Cunning cat acoustics

Turns out cats can embed a cry within their purr...
18 December 2020

Interview with 

Karen McComb, University of Sussex


someone stroking a cat


Animal behaviour expert Karen McComb told Katie Haylor about some of the cunning ways cats can communicate with us humans...

[Purr with high pitch within it]

Karen - Basically he used to wake me up with this very annoying purr. And in the summer he would do it from about 5:30 AM. You know, sit on the bed, really close to my face and do this purr. And I thought, "why is that purr so hard to ignore?", because you try and sort of pull the covers up and he didn't give up. And I just, I couldn't bear it. It was sort of really grating on my nerves. So I would eventually, you know, just get up and feed him.

There were other people who had these sorts of cats that did this very particular thing, particularly in the morning. I thought, "well, let's do a study on this and actually try and work out what it is about the purr that humans are finding so difficult to ignore". It turned out that there was actually a cry embedded in that purr.

So we think of the purr as a sort of pleasant, you know, low frequency noise, humans generally find that quite comforting.


But we were able to show that in the solicitation purr there was a high frequency noise about the frequency of both a cat cry and a baby cry -

[purr with high frequency within it]

- superimposed on the pattern of the purr. We had solicitation and non-solicitation purrs from 10 cats, one type of each, from each cat. And so we were able actually to play these back to humans, we were able to get humans to rate the pleasantness and urgency of the purrs. And at the same time, we were able to measure the acoustic characteristics of the purr. So we were actually able to zoom in on the ones that they find unpleasant and urgent and what acoustic characteristic was associated with that. And it was this high frequency peak embedded in the purr. And we were even able to remove the high-frequency peak and show that the urgency decreased and the pleasantness increased.

It seemed that cats were using this high frequency juxtaposition to, or it was having the effect, functionally, of tapping into human tendencies to nurture babies and to be very sensitive to the frequencies of a baby cry. It wasn't just that you learnt that that was something that meant they wanted action. It was actually, you were responding at an innate level to what they were doing. In fact, you weren't even aware of what they were doing, except that you find it unpleasant.

Katie - Do we understand that well how cats actually produce purrs? Is it quite complicated to have your general purr and then to layer a cry on top of it?

Karen - Cats produce purrs at a frequency that they shouldn't be able to produce really. I mean, it's in the mid twenties in terms of hertz. Now I would expect that frequency in an elephant, which has huge vocal folds and the fundamental frequency of the vocalisation is related to the size and the mass of the vocal folds. So usually when you vocalise, you force air from the lungs over the vocal folds, and they start vibrating at their own natural frequency. Cat vocal folds are going to be so small that there's no way that they would be giving you a fundamental frequency of 26 hertz, for example. But, instead of doing that normal voicing, when they produce a purr, cats are actually twitching their vocal folds, it's a muscle action that's allowing them to vibrate the vocal folds at that very low rate. The thing about that is it actually still then leaves the opportunity to add voicing to that mechanically produced purr. And so it seems with the solicitation purr that they are forcing air through the vocal folds at the same time, and the vocal folds are then able to vibrate at the much higher frequency of a cat cry. And that, as I say, interestingly is very similar to your baby cry. I mean, sometimes when you hear a cat cry, it's easy to confuse it with a baby cry anyway. So they just happen to be, you know, spot on.

Katie - There's a stereotype, isn't there, that cats have a very uncanny sense of timing. As you were telling me about the purr, one of my kittens jumped up onto the table, headbutted the microphone and purred into the microphone. Did you hear that?

Karen - I didn't!

Katie - I couldn't have made that up...

I think I'm a sucker, to be honest. I give my cats so much attention! But has there been any science done on the other side of the coin? Is it useful for me to talk to my cats? Or is that just me being a bit eccentric? Do they appreciate a vocal interaction with humans?

Karen - I think they do. I've always talked to my cats and I have a particular sort of undulating, frequency-modulated voice that I use with them. And that's the voice I would use with a cat in the street as well. I do think that they are more responsive to that. They learn that that's a positive social invitation from a human. Dogs - there's been work on dogs showing that they're more likely to respond to that sort of frequency-modulated type speech. Again, it's a bit like the way we speak to babies. Really

Katie - Another communication method - I noticed when I was about 10 by spending a lot of time with my big ginger tom who's sadly now passed on - is this idea of blinking. And you've also worked on cats communicating with humans via blinking. Can you tell us what you've shown?

Karen - Yes. Well, like you, I have always communicated in this way with my cats because actually it's something that cats do and they sort of teach you to do it inherently. If you live with cats, you notice that sometimes they narrow their eyes at you and then they close them completely and blink. And if you do this back to them, they respond in kind. So you can set up a really nice conversation in that way. Other people through their own pets were becoming aware of this, but there'd been no work showing, first of all, that humans can stimulate cats to do this back at them by slow blinking themselves. Or any work actually showing what the function of the slow blink was, you know, whether it really was, we get the impression that it's a positive form of communication, but how would you show that?

So we set out - I had a PhD student who worked on this - Tamsin Humphrey. We showed not only that cats were more likely to slow blink, and this was at an experimenter when the experimenter blinked at them. But also if the experimenter had gone through a slow blinking session with them, rather than just sitting with a neutral face, then at the end of the experiment, the cat was more likely to approach and sniff the experimenter's hand. It was seen as a positive by cats when humans, even humans that they didn't know, did this with them. And we had a parallel set of experiments getting humans to do this with their own cats. And of course the cat was more likely to respond when they did that. Then if the human was just sitting with a neutral face.

Katie - On some sort of very basic level, if I close my eyes, when I'm in front of someone, I'm trusting that they're not going to eat me or fight me or something. Is it kind of a lowering of defenses type thing?

Karen - I've always thought that. It's sort of a, "I'm cool. You're cool. So I can take my eyes off you for a minute. And I feel comfortable in your presence". I mean, it's also got very interesting parallels with the genuine smile and humans.


Audio clips of purrs are from the Current Biology below.


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