Source: BBC

Most animals hate the cold. When winter comes around many species burrow underground to hibernate or migrate to lower latitudes where conditions are warmer.

But a few strange creatures do the opposite. They actually embrace the freezing conditions.

We are still unravelling the mysteries of these amazing animals that freeze. For one species in particular, doing so could prove significant. Several scientists are trying to work out how the Arctic ground squirrel (Spermophilus parryii) became the only known warm-blooded mammal to be able to tolerate subzero body temperatures. Solving the mystery could hold the key to freezing human organs for transplant without damaging them.

It might even provide a boost for the controversial field ofcryonics, in which human corpses are put into deep freeze in the hope that they can be returned to life with future medical advances.

A march fly (Bibio sp.) frozen in hoarfrost (Credit: imageBROKER/Alamy Stock Photo)

Subzero temperatures are a problem for all living things because water expands as it freezes to become ice. “When water in an animal’s cells freezes, the ice crystals that form expand and physically rupture the cell, causing death,” explainsDavid Denlinger, an entomologist at Ohio State University in Columbus. To survive freezing temperatures, animals must find a way to prevent ice from forming inside their cells.

Cryoprotectants, much like the antifreeze you put into your car radiator, prevent ice forming by reducing the freezing point of water

The most famous freeze-tolerant species is probably the wood frog (Rana sylvatica), which can survive subarctic temperatures for weeks at a time.

“Freeze tolerant organisms use various “tricks” to limit the damage that occurs when ice forms within tissues,” explains Jon Costanzofrom the Laboratory for Ecophysiological Cryobiology at Miami University in Ohio. “Our team was the first to examine the extreme freeze tolerance in wood frogs living in subarctic Alaska,” he says. “Those frogs all survived freezing to -14C!”

The frogs are able to cope with freezing temperatures by producing “cryoprotectants” – substances that prevent ice crystals from forming inside their cells.

Cryoprotectants, much like the antifreeze you put into your car radiator, prevent ice forming by reducing the freezing point of water.

“Cryoprotectants act by lowering the body’s freezing point, preventing water freezing at temperatures well below 0C,” explains Denlinger. When a cryoprotectant chemical dissolves in water, it forms strong bonds with water molecules. Water molecules that are bonded to the cryoprotectant can no longer bond with other water molecules to form ice, meaning that the water can be cooled to subzero temperatures without freezing.

A wood frog (Rana sylvatica) (Credit: John T. Fowler/Alamy Stock Photo)

Wood frogs produce urea, glucose and glycogen to act as cryoprotectants in response to subzero temperatures, and researchers have now identified freeze-induced genes that are responsible for transporting glucose into cells.

The red flat bark beetle from Alaska can supercool its body fluids to -50C

Freezing vertebrates and insects also produce antifreeze proteins, which bind to ice crystals and prevent them growing. One compound, antifreeze glycolipid (AFGL) is used as a defence against freezing in organisms ranging from plants to beetles, andin 2014 scientists identified the same glycolipid as an antifreeze compound in the wood frog.

Most impressive among freezing animals may be the invertebrates. A host of arthropods from cockroaches to caterpillars can tolerate freezing temperatures for days at a time.

Many of these species use cryoprotectants and antifreeze proteins to protect their cells, and Denlinger says that body fluid freezing points (also called supercooling points) as low -25C are not uncommon.

One truly remarkable beetle, the red flat bark beetle (Cucujus clavipes puniceus) from Alaska, can supercool its body fluids to -50C. But there is huge variation between individuals – some can tolerate body temperatures as low as -100C.

By dehydrating themselves, Antarctic midges are able to survive temperatures as low as -20C

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