Squirrels get lots of attention from humans, but not always much appreciation. We tend to dwell on the stolen birdseed or the occupied attics, yet squirrels have a long, mostly harmless — and often entertaining — history of living in our midst.

That's the focus of Squirrel Appreciation Day, founded in 2001 by North Carolina wildlife rehabilitator Christy McKeown. Squirrels are widespread and widely beloved, and despite their penchant for mischief, they generally don't inspire the same scorn as other urban wildlife like rats, pigeons or opossums.

Squirrel Appreciation Day is observed Jan. 21 every year, but much like Elephant Appreciation Day, Penguin Awareness Day and similar informal holidays for animals, it's pretty decentralized. As McKeown explains on her website, "while there are no official events scheduled, you can help celebrate by putting out extra food for the squirrels." She also warns, though, that "too many treats will lead to health problems." (And feeding any wild animal on a regular basis could make it dependent.)

The holiday is mainly focused on bushy-tailed tree squirrels common in the Eastern U.S., and it's held in midwinter since that can be a fallow time of year for them. But even if you don't feed any squirrels this year — maybe they've already fattened up on birdseed and drywall — it's worth taking time to appreciate them. So in honor of Squirrel Appreciation Day on Jan. 21, here are 21 noteworthy squirrel facts:

1. There are more than 200 squirrel species worldwide, from tree squirrels and flying squirrels to chipmunks and marmots. They're all in the Sciuridae family, which is native to every continent except Australia and Antarctica.
 
2. Squirrels range in size from the five-inch African pygmy squirrel to the three-foot Indian giant squirrel.
 
3. Squirrels have four front teeth that grow continuously, at a rate of about six inches per year. This helps their incisors endure the constant gnawing.
 
4. The NASDAQ stock market was briefly shut down in 1987 and 1994 due to squirrels chewing through power lines.
 
5. In 2005, a pack of squirrels in Russia reportedly killed a stray dog that was barking at them. They may have been starving due to a pine cone shortage.
 
6. Adult squirrels normally live alone, but they sometimes nest in groups during severe cold spells. A group of squirrels is called a "scurry" or "dray."
 
7. When squirrels hide food for winter, they often dig fake holes to fool would-be thieves. To make sure they don't fool themselves, they lick their food before burying it, leaving a scent they can later detect even under snow.
 
8. All tree squirrels belong to the genus Sciurus, which comes from the Greek words "skia" (shadow) and "oura" (tail). The name reportedly reflects tree squirrels' habit of hiding in the shadow of their long, bushy tails.
 
9. The eastern gray squirrel is the most common tree squirrel species in the U.S., and humans have helped introduce it not only to western North America, but also to Europe and South Africa.
 
10. The eastern gray has become a pest in the U.K., where it threatens the survival of smaller, native red squirrels. This has made it popular for Britons to eat gray squirrels, part of a broader international trend of eating invasive species.
 
11. There's also a rich history of eating native squirrels in the U.S., where they've long been used in dishes like Kentucky burgoo and Brunswick stew. Squirrel meat has fallen out of favor lately — especially that of flying squirrels, which are relatively rare — but many Americans still hunt and eat eastern grays.
 
12. Tree squirrels mostly eat nuts, seeds and fruit, but they are omnivores. Gray squirrels, for example, have been known to eat insects, snails, bird eggs and animal carcasses when other food is scarce.
 
13. Better hope those carcasses aren't too rancid, though — squirrels, like many rodents, can't vomit. (They also can't burp or experience heartburn.)
 
14. The average adult squirrel needs about a pound of food per week.
 
15. A 2010 study found that some squirrels collect old rattlesnake skin, chew it up and then lick their fur, creating a kind of "rattlesnake perfume" that helps them hide from the smell-dependent predators.
 
16. All-black or white tree squirrels may look like distinct species, but in most cases they're actually just color variations of gray squirrels.
 
17. An eastern gray "rally squirrel" became an impromptu mascot for Major League Baseball's St. Louis Cardinals when it ran onto the field during the 2011 playoffs. The Cardinals went on to win the World Series.
 
18. Flying squirrels can't really fly — they just use flaps of skin between their limbs to glide through the air — but it often seems like they can. Their acrobatic leaps between trees often span up to 150 feet.
 
19. Red squirrels are solitary and highly territorial, but in some rare cases they've been known to adopt orphaned pups of their relatives.
 
20. Marmots are celebrated as weather forecasters in the U.S. and Canada, but their skills are a bit overhyped. Punxsutawney Phil's predictions were mostly wrong between 1988 and 2010, for example, while a study of Canadian groundhogs found their success rate was only 37 percent over 30 to 40 years.
 
21. Squirrels communicate using complex systems of high-frequency chirps and tail movements. Studies have also found they're capable of watching and learning from each other — especially if it relates to stealing food.

If you do feed squirrels for Squirrel Appreciation Day, be careful. They're known to bite fingers, and they're notoriously feisty when food is up for grabs — as this video shows:

To be fair, though, they are capable of sharing food when there's enough to go around:

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