Staying warm: How do critters do it?

Rick Spitzer
Special to the Daily
Thick waterproof fur and a layer of fat is what keeps the beaver warm in cold water.
Rick Spitzer | Special to the Daily |

EAGLE COUNTY — The animals in the Rockies around us have developed many adaptations to survive in this environment. Food is the fuel to keep an animal’s body warm in the winter. A high energy demand is what drives small mammals and birds to use foods such as seeds, fruit, buds and insects rather than grasses or leaves. Water is important to circulate that warmth. The adaptations are often associated with how well an animal can obtain or manage enough food and water to survive.

There are basically four ways that the warm blooded wildlife in the Rockies manage the winter: migration, hibernation, insulation and managing heat.


A number of animals do the same thing a few of our neighbors do — they pack up and leave! Some of the birds that are around in the summer have no food supply available in the winter, so they head south. Hummingbirds that feed on flower nectar travel thousands of miles to get to their winter grounds in southern Mexico. There are also some bird species that live north of us in the summer and spend winter in the valleys of Eagle County.

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Some of the large mammals also migrate, but shorter distances. Some elk may move many miles from high terrain such as the alpine tundra down to montane valleys during the winter. Bighorns move to lower elevations and south-facing slopes. Mountain goats tough it out in the same area all winter, but they seek shelter on the lee side of the peaks and on south-facing slopes.


Small mammals have shorter fur than large animals. Small mammals also have a high surface area to mass ratio. Because of those two factors small animals tend to loose body heat more quickly than large animals like elk and bears. In addition, for many small mammals it is difficult to find food and water in the winter so their strategy is hibernation, a very deep sleep. These small mammals move underground or into nests that insulate them from the surface temperatures to hibernate. The hibernation in this area may last for 200 days, from early October to early April.

Animals that hibernate add body fat during the short summers and very slowly burn that fat for energy and body maintenance during their deep winter sleep. Movement burns energy more quickly, so they sleep to conserve that energy. If they fail to put on enough fat during the summer, then they will not survive the winter. During hibernation, some animals may lose as much as 40 percent of the total body weight.

Animals that truly hibernate are difficult to wake. Some store food in their dens or burrows and may wake for short periods to eat. The body temperature, respiration and heart rate drops. The body temperature may drop from the upper 90 degrees to as low as 30 or 40 degrees. Heart rates may be as high as 150 beats per minute in summer and drop to a low of 15 beats per minute in hibernation. Respiration rates may drop from one every second or so to a low of one or two a minute, and it may even stop for minutes at a time.

Bears? Some literature says that bears and some other animals do not truly hibernate because they wake often and move around in their dens. There is no major drop in body temperature and they are alert to what is going on around them. They will respond, if disturbed. Females also have their cubs during the winter and do take care of them. Bear respirations drop from 10 breaths per minute to a low of one breath every 45 seconds. Heart rate may drop from 50 beats per minute to 10 per minute. Before they move into their dens they may consume an incredible 15,000-20,000 calories per day, which is more than twice their summer intake. (A Big Mac is around 700 calories.) Bears can survive for as long as 100 days without eating, drinking, urinating, defecating or exercising.


Small animals that do not hibernate lose heat quickly because of their short hair and high surface area to body mass ratio and must remain very active to stay warm. To do that they need a high metabolism. Pikas, pocket gophers and mountain shrews have heart rates up to 1,200 times a minute and burn a lot of energy to stay warm. To keep that going they must consume their own weight in food each day to survive. A pika stores food during the summer under the rock piles in their habitat. Pocket gophers and shrews dig through the soil (if it is thawed) or tunnel through snow to find the food they need. They may need to eat something every hour to survive!

Large animals need to deal with snow during the winter. Snow covers grasses and shrubs and make it difficult to move around in the environment. Herd animals move to south-facing slopes, and when they encounter deep snow, a lead animal will break the trail for others to follow.

Elk, mountain goats and bison are grazers and need to clear the snow with their hooves and muzzles to get to the grasses. They eat shrubs, bushes, bark and twigs during the winter months. Deer and moose tend to eat the tender parts of shrubs and bushes which may be above the snow line. They may also eat bark, pine needles and twigs during the winter months.


