Our fingers and toes get pruney when wet. It happens to even the smoothest among us. And it happens to each of us in roughly the same way. There are wildly different ways wrinkles could occur on a surface, but pruney fingers have a particular look. Mine in this link illustrates the pruney signature.
This universality already suggests that there could be a good evolutionary reason for pruney fingers.
... unless. What if pruney-ness is an incidental side effect of something? What if, say, water is absorbed into the skin, and those wrinkles are simply the physical result?
But why would water absorption lead to wrinkles with that signature shape? And why would water absorption lead to wrinkles on the finger tips but not all the other spots on the body? And why would water absorption lead to wrinkles at all, given that water absorption should generally lead to swelling and consequently taut skin?
No, water absorption can't explain it.
And, if water absorption for some reason did lead to pruney fingers by accident, then wouldn't this be bad for our grip? Why wouldn't natural selection devise a solution, so that the wetness common in a primate's life -- from rain or dew -- doesn't undermine the smoothy goodness of our non-wet, non-pruney grip?
It doesn't add up. Pruney fingers don't seem like a side effect.
And, in fact, it has been known since the 1930s that nerve damage to a finger abolishes the pruney response. Pruney fingers are neuronally modulated. That's even further reason to suspect that our prunes are adaptive.
So what might they be for? Why are pruney fingers good? They must, one suspects, somehow help -- not hurt -- our grip in wet, rainy or dewy, conditions.
"They're rain treads." This was my graduate student Romann Weber answering. In dry conditions race cars and shoes get the most traction with smooth, treadless soles. That's why race cars have smooth treads. Treads are only useful when the ground becomes complex, and especially when it's wet.
In order for a hand to reach out and grip a wet surface without hydroplaning, it needs a way to efficiently remove the water between the skin and the surface it is trying to grip. The best way to quickly move water tends to be via channels, the stuff of arteries and rivers. The fingertips need channels, or tubes, out which the water can quickly squirt, so that the entire fingertip can touch the surface.
The question we set out to answer was this: If pruney wrinkles are rain treads, then what should they look like?
When fingers press down, they touch first at a point, and the surface of contact gradually expands. One can think of this first point as the peak. And, more generally, one can think of fingertips as having the topography of a mountain.
Fingertips are like mountains. And mountains, too, must remove water off their backs. How do they do it?
Mountains come equipped (after eons of sediment getting washed away) with drainage networks that efficiently channel the water away. And mountains -- convex topographies of the kind analogous to our fingertips -- have wrinkles of a very particular kind.
The photograph in this link shows (on the left) an illustrative example of the relevant kind of mountain (with my finger shown again, now on the right).
To understand the key signature of the mountain's wrinkle shape, think of it as a neuron. At the middle-top is the neuron's (ahem, mountain's) "body" or "soma." In this case there are three really thick dendrites branching below it, and each of these branches further. Each of these segments is not a channel, but, rather, the divide between the channels. The divides, in their entirety, are tree-like, like a neuron. The channels, on the other hand, don't intersect one another at all (which is quite unlike the structure of drainage networks on non-mountain topographies).
We find that same key wrinkle shape in our pruney fingers, as an examination of my earlier fingers can attest. For more detail, I recommend reading our (me, Romann Weber, Joseph Palazzo and Ritesh Kotecha) very short article in Brain, Behavior and Evolution, currently freely available here.
Our pruney fingers may be a crucial part of our primate repertoire: Once primates went the way of finger nails rather than claws, treads were needed where claws may have sufficed before.
And there are obvious applications -- better rain treads for shoes, and potentially for tires. There are at least two aspects of our biological treads that are more clever than what we've currently got on our shoes and tires.
First, our pruney treads appear only when they're needed, dynamically transforming from race-car smooth to wet-conditions wrinkley as the weather conditions warrant.
And second, once our pruney treads have squirted out the water and a grip is completed, our entire finger surface -- including the channels -- touches the gripped surface. This is because our tread's channels are squooshy. Our shoe and tire treads, on the other hand, are more rigid, and don't fully contact the ground; instead, part of the shoe or tire remains lifted up off surface. That's potential surface contact -- better grip -- that it's not getting.
Far from an embarrassing mistake, wet wrinkled fingers are yet another testament to biology's brilliance.
And I want that tech on my shoes.
Mark Changizi is Director of Human Cognition at 2AI, and the author of The Vision Revolution (2009) and his new book, Harnessed: How Language and Music Mimicked Nature and Transformed Ape to Man (2011).