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10 Questions Still Baffling Scientists

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Science has done a terrific job of answering some of the world’s most difficult questions, but certain mysteries still elude researchers. How does gravity work? Can your pet fish really predict an earthquake? Why do we yawn so much? Here’s what we don’t know and how close we are to figuring it out.

1. Why Do We Yawn?

Theories about why we yawn are as common as grumpy toddlers at nap time, but two explanations seem plausible after experimental tests. One is that yawns help cool the brain and optimize its performance. Psychologists at the State University of New York at Albany say it explains why we yawn when we’re drowsy: Like the fan in a computer, the yawn kicks in when our performance starts lagging.

But if yawns are our brains’ way of kick-starting their efficiency, why is yawning contagious? The brain-cooling camp suggests that it’s a way to maintain group vigilance and safety. When a member of a pack yawns, signaling that he is not functioning at his best, the whole group may need to yawn for a collective cognitive boost.

That’s not the only theory floating around, though. Another explanation contends that contagious yawning builds and maintains empathy between yawners. A sympathetic yawn signals an appreciation and understanding of someone else’s condition and subconsciously says, “Me too, buddy.” So which story is the accurate one? Scientists aren’t ready to declare a winner yet—they need a little time to sleep on it.

2. Why Do People Spontaneously Combust?

Here’s what we know: Humans really do spontaneously combust. One of the first people recorded to have gone up in smoke is a poor Italian knight who burst into flames after drinking strong wine in the mid–17th century. The cause of the mysterious fireworks befuddles scientists, but they’re certain that each instance is less spontaneous than it seems. Over centuries, 120 cases of spontaneous human combustion have been reported, but because most of the cases involve smokers, a common hypothesis is that an outside flame is involved. The theory is that a cigarette scorches the skin and breaks it deep enough to force body fat to seep rapidly from the wound into burning clothing; together they act like candle wax and a wick.

It’s far more probable than the competing idea—that methane gases build up in the intestines and are sparked from inside the body by a mix of enzymes. But there’s a problem with testing both theories: Researchers can’t just walk around setting people on fire. They may have found a substitute that will answer the question, though. Pig tissue combusts in a way that’s consistent with the “wick effect,” and samples are far easier to obtain. Who knew bacon would help solve the mystery of one of Spinal Tap’s drummers?

3. Why Do Placebos Work?

When a new drug enters clinical trials, researchers need a control group against which to compare its effects. Members of this group are given what they’re told is the drug but is actually a pill containing no active ingredients, a placebo. Frequently, though, the control subjects feel the drug’s effects. Or at least they say they do. What actually happens to placebo poppers is still unsettled. Some studies have found objectively measured effects that are in line with a real drug’s results. Others have found that the benefits are only subjective; patients said they felt better after taking the placebo, regardless of their actual improvement. This mixed bag of evidence could support any number of explanations. There could be an actual physiological response, Pavlovian conditioning (a patient expects to feel better after medicating), positive feelings from patient-doctor interactions, an unconscious desire to “do well” in a clinical trial, or even a natural improvement in symptoms.

Whatever the cause, pharmaceutical companies are keen to figure out the placebo effect given its potential to throw clinical trials into disarray. Real drugs often can’t compete against the effects of fakers, and about half get scrapped in late-stage trials. For the researchers who’ve spent nearly 10 years trying to bring their drugs to market, that’s a bitter pill to swallow.

4. What Was Life’s Last Universal Common Ancestor?

A whale and a bacterium or an octopus and an orchid don’t seem to have much in common, but deep down they’re all the same. Research reveals that most of life’s tiniest components, like proteins and nucleic acids, are nearly universal. The genetic code is written in the same way across all organisms. A small core of genome sequences is also similar across major branches of life’s family tree. All this suggests that every living thing made of cells can trace its lineage back to one source, a universal common ancestor.

In theory, this idea makes a lot of sense. Getting this ancestor to show up for a paternity test is tougher. Scientists estimate that the last universal common ancestor (LUCA) split into microbes and later eukaryotes (animals, plants, and the like) around 2.9 billion years ago. The fossil record from that era is scant, and by now, the genes that have traveled down the family tree have been lost, swapped, or shuffled around.

