Frogcicles

By Bob Dispenza, Park and Education Manager, Allen County Parks, Fort Wayne, IN

During this winter’s cold weather, you’ll often hear people say they’re “freezing.” It’s the time of year when my wife’s “freezing” feet seem to find their way onto my back to warm up. But we don’t know what real freezing is – not like some of our native frogs do. They will actually freeze solid, which would kill most animals. Our little “frogcicles” can thaw out and live again in the spring.


Gray tree frog photo by Natalie Haley
Stars of “It Came From Beneath the Ice” (freezable frogs in our area):

• Eastern gray tree frog (Hyla versicolor)
  
• Cope’s gray tree frog (Hyla chrysoscelis)
  
• Spring peeper (Hyla crucifer)
  
• Chorus frog (Pseudacris triseriata)
  
• Wood frog (Rana sylvatica)

Theses frogs are unable to dig down below the frost line, can’t migrate, and live on land or in shallow water that disappears or freezes completely in winter. They can’t fight (by keeping warm), and they can’t run away (by migrating). But they can adapt. Without the ability to survive freezing, they would not make it through the cold weather.

Several adaptations make all this possible. The goal is to get the water out of the cells, and then get it to freeze in a controlled fashion at the highest possible temperature. Water freezing inside the cells will kill them, but frogs can survive water freezing in areas outside the cells (called extracellular spaces). Condensation nuclei (chemicals, proteins and bacteria that ice will form around) are found throughout the extracellular spaces, such as the digestive, circulatory and lymphatic systems and skin tissues.



Wood frog photo from River Bend Nature Center
Frog skin, being so moist, begins to freeze as soon as the frog’s surroundings do. As the frogs start to freeze, the heart keeps beating, distributing glycerol and glucose around the body, and these chemicals move into the cells. This allows most water to leave the cells and freeze in the extracellular spaces, while the remaining water inside the cells remains unfrozen. Glycerol (sugar alcohol) and glucose (sugar) keep some water in the cells, but the solution freezes at a much lower temperature than before, like salt makes water freeze at a lower temp on the road. These chemicals, called cryoprotectors, also provide ready energy that doesn’t require oxygen, since the blood freezes also. Glycerol is chemically related to ethylene glycol, which we use as antifreeze in our cars. While ethylene glycol is poisonous, glycerol is not.

As freezing continues, the heart stops, breathing stops, and brain activity most likely stops. The frogs are technically dead, though clinical death is an irreversible state, and frog freezing is quite reversible.

Frogs can freeze down to -15ºC (5ºF), and even lower in northern parts of their range. Up to 80% of the water in their bodies can turn to ice. They don’t always survive – it can get too cold. Some can only survive freezing for short periods of time (hours), while others can tolerate weeks as frogcicles.


Spring peeper photo from Ohio Frog & Toad Calling survey
As temperatures warm in the spring, the frogs begin to thaw, and heartbeat, breathing and muscle movement start again spontaneously. About 12 hours after thawing, the frogs have mostly recovered and resume normal frog behavior (croaking, eating, mating, singing songs about being green, and searching for princesses). They can take first advantage of freshly-thawed vernal pools, and begin mating activities even before all the ice melts.

In the spring and summer, frogs croak to attract a mate. In the winter, some frogs “croak” to survive.

For more information:
Laboratory for Ecophysiological Cryobiology: Wood Frog Freezing Survival
Nature North: Frozen Alive
Nova Ask the Expert: Frozen Frogs
National Geographic Anti-freeze-Like Blood