Dog Growls Reveal Size to Human Listeners

June 3, 2008 -- Few animal listeners, including humans, miss the basic "back off" message of a dog growl, but new research shows such sounds also tell us how large and heavy the growler is.

The study, which is published in the Journal of the Acoustical Society of America, presents the world's first evidence for interspecies size assessment based solely on vocal signals.

Lead author Anna Taylor told Discovery News "it is the depth of the growl that provides us with the greatest cue."

Taylor, a psychologist at the University of Sussex, and her colleagues recorded growls emitted by 30 privately owned domestic dogs of 22 different breeds. An experimenter would get a dog to growl by showing up at the dog's residence, approaching the canine and then staring into its eyes, which is perceived as an intimidating move and elicits a defensive growl.

The researchers then played these growls back to over 50 human listeners, who were asked, "What is the size of this dog?" During one part of the experiment, the listeners also heard growls that had been acoustically altered to reflect sounds one would expect to hear from larger or smaller canines.

In virtually all cases, the listeners correctly guessed the general size of the dog, be it a 77-pound Rottweiler or a 2.2-pound miniature Dachshund, based on just the animal's growl. The researchers were also able to predict how the test subjects would rate the artificially manipulated growls.

Taylor explained that dog growls result from two acoustic features: formants and fundamental frequency. Formants, she said, are the resonant frequencies of a voice.

"These frequencies are determined by the physical properties of the vocal tract (size and shape). Because the vocal tract itself is directly linked to the size of an individual, this means that the distribution of formants is an acoustic correlate of body size."

Fundamental frequency, on the other hand, is the frequency of vibration of the vocal folds.

"This depends on several factors, including the mass, length and tension of the vocal fold," she said.

The fundamental frequency is perceived as the pitch of a vocalization. Since vocal fold characteristics are not linked to the individual's body size, the pitch is not a good predictor of how big the individual is.