The wood frog’s activity is limited to a few short summer months from May to September before they burrow into their hibernacula under the leaves and snow. There are four common North American hibernating frogs. All tolerate being frozen with extensive ice formation in the body cavity and in the spaces between the cells but there are no ice crystals which form within the cells. The frogs use alcohols and sugars to protect the tissues when freezing.
In one day they can change themselves to become frost-tolerant. Many insects, which use similar techniques, actually perfuse their tissues prior to freezing in a sort of cold-hardening process as they accumulate the alcohols and sugars in anticipation of freezing.
In wood frogs, when the ice actually begins to form, even in a tiny amount on the skin, the hormone adrenaline is triggered to be released from the adrenal glands in a massive amount. Similar to the fight or flight response in all animals (including humans) the adrenaline is released and the immediate response in the frog is increased heart rate, rise in blood glucose concentrations and a redistribution of blood flow to the muscles. When the adrenaline circulates to the liver, enzymes there change the liver’s stores of glycogen to glucose. Consequently, a quick rise in the frog’s blood glucose acts like antifreeze within the cells.
Amazingly, it is just the opposite that is happening outside the cell. Glucose not only acts as a protectant from ice crystals in the cells but it also helps draw water from the cells, displacing the water outside the cells. Special proteins act as ice-nucleating agents to actually promote ice crystal formation outside the cells in the fluid, which osmotically causes withdraw of water from within the cells. Within about 15 hours the frog is basically frozen solid except for the insides of the cells. The frog’s heart and breathing stop and they are then metabolically inactive, therefore, the lack of oxygen is not harmful.
This magical chemistry feat is another testament to the ingenuity of animals that survive in a world nearly incomprehensible to humans and our limited physical adaptations.
