How are you spending January? When winter arrives, humans endure the cold by changing our environment. Some flee to warmer climates but most of us turn on the heat and bundle up in extra layers of clothing. How, I wondered, do other animals survive in the cold and snow? Snuggled in front of the fire in my slippers and under a blanket, I read Winter World (Harper Collins, 2003) by Bernd Heinrich. Heinrich, professor emeritus of biology at the University of Vermont, spends time at his homes in Vermont and western Maine, observing and chronicling the wonders of the natural world. Traveling through the winter landscape with Heinrich, I learned that animals use a variety of skills to survive winter. In this article, we will explore animals’ main winter survival techniques and examine the strategies of a few particular animals.
DANGERS OF WINTER
Animals face two dire circumstances in cold weather.
- Scarcity of food and water: Finding nourishment is paramount for animals in winter. The landscape is barren and water can freeze so finding food is an ongoing struggle. Seeds and insects have vanished and small animals go underground, making it hard for predators to find them.
- Cold temperatures: Seventy percent of an animal’s cells are water. As temperatures turn cold, the cells that provide an animal its energy have trouble functioning. When an animal’s internal temperature goes below freezing, the water in the cells freezes and the animal dies.
WINTER SURVIVAL METHODS
Animals use three survival strategies to avoid freezing to death. The can rely on one or a combination of these techniques:
- Migration: moving away from the cold
- Dormancy: lowering body temperatures and slowing body functions to conserve energy
- Adaptations: employing techniques, including evolutionary structures, to survive cold temperatures
MIGRATION
Some animals change their habitat by moving to a new area when cold weather approaches. Traveling by land, air, or sea, many animals move great distances to a warmer destination. Successive generations of Monarch butterflies fly up to three thousand miles from the U.S. to their winter home in Mexico. Caribou in Alaska walk more than 2,000 miles to their winter home. Humpback whales swim from the icy Alaskan waters to the tropics of Hawaii, making a round trip journey of up to 10,000 miles. Not all migrations, however, are lengthy. Travelers, such as mule deer in Montana and birds that live high in the mountains in the summer, make shorter “altitudinal” migrations to lower elevations to find warmer temperatures and less snow.
DORMANCY
Some animals go dormant or hibernate through the winter. During hibernation, the animal curls up in a ball to conserve body heat and stay warm. Its body temperature and respiration decrease and its heart rate slows down. By using less energy to stay alive, the animal no longer needs to seek food but can live off the fat reserves it has stored in the fall. Hibernation occurs over a long period of time and the type of hibernation varies. Rodents, such as groundhogs and chipmunks are true hibernators. They burrow into the ground where their body temperatures and heart rates drop to extreme lows and their breathing slows. In true hibernation, the animal cannot be awakened. Torpor is a shorter version of inactivity. Raccoons and gray squirrels stay active during the winter and go into a torpor or an occasional sleep when temperatures turn frigid. Birds were often thought to either migrate to a warmer environment or were resistant to cold. Scientists now know that some birds, such as nighthawks, chickadees and some hummingbirds can enter in and out of torpor as temperatures demand. Brumation is the term used to define the dormancy of reptiles such as turtles.
ADAPTATION
Staying warm
Do you notice how racoons seem to be much larger and deer seem to be darker in winter? They, like other mammals, grow a thick, dense coat in winter to cope with colder temperatures. Aquatic animals like otters, minks, and muskrats have a double layer of fur, with fine hairs near the body covered by the longer, outside hairs. They rub body oils on their fur so water will not soak in. Deer and moose have winter coats with hollow hairs. The trapped air adds a layer of insulation. Bird feathers are not just for color and flight. Made of keratin, they are designed with rows of barbs attached to a single shaft. They interlock to create a nearly solid outer surface around the bird. Down feathers under their outside feathers provide an additional layer of insulation. Birds also rub oils along their outside feathers so rain will slide off rather than soak in. After enjoying a high protein diet when young so they grow fast, overwintering adult birds switch to a high energy diet of seeds and fruits. Birds fluff up their feathers to trap air which adds a layer of insulation. They tuck their heads under their wings when roosting and shiver to stay warm.