But some features of proteins and nucleic acids encoded by these genes—such as their three-dimensional structure—have been preserved throughout time. A survey of these molecular traits offers a glimpse at what the last universal common ancestor might have looked like. Researchers have found that tiny organelles (specialized subparts of cells) as well as their associated enzymes are shared by all major branches of life, meaning that they must have been present in the last universal common ancestor. This and other evidence suggests that the LUCA was as complex as a modern cell—which doesn’t make our forebear all that visually impressive. But on the plus side, until scientists get to the bottom of this question, we can all save money on Father’s Day cards for the granddaddy of all life on Earth.

5. How Does Memory Work?

For a long time, neuroscientists thought a memory was stored in a scattered group of neurons in either the hippocampus or in the neocortex. Last year, researchers at MIT proved that theory for the first time by causing mice to remember or forget an event by activating or deactivating the associated neurons.

It’s an essential piece to the puzzle, but to recall a memory on its own, the brain has to activate the correct assortment of neurons. And how exactly the brain pulls off that trick isn’t fully understood. Studies on rodents and brain imaging in people suggests that some of the same neurons that the original experience affected are involved. In other words, remembering something may not just be a matter of grabbing it from its storage space but re-forming the memory each time it’s pulled out.

6. Can Animals Really Predict Earthquakes?

The idea that our furry and feathered friends could warn us about impending doom is a nice one, but it’s been hard for scientists to prove. Pet owners have noted how their animals acted funny just before an earthquake since the days of ancient Greece. There’s no shortage of reports, but almost every one is anecdotal, based on opinions of what’s “normal” and “funny” for an animal. And the stories are generally reported long after the fact.

It’s not out of the question that animals may sense and react to some environmental change that we don’t notice—anything from seismic waves to changes in electric or magnetic fields. However, it’s not clear that earthquakes even produce such precursors. Plus, whatever the proposed cause, it’s nearly impossible to test. If we can’t predict earthquakes, we don’t know when to observe animals, and it’s even more difficult for researchers trying to reproduce the experiment later. The few “lucky” cases where quakes happened during animal experiments provide conflicting evidence. If you’re going to rely on a cat for earthquake advice, consult one with a degree in seismology.

7. How Do Organs Know When to Stop Growing?

Every mammal starts out as a single cell before growing into trillions of them. Usually, there’s tight control over the number and size of cells, tissue, and organs, but sometimes things go very wrong, resulting in anything from cancer to a leg that’s larger than its partner. So what’s sending the “stop growing” signal?

Four proteins that make up the core of what’s known as the Salvador-Warts-Hippo signaling pathway appear to help regulate growth for a number of organs. Shutoff signals sent down the pathway deactivate the protein that promotes growth, but that’s where scientists’ knowledge stops. Where these signals originate and which other elements are affecting SWH is unknown. Scientists continue to learn how to manipulate the pathway, discovering new triggers and working their way to the source, but there are still a lot of mysteries—including how we may be able to “turn off” cancer.

8. Are There Human Pheromones?

Can you actually smell someone’s fear? Or sniff out a rat? Plenty of animals communicate with chemical signals called pheromones, but whether humans are part of that club is a contentious issue. There’s some evidence of people making behavioral and physical changes in response to chemosignals, but scientists haven’t been able to figure out which chemicals trigger these responses. And despite what the labels on pheromone-infused colognes and hair gels will tell you, no compound has been identified as a human pheromone or linked to a specific response.

Moreover, if people are giving off pheromones, scientists aren’t sure how others are detecting them. Many mammals and reptiles have what’s known as a vomeronasal organ that detects pheromones. While some human noses contain the tiny organ, it may not be functional; sensory neurons have little or no connection with the nervous system. So for now, the answer to this question remains “maybe.” And that uncertainty truly stinks.

