In books and movies like Call of the Wild, huge, vicious huskies pull sleds for hundreds of miles over the Alaskan tundra. A Moment of Science wonders, what kind of dogs are sled dogs?
Most champion sled dogs are not pure husky: in fact, the "Alaskan Husky" isn't really a breed at all: it's a mix of various breeds. Although not pure bred, every sled dog must be carefully bred for life on the trail.
The dogs need heavy coats to protect them from the cold, and tough feet to prevent injuries. Unlike the tough dogs in the movies, dogs with softer feet wear booties when they run.
The dogs also need compact bodies to maximize endurance. The sled dogs' remarkable endurance is thanks to their large heart-to-body ratio, and their ability to carry oxygen from the lungs to the muscles, which is about three times that of a human being.
On film, large dogs seem powerful, but their size decreases their ability to use oxygen efficiently. In competitive dog-sledding, dogs usually weigh less than 55 pounds and can comfortably trot over 70 miles a day.
A sled dog also needs the right temperament. They must love running and be sociable and obedient. Like wild dogs, each team has a hierarchy, but the dogs struggle for position by taking aggressive or submissive postures such as squealing and rolling. Real dogfights are rare because the dogs are carefully introduced and trained before going out on the trail.
The careful crossing of northern dogs, like Malamutes, with southern breeds like Greyhounds produces dogs with the right stuff to be sled dogs.