Monkeys, baboons, squirrels and rhino bark, so the phenomenon is not peculiar to dogs. The chirping of birds, tape recorded and played back at slower speeds, sounds surprisingly similar to the bark of a dog. Barking is usually a warning that someone or something is approaching. The alarm does not indicate whether the person approaching is friend or foe, but merely acts as an alert, ”Halt, who goes there?” When this individual has been identified, the behaviour changes, and if the subject is seen as a friend then the dog may whimper and wag its tail. If the visitor is considered potentially hostile then the dog will growl and become threatening.