If you ever soak in water for more than five minutes, likely you’ve noticed that your hands and feet become wrinkly. If you’ve heard of osmosis, the process by which water diffuses from an area of high concentration to an area of low concentration through permeable membranes, you might think this explains how pruny fingertips come about. A long held scientific explanation for the wrinkly fingers and toes people acquire in water went like this: osmosis causes water to move from the body of water (higher concentration) into the body (lower concentration) which swells up; the layers of skin cells in the epidermis are connected to each other via proteins about every millimeter or so, making some of this skin stay in place and the rest bloat forming wrinkles. Under close inspection, however, this theory doesn’t hold water.
First of all, an odd fact for your consideration: people with sympathetic nerve damage to their hands or feet don’t get wrinkly hands or feet in water, respectively. Putting extremities in a tub of water to see if they wrinkle or not is actually a simple, non-invasive diagnostic technique to assess nerve damage. Should the paralysis or numbness be non-permanent, the ability to get pruny returns when the nerves heal (Changizi et al., 2010). Secondly, pruny digits don’t really seem to bloat, do they? A quick bath proves they actually shrivel; not what you’d expect from simple osmosis. Third glaring flaw with the traditional explanation is that the effect is localized to hands and feet. You don’t get wrinkly shins or a bloated face from swimming. This has been theorized to be due to high levels of spongey keratin in the dead skin that builds up on hands and feet, but babies get pruny, too, without such callouses. So what is actually going on? Because nerds are awesome people obsessed with weird stuff like this, there’s a scientist named Dr Einar Wilder-Smith, of National University Singapore, who has spent over a decade studying the phenomenon of aquatic wrinkling and has figured out how it happens. Building upon this work, a team of scientists in Idaho has figured out why it happens.
The how: the glabrous (bald) skin of hands and feet are full of sweat glands that make them more permeable to water than other parts of the body’s skin. More water in this type of skin causes dilution via diffusion of normally highly regulated levels of salt within the skin cells. This causes destabilization of the skin cell membranes which in turn causes increased firing of sympathetic nerves. The sympathetic nerves help control blood vessel diameter and interact with glomus bodies in the extremities, which are tiny specialized organs involved in sensing and regulating temperature. The glomus bodies react to vasoconstriction triggered by the sympathetic nerves by further reducing the volume of blood allowed into finger and toe tips. This makes your fingertips and toes shrink in volume. The shrinking occurs unevenly due to the protein connections between skin layers mentioned earlier, making them become all wrinkly.
The why: aquatic wrinkling isn’t random. Evolutionary biologist Dr. M. Changizi and his team showed in 2010 that the grooves of aquatic wrinkling resemble and function similarly to watershed tracks, funneling water off akin to rain tire tread. Human grip upon submerged objects is significantly improved when hands are pruny. Thus there is an evolutionary advantage to wrinkly fingertips that can be extrapolated to account for feet as well since such wrinkles should make it easier to keep one’s footing in rain slicked terrain. Mystery solved.
To learn more about the details of this advantageous soggy phenomenon, check out the following:
Wilder-Smith & Chow, 2003 DOI: 10.1002/mus.10323
Changizi et al., 2010 doi: 10.1159/000328223