Monday, September 15, 2008 5:42 AM
Tail wagging is a behavior closely associated with dogs, who seem to engage in it more than other animals. The exact reason why dogs wag their tails may not be entirely clear, but it does seem to be linked with a desire to communicate information to other members of the pack. Being inherently social animals, dogs wag their tails to provide social cues. In combination with other body language, owners can use it as an indicator of how a dog feels about a situation.
It appears that dogs wag their tails in social situations only, and do not do so alone. It may be that dogs wag their tails in response to stimuli which they experience.
Source from http://www.wisegeek.com/why-do-dogs-wag-their-tails.htm
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