Sled dogs, known also as sleigh dogs, sledge dogs or sleddogs are a group of dogs that are used to pull a wheel-less vehicle on runners (a sled or sleigh) over snow or ice, by means of harnesses and lines. It is not certain when this unique form of transport was first thought of by humankind nor where it originated, but it may well have been in eastern Siberia, whose tribes have a long history of nomadic winter travel. Several distinct dog breeds are in common use as sleddogs, although any medium-sized breed may be used to pull a sled. Purebred sleddog breeds range from the well-known Siberian Husky and Alaskan Malamute to rarer breeds such as the Mackenzie River Husky. Dog drivers, however, have a long history of using other breeds or crossbreds as sleddogs.