Frog Hibernation: How Frogs Survive Winter

I learned about how frogs survive winter hands-on from my outdoor pet toad. Frogs and toads can survive the harsh winter months and emerge in the Spring, nutritionally deprived but otherwise unscathed. But the question most people wonder is how these cute, squishy, fragile creatures endure the extreme cold and lack of resources that come with the trials of Winter? The answer is: hibernation.

Frogs and toads survive Winter by hibernating. Generally, both species bulk up during the year and in the Fall, toads burrow underground, and aquatic frogs head below freezing water. During Winter, their metabolism and heart rate slow down while they live off stored body reserves until the Spring.

While it might sound easy enough in theory, hibernation is far from a simple winter-long nap. It takes significant preparation and unique metabolic adaptations that make this process biologically possible for frogs to survive harsh winters. 

This article will discuss everything there is to know about frog and toad hibernation, how you can help your local frogs and toads hibernate as well as amazing facts about how these creatures survive Winter.

Quick Frog And Toad Hibernation Facts

Before we dive into how frogs and toads hibernate, here is a quick overview of how they survive winter depending on the type of frog:

Type of FrogEnvironmentFreeze TolerantMethodLocations
Aquatic FrogsWaterNoLayBelow Freezing Water
ToadsLandNoBurrowIn Soil Below Frost Line
Tree FrogsLandYesHideUnder Leaf Litter

Here are some frog and toad hibernation facts:

  • Toads hibernate on land below the frost line which is about 1 meter (3 feet) underground
  • Toads may hibernate in groups located in “communal hibernation sites”
  • Aquatic Frogs hibernate below freezing water
  • Tree Frogs hibernate under leaf litter
  • Tree Frogs can be cooled down to -7°C or 19°F for weeks and survive
  • Some tree frogs can stay frozen for eight months up to a year
  • Up to 65% of tree frog’s total body water freezes in Winter

Frogs Prepare to Hibernate All Year

Hibernation is an extensive process that many animals go through in order to have the best chance of survival during the winter months. This is why frogs have the physiological and behavioral adaptation of hibernation. A frog that does not properly prepare before the onset of winter will not survive until the Spring. This is a cruel but true fact of the animal kingdom.

Much of a hibernation’s success relies on the frog obtaining enough life-sustaining nutrients and choosing the proper location to hibernate. Without taking these key steps, they are doomed to face the harshness of Winter. 

The first step of preparing for hibernation is knowing when the preparation process must start. Frogs instinctively know what to do depending on the time of year. In the Spring, directly following their hibernation, frogs generally focus on mating, which typically occurs in the Spring in North America. After that, most of their time is spent avoiding being eaten and eating to build reserves to hibernate.

Let’s discover the complete hibernation process from bulking up all year to finding the ideal place to hibernate, to emerging from hibernation in the Spring.

Frogs Bulk Up to Create Reserves For Hibernation

When the mating season is over, frogs generally use most of their energy on stocking up nutrients necessary for survival as a food reserve during hibernation. As carnivores, most frogs feed on insects and invertebrates, such as insect larvae, spiders, slugs, and worms.

Frogs and toads generally eat anything that is moving and can fit into their mouths. The U.S. Department of Agriculture estimates that a single adult toad can eat 10,000 insect pests over the course of an average summer in order to have the nutrient storage necessary for Winter. 

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I used to watch my skinny toad emerge from hibernation in the Spring, and the same very large, round, fat toad burrow to hibernate in the Fall. My toad spent all year eating more calories than his body needed to bulk up for the Winter.

As the cooler months of September and October approach, frogs generally switch to focus on finding the perfect hibernation location.

Where Frogs Hibernate During The Winter

Where frogs hibernate during the winter generally depends on the type of frog. Aquatic frogs hibernate underwater, toads hibernate underground and tree frogs hibernate on land under leaf litter. Frogs in temperate climates do not hibernate, but instead, they estivate during the dry season.

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Aquatic Frogs Hibernate Underwater

Leopard Frogs, Green Frog, Mink Frog, Pickerel Frogs, Bullfrogs, and their tadpoles hibernate below freezing water. Aquatic frogs generally sit, float, or slowly swim at the bottom of ponds. Oxygen levels are high below 4°C or 39°F and provide a great environment for them to hibernate during Winter.

As their name indicates, aquatic frogs need to be in the water in order to thrive and survive. So there is not a better place for them to spend winter than underwater. Aquatic frogs can breathe through their skin and enjoy the high oxygen levels of cold water.

Once the frozen water warms up, the frogs become active again and continue their growth if they were tadpoles or pollywogs. Frogs do not burrow in the mud like toads because they require direct contact with water to live.

Toads Hibernate on Land

Toads hibernate on land, generally 1 meter or 3 feet below the frost line. Toads hibernate in rodent tunnels, natural crevices, under rocks, and prefer soft sand to create their own burrows. Toads have been found to burrow in groups in communal hibernation sites if locations are scarce.

One common misconception about toads is that they hibernate near water sources. Toads are less aquatic than frogs and aren’t typically found in water past their tadpole stages. Instead,  toads dig a deep hole or burrow underground to spend the Winter. I used to watch my toad do this every Winter and it was a cool sight to see.

