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Flying squirrel

Flying squirrels, scientifically classified in the tribe Pteromyini within the family Sciuridae, comprise approximately 57 species of gliding rodents distributed across 15 genera, making them the most diverse lineage of gliding mammals.^[1]^[2] These small to medium-sized squirrels, ranging in body mass from 13.5 g in pygmy species to over 2,700 g in giants like those in the genus Petaurista, are distinguished by their patagium—a furred, parachute-like membrane of skin extending from the wrists to the ankles, and sometimes including a tail membrane, which allows them to glide distances of up to 150 meters or more between trees with remarkable control and agility.^[1]^[3]^[4] Some species, particularly in North America, exhibit biofluorescence, glowing pink under ultraviolet light.^[5] Unlike true flight, this gliding adaptation enables efficient arboreal locomotion, sharp turns, and soft landings, supported by their large eyes adapted for nocturnal vision and a slender, furry body with a bushy tail for stabilization.^[3]^[6] Primarily nocturnal and strictly arboreal, flying squirrels inhabit a wide array of forest ecosystems, from tropical lowlands and subtropical broadleaf woods to temperate, boreal coniferous forests and even high-elevation habitats above the tree line in the Himalayas, spanning Europe, Asia, and North America.^[1]^[6] Their global distribution reflects an Old World origin with a single colonization event into the New World, where North American species like the northern (Glaucomys sabrinus) and southern (Glaucomys volans) flying squirrels occupy coniferous, deciduous, or mixed woodlands, often preferring mature forests with dense canopies for nesting in tree cavities or dreys.^[7] In Asia, where the majority of species occur, they thrive in moist temperate and pine-dominated forests at elevations from 100 to 4,500 m, though habitat fragmentation poses significant threats to many populations.^[6] These squirrels play key ecological roles as seed dispersers, mycorrhizal partners through their consumption of fungi, and prey for predators like owls and martens.^[3]^[6] Their diet is predominantly herbivorous and mycophagous, consisting of nuts, seeds, fruits, leaves, buds, bark, flowers, and hypogeous fungi (truffles), with occasional insects, bird eggs, or sap supplementing the intake, varying by species and season—northern species, for instance, rely heavily on lichens and fungi in winter.^[3]^[6] Reproduction typically occurs in tree hollows or nests, with most species producing one or two litters per year of 1–6 altricial young after a gestation period varying by species, typically 37–46 days, though breeding timing aligns with resource availability, such as rainy seasons in tropical regions or spring in temperate zones.^[3]^[8]^[6] Socially, they often form winter aggregations in shared dens for thermoregulation, exhibiting behaviors like vocalizations and scent marking, but many face conservation challenges from deforestation, climate change, and poaching, with several species listed as vulnerable or endangered.^[3]^[6]

Physical characteristics

Body structure and gliding adaptations

Flying squirrels are distinguished by their patagium, a fur-covered gliding membrane that extends from the wrists to the ankles, and sometimes including a tail membrane, enabling controlled aerial descent between trees. This membrane is supported by a styliform cartilage, a specialized cartilaginous extension originating from the pisiform bone at the wrist, which projects outward to tension and shape the patagium during gliding.^[9] The patagium's edges are fringed with dense fur, which reduces turbulence and enhances aerodynamic stability by minimizing wingtip vortices.^[10] Limb modifications further adapt flying squirrels for gliding, including elongated forelimbs relative to non-gliding squirrels, which increase the span of the patagium for greater lift.^[11] At the wrists, cartilaginous processes, including the styliform element, allow precise control of membrane camber through muscle action, such as contraction of the palmaris longus.^[12] The hindlimbs feature similar extensions at the ankles, contributing to the overall membrane support. In contrast to non-gliding squirrels, which lack these extensions and have shorter, more robust limbs suited for terrestrial scrambling, flying squirrels exhibit slender, elongated bones that prioritize aerial efficiency over ground propulsion.^[13] Their tail is notably flattened and bushy, functioning as an aerodynamic rudder to steer and stabilize during glides, a trait absent or less pronounced in non-volant relatives.^[14] The skull of flying squirrels shows adaptations for their nocturnal, arboreal lifestyle, including enlarged orbits that accommodate large eyes for enhanced low-light vision.^[3] These eye sockets are proportionally larger than in diurnal non-gliding squirrels, reflecting convergent evolution with other night-active mammals to maximize photoreceptor density.^[15] Dentally, they retain the characteristic Sciuridae formula with prominent, chisel-like incisors for gnawing hard arboreal foods like nuts and bark, but without unique modifications distinguishing them from non-gliding squirrels beyond overall cranial elongation for lightweight construction.^[16]

Size, appearance, and variations

Flying squirrels exhibit a wide range of body sizes across their approximately 56 species, with head-body lengths varying from as small as 7-10 cm in pygmy flying squirrels (Petaurillus spp.) to over 50 cm in giant flying squirrels (Petaurista spp.).^[17]^[18] Weights correspondingly range from about 20-25 g in the pygmy species to 1.5-3 kg in the largest giants, reflecting adaptations to diverse ecological niches within the tribe Pteromyini.^[19]^[20] Their appearance is characterized by soft, dense fur that covers the body, providing insulation and a sleek profile enhanced by the patagium, the gliding membrane spanning from wrist to ankle. Coloration typically features muted tones on the dorsum, such as grays, browns, or blacks, contrasted with paler white or cream underparts, though patterns can include subtle striping or spotting in certain taxa.^[21]^[22] Sexual dimorphism is generally limited, with females often slightly larger than males in body mass and size in several species, potentially linked to reproductive demands, but sexes are otherwise similar in pelage and form.^[23]^[24] The tail is a prominent feature, broad and flattened with bushy fur that gives it a feather-like appearance, aiding in overall aesthetics and balance; its length varies from 70% to 120% of head-body length, being relatively shorter in smaller species like Glaucomys (70-80%) and longer in larger ones.^[25]^[26] Regional variations in appearance are notable between North American and Asian species. North American flying squirrels (Glaucomys spp.) display subtle, camouflaging grays and browns with minimal markings, suited to temperate forest understories. In contrast, many Asian species exhibit bolder colorations, such as the mahogany-red dorsum and pale underparts of the red giant flying squirrel (Petaurista petaurista), or dark rufous with black accents in other Petaurista taxa, reflecting greater diversity in tropical environments.^[21]^[27]^[28]

