Red-cheeked Flying Squirrel
(Hylopetes spadiceus, Blyth 1847)



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Figure 1. Red-cheeked flying squirrel (Hylopetes spadiceus) climbing up a silverback (Rhodamnia cinerea) trunk in Nee Soon Swamp Forest after emerging from the nest hole (Nick Baker, permission pending).























Introduction

The red-cheeked flying squirrel is a gliding mammal that is found in Singapore and most of Southeast Asia [1]. First discovered and named in an expedition to Burma (present-day Myanmar) by Edward Blyth in 1847, the red-cheeked flying squirrel has remained fairly unstudied, with little known about its specific reproduction, diet and ecology [2]. It is nocturnal and lives in primary forest, nesting in small holes in the trunks of tall trees, coming out and gliding from foraging patch to foraging patch at night [3]. Despite being known to man since Blyth’s discovery, the red-cheeked flying squirrel was first discovered in Singapore in October 1996 and is critically endangered within Singapore [3].
While the red-cheeked flying squirrel is not actually able to fly (despite its name), it is able to glide from tree to tree using the patagium, a furry parachute-like membrane that resembles wingsuits used by BASE jumpers (though in actual fact, wingsuits were designed after them and are sometimes known as “flying squirrel suits”).

Figure 2: The patagium, used by flying squirrels (including the red-cheeked flying squirrel) to glide from tree to tree.
Figure 2: The patagium, used by flying squirrels (including the red-cheeked flying squirrel) to glide from tree to tree.



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Taxonomic Information



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Figure 3: An artist’s impression of Hylopetes spadiceus ([2], image by Peter Schouten).



TaxonomyThe red-cheeked flying squirrel is classified as a mammal (Mammalia) and a rodent (Rodentia), with a single pair of continuously growing incisors in the upper and lower jaws. In particular, as a squirrel, it falls into the grouping of Sciuridae. The genus Hylopetes is for Southeast Asian flying squirrels (flying squirrels belong to the overall tribe of Pteromyini) and the specific species epithet is spadiceus.

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Figure 4. The red-cheeked flying squirrel was first described as Sciuropterus spadiceus (The Journal of the Asiatic Society of Bengal, 1847).

Names

The red-cheeked flying squirrel was first described as Sciuropterus spadiceus by Blyth [4], and later modified to Hylopetes spadiceus in 1908 when Sciuropterus was split into Hylopetes, Petinomys and Glaucomys by Thomas Oldfield [5]. It is also referred to as the amoenus (Miller, 1906); aurantiacus (Wagner, 1841); belone (Thomas, 1908); everetti (Thomas, 1895); harrisoni (Stone, 1900); sumatrae (Sody, 1949) and caroli (Gyldenstolpe, 1920). Currently, everetti and sumatrae are recognized as subspecies.

Etymology

The species epithet of spadiceus refers to the colouration of the fur: spadiceus means "light brown" in Latin.

Type Locality“Arracan”, from Arakan, Burma [6]. Details on the holotype and the availability of paratypes could not be located.
SubspeciesIt has 3 subspecies: H. s. spadiceus, H. s. everetti and H. s. sumatrae. The two additional subspecies are normally defined by location: everetti is found on the island of Borneo, both in the side that is Eastern Malaysia and the Kalimantan area that is Indonesia. As its name suggests, sumatrae is found on the Indonesian island of Sumatra, particularly on the northern side of the island.

Phylogenetics

Phylogenetically, the flying squirrels are classed together as Pteromyini, with Hylopetes closely related to two other genuses but still forming 3 distinct clades based on bacula, foot pad, wrists, teeth and ankle analysis: the Eoglaucomys, the Glaucomys and the Hylopetes. Various studies have put between 5-9 species within the genus of Hylopetes, though the leading authority in the field, Richard Thorington, estimated it more conservatively at 5 species [7]. Morphological characteristics and mitochondrial cytochrome b gene sequence analysis suggests that Hylopetes and other New World flying squirrels in Glaucomys (found in North America) share a close relationship, while also related to Petinomys [7]. Data suggests that Glaucomys and Hylopetes diverged about 28.6 million years ago, while Glaucomys and Petinomys diverged slightly further back, a 29.2 million years ago.

Petinomys and Hylopetes are sympatric through most of their ranges and morphologically similar, hence they are estimated to have diverged about 2.2 million years ago in Southeast Asia. As such, the exact phylogenetic analysis is still up for debate and more research is still ongoing in this area.

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Biological and Ecological Information


Description


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Figure 5: Sketch of the red-cheeked flying squirrel's body plan and rough dimensions (http://www.planet-mammiferes.org/Photos/Rongeur/Sciuromo/Petaure/HyloSpa1.jpg, permission pending).

