Animals in the wild don’t have access to a cozy fireplace or a warm house, and have to use very distinct survival strategies when the temperature drops during winter. Virginia is a state with both humid summers and cool winters, and animals that were once comfortable in the warm southern climate must adapt to a quick shift in temperature. Depending on their species, mammals may try everything from reducing their movement and activity to falling asleep for months. Reptiles face an even greater danger from the cold, and adjust their bodies and behavior more dramatically. This article covers torpor, hibernation, and brumation, the methods animals use to survive the world of winter, opens a new window.
The Shallow Sleepers: Winter Torpor
Mammals that don’t fall asleep for a long time during winter and merely reduce their activity level are often defined as being “in torpor.” Opossums are considered a "living fossil", opens a new window because their form is quite similar to how it was in dinosaur times, and they never evolved a hibernation behavior. They can survive our winters through a state of torpor, though. Opossums will stay mainly in one “home den” during winter and reduce their movement, but will still leave to feed and are vulnerable to frostbite, opens a new window during the coldest days. Opossums are most likely to try to den inside people’s houses, opens a new window during winter in search of warmth.
Opossums are not the only animal to use torpor as a winter survival strategy. Skunks, opens a new window will also enter torpor during winter and will mostly stay in their dens. Their winter dens are communal, opens a new window, and the winter is their mating season. This allows their babies to be born in May-June, when the weather is easier for the young to survive. Raccoons enter torpor in a similar manner to skunks, and their mating season, opens a new window is at roughly the same time. Raccoons will still leave their dens to eat during this period, but are much less likely to move long distances or switch dens. Even white-tailed deer, opens a new window use a form of torpor, although their method is different from smaller mammals. Since they can’t make a warm den in a tree trunk or hole, they must rely solely on their winter coat and fat reserves to survive. The deer’s metabolism and heart rate remain the same, but their activity level and movement fall sharply once the temperature is reduced. They will try to seek out a "yarding", opens a new window area that is warmer and less windy during winter, which will often be a south-facing place convenient to their home range.
The Long Sleep: Hibernation and Deep Torpor
The animals listed above reduce their activity level but generally don’t fall asleep for days at a time. Animals that enter deep torpor and true hibernation use long rest periods and reduced metabolism as their key survival strategy. Black bears, opens a new window are well-known deep sleepers. A black bear’s strategy is to eat, opens a new window massive amounts of food during summer and fall to build up enough fat to survive its period of winter inactivity. In Virginia, bears will enter their winter dens between October, opens a new window and January. Black bears will not eat during their hibernation, and their bodies create a “fecal plug”, opens a new window made of hardened plant matter that prevents defecation during sleep.
Black bears are sometimes not considered a “true hibernator” because their body temperature, opens a new window doesn’t drop much during the rest period, but they display most other characteristics of deep, efficient hibernation, including dropping their heart rate and metabolism. Bears will retain most of their muscle mass, opens a new window and maintain their circadian rhythm during hibernation, allowing them to resume normal activity quickly when the time to wake comes. Black bears are not in a “locked” sleep during hibernation and can be easily woken, opens a new window up; if you ever encounter a hibernating bear, do not approach or wake it! Virginia’s bears will end their hibernation on their own between mid-March and early May.
Another common animal in our area known for deep sleep during winter is the woodchuck, opens a new window. Unlike with bears, there is no controversy as to whether the woodchuck is a “true hibernator” or not. The woodchuck’s body temperature, opens a new window falls nearly 60 degrees during hibernation, and its heartbeat and breath rate slow dramatically. Woodchucks must stay asleep during hibernation as long as possible. If they wake up too early or often, they will lose more of their body fat, opens a new window because their temperature rises while awake. However, they will occasionally wake up during their hibernation to relieve themselves occasionally since they don’t have a fecal plug like a bear. Virginia woodchucks usually hibernate from October to February.