Bioluminescence and fluorescence

In 2019, researchers discovered that the fur of New World flying squirrels in the genus Glaucomys exhibits vivid ultraviolet (UV) fluorescence, emitting a bright pinkish glow when exposed to UV light. This phenomenon was observed across all 108 examined museum specimens from both recognized species—the northern flying squirrel (Glaucomys sabrinus) and the southern flying squirrel (Glaucomys volans)—spanning regions from Canada to Central America, and consistent in males, females, and across historical collections dating back to the 19th century. The fluorescence is most pronounced on the ventral (belly) side of the body and appears as a passive response to UV excitation, with no evidence of bioluminescence, which involves self-generated light through biochemical reactions.^[5] The chemical basis for this fluorescence lies in the presence of porphyrin compounds accumulated in the fur. These organic molecules, known for their role in various biological pigments, absorb UV light and re-emit it as visible pink light; analyses confirm porphyrin accumulation specifically in the skin appendages, including fur, of these squirrels. While the exact origin of these fluorophores remains under investigation—potentially linked to dietary intake or genetic factors—similar porphyrins have been detected in other mammals, though at levels insufficient to produce noticeable fluorescence. Subsequent mass spectrometry studies on fur extracts have identified multiple potential contributors, but porphyrins stand out as the primary agents in Glaucomys.^[29]^[30] Potential adaptive functions of this fluorescence may enhance survival in the squirrels' nocturnal lifestyle within dimly lit forest canopies, where UV light from stars and the moon could activate the glow. Hypotheses include intraspecific communication, such as signaling during mating or social interactions among conspecifics; UV camouflage to blend with fluorescent forest elements against UV-sensitive predators; or aposematic signaling to deter threats. These roles are speculative but align with the squirrels' low-light activity patterns, potentially making the trait more visible to conspecifics than to diurnal observers.^[5] Post-2019 research has confirmed the absence of this fluorescence in Old World flying squirrels across genera like Petaurista and Pteromys, with no pink UV emission reported in examined Asian and European specimens. Ongoing genomic studies aim to identify genes regulating porphyrin production and accumulation in Glaucomys fur, comparing them to non-fluorescent relatives to elucidate the trait's evolutionary origins.^[31]

Taxonomy and phylogeny

Classification and genera

Flying squirrels are classified within the tribe Pteromyini, part of the subfamily Sciurinae in the family Sciuridae, which encompasses all squirrels.^[32] This tribe diverged from other squirrel lineages, particularly the tree squirrels of tribe Sciurini, approximately 30–40 million years ago during the late Eocene to Oligocene epochs, based on molecular clock analyses of mitochondrial and nuclear DNA sequences.^[33] The Pteromyini comprise approximately 56 extant species distributed across 15 genera, reflecting a diverse array of gliding adaptations primarily in forested habitats of the Northern Hemisphere.^[1]^[34] Key genera include Glaucomys, which is endemic to the New World and contains two recognized species (northern and southern flying squirrels) adapted to North American temperate forests; Petaurista, featuring 14 species of large Asian giant flying squirrels known for their impressive size and glides exceeding 100 meters, following recent taxonomic revisions; and Pteromys, with two Eurasian species that inhabit boreal and temperate woodlands.^[7]^[35] These genera exemplify the tribe's morphological and ecological variation, from small nocturnal gliders to larger forms with specialized patagia.^[19] Molecular phylogenetic studies, utilizing cytochrome b and other genetic markers, reveal two primary clades within Pteromyini: a Holarctic clade dominated by Glaucomys and closely related forms, and an Oriental clade encompassing most Asian genera such as Petaurista, Pteromys, and Hylopetes.^[7] Debates persist regarding subgeneric divisions, particularly in the Oriental clade, where morphological similarities and limited fossil calibration complicate precise boundaries between genera like Petinomys and Aeromys.^[19] The nomenclature of flying squirrels traces back to early 20th-century morphological assessments, notably Oldfield Thomas's 1915 analysis using the baculum (penis bone) as a taxonomic character to delineate squirrel groups, including preliminary distinctions among pteromyine forms. Subsequent refinements in the 2000s, driven by genetic data from studies like those employing mitochondrial DNA sequences, have clarified monophyly and intergeneric relationships, overturning some earlier subfamily elevations to tribe status and integrating flying squirrels more firmly within Sciurinae.^[33]

Living species diversity

Flying squirrels, belonging to the tribe Pteromyini within the family Sciuridae, encompass approximately 56 extant species distributed across 15 genera.^[1] This diversity reflects ongoing taxonomic refinements, with recent discoveries expanding the known roster; for instance, two new woolly flying squirrel species, Eupetaurus tibetensis and Eupetaurus nivamons, were described from the eastern Himalayas in 2021 based on morphological and genetic analyses of museum specimens, alongside the description of a new genus Priapomys with P. leonardi. A 2025 study further revised the genus Petaurista, recognizing 14 species including the newly described P. nujiangensis from northwest Yunnan, China.^[34]^[36]^[1]^[35] Notable examples include the northern flying squirrel (Glaucomys sabrinus), native to North America, where certain subspecies such as the Carolina northern flying squirrel (G. s. coloratus) are classified as endangered due to habitat fragmentation and loss. In Eurasia, the Siberian flying squirrel (Pteromys volans) represents a widespread species across boreal forests, though it faces regional vulnerabilities from logging. Among the largest is the red giant flying squirrel (Petaurista petaurista) of Southeast Asia, which can reach weights of up to 2 kg and exemplifies the size variation within the group.^[28] Patterns of diversity highlight high endemism in the Indo-Malaya biodiversity hotspot, where over 30 species occur, many restricted to specific islands or forest types in regions like Borneo and Sumatra. Sexual dimorphism in body size is evident in several genera, such as Petaurista, where females are often larger than males, potentially linked to reproductive demands and foraging efficiency.^[37]^[38] Conservation assessments by the IUCN indicate varied statuses across the group, with about 20% of species threatened; examples include the critically endangered Namdapha flying squirrel (Biswamoyopterus sp.), impacted by deforestation in India's Arunachal Pradesh, underscoring broader threats like habitat loss and hunting in tropical ranges.^[39]^[40]

