Today’s post comes from David LeGros, one of our Algonquin Provincial Park naturalists.
As the crisp days of fall grow colder and the occasional dusting of snow whitens the landscape, we know that winter is just around the corner. The countless songbirds of our forests avoid our cold winters and lack of food by migrating south.
Other animals adapt to cold conditions and may develop a thicker coat of hair or feathers. We believe some have the enviable ability to sleep through the long Ontario winter by hibernating.
What is hibernation?
Hibernation involves the animal reducing its metabolism to a point where it uses very little energy; even their heart rate and body temperature drop and they usually do not eat. Many animals do this: from bears to jumping mice, to turtles and snakes, and even many insects remain dormant during the winter.
Cold-blooded animals (that cannot produce their own body heat through their metabolism), such as reptiles and amphibians, must hibernate in Ontario; It’s too cold to be active: they would freeze and die. To survive this difficult season, they must make sure they find the right place.
For most frogs, the right place is underwater. Many times we think that the water, in the winter season, is very cold. But as we discovered with the ice cube tray in the freezer, water is liquid until it freezes. If the water is still liquid, then it is above freezing, but not by much!
Many of Ontario’s frogs, such as the bullfrog, green frog, mink frog, and pickerel frog, hibernate in water. Many think that these frogs hide in the mud at the bottom of a pond or river to “stay warm” and remain hidden from predators.
But it turns out that to survive the winter under the ice, they need to stay away from the mud because…
…these frogs breathe through their skin!
Aquatic frogs, like this green frog, spend the winter underwater. As long as the water doesn’t freeze, neither will the frog. The only problem is making sure they get enough oxygen to survive.
Very cold water (4°C) contains more oxygen than warm water, which is good news for frogs. They can simply absorb the small amount of oxygen they need directly from the water through their permeable skin.
If they were buried in mud, their oxygen supply would be quickly depleted, so they remain exposed on the bottom, or perhaps nestled between rocks, logs or roots, to get a constant supply of oxygen from moving water.
Toads commonly found on trails and gardens, such as the American toad, hibernate in the ground. Forests and fields certainly freeze, and even the ground freezes, so toads must find a place to escape the frost.
American toads must find a place on land to hibernate. To do this, they must dig into the ground, often more than 50 cm, to reach below the frost line.
To do this, they dig! Toads have special hardened bumps on their hind legs that help them dig into the ground. They must dig deep, often more than 50cm into the ground, to reach below the frost line, where they will overwinter.
Only once the soil temperature rises in the spring will they know it’s the signal to re-emerge.
Some other frogs deal with low temperatures a little differently: they’ve found a way to turn cold! Our wood frog and the three tree frog species found in Ontario (gray tree frog, spring frog and chorus frog) are actually frost tolerant.
Before we get visions of frogs, know this: These frogs freeze differently than, say, a hot dog. These tree and wood frogs hibernate in leaf litter or under bark, somewhere that is not truly insulated from freezing temperatures.
Once temperatures drop to -5°C, small ice crystals form in the body, freezing approximately 40% of the body’s water content.
Tree frogs, like the gray tree frog shown above, hibernate under dead leaves or tree bark and are exposed to freezing temperatures. They often freeze and their hearts and other bodily functions stop.
In this condition, the frog no longer breathes, no blood flows, and there is no heartbeat; It’s like she’s dead. Once spring arrives, the frog thaws (sometimes in as little as a day) and can hop away.
Other animals cannot freeze or cheat winter in this way; Ice crystals severely damage cells, usually causing death.
So the next time you see a frog in spring, know that it had to survive some tough conditions the previous winter, and that’s something to sing about!