Winter is Coming: How Do Frogs Avoid Freezing?

Yesterday while shopping at the mall, I noticed that some stores had already begun putting up holiday decorations. The racks were filled with sweaters and down coats. We humans (at least in the northeast U.S.) are preparing for winter. But what about our amphibian friends? How do they prepare for winter? After all, frogs would seem vulnerable to extreme cold with their thin skins and their need to constantly stay moist.

Actually, we don’t need to worry about the frogs. They are well-equipped to deal with the cold weather, even with Arctic temperatures.

Frogs are ectothermic, which means that they rely on their environment to regulate their body temperature. Birds and mammals, including humans, are endotherms. We generate heat chemically and internally by breaking down food. The bodies of ectotherms reflect the air, ground, and water temperatures around them. One advantage that ectotherms have over mammals is that they can survive for long periods without eating.

In the fall, frogs first need to find a place to make their winter home, a living space called a hibernaculum that will protect them from weather extremes and from predators. The frog then “sleeps” away the winter by slowing down its metabolism. When spring arrives, it wakes up and leaves the hibernaculum, immediately ready for mating and eating.

Aquatic frogs and toads such as the leopard frog and American bullfrog usually hibernate underwater in streambeds or on pond bottoms. Because aquatic frogs need oxygen, they lie just above the mud, or only partially buried in the mud, so they are near the oxygen-rich water. They may even occasionally slowly swim around.

Terrestrial frogs and toads typically hibernate on land. Those frogs and toads that are good diggers like the American toads burrow deep into the soil, safely below the frost line. Other frogs, such as the wood frog and the spring peeper, aren’t good diggers and so must scout out their winter homes in deep cracks and crevices in logs or rocks, or they might dig down into the leaf litter.

These frozen peepers and wood frogs might look dead; their hearts have actually stopped beating. But the partially frozen frogs aren’t dead. Instead, they have a kind of natural anti-freeze in their bodies. Ice crystals form in their organs and body cavity, but a high concentration of glucose in the frogs’ vital organs prevents freezing. When spring approaches and its hibernaculum warms up above freezing, a frog’s frozen body will thaw, and it will come back to life.

As you go about preparing for winter, think of the frogs with their amazing adaptations for survival, safe in their winter homes, waiting for spring.

Here’s a video from YouTube about the hibernation of a wood frog. It’s pretty amazing—take a look!

This is a partial repost of an earlier post from December 2010. Most of the information from the post came from an article in Scientific American, How Do Frogs Survive the Winter? by Rick Emmer.


Back to School: Thoughts about Grow-a-Frog Kits in the Classroom

Tadpole Metamorphosizing into Frog

It’s back-to-school time again, a time when teachers may be planning their life science classes. What better way to illustrate an animal’s life cycle than by teaching kids about the remarkable transformation of a tadpole into a frog?  Unfortunately teachers may decide to use grow-a-frog kits to teach children about metamorphosis.

I have to admit I have mixed feelings about advising teachers not to raise live amphibians in the classroom. As a child, my love of animals was fostered by the various animals I kept at home, including fish and turtles. I probably would have enjoyed having a live frog in the classroom.

But the authors of an article in the Herpetological Review, “Considerations and Recommendations for Raising Live Amphibians in Classrooms,” remind us that the world is no longer a simple place: innocent acts like catching tadpoles and releasing them later into local ponds are much more complicated than they used to be.

Releasing live  frogs “grown” in the classroom into the wild can potentially harm native amphibians (even if the animals are native to the area) by possibly spreading infectious diseases, such as the deadly chytrid fungus, a disease that has wiped out entire frog populations. It may also introduce species (such as bullfrogs) that might become invasive and disrupt local amphibians.