Fossil species and evolutionary history

The fossil record of flying squirrels (tribe Pteromyini) is sparse and primarily consists of isolated dental remains, with the earliest purported evidence dating to the late Eocene (approximately 36.6–35.8 million years ago) from the genus Hesperopetes in North America and Asia, though its assignment to gliding forms remains debated due to the absence of postcranial evidence confirming adaptations like the patagium.^[41] The oldest undisputed flying squirrel fossils appear in the early Miocene (around 23 million years ago) in Europe, including the small-bodied genus Blackia, such as B. miocaenica, known from dental specimens in deposits like those in Sansan, France, indicating early diversification among diminutive arboreal forms.^[42] These early records suggest an initial radiation tied to the expansion of forested habitats during the Oligocene, following global cooling and the proliferation of angiosperm-dominated woodlands that favored arboreal lifestyles.^[41] Key fossil genera from the Miocene provide insights into the evolution of gliding adaptations from non-gliding arboreal squirrels within the subfamily Sciurinae. Notably, Miopetaurista, documented across Eurasia from the early to late Miocene (approximately 23–5 million years ago), includes species like M. neogrivensis, whose 11.6-million-year-old skeleton from Catalonia, Spain—the oldest known for the group—preserves diagnostic postcranial features such as elongated tarsal bones and a styliform styloid process, evidencing an early patagium for controlled gliding.^[9] This genus, often large-bodied and resembling modern giant flying squirrels, highlights a phylogenetic split from tree squirrels (Sciurini) around 36.5–24.9 million years ago, with Pteromyini diverging from ground squirrels (Xerinae) near the Eocene-Oligocene boundary, approximately 50 million years ago, amid the broader sciurid radiation.^[41] The Miocene saw peak diversity, with up to 10 Miopetaurista species and others like Albanensia, reflecting adaptations to diverse Eurasian forests before a Pliocene contraction linked to climatic shifts.^[43] Significant gaps persist in the fossil record, particularly for soft tissues like the patagium, which rarely fossilize and leave gliding adaptations inferred mainly from skeletal proxies in rare articulated specimens.^[9] Molecular clock analyses, calibrated with fossils such as Miopetaurista, estimate the crown-group age of Pteromyini at 25–30 million years ago, aligning with late Oligocene diversification of subtribes (Pteromyina and Glaucomyina) around 27–18 million years ago, though earlier Eocene origins cannot be ruled out pending better preservation.^[41] These estimates underscore the challenges of integrating fragmentary fossils with genetic data to reconstruct the transition from climbing to gliding in this clade.^[44]

Distribution and habitats

Global range and biogeography

Flying squirrels, belonging to the tribe Pteromyini within the family Sciuridae, exhibit a global distribution primarily confined to the Holarctic and Oriental biogeographic realms. Their range spans North America, Europe, and Asia, with the highest diversity concentrated in South and Southeast Asia, where over 40 extant species across 14 genera are found. In contrast, three species occur in North America (Glaucomys sabrinus, G. volans, and G. oregonensis)^[45], and a single species (Pteromys volans) inhabits northern Eurasia from Finland to Japan and Korea. These squirrels are notably absent from Africa south of the Sahara and from Australia, reflecting the broader Sciuridae family's limited penetration into Australasian and sub-Saharan African regions due to historical dispersal barriers and ecological constraints.^[46] Populations display marked disjunctions, with isolated North American lineages contrasting sharply against the species-rich assemblages in Southeast Asian hotspots, where more than 30 species thrive amid tropical forests. Island endemics further highlight these patterns, such as the Japanese flying squirrel (Pteromys momonga) restricted to Honshu and the Taiwan giant flying squirrel (Petaurista grandis) on Taiwan, resulting from Pleistocene sea-level fluctuations that promoted speciation on isolated landmasses. Genetic analyses indicate that such disjunct distributions stem from ancient vicariance events, with low gene flow between continental and insular forms.^[46] The biogeography of flying squirrels is shaped by historical expansions and barriers, originating in Europe during the Oligocene-Miocene transition around 23-18 million years ago (Ma), followed by mid-Miocene dispersals into Asia and North America. The Bering Land Bridge played a pivotal role in Pleistocene migrations, enabling faunal exchanges between Eurasia and North America during interglacial periods, as evidenced by fossil records of genera like Miopetaurista across these continents. Post-glacial recolonization from southern refugia in Southeast Asia repopulated northern latitudes in Eurasia after the Last Glacial Maximum, supported by phylogeographic studies revealing haplotype diversity consistent with northward expansions from tropical holdouts. Uplift of the Tibetan Plateau and intensification of Asian monsoons acted as barriers, limiting further westward spread and confining much of the modern diversity to eastern and southeastern Asia.^[46]

Preferred environments and adaptations

Flying squirrels primarily inhabit old-growth forests characterized by dense canopies that provide vertical stratification for arboreal movement, with North American species favoring coniferous-dominated boreal and mixed woodlands, while many Asian species occupy tropical and subtropical broadleaf forests.^[26]^[47] These environments offer interconnected tree layers essential for their lifestyle, as evidenced by studies showing higher population densities in mature stands with multi-layered canopies compared to younger or fragmented forests.^[48] Their altitudinal range spans from sea level to elevations exceeding 4,000 meters, particularly in the Himalayan region where species like the woolly flying squirrel (Eupetaurus cinereus) thrive in high-altitude coniferous forests up to 4,800 meters.^[49] Adaptations such as denser, woolly fur in montane populations provide insulation against cold temperatures at higher elevations, enabling survival in climates with subzero winters and reduced oxygen levels.^[50]^[51] Microhabitat requirements include hollow trees or snags for nesting, often in live or dead trees with cavities, alongside abundant sources of mast such as seeds and nuts from mature trees; these features support their denning and resource needs, with populations showing sensitivity to forest fragmentation that disrupts such structures.^[48]^[52] Downed woody debris and moist soils further enhance habitat suitability by maintaining humidity and fungal resources in the understory.^[53] Evolutionarily, gliding adaptations have enabled flying squirrels to exploit discontinuous forest patches by facilitating efficient travel between isolated trees, a trait that likely arose in response to fragmented arboreal habitats during their phylogenetic divergence.^[54] Additionally, in North American flying squirrels, ultraviolet fluorescence in their fur, which glows pink under UV light prevalent in dim forest understories, may aid in intraspecific communication or camouflage within low-light woodland environments.^[5]^[55]

