ANSWER:

Frogs do hibernate.

More Info: In order to avoid severe temperatures, frog species employ one of two strategies: freeze avoidance or freeze tolerance.  Freeze avoidance species can tolerate low temperatures, but they will die if they freeze.  Freeze tolerance species undergo reversible extracellular freezing.

Hibernation Tactics: Freeze Avoidance

As an ectotherm, the frog cannot self-regulate its body temperature and therefore depends on external forces to do so.  When the temperatures begin to fall, so does a frog’s body temperature.  The frog’s metabolic rate will begin to slow in response to cold temperatures, and it must find a place to wait out the winter before it freezes.

Aquatic Species

Many frog species choose to remain in the water during hibernation.  They will lightly cover themselves with mud at the bottom of the pond to escape being seen by predators.

But if a frog breathes through lungs and can drown if too much water enters the lungs, how can they survive extended periods underwater?  The state of torpor that hibernation creates requires less oxygen intake so they only need to periodically replenish oxygen reserves. If they are unable to get air such as in instances where a pond has frozen over, they have the ability to breathe through a process called cutaneous respiration.  Their skin is highly vascularized, and when moist, allows oxygen to diffuse easily.  The frogs only bury themselves enough to remain hidden but not enough to cover the skin and block respiration as the mud holds very little oxygen.  [1]

They must also choose their hibernaculum carefully.  For example, frogs that choose a stagnant pond or a swimming pool are at a higher risk of freezing or drowning than frogs that choose fast flowing water sources.  This is because stagnant water carries less oxygen than flowing water having the potential to become anoxic or hypoxic.  Stagnant water will also freeze over more readily giving the frog no way to get fresh oxygen. [2]

Terrestrial

As the name implies, terrestrial frogs will normally hibernate on land.  Terrestrial frogs must find hibernaculum that will not freeze such hollowed tree roots or natural crevices that are far below the frost line.

 

Hibernation Tactics: Freeze Tolerance

There are several species of freeze-resistant frogs, most notable the wood frog. So how do these species differ when it comes to hibernation? Rather than hiding from frost, freeze-resistant frogs can actually ‘freeze’ during the winter, and thaw out in the warmer weather.

Freeze-tolerant frog species do not need to hibernate far underground to avoid frost.  In fact, they simply find a log, rock, or any leaf litter to hide beneath and allow the temperatures to transform their bodies.

When temperatures drop and freezing begins, water is drawn out of the cells of the frog’s vital organs into extracellular spaces such as between the skin and muscles.  Here the liquid can crystallize without damaging the organs. [3] As soon as ice crystals begin to form in the peripheral tissues, this triggers liver glycogenolysis, which produces high concentrations of glucose that will function as a cryoprotectant to be distributed to the vital organs, such as the heart and brain tissue, through the bloodstream. Breathing, heartbeat, and most other vital bodily functions will then cease.  This entire process usually occurs in the first 24 hours after freezing begins. [4]

The freezing process must happen slowly while the body temperature is still high in order to allow the water to leave the vital organs before freezing. If freezing happens too quickly, the water in the organs will freeze, shut down, and the toad will freeze to death.

Physical Characteristics of a Hibernating Frog

A frog in hibernation looks physically different from an active frog.  First, its stance alters when it enters hibernation.  An active frog that sits upright with its legs tucked in by its sides will suddenly appear more sprawled and flattened during hibernation.  This hibernating stance is called a four-point-stance.  Next, during hibernation the frog’s eyes are covered with a nictitating membrane and pulled in closer to the head to protect them from debris.  And finally, the skin appears reddish due to the blood flow to the skin that is needed to facilitate its cutaneous respiration. [5]

 

Resources

[1][5] Henderson State University
How a Frog Hibernates
https://www.hsu.edu/pictures.aspx?id=1263

[2][3] Kentwood D. Wells
The Ecology and Behavior of Amphibians
Chicago: University of Chicago Press, 2007. Print.

[4] Journal of Climate Research; Jack R. Layne, Jr
Adaptations of frogs to survive freezing
1995; Volume: 5; Pages: 53-59
http://www.int-res.com/articles/cr/5/c005p053.pdf

EPA
Wood Frogs: Species Profile
http://www.epa.gov/region1/ge/thesite/restofriver/reports/final_era/B%20-%20Focus%20Species%20Profiles/EcoRiskProfile_wood_frog.pdf

Feder, Martin E..
Environmental physiology of the amphibians
Chicago [etc.: University of Chicago Press, 1992. Print.
http://books.google.com/books?id=oaS-OpEjPtUC&pg=PA264&dq=frogs+come+out+of+hibernation&hl=en&sa=X&ei=7cb_UfP7LrPc4AOmxoCgAg&ved=0CE8Q6AEwBA#v=onepage&q=frogs%20come%20out%20of%20hibernation&f=false

Annu. Rev. Ecol. Syst. Kenneth B. Storey and Janet M. Storey
NATURAL FREEZING SURVIVAL IN ANIMALS
1996. 27:365
http://http-server.carleton.ca/~kbstorey/pdf/322.pdf