Some birds aggregate or flock together to share warmth. Ducks, geese and other water birds have an extra layer of fat for protection and also rub oils along the outside of their bodies so water slides off their backs. Water birds keep their feet and legs warm through a system called counter-current heat exchange. Their veins and arteries are close to each other so that warm blood leaving their bodies warms up the cold blood returning to their bodies. Flocking also is a defense against predators, at least for those away from the edges of the flock. Lookout birds alert the others of an encroaching hawk or other predator. Â Aggregating also is common among other animals. Bats often gather in a cave to keep warm and flying squirrels huddle together to conserve body heat. Lady bugs, stink bugs and garter snakes are a few of the creatures massing together in crevices or under rocks to avoid freezing and predation.
Finding food
Animals that stay active in the winter, such as coyotes, bobcats, owls, and hawks need to keep eating. They hunt for other living creatures but also scavenge on carcasses of dead animals. The digestive enzymes of deer and moose change so they can feed on twigs and bark. They will also dig through snow or duff looking for left-behind acorns and other nuts. Birds that stay through the winter such as chickadees, woodpeckers, sparrows, and finches look for berries that hang on into the winter and search for overwintering insects in tree bark and crevices. They look for seeds in forbs and grasses left standing or fallen to the ground. Robins, often thought of as the harbingers of spring, actually do not migrate. Rather they change their dietary requirements from earthworms and insects to fruit, and leave suburban tracts for forested areas. They switch back to an earthworm diet and appear in yards when the soil warms and softens in the spring and worms are available.
Conserving energy
Deer and other mammals restrict their movements, staying near sources of food and water. They look for tree cover to enjoy milder temperatures, protection from wind, and less snow. Most salamanders of the Appalachian Mountains spend the winter underground, moving little and burning as few calories as they can. The red-backed salamander, on the other hand, is active during the winter and stays near ant colonies to have a steady food supply. Fish are cold blooded so do not have to generate heat to stay warm. They do slow down and find habitat that does not require them to swim fast or hard. In deep lakes, they cluster in schools in the deepest part where there is more oxygen and the water warmer. Their metabolism slows down, reducing their need for food and oxygen. In shallow water, ice and snow can block sunlight, reducing oxygen levels which can be deadly.
Burrowing and Tunneling
Many species go underground to escape the cold. Mice and voles burrow under leaf mold or into the soil. They sometimes live in cavities of rotting stumps. In these subterranean tunnels, they feed on hiding insects. Many species take shelter from cold and wind by burrowing into thickets and dense shrubs. Beavers cut down trees and pile branches into the water. They cover their lodge with more sticks, mud, and rocks and use an underwater entrance. Here they are protected from predators and, like other water mammals, their double layer of fur keeps them warm. They feed on the twigs and bark gathered in the den. Snow has great insulating properties. Ruffled grouse tunnel into the snow at night for shelter and to stay warm. In snowy areas, moles, voles and other small animals do the same. In their snow tunnels, they feed on seeds and grasses left behind and are insulated from the cold. They are also somewhat, but not completely, protected from predators. Larger animals can struggle to find food in the snow but have adaptations that enable them to find their hidden prey. Coyotes and foxes locate voles by sound and pounce with their front paws through the snow, crashing through the tunnels and trapping their prey. Great grey owls can hear a vole’s movement under the snow from thirty feet away. They take a diving plunge and can drive through crusted snow with their balled up feet.
Tolerating the cold
Many species have evolved to tolerate winter weather. In addition to changes in fur and undercoats, the size of an animal plays a role. Animals living in very cold climates are often very large – think of polar bears. Small animals, like the pika, a relative of the rabbit, have small ears and appendages close to the body that resist frostbite. The fur of some animals, such as snowshoe hares, weasels, and Arctic foxes turns from brown to white in the winter. The white color offers camouflage from predators and is also a thicker, better insulator.
