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They aren’t moving. They’re not responding to touch or light. Their hearts aren’t beating. They’re no longer breathing. Their skin is ice-cold and hard to the touch. By that description, you probably don’t think I’m describing living things. However, there are some animals that survive like this because of a process called freeze tolerance.

Unlike mammals, whose bodies are always working to maintain a constant temperature, cold-blooded animals (ectotherms) are at the mercy of their environment. As a result, when the temperature plummets, they have to come up with a physiological means to survive the bitter cold winters.

Bugs and amphibians are the freeze-tolerant animals researchers study most often. Among amphibians, a few select frogs show a clear freeze-tolerance response. Large amounts of their bodily fluid freeze into ice particles to help them make it through the winter.

The physiological changes that prepare freeze-tolerant animals for cold weather begin as the temperature drops, but they don’t happen overnight. These animals distribute substances called cryoprotectants, which allow them to get colder than freezing while shielding them against dehydration and other cell changes that can occur when ice forms. Once frozen, the animals can withstand temperatures ranging from 32 to -22 degrees F, depending on the species. Freeze tolerance can last from weeks to months. Cycling through this process is a must for any frog that wants to see the next summer.

However, thawing may be just as important as freezing. Freeze-tolerant animals likely go through several freeze-thaw cycles, during which the distribution of cryoprotectants and tolerance to the cold increases. This suggests that the cryoprotective process isn’t finished after the first freeze and is a cumulative process as the winter grows colder. When spring arrives and the weather returns to consistently above-freezing temperatures, the animals thaw, their hearts start beating again and they continue on without negative effects.

Scientists want to learn more about freeze tolerance from the animals that do it so well. In case you missed it, humans can’t freeze and thaw themselves. By studying the physiological changes in freeze-tolerant species, researchers can apply this work to mammals, such as by storing organs for later use and keeping cell lines healthy while in storage. Studying an “ideal” organism that has overcome a functional problem allows scientists to better understand those problems in mammals. Therefore, comparative physiologists continue to research the unique traits of various organisms to understand how evolution has already handled these challenges.

Brian Stogsdill is a graduate student at Wright State University, in Dayton, Ohio, currently finishing his doctorate in biomedical science. Stogsdill researches freeze tolerance and aquaglyceroporins (a family of proteins that conduct water).