Virginia’s native bat species also hibernate during winter. Bats like northern long-eared bats, opens a new window, little brown bats, opens a new window, and big brown bats, opens a new window all seek out areas to hibernate in. Little brown bats and northern long-eared bats tend to hibernate in large caves with many other bats, while big brown bats hibernate in both caves and smaller locations, such as attics, opens a new window. Smaller bats, such as little brown bats, rest all winter, whereas big brown bats may be active for short periods when the temperature is warmer. Bats, opens a new window that are still active and flying during the cold may be affected by white-nose syndrome, opens a new window, a fungal disease which affects bats’ ability to sleep properly, as it afflicts them with irritating growths on the nose. This disease causes many bats to die of exhaustion and starvation and is a major threat to the survival of bat colonies. In some species, white-nose syndrome has killed 90% of the population, opens a new window, showing how important a successful hibernation is to bats.
Brumation: A Cold-Blooded Rest
Low temperatures are very dangerous to cold-blooded creatures such as reptiles and amphibians. Their dormancy period is called brumation, opens a new window, and they use it with a variety of strategies to survive the winter. You’ll notice that copperheads, opens a new window seem to disappear by November, opens a new window, and bites recorded after October are very rare. This is because they have started brumation as a means of coping with colder temperatures. They will head for a place that offers protection from cold winds and restrict their activities and movements as much as possible. In forests or other wilderness environments, they may seek out communal dens, opens a new window with other copperheads or other species of snakes for warmth. Copperheads living close to humans may have to rely on wood piles, opens a new window or leaf litter, so be very careful when moving around wood and leaves during winter! In some places, the communal dens of copperheads may include timber rattlesnakes, opens a new window, although these are not present in the Fredericksburg area. Some snakes may gather in groups of up to 100 individuals, opens a new window in caves to better survive the cold.
Snakes are not the only reptiles that must brumate to survive the winter. Box turtles, opens a new window have their own method of brumation. They will start to eat less and reduce their activity level in October, spending most of their remaining effort digging a burrow. Cold-blooded animals are distinct from mammals, which always tend to eat more before their hibernation/torpor phase. They will usually lose 6-7% of their body weight, opens a new window during the brumation cycle because they eat nothing during brumation. Unlike hibernating mammals, reptiles are still awake and have a normal circadian rhythm during brumation, although they may sleep more because the night is longer during winter. Box turtles, opens a new window may briefly emerge from their burrow to drink water during brumation, but will avoid other activity as much as possible. Turtles will slow their respiration so drastically that they breathe through their cloaca (i.e. "butt breathing", opens a new window) to conserve energy. Cloacal breathing is used because this is the area on the turtle’s body that has the most blood vessels, making it ideal for the transfer and conservation of energy.
Although their activity is greatly reduced, the reptiles listed above are still partially awake and can move their bodies during their brumation phase. Amphibians that use brumation may alter their behavior even more radically than reptiles to survive because their skin is much more permeable than reptile skin. Bullfrogs, opens a new window typically head for the bottom of a pond (where the water is warmest) to survive the winter, but, unlike lake-dwelling reptiles, they always leave part of their skin exposed to the water so they can breathe through it. Amphibians breathe through their skin because their lungs, opens a new window are not as efficient as those of reptiles or birds. Wood frogs, opens a new window brumate on land instead, using leaf piles and cracks in rocks and trees for shelter. Because they don’t take shelter in the water, their bodies become partially frozen during the colder weather. They use the urea, opens a new window and glucose in their bodies to create a natural antifreeze that prevents frostbite and cold death.
Salamanders, opens a new window are the other main type of amphibian in Virginia, and they use similar strategies to frogs to survive the cold. Shenandoah salamanders don’t leave their home range, but do seek out moist cracks, opens a new window in the rock to brumate. They rest from October to April and, like wood frogs, rely on urea and glucose to prevent death by frost. Yellow spotted salamanders, opens a new window use a different strategy that involves hiding deep in the earth to survive the cold. They typically seek out an abandoned mammal burrow and stay at the bottom of it until March. They have their own natural antifreeze to help them survive severe cold, like that of Shenandoah salamanders.
Whether they be reptiles, birds, or mammals, Virginia’s animals all have their own distinct tricks for surviving the winter cold. If you’re curious about them, you can learn all about them from CRRL’s list of books on animals in Virginia. Learn about Virginia’s wildlife at your library!