9. What’s the Deal With Gravity?

Of the four fundamental forces of nature, gravity is the runt of the litter. It holds the universe together, but it’s weaker than its three siblings: electromagnetism, weak nuclear forces, and strong nuclear forces. How much punier is it? The next step up, weak nuclear, is 10^26 (100,000,000,000,000,000,000, 000,000) times stronger. Gravity’s relatively feeble pull makes it hard to demonstrate with small objects in the lab.

Gravity doesn’t play well with the other forces either. Try as they might, scientists can’t use quantum theory and general relativity to explain gravity on small scales. And this incompatibility leaves us short of physicists’ grandest goal: a unified theory of everything.

Worse still, scientists can’t even figure out what gravity is made of. The other fundamental forces are all associated with particles that help carry them, but no one’s been able to detect the gravitational particle—the hypothetical graviton—even with the most super of supercolliders! And while some scientists are frustrated by its elusive nature, others know it’s just gravity’s way—the force has a reputation for bringing us down.

10. How Many Species Are There?

Taxonomists have been finding, naming, and describing species in an organized manner for more than 200 years, and they’re probably nowhere close to being finished. It’s not that they’re slacking off on the job either. In the last decade alone, scientists have reported more than 16,000 new species per year; in total, they’ve cataloged 1.2 million. It’s anybody’s guess how many are left undiscovered, though. Going out and finding every single species would take the 300,000 working taxonomists a lifetime, so they have to make educated guesses.

Making these kinds of extrapolations presents serious logistical hurdles. Biodiversity hotspots often fall in developing countries, which suffer from a shortage of taxonomists. Furthermore, up to 80 percent of the planet’s life may be hiding out in hard-to-reach places under the sea.

Given these troubles, it’s no wonder there’s a wide variance in expert guesses of how many species are left undiscovered. The most recent ballpark figures place the number between five and 15 million species, which makes the odds of someone discovering a unicorn just slightly better than we’d even dared to dream.

This story originally appeared in mental_floss magazine.

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10 Fab Facts About George Harrison
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You probably know George Harrison as a Beatle, the lead guitarist of the most famous band in the world. We’re guessing that there’s a lot you don’t know about the youngest of The Fab Four, who was born on this day in 1943.

1. HE WAS ONLY 27 WHEN THE BEATLES BROKE UP.


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George Harrison turned 27 on February 25, 1970, less than two months before Paul McCartney told the world he had no future plans to work with the Beatles. It had been 12 years since Harrison had joined John Lennon’s band, The Quarrymen—shortly after McCartney, his Liverpool schoolmate—in 1958.

2. HE INVENTED THE MEGASTAR ROCK BENEFIT CONCERT.

Before Harrison organized the 1971 Concert for Bangladesh, there were performances for charity, of course. But when his friend, the great Indian sitar player Ravi Shankar, told him about the plight of Bangladeshi refugees, victims of both war and a devastating cyclone who now faced starvation, Harrison felt compelled to devote himself to the cause. He recruited stars like Eric Clapton, Bob Dylan, Ringo Starr, Billy Preston, Badfinger, and Leon Russell, and together they played two sold-out shows at Madison Square Garden on August 1, 1971. Harrison then arranged for the release of a concert album and film. The ventures had raised more than $12 million by 1985, and profits from sales of the movie and soundtrack continue to benefit the George Harrison Fund for UNICEF.

3. HE WROTE “CRACKERBOX PALACE” ABOUT HIS QUIRKY MANSION.

Harrison nicknamed his 120-room Friar Park mansion “Crackerbox Palace” after a friend’s description of Lord Buckley’s tiny Los Angeles home. The 66-acre property, about 37 miles west of London, was first owned by Sir Frank Crisp, a lawyer who lived there from 1889 to 1919. Harrison bought the estate in 1970—and quickly penned “The Ballad Of Sir Frankie Crisp,” which appeared on his first solo album, All Things Must Pass, also in 1970.

Friar Park was a strange place, with gnomes, grottos, a miniature Matterhorn, and lavish gardens, which Harrison loved to tend. According to the Victoria County History website, the house itself “is an architectural fantasy in red brick, stone, and terracotta, mixing English, French and Flemish motifs in lavish, undisciplined profusion.”

