogs will stay frozen for up to eight months!
In early spring, when the weather begins to turn warmer, her heart begins to beat again. Her brain starts to show activity, and she starts to move. The defrosted frog will be perfectly healthy. She will be one of the first arrivals to the newly formed vernal pools, ready to mate and start the life cycle all over.
What’s happening
So how do wood frogs survive something that would probably kill most other animals? Is it truly a superpower with no explanation? While it would be fun to think frogs have superpowers, scientists have studied this amazing phenomenon and have an explanation. When freezing temperatures arrive and the tiniest amount of ice begins to form on the outside of the moist frog, a change is triggered inside the frog. Its liver begins to release a thick, syrupy substance called glucose.
As the frog begins to freeze, glucose moves into its tiny cells and water moves out. This prevents sharp, deadly ice crystals from forming inside the cells. The water in between the cells freezes, while the insides of the cells are full of protective glucose and do not freeze. In the span of about 15 hours, the frog is basically frozen solid, full of both ice and protective glucose antifreeze.
This ability to freeze and thaw allows the wood frog to survive winters in places as far north as Alaska! While scientists have figured out how wood frogs survive this freezing and thawing, there is still a question: What happens to trigger the wood frog’s heart to start beating after it has been still and frozen all winter? Scientists continue to try and unravel this mystery.
Take a walk
Right now, it’s cold outside and wood frogs are hunkered down and frozen beneath leaf litter. Can you think of a place where they might be hidden and frozen right now? In the spring, plan to take a walk near the area you are imagining. Are there vernal pools? Maybe your hunch about where to find them will be right and you will find some of these fascinating frogs.
____________
Follow Willy's Wilderness on Facebook for more kid-friendly nature stories and activities.