The ever-changing cycle of seasons is one of the pleasures of living in a temperate climate. From the emergence of tender life in spring and the abundance of summer, to autumnal ripening followed by the long, cold winter, each season has its unique conditions. Survival demands that plants, animals, fungi, and even bacteria are adapted to survive all the seasons, and, for most, winter is the most difficult. This might seem obvious, but the implications are important.
Because of the tilt of the northern hemisphere away from the sun in the winter, we experience shorter days and lower temperatures, often below the freezing point of water. Water is essential to life, the fluid matrix of living cells. When ice forms inside cells it not only slows down their functioning, but the sharp ice crystals can pierce cell walls, destroying them. To avoid this, many plants and animals have developed ways to protect themselves from the cold, by reducing the amount of water in their cells during winter or producing some kind of antifreeze to prevent the formation of sharp crystals.
Trees and shrubs, with persistent woody stems that live for many years, have ways to cope with winter. In late summer, they respond to longer nights by ceasing growth of tender new leaves. Winter-hardy plant cells contain as little water as possible. Deciduous trees form hardy buds in summer and shed all their leaves in the fall. The buds stay closed until it is safely spring; they need warm weather and lengthening days before their new leaves and flowers start to emerge.
Evergreen leaves have a waxy coating that prevents them from losing water. They keep their needles all winter, and during mild spells these evergreen leaves carry out some photosynthesis. Evergreens are also shaped so that they can safely carry or shed a load of snow and ice. They have branches that angle downwards, strong attachments to a single trunk, and flexible twigs, all of which help them emerge unscathed from winter storms. The oily resins that make evergreen needles smell so good (and less tasty to herbivores) also act as antifreeze.
Herbaceous (non-woody) plants also survive the winter in several different ways. Annual plants complete their lives in one growing season and overwinter as seeds, which are dry and have thick coats. Biennial plants like mullein or evening primrose form a flat rosette of leaves and a food-storing root in the first year, then flower, go to seed, and die in the second. Many hold their seed heads on stiff stalks that release their seeds gradually throughout a windy and snowy winter. Perennials like asters die down to the ground, but their roots survive, and we often see their seed heads above snow, too. The seeds of all are important food sources for birds, mammals, and insects, and those that aren’t eaten, or are eaten but not digested, get dispersed far and wide.
Like plants, animals also have a variety of ways of coping with winter. Animals have four basic strategies for surviving a season of cold and want. Some leave and go to a more suitable place, or migrate. Others become dormant, sleeping for days or weeks but waking and becoming active at times. A small number of mammals hibernate, entering a deeply torpid state for the whole winter. Many, including humans, remain active throughout the winter. And a few, like bats, migrate and hibernate.
Most of those animals in the Northeast that migrate in winter are birds: two-thirds of our bird species leave in the fall and return in the spring. A few large insects also migrate. No individual insects live long enough to go south and come back, but in the past fifty years we have learned that the monarch butterflies that hatch here in late summer will winter in Mexico, and their offspring or the next generation returns. Green darner dragonflies migrate in huge numbers and are often spotted by hawk watchers on mountaintop observatories. Young dragonflies migrate back in the spring.
Hibernation (using the word in its broad sense) is something many animals do, each in its own way. Because their body temperature is close to that of their environment, most invertebrates, amphibians, and reptiles remain inactive during a northern winter. Frogs and toads cope in different ways. Pond-dwelling frogs burrow into muddy pond bottoms where the temperature remains above freezing. Toads dig themselves backwards down into soft earth, just deeper than the frost. In a year with deep snow cover in the woods, they may not need to dig far, but in a cold and snowless winter they must go deeper to avoid being killed by freezing. The most hardy frog species, including spring peepers, wood frogs, gray tree frogs, and chorus frogs, endure winter in a way that seems fantastic. They use sugar as antifreeze. As the temperature falls to the freezing point, the frog’s liver very rapidly produces glucose, flooding the frog’s organs with it and extracting most water from the cells via osmosis. Ice crystals do form but harmlessly, in the spaces between the tissues. These frogs, which overwinter in leaf litter, can wake up and become active during thaws, then refreeze with no ill effects.
Turtles stop eating in October and dive to river bottoms or burrow in pond mud. Painted turtles can live through the entire winter without breathing. They have evolved unique metabolic ways to survive more than half of every year without access to air or food. Garter snakes, which don’t socialize otherwise, may slither off en masse to convenient nearby hibernacula, hollows in gaps under stones or even deep in manure piles. They can’t dig, so they must find available spaces below the frost line. Spring’s warm weather will bring out the turtles and snakes, and also the plants and invertebrates that serve as their food sources.
Few mammals hibernate, for it is a very specialized strategy. Here in New England, our hibernators are bats, jumping mice, woodchucks, and, according to most biologists, black bears. The first three lower their body temperature and metabolism drastically. A hibernating woodchuck’s body temperature drops from 96.8° to 38° F, its heart rate drops from around 130 beats per minute to as low as four beats per minute, and its breaths drop from forty per minute to just one every five to six minutes. The very large and well-insulated black bear only lowers its body temperature about ten degrees to 88°F, and it wakes easily in winter, but its digestive system shuts down completely as it lives off its fat reserves, and it does not eat, urinate, or defecate.
Dormancy is more intermittent and restless than hibernation. Chipmunks, raccoons, and striped skunks all sleep through most of the winter. Raccoons put on a large amount of fat and sleep through cold spells. Skunks sleep for weeks at a time in their dens, and several females may huddle together for warmth. Both skunks and raccoons will go out to forage in milder weather. Chipmunks stay out of sight in their deep burrows but wake to eat from their big caches of seeds.
The fourth strategy – that of all the birds and mammals we see regularly, or whose tracks and sign we find in winter – is to resist the cold and remain active. Because mammals and birds maintain a constant body temperature, which takes a lot of energy, finding food is a key focus of their lives in winter. Many change their food sources, from insects or green leaves to whatever else is available, including berries, buds, twigs, and seeds. Keeping warm and conserving energy is another necessity. Beavers stay warm and active inside their lodges, using underwater entrances to reach their food supply – branches cached beneath the ice. Freshwater fish, mud puppies, insects and crayfish in deep lakes and swift brooks are safe from freezing and lead active winter lives in the cold water these habitats.
When we go for walks in winter, we see bare limbs and evergreen boughs, sturdy weed stalks and dry seed heads, the signs of plants in winter mode. And we find the tracks of deer, snowshoe hare, squirrels, mice, and many other animals, all doing what it takes to find food, shelter, and sources of water in a season of hardship.
Heinrich, Bernd. Winter World: The Ingenuity of Animal Survival. New York: Harper Collins, 2003.
Holland, Mary. Naturally Curious. North Pomfret, Vermont: Trafalgar Square Books, 2010.
Levine, Carol. A Guide to Wildflowers in Winter. New Haven: Yale UP, 1995.
Marchand, Peter. Life in the Cold: An Introduction to Winter Ecology. Third Edition. Hanover, NH: UPNE,1996.
Stokes, Donald. A Guide to Nature in Winter. Boston: Little, Brown, 1976.