The red-cheeked flying squirrel is a medium-sized squirrel with blackish or grey-brown upper parts, with rust-coloured, buff or yellow tips, especially in the midline. The underparts of its body are white on grey under-fur, with a faint orange tinge. Its gliding membrane has a thin white margin. Its tail is dark, slightly orange-brown with buff under-fur, with a distinctly orange or chestnut colouring at the base. Its cheeks are orange-brown or grey [2].
SizeBased on 3 specimens [7]:l Female: 146.44mm (HB: head and body), 129.1mm (T: tail), 78.0g (M: mass)l Male: 142.00mm (HB), 118.1mm (T), 70.9g (M)l Undetermined: 147.9mm (HB), 125.8mm (T), 75.9g (M)
In general, they are believed to grow up to 180mm (HB), 160mm (T) and weigh up to 157g [8].

Distribution

The red-cheeked flying squirrel is found in Southeast Asia, from southeastern Burma, as well as western and southern Thailand, through the Malay Peninsula to Singapore (H. spadiceus spadiceus), as well as on the Indonesian island of Sumatra (H. spadiceus sumatrae). It is also found in southeastern Vietnam (H. spadiceus spadiceus), with a subspecies on the northeastern areas of Borneo in Indonesia in tall and secondary forests (H. spadiceus everetti) [2].

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Figure 6. Region in which the red-cheeked flying squirrel is found (IUCN Red List, permission pending).

Crucially, within Singapore, the red-cheeked flying squirrel has been spotted in the Bukit Timah Nature Reserve, as well as the Nee Soon Swamp Forest area of the Central Catchment Nature Reserve.

Habitat

The red-cheeked flying squirrel lives in primary vegetation, though it has also been able to survive in secondary vegetation, as seen in the isolated areas that it still can be found in on mainland Singapore [9]. However, such abilities are limited and the situation is precarious, highlighting the importance of preserving primary and even secondary forest in Singapore.

Ecology

The red-cheeked flying squirrel is known to nest in tree holes about 32 millimetres wide and 0.3-3.3 metres above the ground [2]. In particular, their nesting holes are generally located nearer the base of medium-sized trees (such as the silverback tree, Rhodamnia cinerea) [9], preferring to either excavate their own holes or use abandoned holes made by other species rather than use natural tree holes. They have also been reported to nest within coconuts [8]. A red-cheeked flying squirrel nest in Singapore's Nee Soon Swamp Forest area was found to have an entrance about 1.31m above ground and measured 3cm wide and 4cm high [9].

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Figure 7. Dry grass plug that blocks the entrance to the hole in the tree. ([8], permission pending)
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Figure 8. A red-cheeked flying squirrel emerges from the tree hole, while another's head can be seen at the hole ([8], permission pending).




















During the day, the nesting hole entrance is plugged with dry grass or some vegetation to block sunlight, and this is subsequently removed at dusk when the nocturnal squirrels move out of their holes and ascend to the canopy to begin foraging [8]. Since they are gliding mammals, it is crucial that they begin their foraging from a tall height to allow sufficient descent time in gliding from tree to tree. Each nesting hole can be shared, as shown in the photographs above that show up to 3 squirrels coming out from the same nesting hole at dusk.

Reproduction

Little is currently known [2], though other flying squirrels are known to reproduce either once or twice a year to live young, which are taken care of by the mother for up to two months.

Diet

Little is currently known [2], but the red-cheeked flying squirrel has been observed eating fruit (specifically, Syzygium chloranthum) in Singapore [9]. In general, flying squirrels are known to be omnivores, eating a variety of fruit, nuts, fungi, seeds and insects [10].

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Field & General Identification


Field Identification

The red-cheeked flying squirrel is often confused with another squirrel that inhabits the same environment and looks fairly similar: the Horsfield's flying squirrel (Iomys horsfieldii).


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Figure 9. The Horsfield's flying squirrel (Iomys horsfieldii), which is often confused with the red-cheeked flying squirrel due to appearance similarities and shared habitats.



A first guide to telling the two apart in sightings would be the size: the red-cheeked flying squirrel is smaller in size (it is actually the smallest flying squirrel found in Singapore!) while the Horsfield's flying squirrel is somewhat larger. Other differences are summarised in the following comparison table [9].


Red-cheeked Flying Squirrel
Horsfield's Flying Squirrel
1.
Patagium has a thin white margin.
Patagium has an orange margin.
2.
Tail is dark brown with an orange-tinged base.
Tail is uniformly grizzled along its length.
3.
Underside is white and marked with grey and orange.
Underside is uniformly orange-tinged.

General Identification

If only skeleton samples are available, there are other ways to distinguish the red-cheeked flying squirrel from other superficially similar squirrels. Flying squirrels usually have relatively large auditory bullae with well developed patterns of septa (the auditory bulla is a hollow bony structure that encloses the middle and inner ear in mammals) [11].

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Figure 10. Underside of posterior half of skulls of selected small flying squirrels showing variation in the shape of auditory bullae: Petinomys genibarbis (a), Petinomys setosus (b), Petinomys vordermanni (c), Hylopetes spadiceus (d), Petaruillus kinlochii (e) (taken from [11]).

Craniodental measurements can also help to distinguish problematic species, though Rasmussen and Thorington Jr. proposed using it in tandem with pelage colouration (the colouration of the fur), which might not be helpful if only the skeleton is available (such as in old museum collections or specimens) [12].


