What function is served by the epidermal or capillary ridges on human fingers, the supposedly unique impressions of which are known as fingerprints?
3$\begingroup$ In this case, it seems that @canadianer successfully identified some positive selection-based answers. But, honestly Mark, I would read my neutralist answer too... too often people "expect" a selection-based answer, when most of the time the answer is simply neutral. $\endgroup$– hello_there_andyMar 15, 2017 at 4:46
3$\begingroup$ It should be noted that fingerprints may not be considered as a reliable unique identifier as they once were. naturalnews.com/… And also that some farmers and some old people can have rubbed out fingerprints. $\endgroup$– Stephan BranczykMar 15, 2017 at 15:37
5$\begingroup$ I find it ridiculous that so many questions like this ask "what is the evolutionary reason…". You could ask whether "whatever is the technical term to describe the feature of the skin that you are referring to" (and it is not fingerprint — do a little work before posting) has any function. But to assume that every feature of an organism is selected for is an intellectual blind alley. $\endgroup$– DavidMar 15, 2017 at 20:37
6$\begingroup$ @StephanBranczyk: Natural news dot com is not a trustworthy source. Google "quackwatch naturalnews". $\endgroup$– R.. GitHub STOP HELPING ICEMar 15, 2017 at 20:41
4$\begingroup$ Fingerprints no longer uniquely identify people. nbcnews.com/id/5053007/ns/us_news-security/t/… You could say fingerprints suffer from hash collisions (computer science and cryptography). $\endgroup$– ChloeMar 16, 2017 at 0:48
I found many plausible claims that fingerprints increase friction. However, the following article claims, at least under their experimental conditions, that fingerprints actually decrease friction with smooth surfaces by reducing contact area.
Fingerprints are unlikely to increase the friction of primate fingerpads.
It is generally assumed that fingerprints improve the grip of primates, but the efficiency of their ridging will depend on the type of frictional behaviour the skin exhibits. Ridges would be effective at increasing friction for hard materials, but in a rubbery material they would reduce friction because they would reduce contact area. In this study we investigated the frictional performance of human fingertips on dry acrylic glass using a modified universal mechanical testing machine, measuring friction at a range of normal loads while also measuring the contact area. Tests were carried out on different fingers, fingers at different angles and against different widths of acrylic sheet to separate the effects of normal force and contact area. The results showed that fingertips behaved more like rubbers than hard solids; their coefficients of friction fell at higher normal forces and friction was higher when fingers were held flatter against wider sheets and hence when contact area was greater. The shear stress was greater at higher pressures, suggesting the presence of a biofilm between the skin and the surface. Fingerprints reduced contact area by a factor of one-third compared with flat skin, however, which would have reduced the friction; this casts severe doubt on their supposed frictional function.
That said, the author does later discuss their potential role in gripping of rough or wet surfaces:
So why do we have fingerprints? One possibility is that they increase friction on rougher surfaces compared with flat skin, because the ridges project into the depressions of such surfaces and provide a higher contact area. Experiments on materials of contrasting known roughness are needed to test this possibility.
A second possibility is that they facilitate runoff of water like the tread of a car tyre or grooves in the feet of tree frogs (Federle et al., 2006), so that they improve grip on wet surfaces. Though there is evidence that friction falls on fingers coated with high levels of moisture (Andre et al., 2008) it is possible that it falls less quickly on fingertips than on flatter skin. Once more, suitable experiments could test this idea.
There seems to be more consensus on the idea that fingerprints are useful for tactile sensation. The following are just some articles which discuss this.
Effect of fingerprints orientation on skin vibrations during tactile exploration of textured surfaces.
In humans, the tactile perception of fine textures is mediated by skin vibrations when scanning the surface with the fingertip. These vibrations are encoded by specific mechanoreceptors, Pacinian corpuscules (PCs), located about 2 mm below the skin surface. In a recent article, we performed experiments using a biomimetic sensor which suggest that fingerprints (epidermal ridges) may play an important role in shaping the subcutaneous stress vibrations in a way which facilitates their processing by the PC channel. Here we further test this hypothesis by directly recording the modulations of the fingerpad/substrate friction force induced by scanning an actual fingertip across a textured surface. When the fingerprints are oriented perpendicular to the scanning direction, the spectrum of these modulations shows a pronounced maximum around the frequency v/λ, where v is the scanning velocity and λ the fingerprints period. This simple biomechanical result confirms the relevance of our previous finding for human touch.
The role of fingerprints in the coding of tactile information probed with a biomimetic sensor.