4. HE LOVED HANGING OUT WITH BOB DYLAN AND THE BAND.

All four Beatles were Dylan fans, and first met him in 1964. But Harrison felt a special bond with him, and spent weeks at Dylan’s Woodstock, New York home in the fall of 1968. The Band was there, too, and Harrison loved the collaborative atmosphere. During this time Dylan and Harrison co-wrote “I’d Have You Anytime,” which appeared on 1970's All Things Must Pass. The two would become bandmates in the Traveling Wilburys, and maintained a close, lifelong friendship.

5. THE "QUIET BEATLE" WASN’T SO QUIET.

"He never shut up," friend and fellow Traveling Wilbury Tom Petty once said of Harrison. "He was the best hang you could imagine."

6. WHEN HE LOST HIS VIRGINITY, THE OTHER BEATLES CHEERED.

The Beatles at the EMI studios in Abbey Road, as they prepare for 'Our World', a world-wide live television show broadcasting to 24 countries with a potential audience of 400 million.
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During the band’s early years, they had extended runs as a house band in Hamburg, Germany, and were paid so poorly (and had to be on stage for so many hours) that they shared a small room in the club’s basement. Hence the witnesses to George’s deflowering, at age 17. "We were in bunkbeds," Harrison recalled. "They couldn't really see anything because I was under the covers, but after I'd finished they all applauded and cheered. At least they kept quiet whilst I was doing it."

7. WITHOUT HIM, THERE MAY NOT HAVE BEEN A MONTY PYTHON'S LIFE OF BRIAN.

EMI Films, Life of Brian’s original backer, withdrew funding for the Monty Python comedy classic just before filming began, scared that the religious subject matter would be too controversial. Harrison, a big fan and friend of the Pythons, set up his own production company—Handmade Films—to fund the project. Why? "Because I liked the script and I wanted to see the movie,” he explained. Harrison not only saw the film, he appeared in it, as Mr. Papadopolous, "owner of the Mount.” Monty Python’s Life of Brian, released in 1979, was a huge hit in both the UK and U.S., and was ranked as the 10th best comedy film of all time in 2010 by The Guardian.

8. HE WAS THE FIRST EX-BEATLE TO SIMULTANEOUSLY TOP BOTH THE SINGLES AND ALBUMS CHARTS.

Harrison began recording the songs that would comprise All Things Must Pass at Abbey Road on May 26, 1970, just weeks after the Beatles broke up. The triple album was released in late November, along with “My Sweet Lord,” the first single from the album. Both the record and the single spent weeks at the top of the Billboard and Melody Maker charts in early 1971, while receiving rave reviews.

9. THE FIRST SONG HE WROTE WAS INSPIRED BY A DESIRE TO TELL PEOPLE TO GET LOST.

Harrison wrote “Don’t Bother Me,” his first first solo composition, while sick in bed at the Palace Court Hotel in Bournemouth, England, in the summer of 1963. It “was an exercise to see if I could write a song,” Harrison said. “I don't think it's a particularly good song ... It mightn't even be a song at all, but at least it showed me that all I needed to do was keep on writing, and then maybe eventually I would write something good." “Don’t Bother Me” appeared on With The Beatles, their second studio album.

10. HE WAS THE FIRST BEATLE TO VISIT, AND PLAY IN, THE U.S.

In the fall of 1963, Harrison traveled to Benton, Illinois to visit his sister, Louise, and her husband, George Caldwell. During his 18-day stay, Harrison also became the first Beatle to play in the U.S.—appearing on stage with The Four Vests at the VFW Hall in Eldorado. He played the second set with the band, taking over lead guitar and singing "Roll Over Beethoven" and "Your Cheatin' Heart."