Behavior and biology

Locomotion and gliding mechanics

Flying squirrels initiate gliding by leaping from a tree branch, typically achieving takeoff speeds of approximately 5–10 m/s, followed by a brief ballistic phase before the patagium—a furred membrane spanning from wrist to ankle—deploys to generate lift.^[56] This deployment creates a cambered airfoil shape, with relative camber averaging 14% of the chord length, enabling lift coefficients with mean values around 2.1 during glides.^[57] Glides cover horizontal distances of 20–60 m for smaller species like the southern flying squirrel (Glaucomys volans), with vertical drops of 20–30 m, while larger species such as the Japanese giant flying squirrel (Petaurista leucogenys) can achieve up to 50 m horizontally.^[58]^[59] Aerodynamically, the patagium functions as a low-aspect-ratio wing (1.0–2.2), producing lift through high angles of attack (35–54°) that often result in stalled flow, balanced by drag coefficients of 0.8–1.0 for stability.^[57] Flight control occurs via active adjustments: forelimbs and hindlimbs alter wing camber and pitch to modulate lift-to-drag ratios (averaging 2.26), while the flattened tail acts as a rudder for yaw and banking during turns.^[56] High-speed videography reveals maneuvers including 90-degree turns around obstacles, achieved through asymmetric limb positioning and lateral accelerations up to 6 m/s², with glide speeds averaging 5–8 m/s.^[57]^[60] Gliding enhances energy efficiency compared to climbing, with non-equilibrium dynamics allowing glide ratios of about 1.9–2.0 (horizontal distance per unit vertical drop), reducing the overall cost of arboreal travel for body masses around 0.1–0.4 kg by minimizing repeated ascents.^[56] Landing involves deceleration through membrane feathering—increasing drag via partial collapse—and a terminal pitch-up maneuver, limiting impact forces to levels survivable without injury.^[61] As nocturnal animals, flying squirrels rely on enlarged eyes for low-light navigation during glides, enabling precise trajectory adjustments in dim forest canopies.^[62]

Diet and foraging strategies

Flying squirrels exhibit an omnivorous diet, with primary consumption consisting of seeds, nuts, and fungi, supplemented by insects, buds, flowers, fruits, and occasionally sap, bird eggs, or carrion depending on the species and region.^[63]^[64] Northern species, such as Glaucomys sabrinus, emphasize mycophagy, relying heavily on hypogeous fungi (truffles) like those in genera Rhizopogon and Gautieria, which provide essential nutrients and are detected through keen olfaction during foraging.^[65]^[66] Southern flying squirrels (Glaucomys volans) are more granivorous, favoring acorns, hickory nuts, and seeds from mast-producing trees, though they also incorporate fungi and lichens.^[63] Foraging occurs exclusively at night, leveraging their large eyes for low-light navigation, and typically covers distances of about 1-2 km in a circular pattern from den sites, minimizing exposure to predators.^[67] These arboreal mammals rarely descend to the ground, instead using gliding to travel between food sources in the forest canopy, which allows efficient access to dispersed resources while reducing energy expenditure and risk.^[63]^[67] Caching behaviors vary by species; southern flying squirrels store nuts and acorns in tree crevices, nests, or dreys for winter use, exhibiting seasonal shifts toward cached hickory nuts when fresh mast declines.^[63] In contrast, northern flying squirrels do not cache fungi but forage widely year-round, adapting to scarcity by increasing lichen intake (e.g., Alectoria and Bryoria species) during heavy snowfall winters when truffle availability drops.^[65]^[68] Their foraging strategies support mutualistic relationships with mycorrhizal fungi, as flying squirrels ingest sporocarps and excrete viable spores in feces, facilitating fungal dispersal and enhancing forest nutrient cycling, particularly in coniferous ecosystems.^[65]^[66] This mycophagy also positions them as important seed dispersers, as undigested nuts and fruits are dropped or cached, promoting tree regeneration. Nutritional adaptations include hindgut fermentation in the enlarged cecum, enabling efficient breakdown of fibrous lichens and fungal tissues, which are staples for northern populations in nutrient-poor environments.^[68]^[67] Overall, these behaviors optimize energy intake while maintaining ecological balance, with diet diversity influenced by forest maturity and seasonal food pulses.^[65]

Reproduction and development

Flying squirrels exhibit varied mating systems across species, with many displaying polygynandrous or promiscuous behaviors where both males and females mate with multiple partners during breeding seasons.^[21]^[69] In North American species like the southern flying squirrel (Glaucomys volans), breeding occurs biannually, peaking in February to March and May to July, influenced by photoperiod and food availability.^[21] The northern flying squirrel (Glaucomys sabrinus) breeds once annually in early spring, from March to late May.^[3] In contrast, tropical species, such as those in Southeast Asia, often breed year-round due to consistent environmental conditions.^[70] Gestation periods typically last 37 to 42 days in northern flying squirrels and around 40 days in southern flying squirrels, resulting in altricial young that are born blind, hairless, and weighing 2 to 6 grams.^[3]^[21] Litter sizes range from 1 to 6 offspring, with averages of 2 to 4 in most species; southern flying squirrels produce smaller litters (2-3) in spring and larger ones (4-5) in summer.^[3]^[21] Females usually produce one or two litters per year, depending on latitude and resource availability.^[71] Newborn flying squirrels remain in the nest, cared for exclusively by the female, who nurses them for 5 to 10 weeks until weaning; eyes open around 3 to 4 weeks, and fur develops by then.^[3]^[21] Juveniles begin practicing gliding skills at about 5 to 6 weeks, achieving proficiency by 6 to 8 weeks, after which they leave the nest—around 40 days for northern species and 8 weeks for southern ones.^[3]^[21] Males do not provide direct care but may defend territories to protect resources for the family unit.^[69] Sexual maturity is reached at 6 to 12 months.^[72] In the wild, flying squirrels have a life expectancy of 3 to 5 years, limited by predation and environmental factors, while in captivity, they can live 10 to 13 years, with records up to 19 years.^[21] Fertility is influenced by diet, as food scarcity can delay breeding or reduce litter sizes through mechanisms like restricted energy intake or disrupted social cues.^[73]^[74]

