Hello! I'd like to know your opinion on feline purring. I vaguely remember reading that the exact mechanism by which cats purr is actually subject to debate. Do you have any current info about this?
I don’t think it’s up for debate anymore, and yes, I can talk about that! The easiest way to explain why small cats can purr is to first talk about why big cats roar, because they’re effectively different modifications of the same structure.
Basically, the ability to purr or roar boils down to the physiology of two structures in a cat’s throat: the vocal folds and the hyoid apparatus. The vocal folds are the flaps of flesh that are used to shape air and produce noise during vocalization. The hyoid is a specific bone, but the entire structure around it is actually comprised of seven small bones. It joins the larynx to the middle ear and supports the voice box. (Bones 1 and 7 aren’t pictured in this drawing because they’re mostly made of cartilage).
What type of sound a cat can make will depend specifically on how much cartilage and bone comprise that third structure in the hyoid apparatus, and how much movement there is in their vocal folds.
Big cats that roar have the ability to stretch their hyoid apparatus down and back because the third bone in the structure is mostly cartilaginous. When big cats open their mouth wide and drop that structure back, it basically opens up their throat to produce chamber in which sound can reverberate (like the bell of trumpet). Big cat vocal folds have a part that is fused together (called a vocal pad) and it’s what produces the sound of a roar, which is then amplified in the space of the open mouth.
In small cats, the third bone in the chain is ossified (hardened into a rigid mass of bone) and that means the structure can’t really move or stretch. In addition, small cats don’t have pads on their vocal folds, which means they all vibrate individually - and that produces the buzzing purr we know and love.
(Interestingly enough, the source I pulled that graphic from mentioned that snow leopards - who can’t roar - do have a highly elastic hyoid structure, but lack the vocal pads necessary for producing the sound).
Vocal tracts and hyoid apparatuses in cats are super cool, but not something to google if you’re faint of heart. Most images that come up on a search are going to be from dissections, and I learned most of this from papers in which the mechanism of a roar was studied by literally running air through the disembodied vocal tracts of deceased animals.
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