Winter Weather Ways
Vol. 7 Issue 1, Winter 2002
By Leslie McCasker
The human animal usually spends the winter in a home with some sort of heating; they put on extra layers of clothing and heavy coats for added warmth when they go outside. When the need for food grips them, they go to the grocer’s store. But what about the wild animals that live around us? The biggest problem for most animals in the winter is finding enough food. Wild animals cope with the changes in weather and availability of food in one of three ways: adjusting, hibernating or migrating.
Most land-bound animals are forced to remain and stay somewhat active during the winter. They must adjust to our changing weather. Many make changes in their behavior or bodies. Cold-blooded animals (i.e., insects, reptiles, amphibians, and fish) must hibernate if they live in environments where the temperature and therefore their own body temperature drops below freezing. Box turtles burrow into the soil or mulch piles. Reptiles like lizards and snakes seek protective cover under rocks, leaf litter and mulch piles. Many others hoard food stores to get them through the winter. Squirrels and mice stash their food in tree cavities, under leaf litter, or in holes in the ground. Still other animals, such as voles, have communal food storage areas underground.
Most animals prepare for winter by undergoing physiological changes -accumulating body fat is the most crucial, a vital insulator for warmth and source of energy. Many of these animals, like deer, squirrels, and raccoons, spend the fall feasting on energy- and fat-rich acorns and other nuts that help them put on an insulating layer of fat beneath their skin. Their sparse summer coat is gradually replaced by a warmer one made up of a dense layer of under fur and a thick surface layer that helps to trap body heat. These species, as well the rabbit, otter, muskrat, fox, and bobcat, remain active throughout the winter, foraging or hunting daily. For other species, such as opossums and skunks, winter activity is temperature dependent. During extremely cold periods they spend their time in their nests or dens, curled up in a semi sleep dormant state. Certain insects and spiders stay active if they are in frost-free areas and can find food, while others are normally active in winter (i.e., winter stone fly, crane fly, and snow fleas).
Hibernation is the practice among certain animals of spending part of the cold season in a more or less dormant state, apparently as protection from cold when their normal body temperature cannot be maintained and food is scarce. This deep sleep allows them to conserve energy and survive the winter with little or no food. Hibernation is caused by a chemical trigger released by the brain when the animal experiences extremes of temperature, lack of food, or decreased amounts of daylight. Most hibernators prepare in some way for the winter. Some store food in their burrows or dens, to eat when they awaken for short periods. Many eat extra food in the fall while it is plentiful, and store it as body fat to be used later for energy.
Hibernators have two kinds of fat: regular white fat and a special brown fat. The brown fat forms patches near the animal’s brain, heart, and lungs. The fat sends a burst of energy to warm these organs first when it is time for the animal to wake up. Hibernating animals are able to store enough food in their bodies to carry them over until food is once again obtainable. They do not grow during hibernation. Their bodily activities are reduced to a minimum; in fact they may have only one or two heartbeats every minute. This energy-efficient dormant stage enables the hibernating animal to have periods of inactivity that last for weeks or even months.
True hibernators go into such a deep sleep that they are difficult to wake and may even appear dead. Their body temperature drops, and their breathing and heart rate drop significantly. For example, the groundhog, or woodchuck, is one of our true hibernators. It spends most of the summer in fields and in tunnels it has dug below. During winter, the groundhog finds it way to the deepest recesses of those tunnels where it will hibernate. A hibernating groundhog’s heart rate slows from 80 beats to 4 beats per minute, and its body temperature drops from 98F to as low as 38F. If its temperature falls too low, it will awaken slightly and shiver to warm up a bit.
If an animal lives in an area where the winter is mild, it may hibernate only briefly, or not at all. However, even when the winter is severe, hibernators may wake up for short periods every few weeks to use their “toilet rooms” and eat if food is available. Animals such as raccoons, skunks, and some chipmunks are light sleepers and are easily awakened. They may sleep during the most severe weather and wake to roam and forage for food in milder weather. Some insects spend their larval stage in a state of hibernation.
Our largest hibernator is the bear. Bears are unique because, unlike other hibernators, they do not eat, drink, or excrete at all while hibernating, which can be as long as six months. Although the quarters are cramped, female bears give birth and nurse their cubs during hibernation. Other true hibernators include the jumping mouse, little brown bat, the eastern chipmunk, and some species of ground squirrels. Unlike bears, bats do not sleep continuously throughout the winter. Instead, their hibernation consists of alternating periods of arousal (wakefulness) and torpor (deep sleep), generally for two week periods. Because each arousal uses a tremendous amount of energy, it is extremely important not to disturb hibernating animals so they will have enough energy to get through the entire winter.
Our winged neighbors have the ability to relocate to more suitable environments when resources like food, water, and cover diminish with the onset of winter. Some birds are able to adjust to these changes and remain in the same environment all year. We call these our resident species. However, other species must change their location in order to survive. Migration is the regular, periodic movement of a species to an area with more suitable environmental conditions. A single round -trip may lake the entire lifetime of an individual, as with the Pacific salmon; or an individual may make the same trip repeatedly, as with many of the migratory birds and mammals. More than one- third of the world’s bird species migrate each fall and spring. The animals may travel in groups along well-defined routes; or individuals may travel separately, congregating for breeding and then spreading out over a wide feeding area.
Various factors determine the initiation of migration. In some cases, external factors such as temperature, drought, food shortage alone may cause the animals to seek better conditions. In many species, migration is initiated by a combination of physiological and external factors. In birds the migratory instinct is related to the cycle of enlargement of the reproductive organs in spring and their reduction in fall. Studies have shown that variation in day length is the chief external stimulus for this cycle: light received by the eye affects the production of a hormone by the anterior pituitary gland, which stimulates growth of the reproductive organs.
Much study has been done on how migrating animals navigate, although the subject is still not well understood. Studies show that salmon depend on their olfactory sense to locate and return to their stream of origin. Herbivorous mammals often follow well established trails and probably also use their sense of smell. Bats, whales, and seals use echolocation to navigate in the dark or underwater. Some whales also appear to take visual bearings on objects on the shore during their migrations.
Migratory birds are believed to use the stars, sun, and geographic features as guides. Night migrating birds are sometimes disoriented by prolonged heavy fog. Day-migrating birds navigate by the sun and make use of geographic features, particularly shorelines. Most migratory birds travel within broad north-south air routes known as flyways.
There are four major flyways in North America: the Pacific, Central, Mississippi, and Atlantic. The space within a flyway that is used by a particular group of birds is called a corridor. The breeding grounds of a bird species are regarded as its home territory. Some migratory birds winter only a few hundred miles from their breeding grounds, while others migrate between the cold and temperate zones of the two hemispheres. Such migration is seriously affected by the increasing rate of destruction of the natural habitats. The longest migration journey is made by the arctic tern, which alternates between the Arctic and the Antarctic.
The monarch butterfly has a north south migration pattern that resembles that of many birds. One monarch population that inhabits the northeastern and mid- western regions averages a flight speed of 12 mph as the butterflies head for winter quarters in Mexico’s Sierra Madra mountains. Monarchs begin their return trip in the spring, but they breed along the way and then die; the new generation completes the journey.