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Mechanism of "Flight"


Strictly speaking, the red-cheeked flying squirrel does not fly, as it is not capable of powered flight as we see in birds and in other mammals such as the bats. It instead glides, beginning from a high branch and leaping off while extending their patagium (a large membrane that resembles a wing) to maximise the air resistance it will face as it is pulled down by gravity. By making small movements and shifting the positions of its legs, it is then able to steer itself mid-glide. Upon landing at its target site (often another branch), the considerably large tail is then used as a brake of sorts to prevent it from overshooting [10].

It is importantly to note that while the patagium seems to be crucial in its movement, the tail is also crucial. The Horsfield's flying squirrel, a very similar species also found in similar habitats was once observed to land short while gliding from one tree to another. This was due to a tail injury, which prevented the squirrel from controlling its body movements as precisely as it was used to.

This method of flight, while not as flexible as powered flight due to the limitations (such as requiring a high branch to begin from, and the inability to go higher than the starting position), also comes with benefits: it is far less metabolically demanding on the squirrel and allows it to maintain a diet that is less energy-rich.





Video: National Geographic's segment on flying squirrels, which also documents their flight.



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Anthropogenic Association


Conservation Status



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Figure 11. Conservation status of the Red-Cheeked Flying Squirrel (Hylopetes spadiceus) on a global scale.

In general, the red-cheeked flying squirrel is fairly extensively found and classified as a “Least Concern” species by the IUCN [1], though threats do exist to its habitat and this may change if the deforestation situation continues in mainland South-East Asia where the red-cheeked flying squirrel is endemic.

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Figure 12. Conservation status of the Red-Cheeked Flying Squirrel (Hylopetes spadiceus) within Singapore (based on the Singapore Red Book's analysis).

Within Singapore, however, the red-cheeked flying squirrel is classified as “Critically Endangered”, due to forest degradation at the Bukit Timah Nature Reserve, which appears to be its primary habitat in Singapore. In addition, rapid urbanisation as a result of population pressures in Singapore have also threatened the natural habitats of the red-cheeked flying squirrel.

Ongoing Research Efforts

Given the charismatic nature of the red-cheeked flying squirrel, it is surprising that it is still relatively unstudied and little is known about it. Most of the problems in studying in (particularly in Singapore) relate to the fact that it is primarily nocturnal and small, making it difficult to spot in the darkness. Better photographic devices and techniques however, alongside improved survey efforts have made quite a difference, as seen in the new locality records published end-2013 [10]. The charismatic nature of the red-cheeked flying squirrel lends itself well to conservation efforts as a photogenic and intriguing mammal that often brings much joy to the public. Increased knowledge of this animal would thus be beneficial in promoting the larger conservation cause in Singapore.

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References


[1] Duckworth, J.W. & S. Hedges, 2008. Hylopetes spadiceus. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 10 November 2014.
[2] Stephen Jackson, 2012. Gliding Mammals of the World. CSIRO Publishing: Collingwood VIC, Australia
[3] Davison, G., P. Ng and H. C. Ho, 2008. Hylopetes spadiceus. Singapore Red Data Book. <http://www.nparks.gov.sg/cms/docs/redbook/aaaHylopetes-spadiceus1aaa.pdf>. Downloaded on 11 November 2014.
[4] Blyth, E., 1847. Proceedings of the Asiatic Society of Bengal for July, 1847. The Journal of the Asiatic Society of Bengal, 16:(2), 851-881.
[5] Oldfield, T. 1908. The Genera and Subgenera of the Sciuropterus Group, with Descriptions of Three New Species. The Annals and Magazine of Natural History, 8:(1), 1-8.
[6] Wilson, Don E. and D. M. Reeder, 2005. Mammal Species of the World: A Taxonomic and Geographic Reference. Johns Hopkins University Press: Baltimore, USA.
[7] Thorington Jr, Richard W., John L. Koprowski, Michael A. Steele and James F. Whatton, 2012. Squirrels of the World. Johns Hopkins University Press: Baltimore, USA.
[8] Ecology Asia. <http://www.ecologyasia.com/verts/mammals/red-cheeked-flying-squirrel.htm>. Retrieved 12 November 2014.
[9] Chua, A. H. Marcus, N. Baker, R. K. H. Yeo and N. Sivasothi, 2013. New Locality Records for Two Species of Flying Squirrels (Mammalia: Rodentia: Sciuridae) in Singapore. Nature in Singapore, 6, 301-305.
[10] National Wildlife Federation. <http://www.nwf.org/wildlife/wildlife-library/mammals/flying-squirrels.aspx>. Retrieved 21 November 2014.
[11] Francis, Charles M, 2008. A Field Guide to the Mammals of South-East Asia. New Holland (UK) Publishers: London, UK.
[12] Rasmussen, N. L. and Thorington Jr, R. W., 2008. Morphological Differentiation Among Three Species of Flying Squirrels (Genus Hylopetes) from Southeast Asia. Journal of Mammalogy, 89(5), 1296-1305.