Frog Free-zees in the Arctic Circle, Why Not Turtles Popsicles?
Did you know there are frogs inhabiting the southern edge of Canada’s tundra and even in Northern Alaska? With temperatures averaging -34°C in winter and 3-12°C in summer, how on earth does a tiny cold blooded amphibian like a Wood Frog (Lithobates sylvaticus) survive? Amazingly, these little guys have evolved a special trick, which they use not only during long periods of hibernation but also off and on during the cold months to survive periods of freezing and thawing.
Frogs are cold blooded or “ectotherms”. Ectotherms are animals that have little to no ability to produce internal body heat, so the heat that drives physiological functions such as, the breakdown of food into energy, comes from external sources like heat from the sun. Ectotherms in cold climates have evolved to slow down everything, even their need to breathe, to use less energy. When warm weather returns, the ectotherm’s metabolism revs up to get on with the business of spring feeding and breeding.
When a frog’s metabolism slows down they don’t need as much oxygen and in some species they don’t even need their heart to beat. But even with these advantages, when ordinary cells freeze, ice crystals form inside causing the cell walls to burst open, resulting in irreversible damage.
So how do frogs survive this freeze? Sugary Cell Antifreeze- Yep that’s right a frog free-zee! High concentrations of glucose in the frog’s vital organs and nucleating proteins in their blood prevents freezing, and unique species like Wood Frogs can go through consecutive freeze-thaw cycles, with no adverse effects.
Kenneth Storey, a professor of biochemistry at Carleton University in Ottawa, has been studying this phenomenon in Wood Frogs for years.
“[Wood Frogs] undergo freeze-thaw cycles all the time,” says Kenneth.
“We have false springs [in Ottawa] all the time where it gets really warm and all the snow melts and then suddenly—bam—the wind comes from the north and it’s back down to minus 10, minus 15 [degrees Celsius], and they’re fine,” he said.
Storey found that Wood Frogs can freeze to a point where they have no heartbeat, respiration or even brain activity. Talk about being suspended in time!
When temperatures warm and the ice melts, the frogs thaw. Water slowly flows back into the cells, blood starts flowing again, and the frog revives. In the lab, Storey said, a frog’s thaw out takes about 20 minutes and another 20 or 30 minutes for its heart to start up again.
“Once the heart starts, it pumps the blood around the animal and the animal starts to revive, then it starts to gulp, then it starts to breathe, then it starts to hop away. So it takes a little while to reactivate after you’ve been frozen down,” he said.
You can read more about this HERE.
But what about turtles? Like frogs, turtles are also ectotherms and their metabolism slows down with cooling temperatures. In the winter, turtles can survive under the ice without air for months at a time, and their heart beat can be as slow at 1 beat every 10 minutes. Impressively, the optimum range for our hibernating Ontario turtle species maxes out at about 4oC, which is the prefect temperature found at the bottom of a pond or lake in the winter.
Sadly, there are few reasons why turtles can’t move as far north as our amphibian friends, including oxygen availability, photo intensity and sex ratio. Currently, the Painted Turtle and Snapping Turtle are the only turtles that have populations extending north of Lake Superior.
Unlike the Wood Frogs, turtles don’t have same special antifreeze in their cells, which means their extremities can easily succumb to damage if frozen. Living, unfrozen cells require oxygen, which means turtles need oxygen throughout the winter, even if it’s just a little bit. If a lake or pond remains frozen for too long the turtle will not be able to surface for air and the available oxygen turtles can absorb from the water will be depleted to levels insufficient for most hibernating turtles. The Painted and Snapping Turtles are lucky as they evolved to live in anoxic environments (low oxygen), and can hibernate a little longer if the ice doesn’t break right away.
It is also suggested that turtles can’t live too far north because the intensity of the sun is weaker the more north you get. Decreasing the photo intensity even slightly can make or break where turtles will live. A turtle’s metabolism is a delicate balance between available foods vs. the energy needed to breakdown and absorb nutrients. Frogs, a much smaller animal, have a metabolic balance at a much smaller scale, which may help them better survive in their northern habitat ranges.
Lastly and probably the most important reason why terrestrial turtles don’t live in the Arctic Circle is how temperature affects the sex ration of their hatchlings. For Painted, Map, Snapping, Musk and Spotted Turtles the warmer their eggs are during incubation the more females and the colder the temperature the more males. As a general rule, the top of a turtle nest is more likely to produce females and the bottom of the nest is more likely to produce males. At a specific northern point, the temperature stays in the “cold” incubation range and all the turtles in the nest will come out male. With few to no females present their population range stops at a distinct northern limit.
Amphibians and reptiles up north are a truly unique phenomenon that took years of evolution to help them survive in such extreme environments. If you see a turtle or frog anywhere in Ontario, especially farther north, please report your findings to Ontario Turtle Tally and FrogWatch Ontario so we can learn more about these elusive northern inhabitants.