Wood frogs produce glucose to survive freezing during hibernation. When ice begins forming in their body, stored liver glycogen is rapidly converted into glucose, which acts as a cryoprotectant.
The glucose helps protect cells in two important ways. It limits the amount of ice that forms inside tissues and reduces the damaging effects of cellular dehydration and shrinkage. Ice forms mainly outside the cells, while the concentrated glucose helps keep the cells from losing too much water.
Wood frogs are extraordinary because they can spend winter partly frozen. Their heartbeat, breathing, blood circulation, and detectable brain activity may stop during freezing, yet they can recover when temperatures rise. Some northern populations tolerate freezing of roughly 65–70% of their total body water.
Urea is also present and contributes to freeze tolerance, which explains why it sometimes appears as an alternative in discussions of wood-frog physiology. However, glucose is the characteristic answer to this quiz question because it is released in large quantities as the frog freezes. The frog may remain frozen for around six months in natural conditions before thawing and returning to activity in spring.