Big or small - you don't even need to look. Credit: Punchstock

If you thought a high-pitched growl would come only from a tiny dog, you could be in for a nasty surprise. Researchers have shown that it isn't the fundamental frequency, or pitch, of a growl that humans use to gauge a dog's size — it's another acoustic property related to the length of the vocal tract.

It was known that within species, formants — a property of a sound wave related to the length of the vocal tract — are used by animals to assess the size of other animals. But it had never been shown to happen between species. Anna Taylor, a doctoral student at the University of Sussex, Brighton, UK, set out to show that formants are used between species as a cue for size by seeing how humans respond to growls from different-sized dogs. Her results are published in The Journal of the Acoustical Society of America.1

Taylor visited the homes of more than 100 dogs, armed with nothing but a microphone, a steely stare, and the dog owners' consent. Taylor made the dogs growl defensively by invading the dog's space and staring it in the eyes. She recorded these snarly responses, 30 of which she went to on to manipulate for her experiment.

That might sound like an unwise experiment for anyone who values their personal safety. But Taylor says that, as an experienced animal behaviourist, she managed to diffuse any encounters before they turned violent.

Size is important

Taylor looked at the formants, and the fundamental frequency of the different growls. Examples of the biggest and smallest formants, and highest and lowest frequencies, can be heard in these sound clips.

A formant is a basic acoustic property, and can be thought of as a resonant frequency of a sound wave in a vocal tract. Dogs have between five and seven formants when they make a noise. In humans, changes in formant come across as different vowel sounds. Different length vocal tracts produce different resonant frequencies, and so the formants take on different values. "Larger vocal tracts belonging to larger animals produce lower formants," says Taylor.

The fundamental frequency is the pitch of a sound. It is a "common fallacy" that pitch can be used to determine size, Taylor says. Pitch is related to the size of the fleshy vocal chords, which can grow to different sizes. The formant, on the other hand, is pretty much fixed.

To test human perception of dog size, Taylor separated out the formants and the fundamental frequency of each dog growl with computer-based acoustic software. She then resynthesised 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 – low to high pitch.

These new growls were played to human subjects in two separate experiments where they heard growls in random order and were asked to assess the size of the dog.

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. When pitch was changed but not formant, the testers estimated dog size more accurately. When asked to say what they thought they were listening for, the testers all thought they were listening for changes in pitch, Taylor says.

The work has sparked interest in the acoustic community. "This step to analyse the vocal repertoire of dogs, especially the subunits of growling, is totally new," says Dorit Feddersen-Petersen, who studies dog behaviour at the University of Kiel, Germany.

"People are using the same acoustic parameters that they use to assess body size from human voices as they use to assess body size from dog growls," says David Feinberg, who studies voices at McMaster University, Hamilton, Canada. This is probably because all mammals have similar anatomy for producing sound, he says.

More than co-evolution?

Taylor thinks that this 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 says.

That people use these cues to assess the size of dogs may be linked to the idea that humans and dogs have been co-evolving for the last 15,000 years, says Feinberg. Feddersen-Petersen suggests the same thing: "This interspecific form of signalling ... must be linked to the close evolutionary history between dogs and humans," she says.

This co-evolution could make it particularly important for humans to pay attention to the size of dogs suggests Feinberg. "While dog may be man's best friend, it may also bite the hand that feeds," he says.