Hibernation for Ontario’s turtles and frogs begin in October when the hours of sunlight shorten and cooler weather begins. Frogs and turtles are often referred to as cold blooded. The more accurate term for these animals is “ectotherms”. Ectotherms are animals that have little to no ability to produce internal body heat, relying instead on their environment for sources of heat to warm up their bodies. Such sources include warmth from the sun or warmth from the water. They rely on this external heat to raise their body temperature so that their internal systems, including respiration, digestion and reproduction function properly. In the winter when external heat sources are not available, an ectotherm has to cool down and their internal systems are slowed, which conserves both oxygen and energy stores. For example, a turtle’s heart can beat around 40 times per minute on a warm summer day but only once every 10 minutes during winter hibernation!
In preparation for winter, turtles and most frogs will move to the bottom of the body of water they live in. This gets them below the frost line where water does not freeze. Cold water becomes dense and sinks to the bottom of a lake or pond, while an insulating layer of ice which is lighter floats and caps the surface of the pond; this creates a protected overwintering pocket for our turtle and froggy friends. Amazingly, if the water were warmer, these creatures’ metabolisms would rev up, using more oxygen and possibly depleting energy stores. If the water were colder, living cells could freeze and die.
Frogs in Ontario such as the leopard frog and bullfrog, typically hibernate underwater. All amphibians including frogs have permeable or absorbent skin that absorbs oxygen from the water. When it’s cold and a frog’s metabolism is slowed down they don’t need as much oxygen, so as long as a frog picks a wetland with a sufficient amount of oxygen trapped in the water, a frog can “breathe” underwater all winter long.
To see footage of a turtle being monitored under the ice
Photos: Shannon Ritchie, Wetland Biologist, Toronto Zoo taking water samples in the field.
Turtles can only stop breathing for a relatively short period of time. In northern latitudes, ice may persist for up to four months. That’s a long time to hold your breath! So, how do turtles get enough oxygen to survive their winter nap? Similar to hibernating frogs, the problem is solved by breathing directly through their skin and absorbing oxygen from the water itself. Turtles absorb oxygen from the water through the skin of their throat cavity, which is lined with shallow blood vessels. A similar type of tissue is present in two thin walled sacs in the turtle’s cloaca (posterior opening for the digestive and reproductive tracts). Turtles are also known to swim to the surface when the ice cracks for a breath of air when needed.
Some frogs, like Wood frogs, will burrow deep into the soil or move to deep cracks and crevices in logs or rocks. These places can freeze so how do frogs survive? Sugary Cell Antifreeze! High concentrations of glucose in the Wood frog’s vital organs prevents freezing. A partially frozen frog will stop breathing, and its heart will stop beating, but when warmed up, the frog’s heart and lungs resume activity—talk about the living dead! The Wood frog is so good at this that it can be found even above the Arctic Circle.
Did you Know? Turtles and frogs will often return to a successful overwintering location year after year. This habit of returning to hibernation sites highlights how these creatures can be affected when changes are made to their home. Localized climatic conditions are changing due to global warming and urban development. These changes are altering the length of seasons, water levels, and the presence of ice. Shallow water is more likely to freeze solid right down to the bottom, freezing hibernating turtles and frogs too! By protecting wetland habitat we can help prevent unnatural changes, and we can ensure that all of our wetland friends have the best chance of making it safely through the cold Canadian winter.
Written by Shannon Ritchie, Wetland Biologist, Toronto Zoo
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