Ecology and conservation

Predators, threats, and interactions

Flying squirrels face predation primarily from nocturnal and arboreal hunters adapted to forested environments. Common natural predators include owls such as the great horned owl and barred owl, martens, and various snakes that ambush them at nest sites or during glides.^[26]^[75] In North America, hawks and weasels also pose threats, particularly to juveniles.^[76] To counter these threats, flying squirrels employ several anti-predator strategies. They remain highly vigilant, often freezing or retreating into tree cavities upon detecting danger, and emit alarm calls including audible "tseep" sounds and ultrasonic vocalizations to alert conspecifics.^[77]^[78] Gliding mechanics allow for erratic trajectories that can evade pursuing predators, enabling quick escapes over distances up to 100 meters.^[79] Anthropogenic threats exacerbate natural pressures on flying squirrel populations. Habitat deforestation has led to substantial forest loss in Asia, where Southeast Asian highlands experienced unexpectedly high rates of tree cover reduction since 2000, fragmenting old-growth forests essential for gliding and nesting.^[80]^[81] The illegal pet trade targets species like Vordermann's flying squirrel, contributing to population declines through capture and habitat disturbance.^[82] Climate change further alters suitable habitats by shifting temperature and precipitation patterns, potentially reducing available old-growth conifer and hardwood forests in both Asia and North America.^[83]^[84] Interspecies interactions highlight flying squirrels' ecological roles and challenges. They compete with invasive or expanding species, such as southern flying squirrels encroaching northward due to human-induced range shifts, leading to resource overlap and parasite-mediated exclusion of northern populations.^[85]^[86] Positively, flying squirrels form symbiotic relationships through seed and spore dispersal; by consuming and excreting seeds from hardwoods and spores from mycorrhizal fungi like truffles, they facilitate forest regeneration and nutrient cycling, benefiting tree communities and associated bird species that rely on dispersed seeds.^[87]^[88]^[89] Flying squirrels also serve as disease vectors in certain contexts. In North America, they harbor ticks carrying Borrelia burgdorferi, the bacterium responsible for Lyme disease, potentially amplifying transmission in forested areas.^[90]^[91] Their role in spreading mycorrhizal fungi to trees, while ecologically beneficial, underscores their position in pathogen dynamics for forest health.^[92]

Population status and protection measures

Flying squirrel populations exhibit varied conservation statuses across their approximately 50 species, with around 20% classified as vulnerable or endangered on the IUCN Red List, while many others remain stable in undisturbed core habitats. For instance, the Carolina northern flying squirrel (Glaucomys sabrinus coloratus), a subspecies endemic to the southern Appalachian Mountains, is listed as endangered under the U.S. Endangered Species Act owing to ongoing habitat fragmentation and loss.^[39]^[93] Population trends indicate significant declines in fragmented landscapes since the 1990s, particularly in regions affected by deforestation and urbanization, though intact forest populations show relative stability. These declines are monitored through non-invasive methods such as camera traps, which offer higher detection rates than traditional live-trapping, and genetic analyses that reveal population structure and connectivity.^[94]^[95]^[96] Conservation initiatives focus on habitat protection and restoration to mitigate these pressures. Key efforts include the designation of reserves like Olympic National Park, which preserves old-growth coniferous forests critical for the northern flying squirrel (Glaucomys sabrinus). Several Asian species, such as the red giant flying squirrel (Petaurista petaurista), are listed under CITES Appendix II to regulate international trade and prevent overexploitation. Reforestation programs in Southeast Asia, including efforts to restore lowland forests, support habitat recovery for gliding species dependent on continuous canopy cover.^[97]^[98] Ongoing research highlights gaps in baseline data, particularly the need for comprehensive surveys in Southeast Asia, where tropical forests harbor high diversity but limited monitoring. Climate models forecast substantial habitat losses, with some species projected to experience over 85% decline in suitable habitat by mid-century, driven by warming temperatures and altered precipitation, underscoring the urgency for adaptive management strategies. As of 2025, recent studies emphasize urgent conservation needs for species like the Siberian flying squirrel amid habitat fragmentation and climate impacts.^[99]^[83]^[100]

