London, May 24 : A doctoral student at the University of Sussex in Brighton, UK, has unveiled an acoustic property related to the length of the vocal tract that enables humans to tell the difference between a big and a small dog just by hearing their growls.
Writing about her findings in The Journal of the Acoustical Society of America, Anna Taylor has revealed that her study casts new light on the formant-a property of a sound wave related to the length of the vocal tract, which is used by animals to assess the size of other animals.
She said that it is the formant of a growl, not its pitch, which is used by humans to gauge a dog's size.
For her study, Taylor visited the homes of more than 100 dogs, made them growl defensively by invading their space and staring them in the eyes. She then recorded the snarly responses.
She manipulated 30 of the growls for her experiment. She looked at the formants, and the pitches of the different growls.
Taylor said that the biggest and smallest formants, and highest and lowest pitches could be heard in the sound clips.
She revealed that a formant is a basic acoustic property, which could be considered to be a resonant frequency of a sound wave in a vocal tract.
According to her, dogs have between five and seven formants when they make a noise.
"Larger vocal tracts belonging to larger animals produce lower formants," Nature magazine quoted her as saying.
Taylor further said that it was a "common fallacy" that the pitch of a growl could be used to determine the size of an animal, as pitch is related to the size of the fleshy vocal chords that could grow to different sizes.
The formant, on the other hand, is pretty much fixed, she added.
During the study, Taylor separated out the formant and pitch of each dog growl with computer-based acoustic software, and then re-synthesised each growl in two ways-first, by making five new versions of each growl each with different formants corresponding to a range of vocal tract lengths from the tiniest dog to the largest; and then making five new versions of each growl by altering the pitch to fit within a range of five frequencies.
The participants heard the new growls in random order in two separate experiments, and were asked to assess the size of the dog.
It was observed that where the formants were changed but not the pitch, the growls that had been manipulated to indicate a longer vocal tract were rated by the testers as coming from big dogs.
Taylor revealed that where pitch was changed but not formant, the subjects estimated dog size more accurately.
She said that all the testers thought that they were listening for changes in pitch.
Taylor believes that the link between formant and size perception might be more widely applicable than just dogs and humans
"Attribution of size based on formant is something we can do for all animals, and possibly all animals can do for each other," she said.