Scientists in Hungary recorded growls by dogs in different situations and analyzed the structure of the calls. The growls recorded during play were very different than the other two calls in that they were shorter and higher in pitch.
In an experiment that was also a part of their study, they allowed a dog to approach a high quality food item (a cooked meaty bone) when alone in a room. Then, they piped in the recorded sound of either a growl made when a dog was threatened by a stranger or a growl made by a dog guarding a bone. They tested 41 dogs in this way and found that dogs were significantly more likely to back away from the bone when they heard the bone-guarding growl than when they heard the threatened-by-a-stranger growl.
Dogs have a huge range of vocalizations and yet much remains to be learned about the differences in meanings and structure of their acoustic communication. This study is one step towards a fuller understanding of the vocal repertoire of dogs.