purrsistence

[sciatrix]

Earlier this week, T got a blood pressure cuff and a cheap stethoscope, to be used to practice taking blood pressure and heart rate counts. Roomie M and I have already been informed that we're probably going to be the model patients, which is fine: it's not as if I haven't signed on to be a living anatomical aid before, and this requires way less effort on my part. But it does mean I have to sit still, and I'm not great at that. So after they tested a blood pressure/heart rate exam on me, I snagged Dent (who happened to be napping nearby) and volunteered him to have his heart rate measured. No blood pressure cuff, but there isn't a reason you can't practice on a cat, right?

It turns out there is certainly a reason not to practice getting heart rates on Dent: he purrs under his breath constantly, just quiet enough not to be heard unless you hold him up to your ear like a conch shell. He was actively annoyed with me the first time I held him up for the stethoscope--he'd been napping! why was I disturbing a nap and a good cuddle?--and still he purred. We took to trying to sneak up on him over the course of the day to see if we could startle him into not purring, in case it was the human attention. Nope--he purrs regardless, quietly and happily, and never, ever, ever stops. Eventually T got a heart rate by waiting until he was asleep and surprising him into pausing.

I also learned this week that cat purrs are apparently almost unique among vocalizations inasmuch as they aren't produced via the myoelastic-aerodynamic theory of laryngeal vocalization: that is, most tetrapod animals make noises by pushing air through vocal folds which vibrate at particular resonances, which means you can replicate the sound if you force the air through an excised larynx from a dead animal. This is also the way that most cat vocalizations work. Purring, though, happens when cats actively vibrate muscles in their larynxes, which is why cats can purr and breathe at the same time. You can vocalize while inhaling as well as exhaling, but the noise never sounds quite the same from inhale to exhale.

As far as I know--and I was listening to Tecumseh Fitch, who is probably the most knowledgeable living expert on the mechanisms of vocalizations across vertebrates--cats are very unusual for being mammals who vocalize this way. There was some thought that perhaps elephants might, too, when they make big infrasonic calls--but nope, it seems that they use the more common MEAD mechanism to make those calls too.

(We had a symposium at work about animal vocalizations, and so I'm perked up and thinking hard. I found out that bats sing, too--including the Mexican free-tailed bats who are so beloved in Austin--and I ought to follow up with that later.)

Comments

[kore]

SINGING BATS. And mice sing too I think?

[siderea]

...you must be new here. :)

[sciatrix]

My mice, Scotinomys, sing like birds, by which I mean they vocalize to one another by producing a series of notes over long distances to communicate. (Distant relatives grasshopper mice also do this, although they produce long howls like tiny scorpion - eating coyotes instead of trills.) Most mice and rats do vocalize to one another, although they mostly use frequencies higher than humans can hear because these high frequency sounds scatter quickly and are harder for predators to hear.)

The bat thing I'd had no idea. More in that one tomorrow, maybe!

[kore]

AW.

[sciatrix]

It's very cute!

[siderea]

So it is the larynx. Huh!

[sciatrix]

Technically they are apparently waving the glottis back and forth at high speeds to vibrate the vocal folds! No rush of air required. It's a remarkably strange vocalization for a bunch of reasons - - purrs are lower frequency than anything a mammal that size ought to be able to produce, and lower frequency vocalizations tend to be associated with displays and territories where you're advertising based on size. But that's not generally what purrs are doing, so... why evolve this weird vocal ability?

(Incidentally, in humans within a gender [and probably more precisely within a particular hormonal profile], you cannot tell body size from the pitch of voice - - but humans sure think you can!)

[siderea]

so... why evolve this weird vocal ability?

I thought I heard the theory that it was therapeutic. That it's a way of imparting vibration, say, to one's kittens, and no, really, vibration can be therapeutic.

Now, I like that theory, but it has the problem of being incongruous with the fact that cats purr when happy or content (Dent is proof that ignorance is bliss, no?) not when worried or concerned.

So I have an alternative hypothesis. I find it fascinating that it's affective, and it is about expressing contentment. Why would a species find it useful to communicate a sense of well-being?

Well, you could use such a thing for the same purpose for which we and other species use expressions of alarm and discontent for. By using it in reverse.

Animals across a wide variety of species evolved vocalizations (and other signaling) of alarm and distress to alert conspecifics. To alert the other members of the pack/herd/flock/troupe/pod to flee, or veer, or circle up to defend the young, or attack. In other words: it's coordinative communication, to elicit socially coherent behaviors.

The problem with this approach is that screaming when you see a predator may be good for your fellows, it's potentially really terrible for you, especially if the predator hadn't quite seen you yet but now knows exactly where you are.

Wouldn't it be neat if a species came up with the approach of making a more-or-less continuous quiet sound, that could fall silent when there was a threat? It would be a lot more discreet a way to signal one's pride – or one's cubs – that now would be an excellent time to shut up and hold still. A kind of deadman's switch – or warrant canary – for a threat alarm.

But the thing is, if a species evolved that, it would have a variety of additional interesting applications. Threat alarms generally do. If you develop the apparatus to make a really big noise that your fellows will recognize as a threat alarm, and it's wired into your HPA axis, well, it's just a half-step to the side to use that in contests for mates and territory. And that gets you aggression displays to conspecifics to contend for social dominance.

You could do it in reverse, too. If instead of an aggression signal, you have a contentment signal, then giving it means you're not contending for dominance, you're acquiescing to the social order as it stands.

[Whups, stopped in the middle of my thought.]

And that solves one of the great problems in being a social organism: how can one trust one's conspecifics enough to cooperate with them, when one is in competition with them for resources? I propose that having involuntary behaviors which betray one's interiority makes an individual more trustworthy. It betrays one's intentions, such that one can't perpetrate deceit. If one is going to attack the dominant member of one's social group in a contest for dominance, one will stop purring first.

[And for another thing!]

I would think this is a particularly hard problem for apex predators. Cooperating enough to say, hunt in packs, is beneficial because more tasty megafauna for everyone, but mighty tricky to pull off when, actually, everyone is a tiger. It's a prisoner's dilemma. Except what makes the prisoner's dilemma the prisoner's dilemma is the prisoners can't communicate, and can't know whether or not other members of the system will defect.

Purring is, I propose, a solution to the prisoner's dilemma: it prevents cats from defecting without warning. It makes cats able to trust cats, so they can tolerate one another's presence enough to cooperate.

[siderea]

(Incidentally, in humans within a gender [and probably more precisely within a particular hormonal profile], you cannot tell body size from the pitch of voice - - but humans sure think you can!)

When you say "pitch of voice" do you mean "random sample of a human speaking" (i.e. average speech pitch) or do you mean a given human's full pitch range, as per singing? (People's singing voices often don't match their speaking voices, for pitch. You can't necessarily tell what "part" someone sings by their speaking voice.)

[hellofriendsiminthedark]

Very good visuals. A+ storytelling.

[moem]

One of our cats had a very bad, no good start (he came from a home where things had gotten badly out of hand, there were 65 cats) and was pretty traumatized, inbred, and not socialized except by cats. For the first six months after we adopted him, we never heard him purr. Once he got started, he did it more and more, and now he will purr away like a happy little engine when you pet him.

[notasupervillain]

Awwwww cats are so good.