BIOLOGICAL WINTER SURVIVAL TECHNIQUES OF SOME COMMON ANIMALS
Surviving winter may sound simple, but it actually involves amazing biological adaptations as shown in the techniques of some animals common to our area.
Black Bears
When we think of hibernation, we usually think of black bears so it is worth examining the biology behind their winter slumber. Bears are not true hibernators but enter a state of deep sleep. This long winter rest is an amazing biological feat. Hibernation is triggered by chemical changes in a bear’s blood. In late summer and fall, bears go on a feeding frenzy and take in about five times more food than normal to add a layer of fat. By late fall, the fat cells secrete a hormone called leptin. Leptin circulates in the blood and affects the appetite centers in the brain, suppressing the bear’s appetite. No longer hungry, the bear knows it’s time to head to the den. Bears make dens under piles of brush, bushes, fallen trees, masses of tree roots, or rocks. They remain in their winter den for up to five months, staying alive on their fat reserves. Humans need water to remove toxins from their blood through urination. The hibernating bear, however, is metabolizing fat and so does not accumulate much urea in its blood and does not need water to flush it out. Urea that accumulates is converted to creatine, which is non-toxic, and nitrogen wastes are biologically recycled back into protein. When spring arrives, the leptin decreases and the bear, hungry again, knows it’s time to leave the den. While hibernating, bears do not undergo an extreme temperature drop. Instead they grow a thick coat of fur before winter arrives to conserve warmth. Pregnant bears give birth to their cubs during hibernation. The mother sleeps while the cubs nurse and grow until the family emerges in the spring. The hibernation cycle coincides with decreasing daylight and food supply in the fall and the increase of both in the spring.
Turtles
Turtle hibernation is called brumation. They are ectotherms, meaning their body temperature drops or rises to match the temperature of their surroundings. The adjustment of their temperature enables aquatic turtles, such as painted and snapping turtles, to spend the winter buried under mud in frigid water and even under ice. Their body temperature drops 90% to about 39o Fahrenheit, or just above freezing and their blood oxygen level drops to near zero. Painted turtles can survive three or four months in this condition. They exist on food they have stored in their bodies and take in oxygen from the water as it passes over their blood vessels in their skin, mouths and cloaca (the terminus of the digestive tract at the hind end). When oxygen is depleted, aquatic turtles switch to anaerobic respiration. While anaerobic metabolism does not require oxygen, it can cause lactic acid to build up in the body. To avoid damage from lactic acid in the tissues, their skeletons and shells release carbon to neutralize the lactic acid.  Terrestrial turtles, such as box turtles, also brumate. Brumation begins when cold weather starts and daylight decreases, generally in October or November. In preparing for brumation, turtles will feed copiously to gain weight before stopping eating and burying themselves underground They use their strong forefeet to dig into soft earth or into hollow logs until they get deep enough to escape freezing temperatures. Insulated in their burrows, their metabolic rate, heart rate, and breathing slow down. Glycogen, a form of glucose, stored in their bodies, gives them enough energy to survive their torpor. This period of inactivity is a not just for survival, but helps regulate hormones, particularly those involved in reproduction.
Frogs
Like aquatic turtles, aquatic frogs hibernate under water in the winter, but cannot slow down metabolism enough to survive buried under mud. Needing oxygen, they stay on top of the mud or partially buried. They even occasionally swim around. Some terrestrial frogs and toads do burrow deep into the soil. Others, such as tree frogs hide in crevices of logs and rocks, under leaf litter, or in abandoned rodent tunnels. When exposed to freezing temperatures, frogs do not die. Their livers produce glucose to increase blood sugar levels and limit the formation of ice crystals. Wood frogs have been found almost completely frozen with no brain activity or heartbeat. When temperatures warm above freezing, they thaw out and their brains and lungs start to function.
