When humans engage in highly strenuous exercise day after day, they start to metabolize the body’s reserves, depleting glycogen and fat stores. When cells run out of energy, a result is fatigue, and exercise grinds to a halt until those sources are replenished.

Dogs are different, in particular the sled dogs that run the annual Iditarod Trail Sled Dog Race in Alaska. This is a grueling 1,100-mile race, and studies show that the dogs somehow change their metabolism during the race.

Dr. Michael S. Davis, an associate professor of veterinary physiology at Oklahoma State University and an animal exercise researcher, said: “Before the race, the dogs’ metabolic makeup is similar to humans. Then suddenly they throw a switch  we don’t know what it is yet  that reverses all of that. In a 24-hour period, they go back to the same type of metabolic baseline you see in resting subjects. But it’s while they are running 100 miles a day.”

Dr. Davis, who studied the sled dogs, found they did not chew up their reserves and avoided the worst aspects of fatigue. He is pursuing the research for the Defense Advanced Research Projects Agency, which gave him a $1.4 million grant in 2003 to study the physiology of fatigue resistance of sled dogs.