I rolled out of bed later than I intended to this morning. I blame the cats.

Our youngest cat, a diminutive calico named Margarita, sprung onto the bed as soon as she heard me start to stir. She immediately started purring — the sound started as a low rumble and rose to a constant vibrato modulated by her breaths — and flopped herself down over my neck. I should have said “OK, kitten, I need to get out of bed” and followed through with that declaration. What I actually said was “Awww. Hi Margarita,” and by that time our biggest cat, Teddy, had settled on my legs and started his own soft, barely audible purrs. I was cat-trapped, and right before I drifted off to sleep again I wondered how the little felines were making those soothing sounds.

Zoologists Dawn Sissom, D.A. Rice, and G. Peters briefly reviewed various ideas about the peculiar sound in a 1991 paper simply titled “How cats purr.” Some kind of soft-tissue vibration — of the vocal chords, soft palate, diaphragm, or parts of the circulatory system — had been previously proposed as the origin of purrs, but there was no consensus among anatomists.

In an attempt to narrow down the possibilities, Sissom and collaborators collected audio recordings of various purring cats — from domestic cats at an animal shelter to a cheetah and cougar, the largest cats known to be capable of purring. Their recordings confirmed that purring is a low-frequency sound that occurs during the entire respiratory cycle of the cat, even as breathing in and out alters the sound of the purr. Where purrs comes from was another matter. Vibrations in the larynx seemed to be key — particular muscles moved folds in the larynx into position and allowed the cats to purr. The fact that a previous study found that cats with paralyzed larynges couldn’t purr bolstered this idea.

Other hypotheses about purrs being produced deeper within the body were not upheld by the team's investigation. Even though you can sometimes feel a cat purr through its body, Sissom and colleagues concluded, the diaphragm and muscles around the ribs have nothing to do with purring outside of allowing a cat to breathe normally. Instead, a cat’s purr is created when the vocal folds of the larynx modify airflow — specifically, “a sudden opening of the vocal folds” while breathing produces the sound which is then conducted to the mouth and nose. And purring does not prevent a cat from making other sounds. A meow, chirp, or yelp can be modified by purring, a fact well known by domestic cat owners with talkative pets.

The mechanism that lets little Margarita purr to her heart’s content is relatively simple. All that seems to be required is a modulation of airflow through her larynx. Given this simplicity, though, it seems strange that some cats can’t purr.

While small cats, cougars, and cheetahs can purr, most big cats seem incapable of purring at all. The 19th century's most famous anatomist, Richard Owen, even separated cats into two groups on the basis of those which could purr but not roar (small cats, the felines) and those which couldn’t purr but could roar (big cats, the pantherines). A bone in the throat called the hyoid seemed to make all the difference. The hyoid provides structure for the tongue, larynx, and the upper part of the sound-producing section of the throat, and is central to sound production in cats. Whereas purring cats tend to have a rigid hyoid bone connected to the skull by a series of other small bones, the large, non-purring cats such as lions, tigers, leopards, and jaguars have more flexible, incompletely ossified hyoid bones which are partly attached to the skull by an elastic ligament. Thus cat relationships could be neatly split on the basis of throat anatomy.

But the details of who can purr and who can’t isn't as simple as Owen delineated. In a review article, G. Peters tabulated that 20 of 36 species of cat have been said to purr, including lions, leopards, and other big cats. (As for the other 16, Peters wrote, there is not yet enough information to know whether they purr or not.) The question is whether the noises made by the big cats within the genus Panthera are true purrs — a sound created by moving air modulated by vocal folds as in smaller cats — or are actually different noises that only vaguely sound like purrs. The “rolling, gurgling growl” female big cats emit while in heat may be a kind of purr, or it may be something else entirely. And, Peters says, big cats might have the ability to purr but simply don’t. Somebody is going to have to make careful, close-up acoustic recordings of these purr-like sounds to better understand how they correspond to purrs of smaller cats, although I imagine finding volunteers for taping tigers in heat is a difficult task.

(As for other mammals, Peters points out that many so-called purring sounds are very different from true purrs. Giant otters have been said to purr, for example, but the sound is so acoustically different from cat purring that the two should not be treated as the same kind of sound. The only other mammals that seem to purr like cats are some of the viverrids — genets, civets, and the binturong. Studies of large-spotted genet purrs appear to show that the creature can purr in the same way a cat does, both in terms of sound production and the details of the sound being emitted. From an evolutionary standpoint, this makes sense — viverrids are evolutionary cousins of cats within a carnivoran subgroup called the feliforms. Not all viverrids purr as cats do, though, and so it is uncertain whether purring in the genet is a shared feature due to common ancestry, or evolved independently.)

Maybe the rarity of purring among big cats has something to do with particular modifications of their larynx. In 1989 anatomist M.H. Hast published a study on the larynges of big cats and found that lions, tigers, jaguars, and leopards had “a large pad of fibro-elastic tissue” near the forward portion of their paired vocal folds. (The exception was the snow leopard, a big cat that has never been heard to roar.) These expansions, in addition to the ability of these cats to lower the larynx thanks to the flexibility of the hyoid bone and its attachments, allowed lions, tigers, leopards, and jaguars to better transfer the energy required to make loud, low-frequency roars. “Combined with an efficient sound radiator (vocal tract) that can be adjusted in length,” Hast wrote, “a Panthera can use its vocal instrument literally to blow its own horn with a ‘roar’.” But these modifications may have left lions, tigers, and their kind unable to purr. The pads of extra tissue on the vocal folds, anatomist G. E. Weissengruber and colleagues hypothesized in a 2002 paper, “would tend to damp such twitchings [of the vocal folds] and thus make it difficult, if not impossible, [for these cats] to purr.”​

Purring is a frustratingly difficult phenomenon to study. We still do not have a detailed understanding of what is actually going on inside a cat’s throat while the sound is being created. Researchers can record purrs to study their acoustic details, and dissections can give us some idea of the soft tissues involved, but how air and anatomy come together to create a purr remains perplexing. And why cats purr is another question altogether. A cat may purr out of contentment, out of fear, or to manipulate other cats into feeling relaxed, among other reasons. Why Margarita loves to jump on the bed every morning and endlessly purr, I may never know, but at least I have a better idea of how she’s doing it. I’m just glad our mischievous kitten can’t roar — a purr is a much nicer sound to wake up to.

The author and this post's feline inspiration. Credit: Brian Switek

References:

Hast, M. 1989. The larynx of roaring and non-roaring cats. Journal of Anatomy. PMCID: PMC1256521

Peters, G. 2002. Purring and similar vocalizations in mammals. Mammal Review. doi: 10.1046/j.1365-2907.2002.00113.x

Sissom, D., Rice, D. Peters, G. 1991. How cats purr. Journal of Zoology. doi: 10.1111/j.1469-7998.1991.tb04749.x

Weissengruber, G., Forstenpointner, G., Peters, G., Kubber-Heiss, A., & Fitch, W. 2002. Hyoid apparatus and pharynx in the lion (Panthera leo), jaguar (Panthera onca), tiger (Panthera tigris), cheetah (Acinonyx jubatus) and domestic cat (Felis silvestris f. catus). Journal of Anatomy. doi: 10.1046/j.1469-7580.2002.00088.x



[This post was originally published at WIRED.]