Pit bulls are medium sized, solidly built, short coated dogs that require little grooming. They have a friendly temperament and are noted for their attachment to their masters as well as for their courage. Although they are short, they have extremely high muscle density and are generally capable of executing a standing four foot vertical jump. Pit bulls have been bred to have an extreme tolerance for pain. For example, the United States Postal Service, which equips its letter carriers with a pepper spray, notes that many capsaicin-based dog-repellent sprays can have little or no effect on an attacking pit bull.