MSU Extension

As temperatures drop, the days grow shorter and perhaps even the snow begins to fall, humans have a multitude of ways to stay warm. We can put on more layers of clothing or a big coat – hats, gloves and scarves can cover areas of skin that may be more exposed to the elements – or we can seek shelter in a warm building. These actions help keep humans safe and healthy during a season that can post varying health risks because of low temperatures. Most animals cannot seek out these same methods of keeping warm and thriving in winter, but they have evolved amazing adaptions to survive through frigid temperatures. Michigan State University Extension explores some of the ways animals cope with winter weather.

According to the National Park Service, there are three major strategies for animals, as well as insects and plants, to survive through cold temperatures: migration, hibernation and resistance (tolerance). Let’s explore each of these in more detail.

Migration

Migration is the movement of a group of animals from one location to another, typically in order to change habitats or living environment. We might often think of birds “flying south” for the winter, but migration can be much more than that. It might involve travel east and west, changes in altitudes up or down a mountain, or even a round trip to multiple locations at different times. The length of time will vary based on the distance traveled. It could take several hours to days or even weeks for animals or insects to complete their journey.

Many factors may impact when the animals “know” it is time to move, including the length of daylight hours, changes in available food and how much energy it takes to stay warm. Examples of migratory animals are Clark’s Nutcracker, elk and mule deer, all found in Glacier National Park in Montana. All three of these animals live high in the mountains during summer, but travel to the mountain base in winter for better access to food and milder weather conditions.

Although migration might seem like a very good option to avoid winters, it places a great deal of stress on animals because it takes so much energy to travel and once they arrive, there is still competition for resources, like food and shelter, with native species.

Hibernation

Hibernation is the second strategy to surviving cold temperatures. Hibernation is long-term dormancy, or inactivity, while “torpor” is the term to describe short-term inactivity. The definition of hibernation from National Park Service’s “Winter Ecology Teacher’s Guide” is “…a physical state where an animal’s body function slows down in order to conserve energy through a season of no food and water, and cold temperatures.” This slower body function is characterized by a decrease in body temperature and reduced respiration, or breathing. The animal will generally curl up into a tight ball to help keep warm, body temperature drops, and respiration and heart rate slow down. These actions reduce the amount of energy the animal must expend to stay alive so it’s able to live off of fat reserves it has developed instead of constantly having to seek out food.

Hibernation strategies exist on a continuum from “true hibernators” to a “deep sleep” and finally an “occasional sleep.” Examples of true hibernators are Columbian ground squirrels and marmots, both of which experience an extreme body temperature drop (90 degrees Fahrenheit normally versus 39 F while hibernating) and very slow respiration (a breath every four to six minutes). Bears, on the other hand, are in the deep sleep category because they do not experience the extreme body temperature drop; instead, they grow a thick hair coat before winter arrives. Raccoons and gray squirrels fall into the occasional sleep category because they generally stay active during the winter except for extremely frigid temperatures.

Resistance

The final evolutionary adaptation is resistance or tolerance of the cold. There are many, many ways this adaption has evolved in different species. Animals that live in cold climates tend to be larger so their body mass-to-surface ratio is higher. For example, cold weather bears like polar bears are larger than bears found in tropical areas like sun bears. Birds will fluff out their feathers to keep a layer of air around their bodies, huddle together to keep warm or roost in tree cavities.

Tiny ears and tails are another adaption that animals have, like the pika, a relative of the rabbit. Small appendages that are close to the body stay warm and resist frostbite compared to having large ears or long tails. Thicker coats of either fur or hair grow in to act as an additional layer of insulation. Mountain goats have very heavy wool undercoats and hollow hairs that keep air trapped close to the body, keeping the cold and wetness out. Similar to mountain goats, moose also grow a coat with hallow hairs to keep them warm.

Shorter daylight hours help to trigger responses of the “master gland” of the body, the hypothalamus, to change behavior or appearance to prepare for the cold. Some animals will increase their food intake to build up fat reserves, allowing them to survive with a decreased food supply. Other animals, such as beavers or red squirrels, create a food cache, meaning they collect extra food when it’s available, store it and then have a supply for the winter.

Snowshoe hares, weasels arctic foxes and ptarmigans all change color as winter approaches. Their fur or feathers change from brown to white, which provides them two major advantages: The new fur or feathers are thicker and act as a better insulator than the brown summer coat, and the color change allows these animals to be camouflaged in the snow to avoid predators and hunt prey.

For more information on winter adaptations, check out the videos and information from the National Park Service.