The AKC breed standard is that eyes may be brown, blue or black; one of each or Particoloured are acceptable (complete is heterochromia). These eye-color combinations are considered acceptable by the American Kennel Club. The parti-color does not affect the vision of the dog. Siberian Husky tails are heavily furred; these dogs will often curl up with their tails over their faces and noses in order to provide additional warmth. As pictured, when curled up to sleep the Siberian Husky will cover its nose for warmth, often referred to as the 'Siberian Swirl'. The tail should be expressive, held low when the dog is relaxed, and curved upward in a 'sickle' shape when excited or interested in something.