Time and again, whenever wood frogs have faced freezing winters, natural selection has favoured individuals capable of surviving temperatures that would be fatal to many other animals. Yet survival in these conditions does not depend simply on avoiding the cold. Wood frogs can survive even when much of the water in their bodies freezes. Their hearts stop beating, and breathing temporarily ceases, but they can recover when temperatures rise. Researchers have discovered that these frogs possess a remarkable range of adaptations that allow them to endure such extreme conditions.
The process begins when ice forms in the spaces between the frog's cells. This draws water out of the cells, creating a risk of dehydration and damage. The frog's liver responds by releasing large quantities of glucose into the bloodstream. Glucose acts as a natural protective substance, helping prevent the cells from losing too much water and reducing damage caused by freezing. The process may appear inefficient: the frog must produce and distribute large quantities of glucose before it can survive the winter. However, researchers believe that this response is closely connected to other processes in the frog's body. The mechanisms that regulate metabolism and prepare the animal for freezing operate together, making the production of glucose an essential part of the overall survival strategy.
It is easy to understand why wood frogs would be at a disadvantage if they could not protect their cells from freezing. During winter, they may remain inactive for extended periods, with very limited opportunities to obtain food. Their bodies must therefore manage stored energy carefully. Researchers have found that wood frogs can tolerate substantial changes in their metabolism during freezing and can restore normal bodily functions when they thaw. This ability is particularly important because the frogs may experience repeated freezing and thawing as temperatures fluctuate.
Yet freezing also creates a separate problem. When cells are exposed to severe stress, harmful chemical reactions can damage their structures. In many animals, such damage can trigger inflammation, which helps the body respond to injury but can itself become harmful when excessive. Studies suggest that wood frogs possess mechanisms that limit the damage associated with freezing and thawing. Researchers have therefore investigated whether the glucose released during freezing serves only as protection against dehydration or whether it also contributes to reducing cellular damage.
One possibility was that frogs with higher glucose levels survived freezing simply because they were larger and had greater energy reserves. To investigate this, scientists compared frogs of different sizes and examined their glucose levels and survival under controlled freezing conditions. The results suggested that size alone could not explain the pattern. Some smaller frogs with relatively high glucose levels survived, while some larger frogs with lower glucose levels did not. Researchers concluded that glucose plays a protective role during freezing, although other physiological mechanisms are also likely to contribute to survival. The findings suggest that this adaptation is not merely a consequence of having more stored energy but is part of a specialised response to extreme cold.