There are stories of mushers who, when considering buying a new Husky, try the dog for the night, letting him sleep on the snow. If the snow is melted there in the morning, the dog is labeled a “melter” and returned. Being a melter would indicate the dog gave off too much body heat and could be overly susceptible to the cold.

Northern breeds have evolved for thousands of years to keep warm under frigid conditions. These breeds are the dogs we tend to think of as Spitz-like, with thick stand-off coats, small pricked ears, and medium to stocky builds. They include breeds such as the Siberian Husky, Alaskan Malamute, and Samoyed as well as mixes of Husky-type dogs.

When the temperature plunged to minus 130 degrees F in the 1973 Iditarod, it was the mushers, not the dogs, who complained. Northern breeds seem to think nothing of sub-freezing temperatures, and seem happiest when the thermometer registers between minus 20 and plus 10 degrees F.

But there are limits. When a dog’s body temperature goes below 98.6 degrees F, cells begin to have problems functioning properly. When her temperature goes above 108 degrees F for a sustained period, cells begin to die. The dog must keep his body temperature within this range.

How do they do it?

Dogs lose heat by way of four heat loss mechanisms: radiation (heat moving from skin to air), evaporation (heat carried off by contact with water, such as through panting), convection (heat carried off by air flowing over the body), and conduction (heat transferred by contact with a cold surface).

Size matters

Since heat is lost from the surface of the body, the more surface area an animal has, the more heat it loses. Larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass and stay warmer in the cold.

To retain heat you want the least body surface area compared to body volume — in other words, you want a sphere. The stocky northern dog is a compromise between the need for a big thickset body and a build that enables it to pull freight or cover ground.

The dog uses his behavior to modify his effective body conformation when it sleeps curled up in a ball, making itself as spherical as possible. Sleeping in a ball also has one more advantage: by tucking the nose under his tail, not only is the bare moist nose protected, but the air the dog inhales is warmed from heat trapped between the tail and body.

Because of their high surface to volume ratio and high circulation, ears lose a lot of heat. In northern breeds they are as small, thick and fur-covered as possible to retain heat and prevent frostbite.

The fat and the furriest

The body also retains heat by increasing its insulation with fat and fur. Fat is far more effective, but also heavier, so fur is a better solution for an active animal. The fur of northern dogs is thick and double coated, consisting of both long, stiff guard hairs and shorter, downy undercoat. Most dogs have a double coat to some degree; northern breeds just have slightly longer and thicker coats, with more abundant undercoat. The bristly guard hairs are somewhat water resistant, preventing snow from seeping into the undercoat. The downy undercoat is actually fine and wavy, forming a dense layer of insulation.

Northern breeds also have a “stand-off” coat, which means the hair doesn’t lie flat to the body. This is achieved because the angle at which each hair follicle is implanted is higher, about 45 degrees in northern breeds compared to less than 30 degrees in sleek-coated breeds. This allows for a thicker layer of insulating fur.

The dog’s legs face special challenges. They must be fairly lightweight for efficient locomotion, since moving a heavy leg takes far more energy than moving a light one. That means they can’t carry much insulating fat. They also can’t carry heavy fur, which could become loaded with ice.

Built-in booties

But don’t their feet get cold running in snow? Turns out that dog feet have several mechanisms that keep them warm. First, their veins and arteries run very close to one another, creating an intricate heat transfer system from warm arterial blood to cold venous blood. Second, they have a type of freeze-resistant connective tissue and fat in their paw pads. The end result is that a dog’s paw is about as good as a penguin’s wing in staying warm!

Many of the dogs that compete in sled races don’t have all the heat-retaining features that traditional northern breeds do because they need to get rid of excess heat, not hoard it. Racing huskies, which must sustain a high rate of muscular activity for long periods, must be built to give off heat. Thus, typical racing huskies do not have particularly thick coats, they often have drop or large ears, and they are seldom more than 50 pounds.

But the dogs native Alaskans relied upon, the dogs that led the polar expeditions, had each of these heat-conserving features. Their lives, and the lives of the people who depended on them, depended on it.

Read related stories on Dogster:

• Tips for Safe Fun with YourDogsin the Cold Weather
• A Criminal Matter? Iditarod Dog Dies After Being Buried in Snow
• Finally, a Happy Ending atIditarod: A Lost Dog Is Found After 7 Days
• 10 Gifts for Dog Lovers Who Also Love the Outdoors

Learn more about dogs with Dogster:

• The 10 Biggest Misconceptions About Guide Dogs for the Blind
• 6 Things to Remember When You Have a Fearful Dog
• Four Things You Should Know About Your Dog’s Growl

Featured Image Credit: Zuberka | Getty Images