After we wrote a few posts discouraging people from buying the Frog-o-Sphere kits, quite a few readers who owned the kits were distressed and emailed us, asking  if they should “let the frogs go.” We wrote back immediately, telling them not to release the frogs into the wild. Instead we referred them to frog care sites and books. If you’ve already purchased a frog kit, it is your responsibility to take care of the frog. Releasing it into the wild helps neither the frog nor the environment.

The problems with the classroom grow-a-frog kits are similar. After the tadpole grows into a frog, the children may become bored with it and the teacher may decide to release it into a local pond. Or at the end of the school year, the teacher may give it to a family, who then releases it somewhere in the neighborhood.  As advised in the article above: “This should never be done, and in fact, it is illegal in several states. No amphibian purchased or received from any commercial or informal (e.g., a neighbor) source may be released into the wild. This recommendation applies whether the species is technically ‘native’ to the region of release, or not.”

If you decide to purchase a grow-a-frog kit (which we hope you don’t), you will be making a commitment to take care of the frog for its natural lifespan.

How can teachers and parents teach children about the life cycle of frogs without raising live frogs? We would suggest contacting a local nature center and setting up a field trip in the spring, where the children can see eggs and tadpoles in the wild.  This past spring while hiking I frequently saw frog egg clusters in ponds and swamps, and it’s fairly easy to see tadpoles as well. A local naturalist will know where to look.

To prepare the children for what they might see in the spring, here are some resources:

The Amphibian Project : Classroom curricula, field projects and hands-on activities, fundraising ideas for students, and links.

Amphibians. 35 min. (Eyewitness DVD) Describes amphibian life cycle and anatomy; behaviors and adaptations; and amphibian characteristics. Grades 5-12

Two frog life cycle plastic sets:

For more information:
Joseph R. Medelson III et al. “Considerations and Recommendations for Raising Live Amphibians in Classrooms,” Herpetological Review, 2009.
The authors of the article above recommend this pamphlet, produced by Partners in Amphibian and Reptile Conservation (PARC) entitled “Don’t Turn it Loose.”

Spring to Life: From Tadpole to Frog

It’s finally Spring, and in our part of the world, we’re ecstatic. We’ve had a rough winter—we had snow and ice on our city streets every day for about 40 days in a row! The crocuses are blooming in my backyard, and outside the city, frogs are springing to life.

In a way, Spring is a good time to think about our “inner amphibian.” After all, mammals are descended from animals that moved from aquatic environments onto the land over three million years ago. As embryos, our heads in the early stages of development look remarkably similar to shark embryos—with gill arches and all. The metamorphosis of frogs is a process that’s not all that different from what all vertebrates go through, but the difference is that most of the development of birds, reptiles, and mammals—such as the growth of the lungs and limbs—takes place inside an egg or inside the mother instead.*


A female frog first lays eggs underwater, sometimes hundreds of eggs, which form into a jelly-like clump called frogspawn, which floats on the water. Most of these eggs become food for other pond life, but some survive.

Tadpoles developing in eggs. © Dan L. Perlman/EcoLibrary.org

The tiny animal inside the egg grows for about a month, then hatches out of the egg. It looks like a small black fish and breathes underwater with feathery gills on each side of its head.

The tadpole’s tail begins to grow; it wiggles its tail to swim. Tadpoles are also called polliwogs. (The word “polliwog” is from Middle English polwygle. Pol means “head” and wiglen means to “to wiggle”). The tadpole eats algae and other plants that grow underwater.

Tadpole. Photo from Wikipedia.


After several weeks, tadpoles begin their metamorphosis. Two tiny bumps appear near the tadpole’s tail—these will grow into back legs.

Two more bumps appear near the frog’s head—these will grow into front legs. Lungs begin to grow inside the tadpole’s body and the feathery gills disappear so that the tadpole will be able to breathe air.

The tadpole now has legs for hopping and walking, lungs for breathing air, but its long tail is awkward on land. Until the tail shrinks and is absorbed into its body, the froglet stays in or near the water.

