I don't know if this has been covered in a previous thread, but what the hell, I'll post anyway.
Below is a link to some great photos of last year's Iditarod. This is commonly called 'the toughest dogsled race on Earth' - over 1,000 miles in total across Alaska.
The sled dogs (which are often Alaskan Huskies - not to be confused with Alaskan Malamutes, Siberian Huskies and even mongrels) are bred and trained for unbelievably hard work in very cold conditions. The teams travel tens of miles per day, with each dog pulling the equivalent of roughly its own bodyweight for hours on end. This is somehow done on a few pounds of fish or meat per day, per dog.
Tremendous stamina and strength. Fantastic!
Dogs really are Man's best friend.