Thomas's Flying Squirrel

Thomas's Flying Squirrel is a large, nocturnal flying squirrel endemic to the island of Borneo, inhabiting tall primary and secondary tropical rainforests. It possesses an expansive patagium — a fur-covered gliding membrane stretching from wrist to ankle — that enables impressive glides between forest trees. It is one of only two species in the genus Aeromys, alongside the Black Flying Squirrel (Aeromys tephromelas). The species was named in honor of Oldfield Thomas (1858–1929), one of the most prolific mammal taxonomists in history.

Endemic to the island of Borneo (Malaysian states of Sabah and Sarawak, Brunei Darussalam, and northern Kalimantan, Indonesia)

Restricted to the island of Borneo: confirmed in the Malaysian states of Sabah and Sarawak (type locality: Baram River region, Sarawak), likely present in Brunei Darussalam, and expected in adjacent regions of northern Kalimantan (Indonesia). Distribution may extend more broadly across Kalimantan, though records remain sparse.

Altitude and depth

Primarily found in lowland forests from near sea level up to approximately 1,200–1,500 m in submontane zones; most records come from elevations below 800 m.

Primarily tall, closed-canopy lowland tropical rainforest and submontane forest with large emergent trees used as gliding launch points. It may also occur in mature secondary forest but strongly prefers intact primary forest with abundant large-diameter trees providing nest hollows.

Adaptations to its environment

The large patagium connecting fore- and hindlimbs allows silent gliding flights of potentially over 100 m, reducing predation risk and locomotion energy costs across a fragmented canopy. Oversized eyes provide excellent low-light nocturnal vision. Long, bushy, semi-prehensile tail acts as a rudder and stabilizer during glides. Sharp, curved claws enable secure gripping on large-diameter trunks and branches.

Listed as Least Concern (LC) on the IUCN Red List. Although its range is restricted to Borneo, the species is considered relatively widespread within suitable habitat, and no quantified large-scale population decline has been documented to date.

Estimated population

No precise global population estimate exists. The population trend is suspected to be slowly declining due to accelerating deforestation rates across Borneo over the past two decades.

Habitat loss and forest fragmentation from commercial logging, conversion to oil palm and rubber plantations, and agricultural expansion are the main threats across Borneo. Localized hunting or snaring may occur, though the species is not a primary target. Forest fires in degraded peatlands of Kalimantan also reduce available habitat.

The species benefits indirectly from habitat protection within established Bornean protected areas, including Kinabalu Park and Crocker Range Biosphere Reserve (Sabah), Danum Valley Conservation Area, Maliau Basin Conservation Area (Malaysia), and various Indonesian national parks in Kalimantan. It is protected under Malaysian and Indonesian national wildlife legislation.

Recommended actions

Expansion and strict enforcement of protected forest reserves in Borneo; development of wildlife corridors to maintain forest connectivity between fragmented patches; targeted camera-trap and spotlighting surveys to better define population size and distribution; reduction of illegal logging and sustainable land-use certification for forest concessions.

Dorsal fur is dark brown to blackish-brown; the ventral surface is paler, typically greyish-white to buff. The patagium is dark-furred on its dorsal surface and paler beneath. Eyes are conspicuously large and dark-adapted. The tail is long, bushy, and roughly cylindrical with dark brown fur throughout.

Size

Head-body length approximately 28–42 cm; tail length approximately 33–46 cm; total length can exceed 80 cm. Patagium span when fully extended may exceed 60 cm.

Weight

Estimated 500 g to 1.5 kg; precise field measurements for this species are scarce, with estimates extrapolated from closely related Aeromys tephromelas.

Distinctive features

Distinguished from the closely related Aeromys tephromelas by its generally smaller body size and strictly Bornean distribution; A. tephromelas also occurs in Sumatra and Peninsular Malaysia and tends to be darker overall. Larger and more uniformly dark than most sympatric Petaurista species, and with a proportionally broader patagium relative to body size.

Presumed largely solitary outside the breeding season, as is typical for flying squirrels. Individuals likely maintain overlapping home ranges but generally forage and rest independently. No evidence of complex social groupings.

