Anyone who has even taken a swim or gone through puberty knows that prolonged contact with water (such as in a pool or ocean, or in your sweaty shoes) will prune up your digits for some length of time.
The popular theory goes thusly, as explained in 2001:
When hands are soaked in water, the keratin absorbs it and swells. The inside of the fingers, however, does not swell. As a result, there is relatively too much stratum corneum and it wrinkles, just like a gathered skirt. This bunching up occurs on fingers and toes because the epidermis is much thicker on the hands and feet than elsewhere on the body. (The hair and nails, which contain different types of keratin, also absorb some water. This is why the nails get softer after bathing or doing the dishes.)
Run that through the Straight Dope Laym-man’s Terms Generator and we get this, from 1987:
Since the underlying tissue doesn’t absorb water, the stratum corneum [very thin, topmost layer of skin] can’t spread out and it buckles like asphalt on the highway in the summer sun.
THE UPDATE! I recall reading several months ago about a new study on why fingers, toes, and sometimes entire soles wrinkle up when wet for long periods. At that time, I did not have a blog, so I probably prattled on about it to my husband as he put his smiles and nods on autopilot and read something political online. However, I was reminded again of this misunderstood phenomenon when I came across a NYT article that blows the absorbed-moisture theory out of the, well, water.
Basically, absorption doesn’t have a damn thing to do with it. In fact, there’s absolutely no absorbing going on at all. It’s all about traction. Like a tire, your fingers can’t grip when wet, but add some water redirecting groves courtesy of the central nervous system and you’re practically Spider Man.
In the study, an evolutionary neurobiologist and his co-authors examined 28 fingers [ed: photos of fingers, actually] wrinkled by water. They found that they all had the same pattern of unconnected channels diverging away from one another as they got more distant from the fingertips.
I like my major scientific studies with a few more than 28 fingertips involved, but this new theory does make a lot of sense, especially when you consider this: nearly a century ago surgeons observed that no wrinkling occurs if a nerve to the finger has been cut.
Fun trick to scare your children: pierce through just the stratum corneum of your fingertip with a (STERILIZED!!1) needle ** and then chase them around the house, moaning. It doesn’t hurt even a little bit, since the topmost layer is only dead skin. As a decorated honors student in junior high, I freaked out my friends on more than one occasion with needles jutting out of the pads of my fingers like a poor man’s Wolverine.
** DON’T DO THIS. It can be done, is all I’m saying.