Bats are the only mammals that truly fly, but they aren't the only ones you might see swooping overhead at dusk. For tens of millions of years, a variety of other furry vertebrates have also been soaring through forests, especially after dark.
Flying squirrels — which actually glide, not fly — date back to at least the Oligocene Epoch, and now come in 43 species across Asia, Europe and North America. They sail from tree to tree on a special membrane between each front and back limb, a trick that has evolved multiple times in history. (Aside from flying squirrels, it's also used by other aerial mammals such as anomalures, colugos and sugar gliders.)
Flying squirrels are mainly nocturnal, but they do sometimes come out during daylight, like this red-and-white giant flying squirrel (Petaurista alborufus) at Foping National Nature Reserve in China. (Photo: Burrard-Lucas Photography)
Gliding through trees by moonlight, these animals can seem like ghosts. Yet their nocturnal mystique is balanced with a doe-eyed charisma, making them valuable mascots for the ancient woodlands where they live. Humans are naturally drawn to cuteness and novelty, so conservationists often rally support for troubled ecosystems by highlighting cute or unusual animals that depend on them.
Even if we rarely see gliding mammals in the wild, it's nice to know they're still out there, patrolling primeval woods as they did long before our own species existed. And since their future hinges on the health of such places, anyone who appreciates these animals must be a fan of native forests, too. To shed a little light on both, here's a closer look at the secret world of flying squirrels:
Those adorable eyes are for night vision.
The Siberian flying squirrel ranges across Northern Europe and Russia, but a population on the Japanese island of Hokkaido is now considered an endemic subspecies, known as Pteromys volans orii. (Photo: harum.koh [CC BY-SA 2.0]/Flickr)
Big, round eyes are one reason why flying squirrels look so cute to humans. But while this trait typically indicates infancy in mammals — like the wide eyes that endear us to babies and puppies — flying squirrels retain their disproportionately plump peepers into adulthood. They evolved big eyes to collect more light for better night vision, an adaptation shared by many nocturnal animals, from owls to lemurs.
They can glow at night.
While we know all species of flying squirrels are active at night, it wasn't until recently that researchers discovered that some also glow at night.
Jonathan Martin, an associate professor of forestry at Northland College in Wisconsin, was coming back from a hike one night when he shined an ultraviolet light at a flying squirrel and saw that it glowed pink, reports Popular Science. Based on that spontaneous discovery, a team of researchers led by Allison Kohler eventually found that all American flying squirrels fluoresce at night.
They also learned the flying squirrels glow more strongly on their undersides. It's still unclear why the squirrels give off a fluorescent effect at all, but the researchers have several theories, including avoidance of predators at night, communication among the squirrels, and navigation of snowy and icy terrain.
Instead of wings, flying squirrels have 'patagia' and wrist spurs.
The furry, parachute-like membrane between a flying squirrel's front and back limbs is known as a "patagium" (plural: patagia). These flaps catch air as the squirrel falls, letting it propel itself forward instead of plummeting. But to make sure the patagia catch enough air, flying squirrels also have another trick up their sleeves: cartilage spurs at each wrist that can be extended almost like an extra finger, stretching out the patagia farther than the squirrel's tiny arms could on their own.
When a flying squirrel wants to reach a tree that's beyond jumping distance, it just boldly leaps out into the night, as captured in the video above. It then extends its limbs, including its wrist spurs, to stretch out its patagia and start gliding. It lands on the trunk of its target tree, gripping the bark with its claws, and often immediately scurries to the other side to avoid any owls that might have seen its glide.
Flying squirrels can glide 300 feet and make 180-degree turns.
A view of a southern flying squirrel (Glaucomys volans) gliding overhead. (Photo: Prattikppf [CC BY-SA 3.0]/Wikimedia Commons)
They may not really fly, but flying squirrels still cover impressive distances in the air. The average glide of a northern flying squirrel (Glaucomys sabrinusis) is about 65 feet (20 meters), according to the University of Michigan Museum of Zoology, or slightly longer than a bowling lane. But it can also go much farther if needed, with glides recorded up to 295 feet (90 meters). That means an 11-inch (28 cm) northern flying squirrel could glide almost the full length of a soccer field, or about as far as the Statue of Liberty is tall. It's also remarkably agile, using its limbs, fluffy tail and patagia muscles to make sharp turns, even pulling off full semi-circles in a single glide.
And such abilities aren't limited to smaller species: Asia's red giant flying squirrel (Petaurista petaurista) can grow 32 inches (81 cm) long and weigh almost 4 pounds (1.8 kg), yet has been seen making nimble glides as far as 246 feet (75 meters).
