By Jennifer Viegas, Discovery News
One dog growl may sound like another to human ears, but a new study shows for the first time that dogs receive specific information in growls that conveys meanings like "get away from my bone" or "back off."
The study, accepted for publication in the journal Animal Behavior, presents the first experimental indication that domestic dogs rely on context-dependent signals when they growl at each other.
The findings add to the growing body of evidence that animal calls are far more complex than previously thought. For example, prior research suggests chimpanzees communicate information about food quality, while birds, prairie dogs, chickens, squirrels, primates and other animals likely share information about predator types.
Of all of these sounds, dog growls are particularly intense.
"A growl is a short-distance warning, not like a bark or howl, which you can hear over a large distance," co-author Peter Pongracz told Discovery News. "When a dog growls, the opponent is near, so he/she can hear clearly that the next few steps forward will not be greeted with a warm welcome."
"The other common usage of growls is during play," added Pongracz, a behavioral biologist at Eotvos Lorand University. "This can be explained by the fact that dogs very often play 'martially' — they wrestle, chase, play tug of war — so these actions and visual displays are accompanied with martial sounds too."
For the first part of the study, Pongracz and his colleagues recorded 20 adult dogs of various breeds growling during the following situations: when a threatening stranger approached, during a tug-of-war game and while guarding a large, meaty bone.
The scientists electronically analyzed the recorded sounds and found that play growls stand out from the other two types because they are, in part, shorter and higher pitched. The computer analysis didn't show any major differences between the other two growls, but dogs picked out the specific meanings immediately during the second part of the study.
For this experiment, 41 adult pet dogs of various breeds were recruited from the databases of the Clever Dog Lab in Vienna and the Family Dog Project in Budapest. The researchers placed a freshly cooked, meaty and juicy large calf bone in a bowl. All of the study dogs found the bone irresistible.
But as the test subject dogs approached the bone, the researchers played back the previously recorded growls through a hidden speaker. The hungry canines only jumped when the bone-guarding growl was played, even though the threatening stranger-associated growl sounded just as menacing to human ears.
While it remains unclear how dogs communicate such precise information, Pongracz said one possibility is that dogs are very sensitive to the emotions of other canines.
"One could argue that a dog faced with a threatening stranger is more 'afraid,' for example, while the dog that is defending his/her bone from another dog is more 'aggressive,'" Pongracz explained, but he hopes future analysis of dog growls will reveal more about how these sounds are structured.
Daniel Mills, a professor of veterinary behavioral medicine at the University of Lincoln, said the study adds to a "growing recognition of the greater complexity of vocalization by dogs."
"I believe that some of the unanswered questions about assessing how vocalization relates to the inner emotional states of dogs will be addressed and, with this knowledge, we will gain much greater insight into the inner lives of dogs and other species," Mills added. "Hopefully this will also lead to a greater appreciation of, and respect for, non-human animals in general."