Starting in 1984, all dogs are examined by veterinarians/nurses before the start of the race, who check teeth, eyes, tonsils, heart, lungs, joints, and genitals; they look for signs of illegal drugs, improperly healed wounds, and pregnancy. All dogs are identified and tracked by microchip implants and collar tags. On the trails, volunteer veterinarians examine each dog's heart, hydration, appetite, attitude, weight, lungs, and joints at all of the checkpoints, and look for signs of foot and shoulder injuries, respiration problems, dehydration, diarrhea, and exhaustion. When mushers race through checkpoints, the dogs do not get physical exams. Mushers are not allowed to administer drugs that mask the signs of injury, including stimulants, muscle relaxants, sedatives, anti-inflammatories, and anabolic steroids. , the Iditarod claims that no musher has been banned for giving drugs to dogs. However the Iditarod never reveals the results of tests on the dogs. Each team is composed of twelve to sixteen dogs, and no more may be added during the race. At least five dogs must be on the towline when crossing the finish line in Nome. Mushers keep a veterinary diary on the trail and are required to have it signed by a veterinarian at each checkpoint. Dogs that become exhausted or injured may be carried in the sled's "basket" to the next "dog-drop" site, where they are transported by the volunteer Iditarod Air Force to the Hiland Mountain Correctional Center at Eagle River where they are taken care of by prison inmates until picked up by handlers or family members, or they are flown to Nome for transport home. According to Iditarod veterinarian Dr. Stuart Nelson, Jr., "Reasons for dropping dogs are numerous. Attitude problems, fatigue, illness, immaturity, injury, being “in heat,” lack of speed and musher strategy, are the more common ones." The dogs are well-conditioned athletes. Training starts in late summer or early fall and intensifies between November and March; competitive teams run before the race. When there is no snow, dog drivers train using wheeled carts or all-terrain vehicles set in neutral. An Alaskan husky in the Iditarod will burn about 9,666 calories each day; on a body-weight basis this rate of caloric burn is 3.5 times that of a human Tour de France cyclist. Similarly the VO2 max (aerobic capacity) of a typical Iditarod dog is about 240 milligrams of oxygen per kilogram of body weight, which is about three times that of a human Olympic marathon runner.