A resource of Biophilia: Pittsburgh, #bioPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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What a snow this week! A frosty world all covered in fluffy white. It’s beautiful to see, but how do our animal neighbors fare in such wintry weather? If you stopped by Phipps’ lagoon over the summer, you probably lost count of the bullfrogs and tadpoles that hopped or swam in and out of view! With so many frogs milling about, have you ever wondered where they go when it’s wintry like this? Do they burrow in the mud like turtles? Swim below the ice like fish? Hibernate like bears? Migrate like birds? Freeze? Let’s explore!

Frogs generally stay put and hibernate in the winter, but their strategy for winter survival depends on whether they are terrestrial or aquatic. Terrestrial frogs can either burrow down into the ground or leaf litter to hibernate or they may seek out shelter in tree holes, fallen logs, or miniature “caves” in rock.

On the other hand, for aquatic frogs hibernating in the water, dissolved oxygen in the water is key to their survival. These frogs, like our bullfrogs in the lagoon, can undergo what is called cutaneous gas exchange, which means they can “breathe” through their skin. Amphibian skin has a higher density of tiny blood vessels than the average vertebrate, and those blood vessels are close to the surface of their skin. This allows oxygen to diffuse through the thin layer of skin to the blood vessels. During the active warm months, the oxygen frogs receive from cutaneous respiration wouldn’t suffice and they have to breathe with their lungs; but it is suitable in the winter when their metabolic rates drop sharply and the frogs move very little. Also, if you visited our lagoon in late summer, you probably saw quite a few bullfrog tadpoles swimming about. Since bullfrogs have a metamorphosis cycle that can last up to two years, their tadpoles must be able to survive the winter in the pond as well.

While overwintering, frogs live off stored energy reserves of lipids and glycogen they built up before hibernating. Lipids are essentially fats, which frogs accumulate during the warm months, and glycogen is a type of stored sugar that frogs accumulate in their livers and muscles. Since the frogs are barely, if at all, active (some aquatic frogs may move a bit while underwater), they burn through this energy slowly—which is important because they might hibernate from as early as October all the way until April.

However, terrestrial or aquatic, a frog’s greatest winter party trick is their freeze-tolerance. As temperatures drop, frog bodies produce glucose to protect their organs from the damaging part of freezing: crystallization. Ice crystals can cause cells to burst or damage their internal structures—neither of which is healthy for the animal. With frogs’ natural “anti-freeze,” they can be frozen to 0 F° (-18 C°) with no ill effects afterwards. Frogs can even undergo multiple freeze-thaw cycles, provided the freezing phase occurs slowly enough to give them time to produce glucose.  

Many other amphibians, like salamanders, survive our winters similarly – either by hibernating on land or in the water. Terrestrial salamanders tend to burrow underground in the winter, while other salamanders will settle into the mud at the bottom of a pond or stream (don't forget the important qualities of water that allow them to do that!) Not everyone hibernates, though; our state’s official amphibian, the hellbender, actually stays active and awake underwater throughout the cold months!

Connecting to the Outdoors Tip: We may not be able to hibernate through the winter, but that means we get to bundle up and explore the wonderland that is Pennsylvania in the winter! This time of you year, you probably won't see those amphibians, but as you walk across the ground and maybe out look out over a snowy pond, just thinkin all the life bursting beneath the surface!

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Resources

NY DEC - Hellbender

Getting through the winter: Green Section Report

National Aquarium—American Bullfrog

U.S. Forest Service—American Bullfrog

Scientific American—Where Do Frogs Go in the Winter? Why Don’t They Freeze to Death?

Jackson and Ultsch 2010: Physiology of Hibernation Under the Ice by Turtles and Frogs

Larson and Barnes 2016: Cryoprotectant Production in Freeze-Tolerant Wood Frogs Is Augmented by Multiple Freeze-Thaw Cycles

Photo Credits: Pexels and Wikimedia user Brian Gratwick CC-BY-2.0