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Why Does Skin Get Pruney in the Tub?

For a long time, scientists thought that pruning of the skin after spending time in the water was simply a matter of fingers being a little spongey. The outermost layer (the stratum corneum) of the outermost layer (the epidermis) of our skin is mostly made up of cells called corneocytes. These cells are filled with keratin, a protein that helps keep the skin hydrated by absorbing water and preventing its evaporation. When you hang out in the pool or the bathtub for a while, the keratin absorbs a lot of water, and the cells swell up. While the thin stratum corneum swells with water, the lower layers of skin that it is attached to don’t, so that outermost layer has to buckle and bend to accommodate its relatively larger size, sort of like a too-big shirt that wrinkles and bunches together when it's tucked in.

Another, more recent explanation is that the wrinkles come from vasoconstriction, or the narrowing of blood vessels. The idea is that hot water makes the blood vessels in the fingers tighten and the surrounding tissue contract, causing the skin to fold.

But the explanation might be more complicated than either one of those potential causes—especially when you consider how the phenomenon occurs in people with nerve damage.

So Unnerving

In the 1930s, two scientists examined a boy whose median nerve was severed, leaving his thumb, index, and middle fingers numb. When they soaked his hand in water, the ring and pinkie fingers wrinkled but the fingers affected by the damaged nerve stayed smooth.

And in 2001, researchers at Tel Aviv University found that nervous system malfunctions caused by Parkinson’s disease also interfered with finger wrinkling. In their study, Parkinson’s patients’ fingers wrinkled less on one side of the body than the other, and wrinkled less overall than the fingers of healthy subjects. Going by the common explanations, the wrinkles were a local phenomenon happening in very small bits of flesh. The involvement of the nervous system, though, suggests that something else is going on.

Getting a Grip

Mark Changizi, neuroscientist and the Director of Human Cognition at 2AI Labs in Boise, Idaho, thinks that the wrinkles’ neural factor is a clue that they’re adaptive. Rather than being a mere side effect of water-logged digits, he says, they’re a functional response to wet conditions: The wrinkles act like drainage networks or tire treads on our fingers and toes, channeling water away and giving skin more contact with, and a better grip on, wet surfaces.

Analyzing the wrinkles on various soaked fingers, Changizi and his team found that they all had similar shapes and characteristics—with disconnected channels that moved away from each other as they got farther from the fingertip—consistent with what is expected in a drainage network. That wasn’t much evidence for Changizi’s hypothesis, but it got the ball rolling. (Update: 11/30/2012, 1:25 pm) While that doesn't seem like much, Changizi points out that the "morphology prediction is actually very strong."

"Of the infinitely many wrinkle patterns that are possible," he says, "[the] drainage hypothesis predicts [the] actual [pattern]."

Since publishing the idea and the initial data last year, he and his team have been looking for evidence of finger-wrinkling in other primates that live in wet environments (they’d already found it happens in Japanese macaques) and are setting up experiments to directly test the wrinkles' effects on grips, While the results aren't ready to be published yet and the pilot studies so far suggest that pruney fingers do help improve grip.

(Update: 11/30/2012, 1:25 pm) Changizi has filled me in on that pilot data. The experiment was conducted Changizi and undergraduate student Joseph Palazzo. They had subjects carry out a timed task of moving objects, including bottles, stones, logs and other items, from one place on a table top to another, and back again. They did this in wet-pruney, dry-pruney (dry objects, and fingers dried after they had wrinkled), wet-nonpruney (wet fingers, but not yet wrinkled) and dry-nonpruney conditions. Wet-pruney performance was better than wet-nonpruney, with the subjects being faster and making fewer mistakes.

Changizi would like to see more behavioral studies like this carried out, and see more data from other species for further tests, but  probably won’t carry out any of these studies himself. "A more sophisticated next experiment would be version-2.0 of this sort of thing, in my mind," he says. "But not my forte." He thinks that other scientists would be much better at that kind of experiment.

