A common frog species takes an unusual approach to winter—one that would kill most other vertebrates.

About This Video

Grade level: 6-13+
Length: 5 minutes
Next Generation Science Standards: LS1.A, LS4.C


The common wood frog (Rana sylvatica) has an unusual strategy for coping with the cold. While most terrestrial frogs and toads burrow deep underground to escape freezing temperatures when they hibernate, the wood frog spends its winters at the surface. The frog tucks itself into shallow depressions, covered only by a blanket of leaf litter. There it undergoes a transformation that would kill most other vertebrates: It freezes solid.

The wood frog possesses a chemical secret that enables it to survive this seemingly traumatic experience. As temperatures plummet, a complex sugar called glycogen circulates throughout the frog’s body. This syrupy fluid prevents ice crystals from forming inside the wood frog’s cells, and keeps them from rupturing when the frog’s internal temperature drops below freezing. By mid-winter, it’s as if the frog is clinically dead. Its heart has stopped beating; its brain activity has ceased; and its body is completely frozen.

When spring arrives and temperatures rise, the frog begins to thaw from the inside out. In a matter of a day or two, the wood frog is able to leap back to life—well before its hibernating neighbors have emerged. This head start gives the species an advantage, providing the wood frog its pick of sites in which to lay its eggs.

Video Discussion Questions

  1. What unique adaptation have common wood frogs evolved to survive freezing temperatures?
  2. What happens physiologically to a wood frog when it comes in contact with ice in its environment?
  3. How might waking up from hibernation before other frogs be advantageous?
  4. Using what you learned from the video, come up with a hypothesis for why salt is put on icy roadways in places that experience below-freezing temperatures in the winter.

Classroom Activities to Accompany This Video


Biomimicry and Biomedicine

NIH scientists in training

Introduction: We can learn a lot about successful design from successful adaptations in nature.  Researchers are trying to find ways to use the wood frog's adaptation for medical applications, specifically for preserving human organs for transplantation.  Currently, organs like hearts, kidneys, and livers must be transplanted within hours after they're harvested.  But if we could find a way to freeze the organs without damaging the tissue or otherwise preserve them for longer before they are transplanted, the availability of viable organs would increase.

Design Challenge Question: How can we keep organs 'fresh' for the longest amount of time?

In this design challenge, students will work in groups of 3-4 to design and test a way to keep an animal organ 'fresh' for the longest period of time.  Each group will be provided with a portion of an animal organ, such as a chicken liver, chicken or cow heart, or cow kidney. Animal organs can be bought in neighborhood or specialty grocery stores.  Divide the organ(s) into equally-sized pieces to distribute to the groups.

  • It is up to you what materials you would like to provide to your students for their designs, but any substances you provide should either be nontoxic (e.g., sugar, ice, salt, water, common food preservatives) or your students should be trained in the proper safety procedures surrounding the handling of them.  

Part 1: 60 minutes

1. After watching and discussing the Return of the Wood Frog video with your students, introduce the concept of biomimicry, or the idea that we can take inspiration from successful adaptations in nature to design solutions in business, engineering, and other fields—here are some examples.

2. Introduce students to Introduction and Design Challenge Question listed above. Each group will be provided with a portion of an animal organ and will be designing a method for keeping it fresh for as long as possible.

3. As a class, brainstorm a list of criteria that could be used to measure organ 'freshness'.  This could include parameters like the organ's color, texture, observations of the organ's physical appearance under a dissecting scope, etc. Circle the criteria that you have the capability to measure in the classroom.

4. Briefly showcase the materials on-hand for the design challenge.

5. Give groups 10-15 minutes to rapidly brainstorm as many possible designs for an organ preservation method as they can. 

6. Then, ask groups to choose one design that they will test and that is plausible given the materials available to them.  Design considerations should include both what substances they will use to keep their organ fresh as well as how they will store their organ (e.g., in a mini cooler). Organs should be stored away from consumable food and drink.

Part 2: 3-4 class periods + daily observations

1. Give your students 1-2 class periods to build and implement their designs.

2. Each day, students should make observations about the freshness of their organs in their science notebooks.

3. After either a specified amount of time (e.g., one week), or after the last organ fails to meet the freshness criteria, have students create a scientific poster summarizing their experience. 

4. Hold a Research Symposium in your classroom for students to share their posters with one another.  Have a class discussion about the various designs and their strengths and weaknesses. 

5. Either as a whole class or in the same small groups, have students iterate on one of the designs shared in the Research Symposium and repeat the test.  What about the design do you want to change and why?  What do you expect to be different about the results based on the changes you plan on making?

6. As a reflection on their experience in this design challenge, ask students to fill in a blank Engineering Design Process diagram to document their process.

Students should follow proper lab safety procedures when handling organs and any other substances.  Make sure to properly dispose of all organs after the end of the design challenge.


Photo credit: NIH


Additional Resources

Antarctic Icefish Adaptations
The wood frog isn't the only animal that has adapted to extreme cold. Learn how the Antarctic icefish has evolved antifreeze proteins in their blood to survive in ice-covered waters.



About the Contributors

Annette Heist is a science writer, radio producer, and a registered nurse working in behavioral health. Ruth Lichtman is a multi-disciplinary visual artist and filmmaker whose work has been featured on The New York Times, The Atlantic, Aeon, and The Huffington Post. Flora Lichtman is a science journalist who has worked for “Bill Nye Saves the World” on Netflix, The New York Times, and Science Friday. She hosts a podcast called Every Little Thing.

This video was produced for bioGraphic, a magazine powered by the California Academy of Sciences to showcase both the wonder of nature and the most promising approaches to sustaining life on Earth.


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