Have
you ever seen a Wood frog? They are hard
to see among the fallen leaves and ferns of their forest floor homes. I’ve only ever seen them twice in the woods,
myself.
But
what is cool about these woodland amphibians is not that they are so good at
camouflage or that they can vary their color, becoming lighter or darker, but
that they can freeze completely solid during the winter, thaw and “wake”
up in the spring and hop away, no problem!
Wood
Frogs, lithobates sylvaticus also known as Rana sylvatica, are common woodland
frogs, found from northern Georgia and northeastern Canada in the east to
Alaska and southern British Columbia in the west. They range throughout the boreal forests of
Canada and the Appalachian forests of the United States and are the most widely
distributed frog in Alaska. But unlike
most ranids, or “true frogs”, which spend most of their lives near or in
water, wood frogs are dwellers of the forest floor.
And
just like other northern frogs that become dormant during the cold season buried
in the soil or leaf litter, wood frogs are freeze-tolerant amphibians that can
tolerate the freezing of their blood and other tissues. But the wood frog has the most northern
distribution of any amphibian on the planet.
During the winter, wood frogs may remain frozen solid for over 190 days
in northern and interior Alaska where winter temperature routinely fall below
-40 °F (-40 °C). Research has shown that
the wood frogs in Alaska and northern Canada have a higher freeze tolerance
than the wood frogs in more southern regions.
This
cold-blooded amphibian can survive these cruel winter temperatures because it can
freeze solid, turning into a “frogsicle”, in an incredible example of
cryobiosis -- the metabolic ability to freeze and thaw to survive adverse
conditions. Wood frogs exhibit selective
freezing and typically have between 35-45%, and sometimes up to 65%, of the
water in their body turned into ice, and this includes the water in its skin,
body cavity and eyes. The wood frog’s
heart and lungs stop, its blood doesn’t flow, and its remaining unfrozen cells
enter a dormant state. Water is forced
out the frog’s cells into the interstitial spaces between them and out of its organs
where it can safely crystallize around mineral and bacterial “seeds”
collected in the frog’s body for just this purpose.
Wood
frogs can overwinter like this because in preparing for winter it accumulates urea,
a component of urine, in its tissues and glycogen is converted to glucose in
large quantities in its liver. The glucose
is then stored within the muscle and heart cells. This helps prevent intracellular ice formation
that would otherwise destroy the cells. Sugary
glucose bonds to the remaining water molecules to prevent them from escaping
the cells, thereby avoiding desiccation through osmotic shrinkage.
The
glucose and urea, act as cryoprotectants and give the frog its ability to freeze
almost completely solid during the winter and thaw out in the spring with
minimal cellular damage. These agents
mix with water and lower the freezing temperature inside the cells, protecting
them from damage. And the urea has an extra
role in suppressing the frog’s metabolism.
Wood
frogs can return to normal body functioning within 24 hours of thawing and can
become active as soon as temperatures rise above freezing. This makes them among the first cold blooded
creatures to “wake” in the spring, this gives them sole access to vernal
or spring pools for breeding. These
small ponds or pools of water are collections of melted snow or spring rain
that usually drain away or evaporate by summer. The brief nature of these seasonal pools prevents
fish or other predators from taking up residence in them, which makes them
ideal nurseries for vulnerable wood frog tadpoles, and these early “waking”
wood frogs have almost no competition for this critical resource.
So
next time you are out in the northern woodlands and forest, look for this amazing
amphibian, but good luck finding them hidden in the leaf litter.
I
hope that you continue to enjoy The Woodsman’s Journal Online and look for me
on YouTube at BandanaMan Productions for other related videos, HERE. Don’t forget to follow me on both The
Woodsman’s Journal Online, HERE,
and subscribe to BandanaMan Productions on YouTube. If you have questions, as always, feel free
to leave a comment on either site. I
announce new articles on Facebook at Eric Reynolds, on Instagram at
bandanamanaproductions, and on VK at Eric Reynolds, so watch for me.
That
is all for now, and as always, until next time, Happy Trails!
Sources
Knight, Jen; “Wood Frogs”,
Appalachian Wildlife Refuge, https://thelaurelofasheville.com/outdoors/conservation/wood-frogs/,
accessed October 4, 2025
Larson,
D. J., Middle, L., Vu, H., Zhang, W., Serianni, A. S., Duman, J., & Barnes,
B. M.; “Wood frog adaptations to overwintering in Alaska: New limits to
freezing tolerance”, Journal of Experimental Biology, [ 2014, vol. 217,
no. 12] pages 2193 to 2200
Petersen, Lee; “Wood Frog
– Lithobates sylvaticus”, August 18, 2020, https://www.lwpetersen.com/alaska-wildlife/wood-frog-lithobates-sylvaticus/#:~:text=Classification-,Identification,larger%20and%20more%20brightly%20colored,
accessed October 4, 2025
Tabler, Dave; “The frog
who freezes solid for the winter”, February 7, 2019, https://www.appalachianhistory.net/2019/02/the-frog-who-freezes-solid-for-the-winter.html,
accessed October 4, 2025
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