The World of Flying Squirrels: Gliding Through the Night

Flying squirrels are captivating creatures belonging to the squirrel family (Sciuridae). Despite their name, these animals don't truly fly like birds or bats, but instead, they are incredible gliders, using a specialized membrane to travel between trees. This article explores the fascinating aspects of flying squirrels, from their unique anatomy and gliding abilities to their diet, habitat, and conservation status.

What are Flying Squirrels?

Flying squirrels are known for soaring anywhere from 150 to 500 feet, sailing from tree to tree to avoid ground predators - but they actually glide rather than fly. Flying squirrels are a tribe (Pteromyini or Petauristini) of approximately 50 species of squirrels in the family Sciuridae. Despite their name, they are not in fact capable of full flight in the same way as birds or bats, but they are able to glide from one tree to another with the aid of a patagium, a furred skin membrane that stretches from wrist to ankle. Their long tails also provide stability as they glide. Anatomically they are very similar to other squirrels with a number of adaptations to suit their lifestyle; their limb bones are longer and their hand bones, foot bones, and distal vertebrae are shorter.

Anatomy and Gliding Mechanism

Flying squirrels don’t have their own means of propulsion, like a bird or bat, but glide using a furry membrane called the patagium that connects at their wrists down to their ankles. When they leap from a tree and spread their limbs, this flap of loose skin forms a square and acts like a hang glider. The direction and speed of the animal in midair are varied by changing the positions of its limbs, largely controlled by small cartilaginous wrist bones. There is a cartilage projection from the wrist that the squirrel holds upwards during a glide. This specialized cartilage is only present in flying squirrels and not other gliding mammals. Possible origins for the styliform cartilage have been explored, and the data suggests that it is most likely homologous to the carpal structures that can be found in other squirrels. This cartilage along with the manus forms a wing tip to be used during gliding. After being extended, the wing tip may adjust to various angles, controlling aerodynamic movements. The wrist also changes the tautness of the patagium, a furry parachute-like membrane that stretches from wrist to ankle. It has a fluffy tail that stabilizes in flight. Flying squirrels can turn by lowering one arm, while a specialized piece of cartilage not found in other gliding mammals extends from the wrist to support the patagium and help them steer. These animals are capable of making 180-degree turns in mid-air to evade flying predators like owls. Moving their hands and feet in opposite directions also helps flying squirrels direct their movements so they land safely on their strong, silent padded feet. Their long, fluffy tails stabilize their flight and flipping those tails up helps them put on the brakes.

Evolutionary History

Molecular studies have shown that flying squirrels are monophyletic (having a common ancestor with no non-flying descendants) and originated some 18-20 million years ago. The genus Paracitellus is the earliest lineage to the flying squirrel dating back to the late Oligocene era. Prior to the 21st century, the evolutionary history of the flying squirrel was frequently debated. This debate was clarified greatly as a result of two molecular studies. These studies found support that flying squirrels originated 18-20 million years ago, are monophyletic, and have a sister relationship with tree squirrels. Due to their close ancestry, the morphological differences between flying squirrels and tree squirrels reveal insight into the formation of the gliding mechanism. Compared to squirrels of similar size, flying squirrels, northern and southern flying squirrels show lengthening in bones of the lumbar vertebrae and forearm, whereas bones of the feet, hands, and distal vertebrae are reduced in length. Such differences in body proportions reveal the flying squirrels' adaptation to minimize wing loading and to increase maneuverability while gliding.

Why Gliding?

Several hypotheses have attempted to explain the evolution of gliding in flying squirrels. One possible explanation is related to energy efficiency and foraging. Gliding is an energetically efficient way to progress from one tree to another while foraging, as opposed to climbing down trees and maneuvering on the ground floor or executing dangerous leaps in the air. By gliding at high speeds, flying squirrels can rummage through a greater area of forest more quickly than tree squirrels. Other hypotheses state that the mechanism evolved to avoid nearby predators and prevent injuries. If a dangerous situation arises on a specific tree, flying squirrels can glide to another, and thereby typically escape the previous danger. Furthermore, take-off and landing procedures during leaps, implemented for safety purposes, may explain the gliding mechanism. While leaps at high speeds are important to escape danger, the high-force impact of landing on a new tree could be detrimental to a squirrel's health. Yet the gliding mechanism of flying squirrels involves structures and techniques during flight that allow for great stability and control.

Species and Distribution

There are about 50 species of flying squirrels ranging across most of North America down into Central America, and stretching from Southeast and Northern Asia into Siberia and Scandinavia. There are three types of flying squirrels found in North and Central America. These are the Northern flying squirrel, the Southern flying squirrel, and Humboldt's flying squirrel. Many other kinds of flying squirrels live across Asia.