90 percent of all flying squirrel species exist only in Asia.
A giant red flying squirrel (Petaurista petaurista) perches on a branch in Sabah, Malaysia. (Photo: vil.sandi [CC BY-ND 2.0]/Flickr)
Wild flying squirrels can be found on three continents, but they aren't evenly distributed. Forty of 43 known species are endemic to Asia, meaning they naturally exist nowhere else on Earth. And relatives of flying squirrels have inhabited parts of Asia for roughly 160 million years, according to new research on flying-mammal fossils that hail from the age of dinosaurs. As the New York Times reports:
The fossils of the new species, Maiopatagium and Vilevolodon, are exquisitely preserved, revealing many details of their anatomy. Winglike sheets of skin stretched from their cheeks to their legs and tails. They also had remarkably flexible shoulders needed to climb up trees and then maneuver through the air during a glide.
Asia has played another key role in flying-squirrel history, according to a 2013 study, with dense forests offering both a refuge and a diversification center. These habitats may have saved flying squirrels during glacial periods, but they also slowly split up and reconnected over time, a process that can spur new species to evolve.
Even if Asian forests did all that, however, many now face growing threats from large-scale deforestation and human-induced climate change, both of which are happening far more quickly than the natural changes endured by ancient flying squirrels. "Based on this work," the study's authors wrote, "we predict a bleak future for the flying squirrels, one which is closely associated with the fate of forests in Asia."
Only 3 flying squirrels are native to the New World.
Flying squirrels exist across a large swath of North and Central America, except for sparsely treed places like deserts, grasslands and tundra. They've adapted to a wide range of forests in dramatically different climates, from Honduras to Quebec and Florida to Alaska. Yet unlike their highly diverse relatives in Asia, all these American flying squirrels hail from just three species. There's the northern flying squirrel and the southern flying squirrel (Glaucomys volans), plus the Humboldt's flying squirrel (Glaucomys oregonensis), identified as a species in 2017 after previously being classified as a subspecies of northern flying squirrel.
Distribution maps for two of North America's three flying squirrel species. (Photo: Darekk2 [CC BY-SA 4.0]/Wikimedia Commons
All three American species are fairly widespread, although some subspecies are relatively rare, like the endangered Carolina northern flying squirrel (G. sabrinus coloratus) or the San Bernardino flying squirrel (G. sabrinus californicus).
If flying squirrels live nearby, we're often oblivious.
Flying squirrels can be difficult to spot in the dark, but they're sometimes betrayed by their eyeshine, like the reddish reflection from this northern flying squirrel (Glaucomys sabrinus) in Ontario. (Photo: PJTurgeon [CC BY-SA 3.0]/Wikimedia Commons)
Most non-gliding tree squirrels are diurnal, or active during the day. And because some species have adapted to city life — like the ubiquitous eastern gray of North America — they're among the most commonly seen wildlife for many people.
But in some parts of the world, including much of North America, flying squirrels are far more common than their daytime visibility suggests. They're widespread not only in remote, wooded wilderness, but also many suburban areas with enough old-growth trees to accommodate a flying squirrel's lifestyle. We just rarely see them because they're active when we tend to be asleep, or at least indoors. Even when we are outside at night, the cover of darkness can hide flying squirrels from us.
If you want to see or hear one, however, there are ways to improve your odds. A flashlight can reveal a flying squirrel's eyeshine at night, for example, as in the photo above. Many species also make high-pitched "cheep" sounds to communicate with each other, often heard within the first several hours after sunset.
Baby flying squirrels need a lot of mothering.
Flying squirrels don't produce body heat until they're about 5 weeks old. When orphaned pups are brought to wildlife rehab centers, they're often wrapped in blankets or heating pads for warmth. (Photo: blu fish design/Shutterstock.com)
Southern flying squirrels are savvy survivors, but they only get to that point with a lot of motherly love. "Female southern flying squirrels give birth to hairless, helpless young that are extremely uncoordinated and incapable of rolling over," explains the University of Michigan Museum of Zoology (UMMZ). "During the first few days of their lives, the young continuously squirm while emitting faint squeaks."
Their ears open within two to six days of birth, and they develop some fur after about a week. Their eyes don't open for at least three weeks, though, and they remain dependent on their mothers for several months. "Females care for their young in the nest and nurse them for 65 days, which is an unusually long time for an animal of this size," the UMMZ adds. "The young become independent by 4 months old unless they are born later in the summer, in which case they usually overwinter as a family."