Froglet with tail. Photo courtesy of www.scienceprojectlab.com


When the tail is gone, the frog has completed its metamorphosis. The young frog will now feed on small insects, caught with their long, sticky tongues. It will eventually move away from the pond and find a safe place to grow.

This Spring, take some time to visit a nearby pond or swamp and see this process yourself. And check out the poster below that Susan designed for Earth Day, inspired by a photo by FROGS ARE GREEN photographer friend Joe Furman. All proceeds go to help our amphibian friends.

*I found this idea in Thomas Marent’s lovely and informative book, FROG.


Wood Frogs Are (Almost) Celebrating Spring

In October, when we wrote our post Winter Turns Frogs into Frogsicles, the wood frogs and spring peepers had settled down for their long (frozen) winter nap. This blog post from The National Parks Traveler, Frogs are a Sure Sign of Spring, But that Doesn’t Mean You Won’t Hear Them Now, reminds us that even though it’s still winter (at least in the Northeast), it’s almost spring for the wood frogs. As the snow melts and the frogs unfreeze in late winter/early spring,  the young frogs have one thing on their minds: the males start calling immediately to potential mates.

I found this lovely video on YouTube by someone called Mysterious Susan (not our Susan though). It does have a mysterious quality as a reminder of the cycle of life.

Also, check out this blog post, “As Winter Wanes,”  in the East Hampton (Long Island, New York) Star about what songbirds, salamanders, and other animals are up to as we approach spring and the daylight hours get longer every day.


Winter Turns Frogs into Frogsicles

This past Sunday, my husband and I went for long hike in Harriman State Park in New York. In late March and early April, the sound of the spring peepers is deafening. But the other day we heard only one or two peepers. I did a little research to find out what happens to the peepers in the fall and winter. What exactly do they do from now until early spring?

Wood Frog

Wood Frog

Frogs and toads have evolved strategies to survive freezing temperatures. Wood frogs and spring peepers actually become a “frogsicle,” as Larry Lyons explains in his article, “All the Frogs Will Soon be Frogsicles” in the Niles (MI) Daily Star. The frogs will soon find a place under the leaf litter or in a crack in a log or rock to settle for their winter nap. They’ll slowly begin to freeze as soon as temperatures reach the freezing point. The frog’s blood stops flowing, its lungs, heart and muscles stop functioning, and ice fills the body cavity. As Niles writes, “We now have a frogsicle in suspended animation.”

About 65% of the frog is frozen. It manufactures large amounts of blood sugar that serve as anti-freeze, preventing ice damage to its organs. When spring temperatures are consistently above freezing, they begin to thaw out and break out in a chorus of frog calls (as mating season begins).

What about other frogs and toads? Toads dig a burrow under the frost line, where they go into a mild state of hibernation. Their metabolism slows down and they no longer need food or water. Aquatic frogs such as green frogs go into what’s called a state of torpor. They descend to oxygen rich deep water, find a hiding place, and don’t move around much until the spring comes.

Perhaps we are more like animals than we care to admit. I know I slow down in late fall and hibernate until spring!


Frog Moon Dance

Well, its a marvelous night for a moondance
With the stars up above in your eyes
A fantabulous night to make romance….

The BBC Earth News reported today that biologist Rachel Grant of the Open University has discovered amphibians around the world synchronize their mating activity by the full moon. The animals use the lunar cycle to coordinate their gatherings, to ensure that enough males and females come together at the same time.

Grant made her discovery while studying salamanders in Italy. She noticed toads all over the road during the full moon. She then collated her data with a 10-year analysis of the mating habits of frogs and toads at a pond near Oxford, England, and with data on toads and newts in Wales collected by colleague Elizabeth Chadwick from Cardiff University, Wales.

This knowledge gives researchers practical information–for example, they will know precisely when to close roads to avoid killing frogs and toads during the mating season. It’s also useful information for FrogWatchers.

Now enjoy this video and imagine a romance—an amphibian romance, that is.