Northern Flying Squirrel (Glaucomys sabrinus): Northern flying squirrels are found in eastern Alaska, the western United States through California, Idaho, Montana, Utah and northern Wyoming, and in the eastern United States in Minnesota, Wisconsin, Michigan, New York and New England. The range of the northern species is restricted to high-elevation forests, while southern flying squirrels are more commonly found in lower elevations. As a distinct sub-species of the more commonly found northern flying squirrel, which thrives in the boreal forests of Canada, the Carolina northern flying squirrel is a species that plays a critical role in determining the health of its environment. “This species is only found in the spruce-fir forest and northern hardwood forests,” Caveny said. If we lose these forests, then we lose these squirrels. In the United States, northern flying squirrels are endangered in Pennsylvania. Northern flying squirrels have large black eyes and thick, soft fur with a rich brown-gray color above and white below. They have belly fur that is white at the tips and gray at the base to the skin. The tail is dark brown above and white below with a flattened appearance. A membrane or folded layer of loose skin exists between their forelegs and hindlegs, giving them the ability to glide through the air between trees. They have been known to cover more than 45 m (150 ft) in a single glide. Northern flying squirrels are slightly larger than the southern flying squirrel at 25-30 cm (10-12 in) in length.
Southern Flying Squirrel (Glaucomys volans): Southern flying squirrels are found throughout the eastern United States, expect for northern New England and the southern tip of Florida, west to Minnesota, eastern Kansas and eastern Texas. Southern flying squirrels have thick, soft, glossy grayish to olive brown fur with white underbellies. The southern flying squirrel can glide over 80 yards.
Other Species: Aside from their patagia, flying squirrels look similar to their grounded cousins, with small rounded faces, prominent ears, and fluffy tails that can be as long as their bodies. Their eyes are large, helping these nocturnal mammals navigate the dark, and their fur color and markings vary by species. Their sizes vary as much as their color. Pakistan’s woolly flying squirrel is the largest gliding mammal at five and a half pounds while Hose’s pygmy flying squirrel of Borneo is the tiniest at just a little more than three ounces.

Habitat and Nesting

They inhabit woodlots and forests of broadleaf and evergreen trees. They make their homes in in woodpecker holes, nests abandoned by birds, or tree cavities in forests, woodlands, and jungles. Southern flying squirrels favor old woodpecker holes as nest sites, but they may build a summer nest of leaves, twigs and bark. About November or December, when temperatures begin to drop sharply, flying squirrels move out of tree cavities and into brooms (clumps of abnormal branches caused by tree rust diseases). Flying squirrels require a forest mosaic that includes adequate denning and feeding areas. Den sites include tree cavities and witches' brooms. Tree cavities are most numerous in old forests where wood rot, frost cracking, woodpeckers, and carpenter ants have created or enlarged cavities. In Interior Alaska, most brooms and cavity entrances have southerly exposures. Nests in cavities are usually located about 25 feet above the ground but may range between five and 45 feet. Flying squirrels excavate chambers in witches' brooms and line them with nesting materials. A dray nest is a ball-like mass of mosses, twigs, lichens, and leaves with shredded bark and lichens forming the lining of the chamber. Flying squirrels build drays entirely by themselves or modify the nests of other species (e.g., bird nests, red squirrel nests). The dray is usually positioned close to the trunk on a limb or whorl of branches with its entrance next to the trunk. In a year's time, a flying squirrel in Interior Alaska may use as many as 13 different den trees within 19.8 acres (8 ha). On a night foray, a squirrel may travel as far as 1.2 miles (2 km) in a circular route and be away from its den tree for up to 7 hours. It may change den trees at night and move to different ones more than 20 times over a year, staying in each for a varying numbers of days. Fairly dense, old closed-canopy forests with logs and corridors of trees (especially conifers) that are spaced close enough to glide between are needed for cover from predators. High quality flying squirrel habitat can be a community mosaic of small stands of varying age classes in which there is a mix of tall conifers and hardwoods.

Diet and Foraging

Flying squirrels are omnivores, which means they eat both plants and animals. They eat according to their environment; they are omnivorous, and will eat whatever food they can find. The North American southern flying squirrel enjoys a varied diet. It eats seeds, insects, slugs and snails, spiders, shrubs, flowers, fungi, and even tree sap. They also like fruits, nuts, and birds' eggs. The southern flying squirrel feeds on nuts, acorns, seeds, berries, some insects, and bird eggs, and will even eat meat if it’s available. Some food is stored in the nest chamber and also in tree crotches. Those observed foraging in the wild in Interior Alaska ate mushrooms (fresh and dried), truffles, berries, tree lichens, and the newly flushed growth tips on white spruce limbs. In spring, summer, and fall the diet is mostly fresh fungi. In winter it's mostly lichens. Flying squirrels are not known to cache fungi for winter in Alaska, but they are known to do so elsewhere in their range. Flying squirrels probably get water from foods they eat and rain, dew, and snow.