We put on coats when it gets colder. Mammals grow them. Their coats become thicker and shaggier during the winter. Mountain goats have a white coat for camouflage. The white hair you see is coarse, thick and long. Some animals such as bighorns, deer and elk have short, stiff outer coats. What you may not see in all these animals is a woolly underfur. All of these layers are hair, but in mammals we call it fur or pelage.

Fur on most animals has two layers. In some animals there is a third, intermediate layer called “awn” hair. Most of the layer that you see is made of what are called guard hairs. This hair is generally coarser and straight and it may be long. In some animals this hair is hollow. Most of the animals coloration is due to these hairs. It tends to repel or shed moisture. Below that layer is an undercoat or down hair that are also called ground hair. This hair is flat, woolly, curly, and wavy. These hairs are also more numerous and shorter. That hair maintains an area of dry air next to the skin to provide a thermal insulation. Animals have the ability to erect these hairs when they get cold to improve the insulation. You experience that when you get “goose bumps.”

Many of our coats and jackets have a down filling. Birds have their own down. Native birds are adapted in many ways for this cold environment. They have a very high metabolism and therefore a higher temperature, as high as 105 degrees. Small birds are at the greatest risk in winter because of a large surface area to volume ratio. A large number of people feed birds in the winter. Bird seed has a high nutritional value for birds in the cold of winter.

Feathers are incredible as insulation, and the oil coating provides waterproofing and additional insulation. Feathers trap air and prevent it from moving and transferring the heat. Birds often fluff their feathers to improve the insulation capability. Ptarmigan have feathers on their legs and feet, but some bird feet are naked. Scales on the legs and feet provide some insulation.

Fatty layers also develop in the fall in birds giving them reserves for winter and also provide some insulation. Think about the fat in your holiday turkey or goose. Fat has a lower thermal conductivity than other tissue.


Our houses have thermostats. So do birds and mammals. It is the hypothalamus in the brain. Warm blooded animals are homeothermic, meaning that they can maintain their body temperature as external temperatures change. When the weather gets colder, they can turn up the heat, as long as they have the food and water that they need. They do this by increasing the metabolic rate in tissues.

An amazing two-thirds of the heat generated in a resting animal is created in the organs of the thoracic and abdominal cavities. The brain also generates some of this heat. Movement using voluntary muscle action generates additional heat, and shivering does the same. The circulatory system helps move that heat to where it is needed.

Small anatomical parts such as fingers lose heat faster due to the surface area to volume ratio. That is why hands and feet get cold first. Skinny legs of animals such as deer are hard to keep warm, but there is a unique anatomical feature that helps out. The arteries in legs run parallel to a set of deep veins. These arteries and veins are close together to create a countercurrent blood flow which helps transfer heat from the outflow of warm arteries to the inflow from the cold, returning blood in veins. The blood going down the leg is cooled and when cold blood reaches lower extremities and paws, it loses less heat. It is amazingly efficient at recycling heat.

In addition, animals have an ability to restrict blood flow in their legs to reduce heat loss. Ducks use that to great advantage! They can maintain a normal body temperature while their feet are in freezing water.

Our homes have a furnace that produces the heat to warm the house. There is actually the equivalent of a furnace in some mammals. Animals that feed on plants such as grasses and leaves digest their foods with the help of microbes. Fermentation also occurs in their gut. All that produces heat which can help maintain body temperature. Blood vessels around the rumen and cecum where this digestion occurs in herbivores helps distribute the heat produced to the rest of the body.

Humans have figured out many ways to stay warm in the winter. We use layers and down filled, or fur-like clothing. Boot and glove heaters keep our feet and hands warm. Our homes and autos have systems to keep us warm. Even with all of that we are still no match to the wildlife when we are out in the elements.

Rick Spitzer is the author of “Colorado Mountain Passes,” published by Westcliffe Publishers and available at The Bookworm of Edwards, City Market, Amazon, and many stores across the state. The book provides photos and text about the history, lore, wildlife and scenery around the passes of Colorado.

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