One study found that 38% of toads hibernate in rodent burrows, 27% under large rocks, 19% under logs or root wads, and 15% under banks adjacent to streams or a lake (Bull, 2006). Another study (Constance 2010) found that toads like to burrow in the following places:

  • Peat hummocks
  • Red Squirrel middens
  • Cavities under spruce trees
  • Decayed root tunnels
  • Natural crevice systems
  • Abandoned beaver lodges
  • Common Muskrat tunnels
  • Spruce-dominated tree stands

Toads search for a location with loose soil they can easily dig at least 1 meter or 3 feet deep. This ensures they hibernate below the frost line to avoid most of winter’s cold temperatures and keep warm. Toad’s feet are made to dig and this task is fairly easy for them.

While most toads are solitary hibernators, there are some instances where toads will actually hibernate in groups if sufficient hibernation locations are scarce. Many will also skip the process of digging entirely and use old mammal shelters or crevices beneath trees and rocks as hibernacula instead.  

If they are unwise with their selection, toads could inadvertently expose themselves to the winter’s chill or risk being prematurely disturbed from the dormant state by outside forces. Toads are not freeze-tolerant and have been reported to die at temperatures between –1.5 to –5.2 °C or 29°F to 23°F (Swanson et al. 1996). Toads generally do not choose their hibernation location until they are ready to enter hibernation, and studies have shown that many actually return to the same hibernation locations annually.

Tree Frogs Hibernate in Leaf Litter And Freeze Solid

Tree frogs like Gray Treefrogs, Cope’s Gray Treefrog, Boreal Chorus Frog, Spring Peeper, and Wood Frogs hibernate in cracks and under leaf litter at the foot of trees to survive Winter. Tree Frogs generally freeze between 40% to 65% of their body’s water content during Winter.

Tadpoles transforming to frogs is remarkable. But nothing is as impressive as a frozen frog whose heart and lungs stop coming back to life as if nothing happens. The steps below highlight the process tree frogs undergo to survive freezing in winter:

  • At the beginning of winter, ice fills the frog’s abdominal cavity and incases its internal organs. The ice crystals form between layers of skin and muscle. At this point, the eyes will also turn white because the lens freezes as well.
  • When the ice crystals are forming, the frog’s liver starts producing large amounts of glucose that flushes to all cells in the body. The elevated blood sugar levels prevent the cells from freezing and bind water molecules inside the cells to prevent dehydration.
  • When the entire process is complete, the frog has no muscle movement, heartbeat, or breathing. For the entire winter, the frog remains a hard lump in the shape of a frog, but it’s still alive, only in a state of suspended animation.
  • In Spring, the frog begins thawing from the inside out. First, the heart will start beating again. The brain will then be reactivated, and finally, the legs will move.

When a frog freezes solid, it will stop breathing, its heart will stop, and the frog will appear dead, but it’s not. You see, when the frog freezes during a period of hibernation, its liver produces large amounts of glucose, thereby increasing blood sugar levels. The elevated blood sugar acts as “antifreeze,” which prevents the formation of ice crystals in the body.

Frogs also have a cool trick to eliminate water from their organs. When they begin freezing, the water in their organs and forms ice outside their body. Up to 65% of the frog’s total body water will freeze during winter. This factor, combined with the “antifreeze,” mechanism protects the frogs’ organs from freezing.

Tree frogs may die if they emerge from hibernation too soon. This often happens when a frog is tempted out of hibernation by a short warm stretch that quickly goes back to temperatures below freezing – a term referred to as “winter kill.” However, if the temperatures remain steady and the seasons start changing, the frog slowly thaws out, the hearts and lungs start working again, and the frog resumes normal activities unaffected.

Frogs in Hot Climates Estivate Instead of Hibernate

Although hibernation is common for frogs during cold Winters in North America and parts of Europe, not all frogs hibernate if they live in consistently warm environments like those in Asia, South America, Africa or Australia. Instead, such frogs may estivate in response to hot or arid conditions.

Estivation is a period of dormancy similar to hibernation but in response to different conditions. Instead of getting out of the cold, frogs that estivate escape the heat. Hot or arid conditions are not ideal for frogs that are cold-blooded and require water and humidity to survive, so estivation is key to survival for frogs in hotter climates depending on the Season.

For example, frogs in temperate climates like Horned Frogs in South America, or African Bullfrogs in parts of Africa burrow to stay hydrated during the Dry season and come back out during Monsoon or Rainy season. Some frogs burrow and form a cocoon of their dead skin around their body to retain humidity. 

The Frog Hibernation Process Starts in The Fall

Once a frog is happy with its location to spend the winter, it will tuck in its limbs, bow its head, and enter a state of hibernation. In order to do this, frogs undergo multiple physiological changes:

  • Thermal Dormancy: Frogs generally biologically operate at very low core body temperatures
  • Behavioral Suppression: Any muscle activity ceases
  • Metabolic Inhibition: The frog’s  metabolic rate slows down 
  • Slower Heart Rate: The frog’s heart rate slows down, some will have no heart rate for long periods of time
  • Production of Glycerol: Frogs produce glycerol that acts as an antifreeze in their bodies

Once all of these changes are complete, the frog will slowly live off of the nutrients it stored in prior months until environmental temperatures increase.