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25 Icy-Cool Facts About Polar Bears
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From starring in Coca-Cola ads to becoming the poster child for climate change, the polar bear is quite the high-profile species. Ursus maritimus is a fascinating animal that roams across the Arctic Circle through Norway, Russia, Canada, Greenland, and Alaska, and there's more to them than the adorable faces you see in children's books and advertisements. Here are 25 facts you should know about the polar bear:

1. THEY'RE THE LARGEST CARNIVORES ON LAND.

A polar bear walks across the snow at sunset.
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Polar bears can weigh more than 1300 pounds and span more than 8 feet, 6 inches from nose to tail, making them the largest carnivores to currently walk the Earth. (Though other bears can grow larger, like Alaska's 10-foot-long Kodiak bear, they're omnivorous, while polar bears prefer an all-meat diet.) The males far outweigh their female counterparts, who may only weigh between 330 and 650 pounds. In general, though, a bear's weight fluctuates significantly throughout the year, with some bears packing on 50 percent more body weight over the course of a successful hunting season, then losing it over the course of their long fasting months.

2. BUT TECHNICALLY, THEY'RE MARINE MAMMALS.

A shot from below of two polar bears swimming in clear blue water
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Because they spend so much of their lives on ice, rather than land, polar bears are the only bears to be considered marine mammals. They hunt, court, and mate out on the ice, spending many months of the year far from land.

3. THEY'RE HIGHER ON THE FOOD CHAIN THAN WE ARE.

A large polar bear opens its mouth in a roar.
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Human beings aren't as high on the global food chain as you might think. Polar bears don't have any natural predators, and their intensely carnivorous diet puts them at the top of the food chain with species like killer whales, according to researchers, while humans fall somewhere closer to the middle. Don't worry too much about getting eaten by one, though—a 2017 study found that during the past 144 years, there have only been 20 fatal polar bear attacks in all of the five countries that have polar bear populations. However, as food becomes more scarce for the bears, humans living in polar territory may soon face more risk from starving bears.

4. THEY'RE LONERS …

A polar bear walks across a large field of ice.
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Other than the two to three years a cub spends with its mother, polar bears are pretty much solitary creatures. Adults spend only a few days a year mating, then go on their own way, spreading out to hunt on their own. They rely on the scent left by the sweat glands on their paws to track other bears, using the smell to sense where potential mates might be headed, among other things.

5. … BUT ARE SOMETIMES WILLING TO SHARE.

A polar bear sleeps cuddled next to her cub.
Paul J. Richards, AFP/Getty Images

Polar bears can play nice with each other sometimes. On occasion, they will hang out together in large groups, especially if there's a big meal that multiple bears can take part in, like a whale carcass. When they do spend time together (in what's called a sleuth), male bears will play-fight with each other, wrestling and swatting at each other without doing any real harm. According to the documentary Polar Bears: Spy on the Ice, polar bears can recognize friends they've met before even if they go without seeing each other for many years.

6. THEY'RE PICKY EATERS.

A polar bear shares food with a cub.
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When food is plentiful, polar bears are very selective about what they eat. They hunt seals, but if there are plenty available to hunt, they won't eat their whole catch. Instead, they'll only eat the energy-rich blubber (up to 100 pounds at a time), leaving the rest of the carcass for other animals to scavenge. When hunting is good, their diet is made up of about 90 to 95 percent fat. When times are lean, though, they'll happily branch out, eating reindeer, rodents, eggs, seaweed, and anything else they can get their claws on. However, because their bodies are so much better at digesting fat than protein, researchers think that if Arctic ice continues to melt and polar bears become unable access the ice (with its blubber-rich seals), they won't be able to get enough calories on land to survive [PDF].

7. THEY SPEND A LOT OF TIME FASTING.

A polar bear sprawls out on its stomach.
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When they're not out on the ice scoping out seals, polar bears spend an incredible amount of time fasting. The female polar bears fast longer than any other mammal species—in Canada's Hudson Bay, pregnant polar bears can fast up to 240 days, or almost eight months. There's reason to think they'll be fasting even longer in the future as sea ice melts, leaving bears with fewer hunting opportunities and less time to accumulate the fat stores needed to get through the lean months. During the 1980s, non-pregnant polar bears spent 120 days fasting between hunting seasons, but researchers now think that the bears will have to go without food longer and longer, fasting for as much as 180 days at a time in the future.