References

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8317177/
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Glaucomys sabrinus (northern flying squirrel) - Animal Diversity Web
Most often found in areas dominated by conifers, northern flying squirrels can also be relatively abundant in deciduous and mixed coniferous/deciduous forests.
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https://animaldiversity.org/accounts/Glaucomys_sabrinus/
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Together, the sister relationship of the Pteromyini with the New World tree squirrels (Sciurinae), the current geographic distribution of the Pteromyini (which ...
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Oldest skeleton of a fossil flying squirrel casts new light on the ...
The patagium is supported by the styliform cartilage which is attached to the pisiform bone. Flying squirrels present an elevated process for articulation with ...
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WING TIP ANATOMY AND AERODYNAMICS IN FLYING SQUIRRELS
Flying squirrels extend wing tips using a styliform cartilage at the wrist, extended by the abductor of the thumb, forming an upward angle, similar to winglets.
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[PDF] Take-off and landing forces and the evolution of controlled gliding in ...
In comparison with other similar-sized squirrels, the forelimbs of flying squirrels are significantly longer, a trait that has almost certainly evolved to ...
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Evolutionary Transformation of the Palmaris Longus Muscle in ...
Sep 9, 2016 · The styliform cartilage was transformed in flying squirrels from an initially superficial and ulnar-derived anlagen into its current form.
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Dec 30, 2022 · The study found no difference in cortical thickness of the scapular glenoid fossa between gliding and non-gliding squirrel-related rodents, ...
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Aerodynamics of the northern flying squirrel (Glaucomys sabrinus)
Aug 6, 2025 · The patagium is covered with dense hair and supported by an elongated tail and limbs; the latter also bear many features adapted for arboreal ...
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[PDF] Skulls of Alaskan Mammals - A Teachers Guide
The size of an animal's orbits (eye sockets) corresponds to the sharpness of their eyesight. Larger orbits indicate better eyesight. The lynx's most acute sense ...<|control11|><|separator|>
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Nov 1, 2023 · Climate and dietary adaptations played a crucial role in shaping squirrel tooth morphology. Environmental conditions can drive morphological ...
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Petaurillus (pygmy flying squirrels) - Animal Diversity Web
Petaurillus is a genus commonly known as pygmy flying squirrels, and consists of three identified species. Petaurillus emiliae is the smallest flying squirrel.
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Indian Giant Flying Squirrel - Petaurista philippensis - Ecology Asia
Head-Body Length : Up to 49 cm. Tail Length : Up to 55 cm. The Indian Giant Flying Squirrel Petaurista philippensis is one of the largest flying squirrels in ...
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[PDF] Phylogenies of Flying Squirrels (Pteromyinae) - Smithsonian Institution
They range in size from the pygmy flying squirrel Petaurillus (24 grams), to the giant flying squirrel Petaurista (1.5 kilograms) (Thorington and Heaney, 1981).
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Patterns of Body and Tail Length and Body Mass in Sciuridae
Tail length is shorter in ground squirrels, longer in tree squirrels, and longest in flying squirrels. In addition, in arboreal squirrels, females have longer ...Missing: appearance | Show results with:appearance
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Glaucomys volans (southern flying squirrel) - Animal Diversity Web
The fur on the underbellies of southern flying squirrels is a white to cream color, while that of northern flying squirrels is grey.
[18]
Flying Squirrel Fact Sheet - CT.gov
Flying squirrels have soft, gray-brown fur on the back and sides, with white underparts, a flattened tail and large, dark eyes for night vision.Missing: appearance patterns morphology regional variations scientific sources
[19]
Sex-specific patterns in body mass and mating system in the ...
Oct 19, 2016 · Earlier studies indicate that flying squirrels may have female-biased sexual dimorphism, with females being larger than males based on body mass ...Missing: coloration | Show results with:coloration
[20]
The evolution of sexual size dimorphism in squirrels - AUETD Home
Aug 16, 2011 · Flying squirrels exhibit a unique lifestyle (gliding), which could require females to evolve larger size to attain functional or reproductive ...Missing: coloration | Show results with:coloration
[21]
[PDF] PATTERNS OF BODY AND TAIL LENGTH AND BODY MASS IN ...
Aug 25, 2008 · Tail lengths in flying squirrels range from 70% to 120% of body length. Glaucomys and Pteromys consistently have the shortest tails (70–80% of ...Missing: appearance | Show results with:appearance
[22]
Squirrel, Northern Flying - Alaska Department of Fish and Game
Flying squirrels require a forest mosaic that includes adequate denning and feeding areas. Den sites include tree cavities and witches' brooms.Missing: biology | Show results with:biology<|control11|><|separator|>
[23]
Petaurista petaurista (red giant flying squirrel) - Animal Diversity Web
Petaurista petaurista is characterized by its large eyes and mahogany-red coloring, though coloration varies with environment. Relative to other squirrels, ...Missing: appearance | Show results with:appearance
[24]
Red Giant Flying Squirrel - Petaurista petaurista - Ecology Asia
p. melanotus), which is dark brown above, orange below, with a black snout, black feet and a black tip to its tail. Other subspecies, for example in parts of ...Missing: appearance coloration
[25]
Ultraviolet fluorescence discovered in New World flying squirrels ...
Jan 23, 2019 · Here, we report the discovery of vivid UV fluorescence of the pelage in Glaucomys, the New World flying squirrels.
[26]
Fur glowing under ultraviolet: in situ analysis of porphyrin ...
May 2, 2022 · Among mammals, recent analyses have identified free-base porphyrins as the compounds responsible for the reddish ultraviolet-induced ...
[27]
Using mass spectrometry to investigate fluorescent compounds in ...
Here we use Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) of flying squirrel fur known to fluoresce under ultraviolet (UV) light
[28]
Photoluminescence in mammal fur: 111 years of research
Apr 10, 2023 · Two classes of luminophore are currently known to cause photoluminescence in mammal fur–tryptophan metabolites, and porphyrin and its ...<|control11|><|separator|>
[29]
Taxonomy browser (Pteromyini) - NCBI
Pteromyini. Taxonomy ID: 337748 (for references in articles please use NCBI:txid337748). current name. Pteromyini. Genbank common name: flying squirrels. NCBI ...
[30]
Science | AAAS
- **Insufficient relevant content**: The provided URL (https://www.science.org/doi/10.1126/science.1083206) leads to a "Page not found" error, indicating the page is inaccessible or does not exist.
[31]
Discovery and description of a mysterious Asian flying squirrel ...
Jul 18, 2019 · The flying squirrels of the tribe Pteromyini (Family Sciuridae) currently comprise 52 species of recent mammals that are placed in 15 genera. A ...
[32]