Reproduction and Life Cycle

Some flying squirrels have only one mating cycle per year but others, such as the Japanese dwarf flying squirrel, have two. The time of year for mating and birth depend on the species as well. Many are promiscuous, meaning that males and females mate with multiple partners. For the southern flying squirrel, there are usually two litters each year with the young born between April-May and August-September. Alternatively, the northern flying squirrel has only one litter each year, with roughly 2-5 young born in May-June. The gestation period is about 40 days, and they have roughly 2-6 young per litter. The mating season for flying squirrels is usually in February and March. When the infants are born, the female squirrels live with them in maternal nest sites. The mothers nurture and protect them until they leave the nest. At birth, flying squirrels are mostly hairless, apart from their whiskers, and most of their senses are not present. Their internal organs are visible through the skin, and their sex can be signified. By week five, they are almost fully developed. At that point, they can respond to their environment and start to develop a mind of their own. Through the upcoming weeks of their lives, they practice leaping and gliding. The life expectancy of flying squirrels in the wild is about six years, and flying squirrels can live up to fifteen years in zoos. The mortality rate in young flying squirrels is high because of predators and diseases.

Nocturnal Behavior

Most flying squirrels are active at night, which means they are nocturnal, since they are not adept at escaping birds of prey that hunt during the daytime. This squirrel is nocturnal and has large eyes that are efficient on the darkest nights. Eye shine color is a distinctive reddish-orange. Flying squirrels can easily forage for food in the night, given their highly developed sense of smell.

Pink Fluorescence

In 2019 it was observed, by chance, that a flying squirrel fluoresced pink under UV light. Subsequent research by biologists at Northland College in Northern Wisconsin found that this is true for all three species of North American flying squirrels. At this time it is unknown what purpose this serves.

Ecological Role

Flying squirrels are important to forest regeneration and timber production because they disperse spores of ectomycorrhizal fungi like truffles. Truffles are fruiting bodies of a special type of fungus that matures underground. They are dependent upon animals to smell them out, dig them up, consume them, and disperse their spores in fecal material where the animal travels. The animal serves to inoculate disturbed sites (e.g., clearcuts, burned areas) with mycorrhizae that join symbiotically with plant roots and enhance their ability to absorb nutrients and maintain health. The flying squirrel's ecological role in forest ecosystems, therefore, gives it economic value. In addition, they may be important prey for a variety of hawks, owls, small carnivores, and furbearers like marten, lynx, and red fox. Management should include retention of other squirrel species in shared habitats.

Predators

Many animals hunt flying squirrels. Owls, hawks, and carnivorous mammals prey on flying squirrels. These predators include tree snakes, raccoons, owls, martens, fishers, coyotes, bobcats, and even feral cats.

Conservation Status and Threats

Almost half of flying squirrel species, such as the southern flying squirrel in North and Central America and the Javanese flying squirrel of Southeast Asia, have stable populations. Their elusive nature makes them hard to study so there’s little data on some species. For those with decreasing populations-such as the endangered smoky flying squirrel of Southeast Asia-threats include destruction of their forest habitats by logging, other types of wood harvesting, and agricultural and residential development, according to the International Union for Conservation of Nature. Hunting and trapping are also threats to species such as Bhutan’s giant flying squirrel and the Northern Chinese flying squirrel. In the United States, northern flying squirrels are endangered in Pennsylvania.

Flying Squirrels as Pests

Flying squirrels become pests only when they take up residence in an attic where they can cause odor and damage with their urine, and keep people awake with their nocturnal activity. They have also been known to use the attics of buildings to build their nests. There are many important steps to prevent a squirrel infestation inside the home. Homeowners should seal all possible points of entry around the house, paying close attention to small openings and cracks around doors and windows. Gnaw marks in wires, insulation and walls may indicate a flying squirrel infestation. If a flying squirrel infestation is suspected, it’s important to contact a licensed pest control professional to inspect the home, identify the species, implement exclusion techniques and trap out any resident squirrels. A thorough inspection beginning about an hour before sunset and into the evening is required to observe activity patterns. This is followed by a day inspection to locate potential entry points. A licensed pest control professional or wildlife specialist will often seal all holes larger than 6 mm with hardware cloth, metal flashing or other appropriate material. To more quickly reduce the population or to remove individuals from within the structure, traps may be used. The professional will be able to recommend the proper course of treatment based on the inspection.

Conservation Efforts

Protecting northern hardwood conifer forests is critical for maintaining potential habitat. Conservation of snags and dead trees is particularly important for nesting. Survey and MonitoringSince the northern flying squirrel has not been documented in Massachusetts for decades, surveys are needed to determine whether small populations still persist. Methods such as nest box surveys, camera traps, and live trapping could provide crucial data (Wells-Gosling & Heaney 1984). The spread of threadworm (Strongyloides robustus), a nematode parasite carried by southern flying squirrels, may threaten the northern species. Monitoring the impact of this parasite and potential mitigation measures should be considered (Pauli et al. Prasad. 2001. Potential changes in tree species richness and forest community types following climate change. Ecosystems 4:186-199.Pauli, J.N., S.A. Dubay, E.M. Anderson, and S.J. Taft. 2004. Strongyloides robustus and the northern sympatric populations of northern (Glaucomys sabrinus) and southern (G. volans) flying squirrels. Journal of Wildlife Diseases 40:579-582.Weigl, P.D. 2007. The northern flying squirrel (Glaucomys sabrinus): A conservation challenge. Journal of Mammalogy 88:897-907.Wells-Gosling, N. and L.R. Heaney. 1984. Glaucomys sabrinus.

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