8. THEY WILL TRAVEL FAR TO FIND DINNER.

Two polar bears walk through the snow.
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The average bear might travel across 100,000 square miles in its lifetime, and that number may be getting higher. In 2013, a bear searcher told the BBC that polar bears were spending 9 to 13 percent more time being active to make up for the fact that the ice they hunt on is drifting faster, leaving them walking on a "treadmill" just to stay within their territory. One bear tracked by the WWF traveled almost 2300 miles from Norway to Russia in less than a year. Due to receding ice, polar bears have to walk farther to find prey, wasting valuable energy. The energy they gain from eating a single ringed seal might not even make up for what they expend trying to find and catch it.

9. THEY CAN SWIM FOR DAYS.

A polar bear swims toward the camera.
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Polar bears are savvy swimmers, paddling at an average speed of 6 miles per hour. And it's a good thing: Due to all that melting ice, polar bears are putting their swimming skills to lengthy use. In 2011, a study reported that a tagged female polar bear swam a total of 426 miles in one nine-day stretch across the Beaufort Sea above Alaska, losing 22 percent of her body weight in the process. Another bear in the study swam for 12 days, though she at least stopped to take some breaks.

10. THEY GET HOT FAST.

A wet polar bear sticks its tongue out.
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You'd think with all that plunging in Arctic waters, polar bears might get chilly occasionally. But since they're built to withstand extreme cold on a regular basis, they actually have the opposite problem: They overheat very easily, and are more likely to die from the heat than the cold. Their two layers of fur and solid layer of body fat (up to 4.5 inches thick) keep their metabolic rate consistent when temperatures reach as low as -34° F. They can sprint up to 30 miles per hour if need be, but much like you wouldn't want to run a race in a heavy ski jacket, polar bears can't spend much time chasing after their prey lest they overheat—a bear's body temperature can rise to feverish temps if they move too fast. On land, they typically only walk at speeds of three miles an hour, and their main hunting technique involves staying very still for hours or days at a time, waiting for a seal to emerge from the ice to breathe.

11. THEY'VE BEEN GETTING IT ON WITH GRIZZLIES.

A polar bear sits with her two cubs.
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In addition to changing their travel patterns and dinner prospects, climate change is altering polar bears' love lives. As the ice-traversing bears are forced to spend more time on the tundra, their habitats are starting to overlap with those of grizzly bears. In some places, the two species are getting more comfortable with each other, with amorous results. In Alaska and western Canada, grizzlies and polar bears are doing more cross-breeding, creating hybrid offspring.

12. THEY GROW A LOT IN THEIR FIRST FEW MONTHS.

A polar bear cub sits on its mother's back.
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At birth, polar bears weigh anywhere from 16 to 24 ounces—about what a guinea pig does. As newborns, they're blind, toothless, and only about a foot long. But by the time they emerge from their den for the first time around four months later, they are substantially larger, weighing between 22 and 33 pounds. In addition to nursing, they'll begin eating solid food around that time, and by 8 months old, they'll weigh 100 pounds or more.

13. THEY HAVE HUGE FEET.

A polar bear swipes its paws in the water.
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In order to balance on ice, polar bears boast giant feet. Their paws can measure up to 12 inches in diameter, acting like snowshoes to spread out their weight on thin ice and deep snow. The bumpy papillae (like the ones on your tongue) on their footpads help grip the ice, keeping them from sliding around. They also have long, curved claws that can measure almost 4 inches—all the better to grab onto slippery seals.

14. UNLIKE OTHER BEARS, THEY DON'T HIBERNATE.

A polar bear leaps into the water.
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While black bears, grizzlies, and other bear species spend each winter denning, forgoing eating, drinking, moving, pooping, and peeing for months on end, polar bears stay active all winter. Polar bears don't need to sleep through the winter, though, because there's plenty of food available to them in the coldest months, when they take to the sea ice to hunt for seals. The only exception is during pregnancy, when a female polar bear digs herself a den and remains sealed inside, surviving off her stores of fat, until her cubs grow large enough to survive outdoors.

15. THEY LOVE TO NAP DURING SNOWSTORMS.

Two polar bears sleep covered in snow.
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Polar bears may not hibernate, but they are happy to lay low when bad weather hits. During the winter, they dig themselves into shallow pits in the snow to protect themselves from wind, sometimes remaining there for days as the snow piles up on top of them like a warm blanket. Sometimes, they take a similar approach to staying cool, digging through the tundra down to the permafrost during the summer to keep from overheating.