Insects
Often we wonder if cold winters will diminish the numbers of some of our unpleasant insects, such as mosquitoes and Japanese beetles. The answer is “no.” Insects have a variety of methods to survive harsh winter weather. Monarch butterflies make their treacherous journey of thousands of miles, but other insects perform less dramatic migrations. Unhappily for farmers, crop pests, such as potato leafhoppers and armyworms, will die in cold northern areas, so move south to warmer states and then move back north when temperatures warm. Other insects stay year-round and overwinter as eggs, larvae, pupa or adults. Praying mantids lay eggs which survive the winter. The adults hatch in the spring. Many insects spend the winter in the larval or pupal stage. Some, such as Japanese beetle grubs, burrow deep into the soil to avoid freezing. Female native bees build nests in plant stems, underground or in the crevices of logs or even buildings. They provision the nests with pollen and lay their eggs before dying. The eggs hatch and the larvae subsist on the pollen until they emerge as adults in the spring. Many Lepidoptera larvae (caterpillars) hide under leaf litter or in hollow logs. To protect themselves from freezing, many larvae replace the water in their bodies with glycerol that acts as an antifreeze. Some insects hibernate as adults in crevices of trees, in leaf litter, and under rocks and logs. Some wasps shelter in the eaves of houses or outbuildings. The Mourning Cloak butterfly overwinters as an adult, building up glycerol in its body to prevent freezing. This is the first butterfly you will see in the spring.
Chickadees
Lively and friendly, chickadees are among our favorite backyard visitors. These tiny birds use a variety of strategies to endure cold winters. Â Chickadees living in the north have evolved to be 25% larger than Southern chickadees, their greater mass offering better cold resistance. All chickadees have a denser layer of down insulating feathers than other songbirds of similar size. They are adapted to and can find food in a variety of habitats: coniferous forests of the north, deciduous forests, marsh land, and often sparsely landscaped suburban yards. Chickadees plan ahead and lay in a stockpile of seeds before cold weather sets in. They have a seasonally-enlarged hypothalamus that allows them to remember where they have stored their caches. At night, chickadees prefer to nest alone in cavities of trees or logs. Without companions to share warmth, they go into torpor, reducing their body temperature by 12-15o and slowing their metabolism as much at 30%. By morning, the fat reserves they accumulated during the day are depleted. They shiver for a while so they can rouse themselves and start foraging again. During the day, they remain warm by exercise and sunning.
HOW YOU CAN HELP
We can help wild animals in winter through our gardening practices by offering nesting and overwintering places.
- Let leaves remain undisturbed in beds
- Allow stems of native perennials to remain in place until spring
- Leave some areas un-mulched and the soil un-disturbed
- Let rocks and logs stay in place
- Build a brush pile or rock pile
QUESTIONS FOR THE FUTURE
Scientists are still unravelling the mysteries of animal winter survival strategies. How can bears and other hibernators stay immobile for so long yet awaken without loss of muscle or bone mass? After burning fat for months, their cholesterol levels rise yet they do not have hardening of the arteries. Can studying the physiological changes of an animal during torpor lead to therapeutic strategies for humans? As warming temperatures alter the blooming time of plants, will insects and migrating birds be affected? How will burrowing animals manage in wet snow that does not provide the insulating power of dry cold pack snow?
Winter eventually ends. The increasing warmth and daylight stir animals out of their dens and migrating creatures return. Buds and insects appear and the summer cycle begins.
Feature Photo: Black-Capped Chickadee by Vickie J. Anderson, CC-BY-SA 4.0
SOURCES:
Butterfly Migration and Overwintering, U.S. Forest Service
The Courage of Birds, (Chelsea Green Publishing), Pete Dunne
History, Purpose and Status of Caribou Movements in Northwest Alaska, National Park Service
How Do Insects Survive Cold Weather, University of Illinois Urbana-Champaign Extension
How do Turtles Survive the Winter?, Carlton College, Cowling Arboretu
How Wildlife Handle Winter, Massachusetts Division of Fisheries and Wildlife
Humpback Whales, National Park Service
Where Do Pollinators Go In Winter?, Xerces Society
Where Do Frogs Go in the Winter?, Virginia Department of Wildlife Resources
Wildlife in Winter, Purdue University Extension
Wildlife Winter Survival Strategies, National Wildlife Federation
Winter World, (Harper Collins, 2003) by Bernd Heinrich
Â