Frogs Emerge From Hibernation in The Spring

Frogs instinctively know when it is time to emerge from hibernation when the soil or environment around them increases to safe temperatures. This usually occurs from March to May in Canada and in the Northern States of the USA.

Because toads do not technically freeze like most frogs equipped with high concentrations of glucose acting as antifreeze, the process of emerging from hibernation is generally quicker for them. They get to skip the  “thawing” process and simply increase their heart rate, metabolic functions and emerge.

Help Your Local Toads Hibernate in The Winter

A hibernaculum, hibernacula, or hibernating is a place where animals, such as toads, can safely spend the winter. You can easily create a hibernaculum to help local toads survive winter in your yard and garden by creating the ideal habitat for them to spend the cold season.

Here is how to create a toad hibernaculum in your yard:

  1. Chose a safe location in your yard
  2. Dig a hole that is at least 1 meter or 3 feet deep
  3. Fill the hole with soft sand
  4. Cover the hole with leaf litter and compost soil

You could also create a toad hibernaculum in a window well if it has the correct environment. In that article, I explain what I did for my toad in a window well. I got to watch it emerge, prepare and head into hibernation every year.

What to do if You Find a Hibernating Frog

If you’re wandering around a winter wonderland and come across what looks to be a frog popsicle, you might wonder what you can do for this animal to help ensure its survival. If you find a frog or a toad hibernating, leave it alone. Do not touch it, pick it up, or relocate it as this generally leads to health issues, premature emergence from hibernation, or the frog’s death.

Relocating a hibernating frog could potentially kill it. If you do find an exposed hibernating tree frog or toad, simply cover it with additional natural sources of heat. Forest debris, such as leaves, bark, and twigs, are great for heat retention and might be enough to keep the toad warm until winter ends. If you find a hibernating frog do not intervene unless absolutely necessary. These creatures have existed for over 200 million years; they can survive without your intervention. 

More About Frog Hibernation

If you were wondering how frogs survive the winter, now you know. These creatures burrow into the sand, hide under leaves, or spend time underwater. Hibernation is essential for frogs to survive the winter months, and luckily for this species, it is a process they do naturally every year. By consuming a sufficient storage level of nutrients and finding a safe hibernaculum, frogs can enter a state of hibernation until the warmer spring months arrive. Although frogs seem like fragile creatures, they’re extremely resilient. 

Learn even more about toads and their hibernation process in these dedicated articles on our blog:

Questions Related to Frog Hibernation

Do Frogs Die In the Cold? Frogs generally do not die in the cold because they hibernate. Tree frogs and aquatic frogs can freeze between 40% to 65% of their bodies’ water content and survive. However, toads are not freeze-tolerant and have been reported to die at temperatures between –1.5°C to –5.2°C (29°F to 23°F).

Do Toads Die In the Cold? Toads are not freeze-tolerant and have been reported to die at temperatures between –1.5 to –5.2 °C (29°F to 23°F). Therefore, toads may die if they are not hibernating far enough below the frost line, or if they are caught off guard below freezing temperatures.

How Long Can a Frog Survive Being Frozen? Some tree frogs like Wood Frogs can survive being frozen for up to 8 months. However, toads are not freeze-tolerant and generally die if exposed to temperatures below –1.5°C to –5.2°C (29°F to 23°F).

How do Wood Frogs Survive Freezing? Wood Frogs can survive being frozen for up to 8 months. 

What Do Frogs Look Like When They Hibernate? A hibernating frog may look dead when it has not yet emerged from hibernation. It may have its limbs wrapped to the sides of its body and have closed or glazed over eyes. However, the frog will naturally defrost and emerge from hibernation when ready.

Sources

Browne, Constance L., Paszkowski, Cynthia A., Herpetological Conservation And Biology 5(1):49-63. Hibernation Sites Of Western Toads (Anaxyrus Boreas): Characterization And Management Implications. Department Of Biological Sciences, University Of Alberta. February 2010

Schmid, WD. Survival of frogs in low temperature, Science  05 Feb 1982: Vol. 215, Issue 4533, pp. 697-698 DOI: 10.1126/science.7058335

Berman, D.I., Bulakhova, N.A. & Meshcheryakova, E.N. The Siberian wood frog survives for months underwater without oxygen. Sci Rep 9, 13594 (2019). https://doi.org/10.1038/s41598-018-31974-6

Bull, Evelyn L. 2006. Sexual differences in the ecology and habitat selection of western toads (Bufo boreas) in northeastern Oregon. Herpetological Conservation and Biology. Vol. 1(1): 27-38

Swanson, D.L., B.D. Graves, and K.L. Koster. 1996. Freezing tolerance/intolerance and cryoprotection synthesis in terrestrially overwintering anurans in the Great Plains, USA. Journal of Comparative Physiology B 166:110–119

Open EDU, Animals at the Extremes, Hibernation and Torpor

National Park Service, Biological Miracle