When it is active

Strictly nocturnal. Individuals emerge from tree hollow roosts after dusk, remain active throughout the night foraging across the canopy, and return to roost sites before dawn.

Diet

Primarily frugivorous and folivorous, feeding on fruits, seeds, bark, leaves, and likely flowers and buds. As with related Sciuridae, opportunistic consumption of insects, fungi, or other invertebrates may supplement the diet, making it a generalist omnivore with a plant-heavy diet.

How it feeds and hunts

Forages arboreal resources by gliding between trees to access distant or patchy food patches. Uses large, sensitive eyes and vibrissae to detect food in low-light conditions. Gnaws bark and husks with strong incisors to access seeds and sap.

Communication

Likely communicates via vocalizations (soft chirps or alarm calls), scent marking from glands on the face and body, and tactile signals. Flying squirrels are generally quiet, but ultrasonic calls may be used that are inaudible to humans. Scent marking likely plays a role in territory advertisement and reproductive signaling.

Interactions with other species

Natural predators likely include large forest owls (e.g., Buffy Fish Owl Ketupa ketupu, Barred Eagle-Owl Bubo sumatranus), arboreal pythons, and civets. It may compete with other large Bornean flying squirrels (Petaurista petaurista, P. elegans) for fruit resources and nest hollows. As a frugivore, it may function as a secondary seed disperser for large-seeded forest trees.

Role in its ecosystem

Functions as a seed disperser and arboreal consumer in the Bornean rainforest ecosystem. Its long-distance gliding ability allows it to link spatially separated food patches, potentially facilitating gene flow and seed movement across larger forest areas than ground-dwelling rodents.

Reproductive biology is poorly documented for this species. Based on related congeners and similar tropical flying squirrels, litter size is likely 1–2 young. In the aseasonal equatorial rainforest of Borneo, breeding may occur throughout the year rather than being confined to a single season.

Raising the young

Young are believed to be born altricial (helpless, with closed eyes) and raised in a nest within a tree hollow by the mother. Duration of maternal care and weaning age are undocumented; in related species, juveniles remain dependent for several weeks to months.

Maturity and lifespan

Age at sexual maturity and wild lifespan are undocumented for this species. Based on related flying squirrels, sexual maturity likely occurs at 1–2 years of age, with a potential wild lifespan of 5–10 years and possibly longer in captivity.

Classification

Mammalia > Rodentia > Sciuridae > Aeromys > Aeromys thomasi

Scientific synonyms

Sciuropterus thomasi Bonhote, 1900 (original combination); sometimes listed as Aeromys thomasi Bonhote, 1900.

Recent studies

Molecular phylogenetic studies of the tribe Pteromyini (Old World flying squirrels) have confirmed that Aeromys represents a relatively early-diverging lineage within this group. Recent biodiversity surveys using camera traps and spotlighting in Borneo have contributed new locality records, though targeted studies on A. thomasi ecology and behavior remain lacking.

Compared with similar species

Aeromys thomasi is most closely related to A. tephromelas (Black Flying Squirrel), with which it shares its large size and dark coloration but differs in range (Borneo-endemic vs. broader Sundaland distribution) and slightly smaller body dimensions. Both Aeromys species are distinct from the larger genus Petaurista in cranial morphology and tail shape. Compared to the Giant Flying Squirrel (Petaurista petaurista), A. thomasi is smaller, darker, and has a less conspicuous bicolored tail.

Thomas's Flying Squirrel can cover over 100 meters in a single glide by expanding its patagium — the equivalent of gliding across a football field without flapping a wing. Despite the name 'flying squirrel,' it cannot generate lift or true flight; it is entirely dependent on gravity-assisted gliding, using body posture to steer. Its namesake, Oldfield Thomas, personally described over 2,000 mammal species and subspecies during his career at the Natural History Museum in London — one of the greatest individual contributions to mammalogy in scientific history.

Where and how to see it

Best observed after dusk in primary lowland rainforest using a red-filtered headlamp to minimize disturbance. Protected areas such as Danum Valley Conservation Area and Maliau Basin in Sabah, Malaysia, offer excellent habitat. Scanning emergent tree trunks and listening for the sound of landing impacts on bark after a glide significantly increases detection probability.

Risks to humans

No notable risk to humans.