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.
A surprising twist to the findings is that, based on prior research, it can now be said that humans are better at predicting the body size of dogs than the body size of other humans when using vocalizations as the clue.
Taylor and her team believe humans are all "hardwired" to predict body size based on formants. As we evolved and developed speech skills, which made our vocal tracts more flexible, this probable genetic predisposition became an "outdated" mechanism when trying to guess the size of other humans based upon what we hear.
Because dogs have not developed speaking skills as we have, their vocal tracts are not as flexible, so "their resonant frequencies still give an accurate picture of their body size," she said.
Tobias Riede of the National Center for Voice and Speech in Denver told Discovery News that the new research "represents an interesting step towards a better understanding of dog-human communication."
"(The scientists) correctly measured some physical aspects of sound production, which correspond to dog size, and attempted to relate them to perceptual abilities of human listeners," Riede said.
The researchers hope future studies will investigate if humans can make similar predictions based upon the vocalizations of other creatures.
The scientists don't know what to expect, as the recent discovery may be somewhat of an animal kingdom fluke due to the unusually close evolutionary relationship people have enjoyed with dogs over the millennia.
Related Links:
Jennifer Viegas' Blog: Born Animal
National Center for Voice and Speech
our sites
video
mobile
shop
stay connected
corporate