16. THEY'RE VERY HARD TO TRACK.

A polar bear wears a tracking collar.
Paul J. Richards, AFP/Getty Images

Considering how far they travel—both walking and swimming—over a given year, you can imagine how hard it is for scientists to track polar bears. By nature, they spend a huge amount of time alone in remote locations. Scientists use boats, helicopters, and low-flying planes to observe them, but that only works in good weather and in certain locations. So recently, they've turned to satellites, fitting bears with non-invasive radio collars and tracking them through high-resolution satellite imagery. It's cheaper than sending out a helicopter, and it lets researchers identify bears even in the most remote areas of the Arctic.

17. THEIR NOSTRILS CLOSE WHILE THEY SWIM.

A swimming polar bear peeks its nose out of the water.
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Polar bears don't have to worry about getting water up their nose. When they swim, their nostrils close to prevent them from breathing in water. They can swim at depths up to 15 feet, and while they typically only dive for a few seconds, they can hold their breath for more than two minutes, enabling them to sneak up on seals resting on ice floes. In 2015, scientists reported observing a record-breaking polar bear dive that totaled 3 minutes and 10 seconds. The hungry bear stalked three seals from afar, swimming almost 150 feet underwater without surfacing for a breath or to reorient himself to the seals' location before bursting out of the water where one of the seals was resting. (Sadly, his prey got away.)

18. THEY CAN TURN GREEN IN CAPTIVITY.

A polar bear in a zoo swims with a ball.
Ina Fassbender, AFP/Getty Images

Though polar bears are sometimes known as the white bear, they aren't white. Their hair is colorless and hollow, and only appears white because of the way light scatters through their fur. (Under that mass of hair, their skin is as black as their noses.) When bears are subject to warmer temperatures in captivity, though, they can take on a bit of a verdant hue. Algae infestations can turn polar bears green, and not just on the outer layer of their fur. The colorful algae grows inside the hollow tube of each hair. This green growth thrives in humid climates, like Singapore, where the bears don't naturally live.

19. THEY'LL NEVER MEET A PENGUIN.

A Bulgarian stamp set featuring a polar bear, a seal, penguins, and a walrus.
State Agency for Information Technology and Communications of Bulgaria, Wikimedia Commons // CC BY 2.5

Though you might see polar bears and penguins together in Coca-Cola ads or on winter-themed pajamas, the two species never mix in real life. They live at opposite ends of the Earth, though they both spend their days in icy waters. Polar bears exclusively inhabit the Arctic, and penguins only live in the Southern Hemisphere. The closest they ever get is when they live in the same zoo.

20. AT ONE ZOO, THEY POOP GLITTER.

Gold glitter on a black background
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At the Assiniboine Park Zoo in Winnipeg, Canada, the polar bears have sparkly poop. In 2014, zookeepers began feeding each of their bears a different color of non-toxic glitter so that they could trace their bowel movements, analyzing the samples to identify health issues, track stress hormones, and generally see how the bears are dealing with zoo life. The colors help the zookeepers label which poop comes from which bear.

21. EUROPEANS HAVE KEPT THEM IN CAPTIVITY SINCE THE 13TH CENTURY.

A 1938, black-and-white photo of a polar bear lying on its back in a zoo
Topical Press Agency/Getty Images

Captive polar bears have piqued public curiosity since as early as the Middle Ages, when the bears were occasionally given to European royalty by Viking traders. In the 1200s, when Henry III kept one in London, it was muzzled and chained but allowed to catch fish and swim in the Thames River. In the 17th century, Frederick I of Prussia kept a defanged and declawed polar bear, staging public fights between it and other large mammals for public amusement.

