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Ever Wonder How and Why You Get 'Pruney' Fingers?


"Could they be rain treads?"

Ever wondered why your fingers and toes wrinkle after a long soak? We did. And we found it actually serves a purpose, according to some scientists.

According to Nature, Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Idaho, thinks they could be "rain treads" that make it easier for us to grip things in wet environments. Nature has more on the study published in the Brain, Behavior and Evolution back in June:

Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed, implying that they are controlled by the nervous system.

"I stumbled upon these nearly century-old papers and they immediately suggested to me that pruney fingers are functional," says Changizi. "I discussed the mystery with my student Romann Weber, who said, 'Could they be rain treads?' 'Brilliant!' was my reply."

. . .

The sides of the finger are like cliffs where water can easily fall away, but the flat part is more like a plateau where water can pool. Wrinkles form on the plateau because "that's where all the work has to be done to channel the water away", Changizi explains.

Some scientists aren't quite on board, saying that the water wrinkles are just the mechanics of the skin reacting with the exposure to moisture.

Either way -- if wrinkles serve a purpose or occur for no reason at all -- what is happening to cause them in the first place? How often have you or your kids asked that question? Well, here's a shortened answer from Dermatologist Laurence Meyer of the University of Utah via Scientific American:

The epidermis, or outer layer of the skin, is made up of cells called keratinocytes, which form a very strong intracellular skeleton made up of a protein called keratin. These cells divide rapidly at the bottom of epidermis, pushing the higher cells upward. After migrating about halfway from the bottom of this layer to the top, the cells undergo a programmed death. The nucleus involutes, leaving alternating layers of the cell membrane, made of lipids, and the inside, made largely of water-loving keratin. The outer layer of the epidermis, called the stratum corneum, is thus composed of these alternating bands.

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.)

While that's a little complex, he's basically saying that layers of dead skin cells on our outermost layer of skin absorb water. Because skin cells underneath this layer are not dead, they do not absorb water and the water-logged dead cells on top wrinkle. Discovery Magazine describes this like so:

. . .since the epidermis is "tied" down to your dermis, [the live cells,] in certain spots, it expands more where it is not "tied" down and this causes your skin to wrinkle.

Watch this video if you want to see it explained:

To recap: Meyer's explanation for the wrinkles on hands and feet are that it's where keratin, the substance absorbing the water, is the most thick; Changizi takes it one step further in noting that the wrinkles are most prominent on the ends of digits -- the first parts to touch a water.

To further test his hypothesis, Changizi is looking into if people with wrinkled fingers are better at gripping in wet conditions. "We began pilot experiments," says Changizi. "The results thus far suggest that, yes, being pruney helps."

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