With the video above in mind, here’s an hypothesis (or, if you prefer, a hypothesis) :
• Do bigger dogs (by and large) have longer, more voluminous vocal tracts? And, if so, might they bark with lower notes?
The hypothesis was broadly confirmed in a comprehensive 1999 experimental study by investigators professor Tobias Riede and professor Tecumseh Fitch, and which was published in The Journal of Experimental Biology, 202, pp. 2859–2867
VOCAL TRACT LENGTH AND ACOUSTICS OF VOCALIZATION IN THE DOMESTIC DOG (CANIS FAMILIARIS)
The team examined x-ray images of 12 dogs of various sizes, measuring the Vocal Tract Length (VTL). They also recorded the same dogs’ growls. (“Growling was induced by staring into the dog’s eyes.”) Subsequent comparisons of the two datasets revealed distinct correlations :
“… we have found clear evidence that vocal tract length is correlated with body size in domestic dogs, despite the apparent variation in skull and vocal tract shape induced by selective breeding. As predicted by acoustic theory, vocal tract length was inversely correlated with the spacing between formant frequencies, which means that formant spacing provides a reliable cue to body size [log(body mass)] in the sample of dogs studied here.”
In other words, not only is it possible to predict (with reasonable accuracy) that a large dog will have a low(ish) ‘voice’ – but the reverse is also true – if you hear a low-pitched bark, its likely to have originated in a big dog.
Note: The study was preceded by another from professor Fitch which found that the same effect applies in rhesus macaques (“… and probably many other species.”)