22. POSING WITH THEM WAS ONCE A POPULAR GERMAN PASTIME.

The orange cover of 'Teddybär' shows a person in a polar bear suit with his arms wrapped around a smiling boy.
Innocences

In the early 20th century, getting a picture with a man dressed in a polar bear suit was a fairly standard activity in Germany, at least according to the many photos found by French photo collector Jeann-Marie Donat. Donat spent 20 years tracking down the vintage photos, taken between 1920 and 1960, for his 2016 book Teddybär. There are several potential explanations for why so many Germans elected to stop for photos with people in polar bear suits (or to dress up as polar bears themselves). Donat suggests that it might trace back to the popularity of the two polar bears that arrived at the Berlin Zoo in the 1920s, while Hyperallergic notes that the costume was created as a Fanta advertising stunt, designed to distract Germans from the horrors of World War II. The photos show people young and old posing next to bears at the beach, in parks, in the street, in the summer and winter, alone and in groups. They all look delighted to get a chance at a polar-bear souvenir.

23. THEY CAN BE … POLARIZING.

Knut and his handler pose for photos lying down on their bellies.
John Macdougall, AFP/Getty Images

Knut, a polar bear cub born at the Berlin Zoological Garden in 2006, was hand-raised by zookeepers after being abandoned by his mother at birth. The cute cub became an instant tourist attraction—the most famous bear in the world, even—and the zoo's attendance rates skyrocketed, netting an extra $1.35 million in tickets when the bear began making twice-a-day public appearances.

But not everyone was psyched about "Knutmania." The young bear's popularity proved to be controversial for animal rights organizations like PETA, whose German spokesperson Frank Albrecht said the zoo should have let the orphaned Knut die rather than continue hand-feeding him, a process that he called a "gross violation of animal protection laws." In 2007, the bear received an anonymous, handwritten death threat from a hater who simply wrote "Knut is dead! Thursday midday." The zoo took the fax seriously enough to assign triple the amount of zookeepers keeping watch over the polar bear during his daily public romp. (Knut continued to live at the Berlin zoo until his death at age 4 from an autoimmune disease.)

24. THEY SOMETIMES GET THE CELEBRITY TREATMENT.

Photographers crowd in front of a barrier to photograph Knut at a zoo.
John Macdougall, AFP/Getty Images

In 2007, celebrity photographer Annie Leibovitz photographed Knut for the cover of Vanity Fair's annual "Green" issue. While Knut appeared solo on the cover of the German edition, he was Photoshopped into an image with Leonardo DiCaprio for the American edition. After his death, the Berlin zoo erected a bronze statue in his honor, and his body was preserved for display at the city’s natural history museum.

25. CHURCHILL, CANADA HAS A UNIQUE WAY OF LIVING WITH THEM.

A green sign in a snowy field reads 'Polar Bear Alert: Stop.'
Paul J. Richards, AFP/Getty Images

Churchill, a town in Manitoba, Canada on the shores of the Hudson Bay, is known as the polar bear capital of the world. During the fall, hundreds of polar bears pass through on their way to their icy hunting grounds on the bay, waiting nearby as the ice hardens for the winter. The locals have adopted unique ways of living with the hungry bears. Many don't lock their doors, so that if someone is running away from a polar bear, they can duck into any doorway. Since Halloween falls right in the middle of polar bear season in town, city employees, police officers, volunteer fire officials, and polar bear conservationists stay on patrol to drive away any bears that might be tempted to go trick-or-treating themselves, using helicopters, sirens, air horns, rubber bullets, and more to keep the bears at bay. Kids, for their part, aren't allowed to wear anything white for the evening.

Churchill also runs a "polar bear jail" for bears that continue to wander into town. Residents are encouraged to call the Polar Bear Alert Program hotline year-round if they see a bear in town, and conservation officers will come and try to scare it away. If shooting loud scare rounds at the bear doesn't do the trick, they trap the bear, or, if all else fails, hit it with a tranquilizer dart and take it to the Polar Bear Holding Facility. The specially-designed compound can hold up to 30 bears and is meant to keep bears that are aggressive or persistently return to the community. When the bay freezes, these bears are transported by helicopter or vehicle onto the ice, where they resume their normal winter hunting routine. With warmer temperatures keeping bears off the ice for longer and longer periods, more towns may soon have to learn from Churchill's strategies for peaceful coexistence.

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