Study Says Fingerprints Aren't For Friction New research in The Journal of Experimental Biology shows that — contrary to conventional wisdom — fingerprints don't increase the friction between the fingertips and the grasped object. Biomechanics researcher A. Roland Ennos explains what fingerprints might actually be for.

Study Says Fingerprints Aren't For Friction

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JOE PALCA, host:

And now, that perplexing and confusing and I'd like to know the answer myself topic, what are fingerprints for? Well, I asked you if you had any thoughts about this. Let's see what people have responded with.

Let's go first to Tony in San Bernardino, California. What are fingerprints for?

TONY (Caller): I think for increased surface area and greater friction to manipulate smaller objects.

PALCA: Okay. That's a good guess. Thank you, Tony. Let's now go to - is it Jose from Tallahassee, Florida?

JOSE (Caller): Yes, it is.

PALCA: What's your guess?

JOSE: Similar to the prior caller, I think it's to increase friction so that we can grip objects since we have evolved away from claws.

PALCA: All right. Well, that's cool. Let's try again number three. Doug from Cincinnati, Ohio, what's your guess?

DOUG (Caller): I believe that fingerprints represent the entire genetic code or individual DNA.

PALCA: Ah, so it's sort of a bar code for human beings on their fingertips. Thanks for that.

Let's try Tom in Kensington, Maryland. What's your guess about the use of fingerprints or why we have them?

TOM (Caller): Well, kind of similar to the first three. I think it's for increased friction. But also I think it helps when your fingers are wet, sort of like tread on a tire.

PALCA: Okay. Let's try one more - thank you for that - and go to Jason in Redding, California.

JASON (Caller): Yeah. Hi. I think that the fingerprints are so that we know when we're feeling something, where it is on our finger, like, for instance, a crack, we can feel the undulations of our fingerprints…

PALCA: So the prints have more sensitivity. Okay. Interesting. All right. So we have a sampling of - thank you very much for that, Jason. We have a sampling of questions, and now we bring in our expert. And I don't mean to put him on the spot, but Roland Ennos is a reader in ecology at the University of Manchester in England. And he has a paper out this week in the Journal of Experimental Biology that looks at fingerprints and friction.

So thanks for talking with us, Dr. Ennos.

Dr. ROLAND ENNOS (University of Manchester): Oh, pleased to meet you, yes.

PALCA: So you - did you hear those proposals?

Dr. ENNOS: I did, yes.

PALCA: And do any of them hit the mark, in your opinion?

Dr. ENNOS: Well, they do. Initially, we did first think that the reason for fingerprints was the idea put forward by most of your - most of your callers, that the fingerprints actually increased friction.

But what we found was that, in fact, fingerprints behave rather more like rubber, as a conventional material. And because the fingerprints - actually mean those - in the gaps between the fingerprints there's - they don't touch the surface, that actually reduces the surface area of contact and so would actually reduce the friction.

PALCA: I see. But I'm - if you're pushing hard - those fingerprints are not very high ridge. I would have thought that, you know, their scale would have been - would only - I mean, they would only work if you were lightly brushing against something.

Dr. ENNOS: Yes. We tried it and we pushed the fingerprints even at high pressures, and still they didn't really sort of touch the surface.

PALCA: Really?

Dr. ENNOS: Yeah. We were - initially, we were looking at a flat surface of Plexiglas.

PALCA: Uh-huh.

Dr. ENNOS: So what we showed is that fingerprints actually reduce - probably reduce friction against flat surfaces.

PALCA: So reduce friction against flat surfaces.

Dr. ENNOS: It would do, yes, because it actually reduces the contact area compared with what it would be if our fingers were flat.

PALCA: Right. Okay. So, okay, this is going a little bit against the form. So what…

Dr. ENNOS: Yes.

PALCA: …would be your explanation of why we would have these things to reduce the friction against flat surfaces?

Dr. ENNOS: Right. Well, there's about three or four possibilities. Firstly, your correspondent suggesting that it would increase friction in the wet, like tires, that is quite possible. And we are going to test that. We'll be investigating the friction of fingers in wet and dry conditions.

PALCA: Really? So describe this experimental setup that you had for testing this Plexiglas.

Dr. ENNOS: Well, what we had was we put our fingers within a tensile, a special sort on tensile testing machine.

PALCA: So you subjected yourself to this trauma?