Two new species of cat-size flying squirrel discovered in the Himalaya
Jun 7, 2021 · Two new species of cat-size flying squirrel discovered in the Himalaya | National Geographic. The Yunnan woolly flying squirrel (pictured) ...<|separator|>
[33]
Eco-Spatial Modeling of Two Giant Flying Squirrels (Sciuridae
Apr 3, 2025 · The two Petaurista species are found in two major biodiversity hotspots, the Eastern Himalayas and Indo-Burma, each with diverse ecosystems and ...
[34]
Sexual dimorphism and variability of craniomandibular morphology ...
The flying squirrels are almost exclusively nocturnal and mainly female-biased in dimorphism (Nandini 2011). The Japanese giant flying squirrel (Petaurista ...
[35]
Finding Hope In Conservation: The Search For A Lost Flying Squirrel
Feb 21, 2019 · As a group, flying squirrels are in trouble with 20 percent globally threatened (IUCN designation Critically Endangered, Endangered, Vulnerable) ...
[36]
New species of giant flying squirrel brings hope to one of the world's ...
Sep 4, 2019 · Both species are listed as critically endangered by the IUCN due to habitat loss and hunting. Almost nothing is known about them, given ...
[37]
Oldest skeleton of a fossil flying squirrel casts new light on the ... - eLife
Oct 9, 2018 · The styliform cartilage, which supports the patagium in all members of the group, attaches to the pisiform and is extended when the wrist is ...
[38]
Early and middle Miocene Sciuridae (Mammalia, Rodentia) from ...
Two MN 7 + 8 localities yielded flying squirrels: Albanensia albanensis was found at Sarıçay, Blackia sp. (most probably Blackia miocaenica) at Sarıçay and ...
[39]
A Miopetaurista (Rodentia, Sciuridae) cranium from the Middle ...
Jul 25, 2022 · This has led to major disagreements regarding the time of divergence and diversification of the group. Although for most mammal clades the ...
[40]
[PDF] the difficulties of identifying flying squirrels (sciuridae: pteromyini) in ...
Mercer and Roth (2003) depict the divergence of flying squir- rels from the Sciurini in the early Miocene. Early Miocene fossils purported to be flying ...
[41]
(PDF) The Evolution and Paleobiogeography of Flying Squirrels ...
Aug 6, 2025 · This study compiles the worldwide fossils of Pteromyini to the species level in order to reproduce the spatiotemporal distribution pattern of flying squirrels.
[42]
Home-range use patterns and movements of the Siberian flying ...
Feb 17, 2016 · The most suitable breeding habitat for the flying squirrel is spruce-dominated mature boreal forest with a mix of deciduous trees that provides ...
[43]
[PDF] Den use and selection by northern flying squirrels in fragmented ...
At the microhabitat scale of den selection snags were not selected over live trees, but flying squirrels primarily used cavities in snags and live trees. Flying ...Missing: adaptations scientific
[44]
Two new species of gigantic woolly flying squirrels discovered in the ...
“Two skins of one of the new species, the Yunnan Woolly Flying Squirrel, were purchased from markets as early as 1973,” said co-lead author Quan Li of the ...
[45]
Woolly Flying Squirrel Eupetaurus cinereus - Palm Oil Detectives
Feb 5, 2021 · Unlike many other flying squirrels, they have powerful jaw muscles and high-crowned teeth, adaptations that allow them to feed on tough conifer ...Missing: altitudinal | Show results with:altitudinal
[46]
Maine Wildlife Park's post - Facebook
Feb 9, 2025 · Their thick fur serves as insulation against the cold, and a layer of fat beneath their skin contributes additional warmth. During the fall, ...
[47]
Home range and habitat selection of southern flying squirrels in ...
These results show southern flying squirrels to be selective in habitat use and sensitive to forest disturbance, not forest generalists.
[48]
Northern Flying Squirrel - Commonwealth of Pennsylvania
Habitat. Northern flying squirrels prefer old-growth boreal forests that contain a heavy coniferous component, moist soils, and lots of downed woody debris.Missing: sources | Show results with:sources
[49]
[PDF] Quadrupedal locomotor performance in two species of arboreal ...
It is possible that the higher than predicted energetic costs of running in flying squirrels result from specific adaptations to climbing such as skeletal.Missing: dentition | Show results with:dentition
[50]
Think pink: Fluorescent pink flying squirrel in UV light at night
Feb 5, 2019 · The reasons for the squirrels to fluoresce pink is still under investigation, but communication and camouflage are two top contenders for why ...Missing: understory | Show results with:understory
[51]
Glide performance and aerodynamics of non-equilibrium glides in ...
In this study, we examine whole-glide performance and time-resolved aerodynamics of the northern flying squirrel (Glaucomys sabrinus), a representative gliding ...Missing: fur | Show results with:fur
[52]
The relationship between 3-D kinematics and gliding performance in ...
Feb 15, 2006 · To do this, I used a pair of high-speed digital cameras to record a portion of the middle of glides by southern flying squirrels, Glaucomys ...
[53]
Flying Squirrels | National Wildlife Federation
Size is another way to tell northern and southern flying squirrels apart. The southern species is smaller, about 8 to 10 inches in length.Missing: patterns morphology variations scientific
[54]
Gliding Behavior of Japanese Giant Flying Squirrels (Petaurista ...
Patagium deployment signals the start of the gliding phase, where aerodynamic forces determine the glide path. In glide phase, angle of attack (α = the angle ...
[55]
How does a northern flying squirrel 'fly'? | The Kid Should See This
Nov 8, 2018 · They maneuver with great efficiency in the air, making 90 degree turns around obstacles if needed. Just before reaching a tree, they raise ...
[56]
Take-off and landing forces and the evolution of controlled gliding in ...
Apr 15, 2007 · In arboreal locomotion, flying squirrels must generate higher take-off forces when jumping from narrow (and hence compliant) tree branches ...Missing: dentition | Show results with:dentition
[57]
Southern Flying Squirrel | Nebraska Game & Parks Commission
When the arms and legs are extended, the membrane forms a wing-like gliding surface. The noticeably large eyes, an adaptation for its nocturnal habits, are ...
[58]
Kansas Mammal Atlas: Southern Flying Squirrel
Southern flying squirrels occur in the eastern half of the United States and Canada, ranging to the Atlantic with its western limit running between central ...
[59]
Woodland Wildlife Spotlight: Southern Flying Squirrel
Aug 18, 2021 · Their wide and flat tail is used for guidance. These adaptations not only allow them to glide from upper to lower branches with ease, but also ...
[60]
[PDF] Effects of Forest Management on Truffle Abundance and Squirrel Diets
Flying squirrels must employ an optimal foraging strategy that allows them to minimize time on the ground and time of exposure to low temperatures. They ...
[61]
[PDF] Interactions among squirrels, mycorrhizal fungi, and coniferous ...
Jul 31, 1988 · of the fungal cycle. When flying squirrels eat sporocarps, they consume fungal tissue containing nutrients, water, viable fungal spores ...<|control11|><|separator|>
[62]
Fly-By-Night Squirrels Favor Truffles and Lichens
Flying squirrels mainly eat lichens and fungi, favoring truffles, and also eat berries, bird eggs, and fat from game carcasses. Lichens are important in winter.Missing: strategies | Show results with:strategies
[63]
[PDF] observations of northern flying squirrel feeding behavior: use of non ...
In areas of heavy snowfall, fruticose lichens. (Alectoria, Bryoria, Letharia, Usnea) are a large part of the diet of G. sabrinus (McKeever 1960;. Maser and ...
