Bullfrog (Rana catesbeiana). Photo by Donna L. Long.

How Cold-Blooded Frogs Survive the Winter and Emerge in Spring

Bullfrog (Rana catesbeiana). Photo by Donna L. Long.
Bullfrog (Rana catesbeiana). Photo by Donna L. Long.

On spring nights in wooded areas, you can hear the varied calls and vocalizations of frogs looking for mates. Some frogs emerge from their winter sleep as early as March. Others come out as the spring days grow warm.

Some frogs have spent the cold winter months hidden from view, hibernating deep within pond mud where frost can’t penetrate. Others were hidden away under rocks and old autumn leaves.

Hiding near the ground’s surface isn’t so remarkable until you remember that frogs are cold-blooded creatures. Frogs, being cold-blooded have body temperatures close to the surrounding air. Frogs to have a variety of fascinating strategies to manage their internal body temperatures and moisture levels.

Green Frog (Rana clamitans) the call sounds like "gunk". Photo by Donna L. Long.
Green Frog (Rana clamitans) the call sounds like “gunk”. Photo by Donna L. Long.

Generally, frogs cease activity when the surrounding air temperature is too cold for their bodies to function. North and Central American frogs are active at atmosphere temperatures between 37 degrees and 96 degrees Fahrenheit. Most frogs species are active during the most “frog-friendly” times of spring and summer.

As autumn sets in and the air temperatures begin to cool, frogs must find ways to survive the coming cold and freezing temperatures. Many frogs go the simple route and just bury themselves in the mud. They will stay in the mud and hibernate until the air temperature is well above freezing. Then they climb out of the mud and start looking for a mate.

But there are other frogs who take a more complicated route. They freeze themselves.

Spring Peepers (Pseudacris crucifer). By Bladerunner8u - wikimedia.
Spring Peepers (Pseudacris crucifer). By Bladerunner8u – wikimedia.

Frozen Frogs

As the weather cools in late summer and early autumn, the body of certain frog’s will produce an abundance of glucose. Glucose or glycogen is the same antifreeze type substance that enables other animals such as some fish, amphibians, reptiles, and insects to survive freezing cold temperatures.

In frogs, the glucose is concentrated in several organs including the heart, liver, and kidneys. Glycose protects these important organs from damage.  The glucose stops the water in the organs from forming damaging ice crystals. These organs are also among the first of the frog’s organs to thaw.

Gray Tree Frog (Hyla vericolor or Hyla chrysoscelis). Photo by USFWS Midwest Region
Gray Tree Frog (Hyla vericolor or Hyla chrysoscelis). Photo by USFWS Midwest Region

Frogs can lower their body temperatures over several hours. Rapid freezing, like putting a frog into a household freezer would probably be cooling a frog too fast. It also takes several hours for a frog to thaw out from a frozen state.

Because they can freeze solid,  the freezing frogs have more choices of where they can hibernate. Freezing frogs can hibernate on land, in leaf litter, or under rocks.

Wood Frog (Lithobates sylvaticus). Judy Gallagher, Wikimedia.

Several North American frogs are freeze tolerant. The wood frog, the spring peeper, the chorus frog, and the gray tree frog are all known to live through subzero temperatures in the winter. These frogs are the earliest of the North American frogs to emerge after the cold weather of winter.

When hearing the peep of Spring Peepers in spring, remind yourself that this little frog was frozen stiff not too long ago.

Studying Frogs: Citizen Science and Nature Study

As early as March, you can find some species of frogs thawing out and becoming active. Spring is also the season of heavy frog mating action. Your local pods and vernal pools can be hotbeds of frog reproduction.

Some environmental centers and organization host frog watching activities. Environmental centers also host “Toad Crossing” volunteer nights. Basically, it is helping frogs and toads cross the road to get to the ponds on the other side. Yes, there is a joke in there somewhere.

If you have Anna Comstock’s Handbook of Nature Study (on Amazon.com), she talks about spring peepers on pages 177 – 180.

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