Part 2: How do Frogs Survive the Winter?

Hibernating Wood Frog Photo by: Travis Wiens

Hibernating Wood Frog Photo by: Travis Wiens

Hibernation is just another fact of life for Ontario’s reptiles and amphibians. Both the shorter days and colder temperatures are cues to our animals to initiate the start of a long winter sleep. For both reptiles and amphibians, a safe site that protects them from winter weather and predators is called a hibernaculum. It’s here that a frog’s metabolism will slow down as their body prepares for the winter. Frogs are cold blooded or “ectotherms”. Ectotherms are animals that have little to no ability to produce internal body heat. Heat drives life’s physiological functions such as the breakdown of food into energy.  To metabolize food, ectotherms rely on their surrounding environment to heat up their body, such as warmth from the sun or warmth from the water. Ectotherms in cold climates have evolved to slow down everything, even their need to breathe, when it gets cold. When spring weather arrives, the animal’s metabolism revs up to get on with the business of feeding and breeding.

Frogs in Ontario such as, the leopard frog and bullfrog, typically hibernate underwater. All amphibians including frogs have porous or absorbent skin that intakes oxygen and water. When a frog’s metabolism is slowed down they don’t need as much oxygen. Frogs will choose a hibernaculum that has a sufficient amount of oxygen trapped in the water, which will be slowly absorbed by the frog’s skin throughout the winter. It’s a common misconception that frogs will hibernate deep in the mud like turtles to insulate their bodies from the cold. In fact, hibernating frogs would suffocate if they dug into the mud for an extended period of time. Frogs need to be exposed to oxygen-rich, clean water when hibernating, unlike turtles, whose metabolism slows down to such low levels that they barely need any oxygen all winter. If you’re lucky, you can see frogs in the winter, under clear ice lying just on top of the mud, partially buried or even slowly swimming around from time to time.

Highly terrestrial frogs, like the American toad, will burrow deep into the soil on land, but safely below the frost line. Wood frogs and Spring peepers are known to seek out deep cracks and crevices in logs or rocks, or just under the leaf litter to compensate for their lack of digging abilities. These terrestrial hibernators can run the risk of freezing if they choose a poor hibernation site. When normal cells freeze, ice crystals form inside the cells. These expanding crystals cause the cell walls to burst open resulting in irreversible damage.

So how do frogs survive?…Sugary Cell Antifreeze! High concentration of glucose in the frog’s vital organs prevents freezing. A partially frozen frog will stop breathing, and its heart will stop beating, but when warmed up, the frog’s heart and lungs resume activity—talk about the living dead!

The wood frog inhabits forests ranging across Canada and even into the Arctic Circle. These little guys are subjected to several freezing episodes that typically last several days. Laboratory studies have shown that wood frogs can survive freezing of up to 65-70% of their body water, a minimum body temperature of -6°with a time period of uninterrupted freezing for ≥ 4 weeks.

For more information about frozen wood frogs click HERE for an interesting article done by Miami University’s Department of Zoology.