Mothers also maintain several secondary nests, notes the University of Georgia's Savannah River Ecology Lab (SREL), where they can flee with their offspring if the main nest site becomes too dangerous. One southern flying squirrel was reportedly seen doing this during a forest fire, even as flames were singeing her fur.
Flying squirrels don't hibernate, but they do hygge.
Despite inhabiting frigid forests in places like Canada, Finland and Siberia, flying squirrels don't hibernate. Instead, they become less active in cold weather, spending more time in their nests and less time foraging. (They do still venture out during winter, though, like the Japanese dwarf flying squirrels in the video above.)
They're also known to deal with harsh winter weather by huddling together. Multiple squirrels sometimes share a nest for this reason, beyond just immediate family members. They can reduce their metabolic rate and body temperature to save energy, according to the SREL, and benefit from each other's radiant heat. Huddling for warmth can be so important, in fact, that flying squirrels are also known to share their nests with other types of wildlife, including bats and even screech owls.
Some flying squirrels are larger than a house cat.
Red-and-white giant flying squirrels can grow 1 meter (3 feet) long from head to tail. (Photo: Burrard-Lucas Photography)
Flying squirrels range in size from a few inches to a few feet, including some of the smallest and largest tree squirrels known to science. Both American species are relatively tiny, for example, while some Asian flying squirrels can be enormous.
Known as giant flying squirrels, these vary from abundant to endangered. The red-and-white giant (Petaurista alborufus) can be more than 3 feet (1 meter) long and top 3 pounds (1.5 kilograms), and it's relatively common in central and southern China. The slightly smaller red giant (P. petaurista) has an even wider range, from Afghanistan and Pakistan to Malaysia and Singapore. Both are listed as species of "least concern" by the International Union for Conservation of Nature (IUCN).
Some other giants are much rarer. The woolly flying squirrel (Eupetaurus cinereus) is known only from about a dozen specimens in the far northern Himalayas, and is deemed endangered by the IUCN due to the clearing of its native pine forests.
There's also the critically endangered Namdapha flying squirrel (Biswamoyopterus biswasi), known only from a single specimen found at India's Namdapha National Park in 1981. It was thought to be the lone member of its genus until 2012, when a related species (B. laoensis) was discovered at a bushmeat market in Laos.
This is not a flying squirrel, but it is a gliding mammal.
Aside from flying squirrels, there are also at least 20 other species of gliding mammals outside the squirrel family, Sciuridae. They inhabit similar forested environments, use their patagia in similar ways and are generally nocturnal; they just evolved their abilities separately, a process called convergent evolution.
Non-squirrel gliders include colugos — also known as "flying lemurs," even though they aren't lemurs and can't fly — and the anomalures, seven African rodents dubbed "scaly-tailed squirrels" despite not being actual squirrels. There are gliding possums, too, a group of marsupials including sugar gliders, the endangered mahogany glider of Australia and the critically endangered northern glider of Papua New Guinea.
Some flying squirrels are attic addicts.
As forests around the world fade to farms and cities, wildlife must adapt or vanish. Many flying squirrels have proven adaptable to human habitats, including both American species, if enough tall trees are left intact. But their resourcefulness also tempts some flying squirrels to share our homes, possibly mistaking attics for huge tree cavities. And that can lead to trouble, as the video above explains.
Ultimately, the key to getting rid of flying squirrels and other rodents is exclusion, or sealing off their entry points, since they or other intruders could otherwise just re-invade. For tips on humanely and effectively parting ways, see this fact sheet by the Connecticut Department of Energy and Environmental Protection, and this in-depth guide on evicting a flying squirrel family. (Don't try to keep them as pets, either — feeding and housing wildlife is generally a bad idea for everyone involved.)
They're one of many reasons why old-growth forests are worth protecting.
Flying squirrels often thrive in primary forests, like this one in coastal Oregon. (Photo: Alaina McDavid [CC BY-SA 2.0]/Flickr)
Forests made flying squirrels who they are, creating environments where gliding skills gave their ancestors an edge. And flying squirrels have helped shape their habitats in return, spreading tree seeds and providing food for native predators like owls.
Flying squirrels only play small roles in big, complicated forest ecosystems, but those ecosystems also happen to be pretty valuable to humans, offering a wealth of natural resources and ecological services like cleaner air, cleaner water and less flooding. We sometimes lose sight of those benefits, and charismatic wildlife like flying squirrels can help us remember not to miss the forest for the trees.
Editor's note: This story has been updated since it was first published in January 2017.