Dr. ENNOS: Yes. Well…

(Soundbite of laughter)

Dr. ENNOS: …not myself. I had a Ph.D. - a student who actually did all the experiments. And he actually put his fingers in there…

PALCA: I see.

Dr. ENNOS: …and then had different sort of squashing forces, the Plexiglas against the fingers. And then the Plexiglas was dragged across these fingers, and then we measured the friction force.

PALCA: I see. And I want to invite our listeners, now that they've given their theories…

Dr. ENNOS: Yeah.

PALCA: …to join us, if they wish, to describe the answers. The number is 800-989-8255. That's 800-989-TALK. So are you satisfied that you've reached the explanation now, or do you think there's - that we have to go further here?

Dr. ENNOS: I think we have to go further. Like all scientists, I'd like to do more research.

PALCA: Mm-hmm.

Dr. ENNOS: But the other options are that it could increase friction on rough surfaces. And so we'll - though initial tests suggest that actually fingers are even worse. Our fingers are even worse at gripping rough surfaces than smooth surfaces.

PALCA: And I'm just wondering, are humans alone in the animal kingdom as - of having these kinds of ridges or bumps on their fingers?

Dr. ENNOS: No. In fact, it's an interesting question. They're common throughout the primates, but rather more interestingly, they're also found on some other creatures which grip onto branches and are arboreal, things like koala bears, which are, of course, marsupials.

PALCA: And…

Dr. ENNOS: And then finally there's the New World monkeys that in South America have - which have prehensile tails, which grip with their tails. They actually have tail prints.

PALCA: So do you worry at all now that - I mean, because from the animal kingdom, I presume that the thinking had been that this was somehow for better gripping, these ridges. And now you've kind of blown that up. It's back to the drawing board for ethologists or physiologists.

Dr. ENNOS: I think it's back from the drawing board, but it's certainly - it must be part of a set - suite of characteristics to improve grip or to prevent wear or to prevent - my favorite theory is that the fingerprints are there to help prevent blisters, in fact.

PALCA: Okay. Well, run past that a little bit. How does that theory work?

Dr. ENNOS: Well, the idea is that in our fingerprints, they need to be soft in order to be - have a lot of friction. If they were just soft, they would blister easily. And so what we have in our fingerprints is beneath the trough of our fingerprints is all reinforced by strong fibers.

But if it was all very hard and reinforced, it wouldn't be able to deform. And so my idea is that the fingerprints are there as part of a suite, so that the -our fingers couldn't stretch, particularly in the direction in which we pull them.

And so the fingerprint can grip on to surfaces, have a nice, good contact area, and yet get reinforced against being blistered. And in fact, you'll notice that when you do get blisters, it doesn't tend to be on the bits which have ridges, like the soles of your feet, your fingerprint - your fingertips themselves or the palms of your hands, but on your heel or the top of your toes or the sort of in between your fingers where there aren't any prints.

PALCA: We're talking with Roland Ennos. He's a reader in ecology at the University of Manchester in England. I'm Joe Palca and this is SCIENCE FRIDAY.

And let's just take one very quick call from William in Kansas City. Welcome to SCIENCE FRIDAY. You're on the air.

WILLIAM (Caller): Hi. This is - I was wondering if the patterns on the fingerprints had anything to do with cellular autonoma that Wolfram, who you mentioned earlier, studied.

PALCA: Really? Cellular automaton, that's really interesting.

WILLIAM: Yeah.

PALCA: Let me ask Roland Ennos if he has the theory about that.

Dr. ENNOS: Cellular automata - I don't know. The fingerprints seem to develop under a series of force fields, when you're in the - as an embryo. And so there must - that seems to be how they're formed. I don't know how cellular automata link with their sort of - that function.

PALCA: Well, if you can figure out a link - I mean, Stephen Wolfram has a lot of money, maybe he'll give you a grant and continue your study.

(Soundbite of laughter)

Dr. ENNOS: I'd love a grant.

PALCA: Anyway, we'll leave it there. Thank you very much, Roland Ennos.

Dr. ENNOS: Okay. Thank you very much.

PALCA: Roland Ennos is a reader in ecology at the University of Manchester in England. He has a paper out this week in the Journal of Experimental Biology -I'm sure you all have that in your mailboxes just now and can go look - that talks about why fingerprints are there. And he's done some interesting experiments.

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