Wood frogs (Lithobates sylvaticus) naturally occur in Canada and the northern and eastern parts of the United States, even in the cold region of Alaska, where they have to survive during harsh winters. It is one of the very few amphibians that has been found living above the Arctic circle. Mind you, these are ectothermic animals, meaning they cannot generate their own body heat and therefore rely on environmental conditions to regulate their body temperature. So, if they are to survive during the cold winters of Alaska, they probably should have a trick up their sleeve. And they do, because these animals are known to go into hibernation for several months and during that period are able to freeze up to 60% of their body. And then when the warm spring sun returns, slowly but surely their vital functions will return as well, one heartbeat at a time. Remember this whenever you tell someone: “I am freezing”. Because these animals can stay literally frozen for at least six months. They should be dead, but evolution has decided otherwise.
A frozen Wood Frog (Lithobates sylvaticus) in its natural habitat.
Normally, cells of living tissue will dry out when temperatures dive below zero, because the formation of ice in tissues draws out the water from the cells. The key to survival for these frogs is the glucose that is stored in their cells and prevents this drying out or desiccation. This is known as cryoprotection and occurs in some northern salamanders as well. Although they have been studied well for this peculiar physiological feature, only recently the first scientist, Don Larson, a graduate student at the University of Alaska Fairbanks, has started looking into the freezing behavior in the wild (and in experimental conditions). He discovered that before entering hibernation, the frogs have experienced several freezing and thawing cycles that enable them to release and store more glucose than if they would not had gone through these cycles. An article in Phys.org highlighted his discoveries (here is also a video available featuring these cute wood frogs).
When the Alaskan winter is coming and temperatures are plummeting, these wood frogs look for a spot on the ground and cover themselves with leaf litter and duff, creating a hibernacula. They will not emerge until at least six months later. In an Alaskan winter temperatures may go down to -20°C and still these frogs will be up and hopping about in the next summer. So Don Larson and co-author Brian Barnes, director of the UAF Institute of Arctic Biology, wanted to see how cold it could get and for how long before these creatures would succumb to the cold. They did so by tracking wood frogs to their natural hibernacula, and using a fenced hibernacula in the Biological Reserve north of the UAF campus. They discovered that wood frogs hibernating in their natural habitat have higher concentrations of glucose in their tissues than frogs hibernating in the laboratory. The importance of glucose is explained by Barnes: “Concentrating sugar inside the cell helps balance the concentration of salts outside the cell that occurs as ice forms. Less water leaves the cell than if sugar was not present and sugar and other cryoprotectants are thought to “hold” water inside the cell”. This is vital for their survival during the winter.
A juvenile Wood Frog (Lithobates sylvaticus)
The frogs in the lab were kept at a constant temperature below zero, whereas the frogs in the field experienced in early autumn freezing during the night and thawing during the day. It seems these freezing and thawing cycles offer the explanation. “In the field in early autumn it’s freezing during the night, thawing slightly during the day, and these repeated freezing episodes stimulate the frogs to release more and more glucose,” Larson said. “It’s not warm enough for long enough for the frog to reclaim much of that glucose and over time it accumulates giving the frog more protection against cell damage”.
This leads to significantly more glucose in their tissues and may well be the reason why these frogs in Alaska can last up to 218 days being frozen at temperatures going as low as -20°C. Frogs collected from other sites in the eastern United States and Canada have been found to only survive in a frozen state for a few weeks and at temperatures no lower than about -7°C. The freezing episodes might be absent over there and prevent the accumulation of glucose. “Whether the extremes in freezing tolerance in Alaska frogs as compared to more southern populations are due to patterns of temperature change during freezing or are due to genetic differences, and thereby represent evolutionary change, awaits further study,” said Barnes.
An Alaska Wood Frog (Lithobates sylvaticus) creates a hibernacula from duff and leaf litter in a spruce forest on the University of Alaska Fairbanks campus in preparation for the long winter freeze. Credit: Uwe Anders
This study primarily has been started to unravel the mechanisms by which these wood frogs are able to survive a frozen state during the winter. However, this is an adaptation that might prove to be useful for humans too, as we have seen in other cases already. One day these wood frogs might help us perfecting human organ transplantation. “If science can figure out how to freeze human organs without damage it would allow more time to reach people in need of organs,” said Larson.