Buried or Frozen: How Animals Survive Winter
“Boys, I need your help.”
It was a cold winter evening and I was walking the boys home from daycare.
“What do you need Mami?”
“I need a topic for Wild City,” I replied.
As much as winter can be wonderful in the city, I was fresh out of ideas.
“Why don’t you write about snakes,” suggested six-year-old Ray.
Lucas, who is three, disagreed, “No see snakes in winter.”
“Where do they go?” asked Ray. “Write about where they go.”
Many animals disappear over the winter. Some, like certain birds and insects, migrate to warmer climates. But what happens to the animals that stay? We sometimes see warm-blood animals, like birds, squirrels, rabbits, and humans. But what happens to cold-blooded animals like snakes, turtles, frogs and insects?
Strategy 1: Go Underground
Garter Snakes are Toronto’s most common reptile and, like most snakes, they spend the winter hibernating underground, perhaps using a mammal burrow, the stone foundation of a building, or an old well. These places are very important to snakes and many will return to the same place (hibernaculum) year after year. Though many snakes hibernate alone, some species will gather by the hundreds in warm underground dens.
American Toads are also a common sight in Toronto. These terrestrial (land living) amphibians also dig deep in winter, as much as 3-4 feet into the soil, to stay safely below the frost line. Other animals like salamanders and centipedes also use this strategy and crawl into tunnels, created by rodents or tree roots, to spend the winter underground.
Strategy 2: Go Underwater
Aquatic (fresh-water living) frogs and turtles have a different strategy. They choose to stay underwater where temperatures are usually between 0 and +4°C.
Turtles dig into the mud at the bottom of ponds to hibernate. But how do they breathe? Snapping Turtles poke their heads out of the mud and absorb oxygen from the water across the membranes of their mouth and throat.
Painted Turtles are even more impressive. They have altered their metabolism to survive without oxygen. If the water is cold, they can stay submerged for three to four months with zero blood oxygen. Just imagine holding your breathe that long!
Leopard frogs and American Bullfrogs also hibernate underwater, but unlike the turtles they do not dig into the mud. They must be near the oxygen-rich water so that they can “breathe” by exchanging oxygen and carbon dioxide through their thin skins. They spend a good portion of the winter lying under the water, but on top of the mud, only partially buried.
Strategy 3: Create Your Own Antifreeze
Not everyone can go underground or underwater, so many animals have to deal with temperatures at which their body fluids would normally freeze. What do they do?
Many insects and other small animals have perfected methods for staying liquid. This is called freeze avoidance. First, they produce special antifreeze proteins in their blood. These proteins bind to early ice crystals and stop them from growing. Second, they build up high concentrations of sugars or sugar alcohols in their blood and tissues. This lowers the freezing point of their body water – the same strategy we use to prevent the water in a car radiator from freezing. In fact, some insects actually make the same antifreeze chemical that we put in the car (ethylene glycol), though the less toxic chemical glycerol is more common. Third, they avoid contact with molecules that cause the formation of ice (ice nucleators) by wrapping themselves in water-proofing. For example, by thickening their waxy cuticle or spinning a water-proof cocoon. Sounds a lot like wearing a water-proof coat doesn’t it? Insects also eliminate ice nucleators from inside their bodies by emptying their guts.
Strategy 4: Freeze Solid (or Almost)
No question: Freezing solid is an extreme strategy and no animal can freeze completely. The formation of ice in body tissues is deadly. Sharp ice crystals puncture blood vessels, pierce delicate cell walls, and collpase internal cell support structures.
Yet some animals like the Wood Frog can freeze almost solid! Because of their amazing ability to do so, Wood Frogs are the only amphibian found north of the Arctic Circle. Unfortunately, Wood Frogs have almost completely disappeared from Toronto, though they can still be seen in the Greater Toronto Area in places like the Kortright Center for Conservation.
Wood Frogs combine freeze avoidance and freeze tolerance strategies so that parts of their bodies remain unfrozen, while other parts freeze solid. Large flat ice crystals form between their skin and muscle and their eyes turn white because the lens freezes. Ice fills their abdominal cavity and forms all around the outsides of their internal organs. This sucks water out of the cells leaving behind a thick syrupy solution so the delicate cells do not freeze (the old antifreeze trick). A partially frozen frog will stop breathing and its heart will stop beating. There really is such a thing as the living dead!
What is interesting about Wood Frogs is that the antifreeze solution they use is glucose; the blood sugar used by all vertebrate animals, including humans. But the frogs can survive glucose levels that are 100-fold (or more) higher than normal, while human diabetics suffer massive injuries at much lower levels.
Next time you visit a wild area in your city, remember all of the amazing adaptations being put to use by the animals hibernating underground, under the ice, or frozen beneath the leaves.
And please don’t step on any frogsicles!