Winter Hibernation Part 1: How do Turtles Survive the Winter?

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Hibernation for Ontario’s turtles begins in October when cooler weather begins to slow their metabolism. Heat speeds up metabolism whereas cold slows it down. A slower metabolism conserves both oxygen and energy stores.  A turtle’s heart beats around 40 times per minute on a warm summer day but only once every 10 minutes during winter hibernation! As a general rule, North American reptiles have a difficult time metabolizing food when their body temperature drops below 16oC.

Turtle will start hibernation begins when water temperatures drop and the hours of sunlight shorten. At this time turtles will move to the bottom of their water body below the frost line where water does not freeze.  Water above the frost line becomes dense at 4oC and sinks to the bottom, while insulating ice made of less dense water caps the top of the pond.   The sunken dense water becomes a perfect microclimate for keeping a hibernating turtle’s metabolism at a constant rate.  Amazingly, if the water were warmer, the turtle’s metabolism would rev up, using more oxygen and possibly depleting the turtle’s energy stores. If the water were colder, the turtle’s living cells could freeze. It is a delicate balance and turtles need to pick the ideal spot to hibernate if they want to survive the winter. They can further protect themselves by burying in the bottom substrate to help insulate and camouflage their bodies.

Living cells need oxygen, and 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 amphibians, such as frogs, the problem is solved by breathing directly through their skin and retrieving 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). Turtles are also known to surface in the winter for air if oxygen levels in the water deplete.

But what if the turtle can’t surface because it’s trapped under the ice? Muscle tissues in both reptiles and mammals are fueled by oxygen-burned carbohydrates, but when the system is oxygen stressed, the muscles operate short-term on an alternative process known as anaerobic glycolysis. In simpler terms, energy is derived without oxygen by burning sugar less efficiently, creating a painful by-product called lactic acid.  All hibernating turtles suffer from some level of lactic acid build up, making them very sluggish in the spring. If the surface is cut off from the atmosphere for too long all the oxygen will be used up and all hibernating life will die, a process known as “winter kill”.

 Once a turtle finds a successful hibernation site they often return to the same location year after year. This habit of returning to hibernation sites highlights how turtles can be affected when changes are made to their habitat. Turtles living in one location are genetically conditioned to be deprived of oxygen during hibernation for a specific length of time. Localized climatic conditions are changing in the GTA due to global warming and the Urban Heat Island Effect. 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 too! The consequences of these changes will have unknown effects on our unique hibernating species. By protecting wetland habitat we can help prevent unnatural changes, and we can make sure that all of our wetland friends have the best chance of making it safely through the cold Canadian winter.

In spring, when days get longer and warmer temperatures start to mix the water column, the bottom layer of water bodies begin to warm up. This activates the turtle’s biological alarm clock and they wake up. Between March and April thousands of turtles once again poke their noses out of the water and fill their lungs with oxygen, successfully ending winter hibernation.