In humans, the tactile perception of fine textures (spatial scale <200 micrometers) is mediated by skin vibrations generated as the finger scans the surface. To establish the relationship between texture characteristics and subcutaneous vibrations, a biomimetic tactile sensor has been designed whose dimensions match those of the fingertip. When the sensor surface is patterned with parallel ridges mimicking the fingerprints, the spectrum of vibrations elicited by randomly textured substrates is dominated by one frequency set by the ratio of the scanning speed to the interridge distance. For human touch, this frequency falls within the optimal range of sensitivity of Pacinian afferents, which mediate the coding of fine textures. Thus, fingerprints may perform spectral selection and amplification of tactile information that facilitate its processing by specific mechanoreceptors.
This paper also asserts a reason for the elliptical nature of fingerprints:
In humans, fingerprints are organized in elliptical twirls so that each region of the fingertip (and thus each PC) can be ascribed with an optimal scanning orientation.
29$\begingroup$ The first paper shows that fingerprints decrease friction when gripping dry acrylic glass, but I guess there wasn't much dry acrylic glass in the environments where fingerprints evolved. It might be quite different when gripping rough or wet surfaces. (But for all I know there might be plenty of research on that as well.) $\endgroup$– N. VirgoMar 15, 2017 at 8:09
17$\begingroup$ @Nathaniel Much in the same way that slick tyres are better than anything on dry tarmac, but you'd want treads on rough or wet terrain. $\endgroup$– SGRMar 15, 2017 at 8:49
1$\begingroup$ Would this mean that people without fingerprints could be measured to have less sense of touch than those with? $\endgroup$ Mar 15, 2017 at 13:31
1$\begingroup$ @Nathaniel I agree. As I said, their experiment is not exhaustive, but literature on this subject seems hard to come by (at least in my admittedly brief search). $\endgroup$ Mar 15, 2017 at 14:35
$\begingroup$ Clarity on the TLA: PC, in "and thus each PC", please. $\endgroup$ Apr 3, 2017 at 3:43
To balance the debate, from a neutralist evolutionary perspective...
There does NOT have to be a direct selective pressure for a trait's contribution to an organism's expressed phenotype.
Three alternative, neutral, explanations:
- "Hitch-hiker" traits: It (the trait) could be a by-product of a more necessary component whose function is directly associated with survival. E.g. we have hair, and it happens to have a colour, the colours perceived in the specific pigmentation of our hair are not strongly selected, one way (blonde) or the other (brunette), at least as significantly as the functionally essential properties of the hair itself: to keep our ancestors nice and warm. This brings us to...
- Ancestrally Inherited Vestigial traits: once evolved, they can take a long evolutionary time "to get rid of". This accounts for traits we still express phenotypically, yet whose original functional purpose is obsolete today. For example, the Coccyx and our defunct visual sensor, a more optimal design is found in squid eyes: the retina should be ahead of the vitreous gap to avoid blind spots, a "glitch" only we measly mammals have to deal with. At least we have a good brain, right? Those pesky, extinction-imminent, tigers have nothing on us. And as for those echidna, still laying eggs? That's soo last-epoch...
- Neutral Evolutionary Regimes: mutations whose inheritance patterns are heavily driven by a force known as genetic drift (wiki). Long story...
A most remarkable trait, in my opinion, is found in the blind cave fish. Why does this fish have eyes, when it can't see?
Have a guess: which of the answers listed above is true: 1., 2. or 3.?
- 2017-03-17: Correction (kudos AliceD): I mixed up reasoning about the mammalian eye example, floaters was not the suboptimal trait, it was blind spots. No blind spot in cephalopod eyes.
2$\begingroup$ In response to your cavefish question, without guessing ;) biology.stackexchange.com/questions/34466/… $\endgroup$ Mar 15, 2017 at 4:57
7$\begingroup$ I think in this case, neutral evolution is highly unlikely, because of the convergent evolution of fingerprints in other species (e.g. Koala and North American Fisher) which appears to be linked to their lifestyle and habitat but it's still important to consider other explanations. $\endgroup$ Mar 15, 2017 at 15:01
3$\begingroup$ @JackAidley You still need to make sure there's no other trait that is linked to fingerprints that's the actual result of convergent evolution, with fingerprints being just a common side-effect of that. $\endgroup$– LuaanMar 15, 2017 at 16:10
2$\begingroup$ Mammalian retina location is a very bad example. It does serve it's purpose so it's promoted. The fact that better solutions exist is meaningless to the evolution process. Switching to a better solution would require un-evolving vision first - that just aint gonna happen. Same with vas deferens being looped over the pubic bone, pointless but "we've invested too much in this solution to scrap it now". $\endgroup$– Agent_LMar 16, 2017 at 15:33
1$\begingroup$ Why wouldn't a squid eye have floaters? You could mention the absence of a blind spot in squid eyes $\endgroup$– AliceD ♦Mar 16, 2017 at 20:53