Complain about seasonal chills all you want, but at the end of the day, you’re basically a walking furnace. Humans are endotherms, meaning that our bodies produce heat internally. We’re also able to maintain a body temperature that stays more or less constant.
So count your blessings; not all animals have these physiological advantages. Apart from a handful of species like the monstrously big leatherback sea turtle, very few reptiles or amphibians are able to keep their bodies at a constant temperature. And since they can’t warm themselves up, these creatures must extract heat from their environment.
But what happens when that environment gets colder? How do frogs, snakes and turtles make it through the winter months in places that see blankets of snow, iced-over lakes and sub-freezing temperatures year after year?
A lot of reptiles and amphibians undergo periods of extreme seasonal inactivity. When the weather gets colder, they may experience decreased heart rates, slowed metabolisms and lower overall body temperatures.
(Scientists disagree over what to call this state. Everyone accepts that it’s some kind of dormancy, but while some experts classify it as hibernation, others refer to it as “brumation.”)
Dormant snakes generally hole themselves up in winter dens. These shelters may take the form of an abandoned rodent burrow, an exposed crevice on a rock face, or some naturally occurring hole underneath a tree. Various snakes have also been known to turn household basements or garages into winter lodges.
In seasonally cold areas like Canada and the northern U.S., snakes must choose their dens with care. Ideally, a winter hangout spot will descend below the local frost line, the maximum depth beneath the ground at which soil freezes.
Hiding out underground is the survival strategy of choice for lots of tortoises and turtles as well. Some species — such as the gopher tortoise — dig their own burrows, but it’s not uncommon to find the shelled reptiles occupying pre-dug, unoccupied rodent holes.
Now a hibernating black bear can sleep for more than 100 days straight without consuming any food or water. Reptiles tend to be more active during hibernation/brumation. When a midwinter warm spell comes along, they’ll use it as an opportunity to crawl up to the surface, bask in the sunlight for a little while and maybe grab a quick drink.
Reptiles are perceived as loners, a reputation that isn’t entirely deserved. Consider the Eastern diamondback rattler, who doesn’t seem to mind cohabitating with gopher tortoises. Members of both species will sometimes go dormant together inside the same burrow.
Likewise, snakes often share their dens with other snakes. Garter snakes are famous for hibernating in huge groups that may consist of hundreds — or even thousands — of individuals. One Canadian den reportedly contained no fewer than 8,000 snakes. Indiana Jones will want to keep his distance.
Kermit the Frog-sicle
As snakes go, garters are remarkably cold-tolerant. One species can even survive the unenviable experience of having 40 percent of the liquid water inside its body freeze solid, but only if it’s allowed to thaw out after a few hours.
Yet the wood frog takes freeze tolerance to a whole new level. A North American native, this cold-weather warrior has the distinction of being the only amphibian in the western hemisphere whose range extends into the Arctic Circle.
Every autumn, wood frogs bury themselves under a thin blanket of leaf litter on the forest floor. Then they’ll remain dormant for up to eight months.
In the process, the heart temporarily stops beating and the frogs enter a state of suspended animation. Left to the mercy of the elements, the amphibians freeze at temperate and polar latitudes. Fortunately, the liver pumps loads of glucose into the bloodstream while urine is retained within the body. All that helps keep the cells from drying out, which is what would normally happen during the freezing process.
Therefore, a full 65 percent of all the water in a wood frog’s body can become frozen and the amphibian will still live to fight another day. Moreover, the frog may be kept frozen at -18 degrees Celsius (or -0.4 degrees Fahrenheit) for as long as 218 days.
Another example of this cool survival strategy can be found in far-north Russia. The Siberian salamander lives in areas that get temperatures of -50 degrees Celsius (that’s -58 degrees Fahrenheit) or lower. To stay alive, it hibernates underneath logs, vegetation and snowbanks. An “antifreeze chemical” in the bloodstream keeps the critter alive when the majority of its body water turns to ice.
What Lurks Beneath the Ice
Terrestrial amphibians with poor digging skills — like the wood frog — tend to either hibernate in preexisting burrows or find shelter at ground level. Good burrowers such as the American toad and spotted salamander proactively dig winter holes for themselves that extend below the frost line.
For aquatically inclined amphibians, there’s another option. Bull frogs are lake and pond denizens who find oxygen-rich bodies of water and spend their winters imprisoned under the surface ice that forms there.
Painted turtles employ a similar strategy. Reptiles generally use their lungs to breathe, but some semiaquatic turtles can also absorb waterborne oxygen through the skin. Overwintering painted turtles do this extremely well — plus they can lower their metabolic rates to the tune of 95 to 99 percent each winter. That’s how they stay alive beneath thick sheets of lake ice for months on end. Sometimes, you can even see them swimming around under the frozen barriers.
But why go through the hassle of skin-breathing when you can turn your snout into a snorkel? In January 2018, the internet was greatly amused by some North Carolina alligators whose noses were seen poking out of a frozen pond. Gators can’t survive being trapped in icy water for much longer than a week or so. For long-term winter lodgings, they construct deep burrows out of waterside mud.