Frogs are amphibians that are capable of living on land and under water. There are also tree frogs which spend most of their time high in the treetops. They have sucking disks on the tip of their toes that enable them to climb vertical tree trunks. They may have a smooth skin or rough and irregular skins.
Frogs are called ectotherms meaning they cannot cool or warm themselves off. They are cold blooded animals meaning their bodies are at the same temperature with regards to the outside temperature. When they cannot dig down deep inside the earth to avoid ice during winters, they tend to freeze.
Many tree frogs freeze in winter and come back to life in the warm months. There are few species of freeze-tolerant frogs like the wood frog, cope’s gray tree frog, eastern gray tree frog, spring peepers and western chorus frog. During the process of freezing or when a layer of ice begins to form on the skin, the wood frog’s liver starts converting sugars stored as glycogens into glucose. The sugar released from the liver is carried through the blood to various organs in order to keep the cells from shrinking and dehydrating. Nearly two thirds of their body water freezes into ice and eventually the heart slows down and comes to a stop. All the other organs also stop to work and the frog ‘seems’ to be dead. But not ‘dead’ altogether.
When the weather becomes warm, these frogs thaw back to life. The so called cryo-protectants (glucose, urea in the urine) have prevented the frogs from dying even though their hearts stopped. The cryo-protectants protect their body cells from dying by resisting shrinkage due to water loss. They also help limit the amount of ice that can actually form in any part of the body.
It still remains an amazing wonder to the scientists as to how their hearts start to work again.