Barking, in addition to whining, howling and growling, is a dog's natural means of communication. Barking is characterized by a series of small, sharp sounds, that tend to vary small in tone or pitch. A dog's bark can signify territorial protection, exertion of dominance, or expression of some need. Typically, barking is "a means of communication triggered by a state of excitement." Being a natural trait, barking is not considered a behavioural problem, until it is produced in excess.