"In terms of the categories of test, then," he says. "There's morphology, behavior, and phylogeny, and at this point we've done the first, poked at the second, and only wondered about the third."

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Big Questions
Why Is Holly a Symbol of Christmas?
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Santa Claus. A big ol’ red-and-white stocking hung by the fire. Nativity scenes. Most classic Christmas imagery is pretty self-explanatory. Then there’s the holly, genus Ilex, which found its way onto holiday cards through a more circuitous route. 

Christmas is kind of the new kid on the block as far as holly symbolism is concerned. The hardy plant’s ability to stay vibrant through the winter made it a natural choice for pre-Christian winter festivals. The Roman feast of Saturnalia, celebrated at the darkest time of the year, celebrated the god of agriculture, creation, and time, and the transition into sunshine and spring. Roman citizens festooned their houses with garlands of evergreens and tied cheery holly clippings to the gifts they exchanged.

The Celtic peoples of ancient Gaul saw great magic in the holly’s bright "berries" (technically drupes) and shiny leaves. They wore holly wreaths and sprigs to many sacred rites and festivals and viewed it as a form of protection from evil spirits. 

Christianity’s spread through what is now Europe was slow and complicated. It was hardly a one-shot, all-or-nothing takeover; few people are eager to give up their way of life. Instead, missionaries in many areas had more luck blending their messages with existing local traditions and beliefs. Holly and decorated trees were used symbolically by new Christians, just as they’d been used in their pagan days.

Today, some people associate the holly bush not with the story of Jesus’s birth but with his death, comparing the plant’s prickly leaves to a crown of thorns and the berries to drops of blood. 

But most people just enjoy it because it’s cheerful, picturesque, and riotously alive at a time when the rest of the world seems to be still and asleep.

NOTE: Holly is as poisonous as it is pretty. Please keep it away from your kids and pets.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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What Are the 12 Days of Christmas?
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Everyone knows to expect a partridge in a pear tree from your true love on the first day of Christmas ... But when is the first day of Christmas?

You'd think that the 12 days of Christmas would lead up to the big day—that's how countdowns work, as any year-end list would illustrate—but in Western Christianity, "Christmas" actually begins on December 25th and ends on January 5th. According to liturgy, the 12 days signify the time in between the birth of Christ and the night before Epiphany, which is the day the Magi visited bearing gifts. This is also called "Twelfth Night." (Epiphany is marked in most Western Christian traditions as happening on January 6th, and in some countries, the 12 days begin on December 26th.)

As for the ubiquitous song, it is said to be French in origin and was first printed in England in 1780. Rumors spread that it was a coded guide for Catholics who had to study their faith in secret in 16th-century England when Catholicism was against the law. According to the Christian Resource Institute, the legend is that "The 'true love' mentioned in the song is not an earthly suitor, but refers to God Himself. The 'me' who receives the presents refers to every baptized person who is part of the Christian Faith. Each of the 'days' represents some aspect of the Christian Faith that was important for children to learn."

In debunking that story, Snopes excerpted a 1998 email that lists what each object in the song supposedly symbolizes:

2 Turtle Doves = the Old and New Testaments
3 French Hens = Faith, Hope and Charity, the Theological Virtues
4 Calling Birds = the Four Gospels and/or the Four Evangelists
5 Golden Rings = the first Five Books of the Old Testament, the "Pentateuch", which gives the history of man's fall from grace.
6 Geese A-laying = the six days of creation
7 Swans A-swimming = the seven gifts of the Holy Spirit, the seven sacraments
8 Maids A-milking = the eight beatitudes
9 Ladies Dancing = the nine Fruits of the Holy Spirit
10 Lords A-leaping = the ten commandments
11 Pipers Piping = the eleven faithful apostles
12 Drummers Drumming = the twelve points of doctrine in the Apostle's Creed

There is pretty much no historical evidence pointing to the song's secret history, although the arguments for the legend are compelling. In all likelihood, the song's "code" was invented retroactively.

Hidden meaning or not, one thing is definitely certain: You have "The Twelve Days of Christmas" stuck in your head right now.

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