Dogs express themselves vocally through a wide variety of sounds such as barks, wines, grunts, yelps, whimpers, growls, howls and chatters (also known as snapping).
There are many reasons why a dog will bark. (1) To draw attention to itself or someone (or thing) else.(2) To warn or challenge another animal (or human) in an aggressive (fearful or dominate) manner, or (3) to greet or entice play.
. As wolves barks are much lower in frequency and not as easily provoked, there is reason to believe that during the process of domestication a loud and easily provoked bark was a desirable feature in breeding dogs. The barking would help the people of the villages to prepare for unwanted intruders (animal predators or unwelcome humans), that’s one theory. Another common dog noise is the wine or whimper. These sounds are normally associated with times of extreme excitement, pain, attention seeking or a sign of submission (in an attempt to exhibit puppy-like innocence and evoke parental like tolerance). The howl is a sound mostly associated with wolves as seen depicted in many familiar scenes with a wolf sitting on a cliffs edge, head thrown back and howling at a full moon. This vocal expression is also associated with hound dogs while tied to the end of a chain connected to a dog house during hunting off seasons.
In wolf packs howls are used to draw pack members together. Once gathered the wolves may hunt, mate or simply howl at the moon (or sing for reasons unknown). In domestic dogs the howl is normally heard during times of loneliness in an effort to draw others near (this is the reason for secluded hound to howl as well). Some humans teach their dogs to ‘sing’ by holding a high pitch note or playing music of some sort. This taps into the dogs’ natural instinct to join his fellow pack members (or family) in song (howl).The final most well noted vocal ability of the dog is the growl. The growl appears in times of play, defense, or challenge.
When growling appears at times of play the sender will engage in the play bow posture and other play suggested forms of communication to assure the receiver that the growl is all in good fun (not a threat). During times of defense of challenge growls are regularly accompanied by curling lips and bared teeth, along with the appropriate ear and eye gestures. In every case, in order to distinguish which message the dog is vocally sending one must consider the other behaviors the dog is displaying, the context in which it is used, the events taking place and the surrounding environment.
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Serpell, J.(1995). The Domestic Dog its evolution, behaviour and interactions with people
Beaver, B. A Guide for Verterinarians
Koler-Matznick, J. (2002). The origin of the dog revisited. Anthrozoos, 15(2), 98-118.: from http://www.canineworld.com/ngsdcs/Orgigin.of.thedog.pdf