[64]
Mating system and reproductive success in the Siberian flying squirrel
Dec 16, 2013 · The reproductive benefit from large female size is one factor related to female-biased sexual size dimorphism in mammals (Ralls 1976) and, ...
[65]
Facts About Flying Squirrels
However, the temperate specieshave a short summer breeding season while those in the tropics breed throughout the calendar. They generally give birth to ...
[66]
https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=3568&context=gbn
[67]
[PDF] Northern Flying Squirrel - Glaucomys sabrinus
Gestation lasts 37–42 days and litter size ranges from 2– 6 young. Young are weaned at about two months and are sexually mature in 6–12 months.Missing: reproduction parental care
[68]
[PDF] Maturation and Reproduction of Northern Flying Squirrels in Pacific ...
Enlargement, turgidity, and redness of genitalia are detectable signs of reproductive activity, whereas reduction, flaccid- ity, and paleness signal inactivity ...Missing: dimorphism | Show results with:dimorphism
[69]
Siberian flying squirrels do not anticipate future resource abundance
Nov 14, 2016 · We observed that flying squirrels reproductive investment did not anticipate future resource availability. Instead, food levels before ...
[70]
Southern Flying Squirrel - Chesapeake Bay Program
Predators. Hawks, owls, domestic cats, snakes, raccoons, weasels and red foxes may prey upon flying squirrels, who avoid predators by staying alert, being ...
[71]
Predators Of The Southern Flying Squirrel - Sciencing
Mar 24, 2022 · A number of other animals will take southern flying squirrels, including foxes (both red and gray), martens, terrestrial weasels, forest hawks and raccoons.
[72]
Southern Flying Squirrel - Maryland DNR
Southern flying squirrels can be found throughout Maryland as well as throughout the eastern United States, southeastern Canada and parts of Central America.
[73]
Ultrasonic Vocalizations Emitted by Flying Squirrels - PMC
Aug 29, 2013 · We demonstrate that flying squirrels produce ultrasonic emissions through recorded bursts of broadband noise and time-frequency structured frequency modulated ...
[74]
How Flying Squirrels Control Their Glide — Biological Strategy
May 12, 2022 · Flying squirrels have membranes and cartilage that help them change lift and drag forces, enabling them to glide more than 100 times their body length.Missing: forelimbs processes stabilizer
[75]
Forest loss in mountains of Southeast Asia accelerates at 'shocking ...
Jun 28, 2021 · By 2019, 42% of total annual forest loss occurred at higher elevations, with the frontier of forest loss migrating upslope at a rate of roughly ...Missing: percentage | Show results with:percentage
[76]
Southeast Asian forest loss much greater than expected, with ...
Jul 3, 2018 · Researchers using satellite imaging have found much greater than expected deforestation since 2000 in the highlands of Southeast Asia.
[77]
Vordermann's Flying Squirrel Petinomys vordermanni
Feb 5, 2021 · Vordermann's flying squirrel is found in the lowland rainforests of southern Myanmar, Peninsular Malaysia, Borneo, and the Indonesian islands of ...Threats · Faqs · Further Information
[78]
Flying squirrel habitats predicted to tailspin under climate change
Jun 11, 2025 · Adapted to an arboreal life, flying squirrels play an important role in pollination and seed dispersal.
[79]
Gliding on the Edge: The Impact of Climate Change on the Habitat ...
Aug 6, 2025 · South and Southeast Asia are considered biodiversity hotspots, yet they face escalating threats from deforestation and climate change.Missing: pet | Show results with:pet
[80]
Anthropogenically-induced range expansion as an invasion front in ...
Feb 27, 2023 · I discuss what factors may enable or limit the expansion of southern flying squirrels (SFS; Glaucomys fuscus) into northern flying squirrel (NFS; Glaucomys ...
[81]
Testing the parasite-mediated competition hypothesis between ...
Strongyloides robustus was detected in 4 squirrel species. •. The highest prevalence of S. robustus was found in red squirrels and northern flying squirrels. •.
[82]
Southern Flying Squirrel - Facts, Diet, Habitat & Pictures on Animalia ...
Basic facts about Southern Flying Squirrel: lifespan, distribution and habitat ... Rodentia. Family. Sciuridae. Tribe. Pteromyini. Genus. Glaucomys. SPECIES.Distribution · Geography · Related AnimalsMissing: characteristics | Show results with:characteristics<|control11|><|separator|>
[83]
Sentinels of Ecological Processes: The Case of the Northern Flying ...
Nov 1, 2012 · The squirrel feeds on truffles and deposits fecal pellets with fungal spores and nitrogen-fixing bacteria across the forest floor (Maser and ...
[84]
[PDF] Squirrels Cannot Live by Truffles Alone - Forest Service
“In the process, the flying squirrel would ensure dispersal of a diversity of fungal taxa, help maintain genetic diver- sity for mycorrizhal fungal species and ...Missing: birds | Show results with:birds
[85]
Flying Squirrels and Diseases: What You Need to Know
Lyme Disease: While primarily transmitted by ticks, flying squirrels can carry ticks that harbor the bacteria causing Lyme disease. Symptoms include fever, ...
[86]
Flying Squirrel Removal - Critter Control
Squirrels are known to carry lice, fleas, and ticks that carry bacterial infections they can give to you. For example, deer ticks carry the Borrelia burgdorferi ...
[87]
Northern Flying Squirrel (Glaucomys sabrinus yukonensis) Uses
Flying squirrels are important to forest regeneration and timber production because they disperse spores of ectomycorrhizal fungi like truffles.Missing: seed | Show results with:seed
[88]
Carolina northern flying squirrel (Glaucomys sabrinus coloratus)
Carolina northern flying squirrel (Glaucomys sabrinus coloratus) · Taxonomy: · Listing Status: Endangered · Where Listed: WHEREVER FOUND · General Information · » ...Missing: IUCN | Show results with:IUCN
[89]
Are habitat loss, predation risk and climate related to the drastic ...
Oct 17, 2013 · The population size of adult flying squirrels decreased from 65 (±11) individuals in 1995 to 29 (±6) individuals in 2009. The number of flying ...
[90]
Comparison of survey techniques on detection of northern flying ...
Dec 1, 2016 · In 2015, we established 3 5 × 5 live-trapping grids (6.5 ha) with 4 camera traps and 4 acoustic detectors systematically embedded in each grid.<|control11|><|separator|>
[91]
[PDF] movement and gene flow of northern flying squirrels across
We trapped squirrels in June and July of 2009 and 2010. We attempted to locate trap sites (Figure 1) in suitable habitat within one flying squirrel home-range ...
[92]
Animals - Olympic National Park (U.S. National Park Service)
Jul 3, 2025 · The park is a rare refuge for species dependent on old growth forests, including some species protected under the Endangered Species Act. ...Terrestrial Mammals · Marine Mammals · Birds · Amphibians and Reptiles<|separator|>
[93]
Back from a century of extinction, conservation proposed for elusive ...
The authors found that the squirrel should occur mostly in medium elevation and lowland areas below 1200 meters in the two countries, but that less than 12 ...
[94]
Global hotspots and knowledge gaps for tree and flying squirrels
Aug 5, 2025 · The tropics, particularly the forests of south and southeast Asia, are hotspots of squirrel diversity; however, this region generates the fewest ...
[95]
Integrating climate and land‐use change scenarios in modelling the ...
Jan 10, 2019 · Scenarios based only on climate change predicted a range increase and a high geographic stability (>50%) for most species, with different, yet ...