Iomys horsfieldiiJavanese flying squirrel

Geographic Range

Javanese flying squirrels are limited to southern Asia. Their native range stretches from the peninsula of Malaysia down through Sumatra, Java, and Borneo. There have been additional sightings of individuals in Singapore, Tioman, and Penang. ("Iomys horsfieldii", 2010; "List of mammal species present in Singapore", 2009)


Javanese flying squirrels are fairly adaptable animals. They are found in a wide variety of habitats. They can live in a variety of forest environments, even those that are degraded. They also inhabit scrub and are found on durian plantations. They shelter in hollow trees. ("Iomys horsfieldii", 2010; "The Terrestrial Mammals of Pulau Tioman, Peninsular Malaysia, with a Catalogue of Specimens at the Raffles Museum, National University of Singapore", 1999)

  • Range elevation
    1000 (high) m
    3280.84 (high) ft

Physical Description

Javanese flying squirrels are mostly orange-brown to brown-gray, with a tail that is a slightly brighter shade on the ventral surface and sides. Body length averages 182 mm and the tail is as long as, or slightly longer than, the body. Like other flying squirrels, Javanese flying squirrels have a thin flap of skin, the patagium, connecting the fore and hind limbs to allow for gliding. The patagium is slightly darker in color than the rest of the body. Javanese flying squirrels also have relatively large eyes and ears and long claws for gripping the trees that they live in.

There is no information regarding sex differences in Javanese flying squirrels, but in the closely related Hylopetes and Petinomys species, males and females are not appreciably different, so it is likely that male and female Javanese flying squirrels are also similar in appearance.

There are three named subspecies of Javanese flying squirrels, though too few specimens have been found to be certain whether these constitute distinct groups. Iomys horsfieldii davisoni has dark brown-gray fur and a dark brown tail mottled with red-brown at the edges and a short tooth-row. Iomys horsfieldii penangensis has lighter, brighter coloration and a longer tooth-row. Iomys horsfieldii thomsoni is very similar to I. h. davisoni, but the tail is uniformly red-brown rather than mottled.

Javanese flying squirrels can be distinguished from their closest relatives by their large size, the orange tinge to the lighter underbelly, and the absence, or near absence of hair on the upper surface of the ear lobe. The very similar Petinomys genibarbis can be distinguished from Javanese flying squirrels by the presence of a 5 mm wart on the cheek, sprouting long black whiskers. ("Mammals of the Indomalayan Region", 1992; "Phylogenies of Flying Squirrels (Pteromyinae)", 2002; "Mammals of Thailand", 1977; Farelli, 2008)

  • Sexual Dimorphism
  • sexes alike
  • Average length
    182 mm
    7.17 in


Javanese flying squirrels are not well studied and their mating system is as yet unknown.

As with other aspects of Javanese flying squirrel behavior, reproductive behavior is unknown. However, in a related genus of southeast Asian flying squirrels, Petaurista, females reportedly have one or two young in a litter, so it is possible that Javanese flying squirrels also produce a small number of offspring. The breeding season for Javanese flying squirrels is unknown. However, they live in a tropical climate, which might allow them to reproduce year round. ("Iomys horsfieldii", 2010; "Mammals of Thailand", 1977)

  • Breeding interval
    The breeding interval of the Javanese flying squirrel is unknown.
  • Breeding season
    The breeding season of the Javanese flying squirrel is unknown.

There is no information available about parental investment in Javanese flying squirrels.


The lifespan of Javanese flying squirrels is unknown. In the related genus Petaurista, the longest recorded life span in captivity is 13.5 years. Squirrels in this genus are larger than Javanese flying squirrels and thus their life expectancy is probably longer, but there is no definitive proof either way. ("Mammals of Thailand", 1977)


Javanese flying squirrels are nocturnal and arboreal. Like other flying squirrels, they are probably able to glide long distances between trees and use their long claws for grip. Almost nothing is known about their social behavior. Members of the related genus Petaurista have been recorded foraging alone or in small groups, so it is possible that Javanese flying squirrels also organize themselves in this manner. ("Iomys horsfieldii", 2010; "Mammals of Thailand", 1977)

Home Range

The home range of Javanese flying squirrels is unknown.

Communication and Perception

Little is known about communication and perception in Javanese flying squirrels. Because they have large eyes and ears (relative to their body size), they probably have well developed hearing and sight, as many arboreal and nocturnal species do. Their sight is probably adapted to work especially well at night, as they are primarily nocturnal. As with similar species, they likely use visual, auditory, and chemical cues in communication, although no behaviors specific to Javanese flying squirrels have been noted. ("Iomys horsfieldii", 2010; Farelli, 2008)

Food Habits

There is no information about the diet of Javanese flying squirrels. However, members of the related genus Petaurista eat fruits, nuts, and shoots, and it is likely that the diet of Javanese flying squirrels is similar, i.e. they are probably herbivorous and frugivorous. ("Mammals of Thailand", 1977)


There is no information about the predators of Javanese flying squirrels. The civets Paradoxurus hermaphroditus and Arctogalidia trivirgata share much of the same range as Javanese flying squirrels, are known to be arboreal, and eat squirrels and small mammals, so they are likely candidates. There are many other arboreal carnivores in southeast Asia, such as Prionodon linsang and Arctictis binturong, that feed on squirrels and small mammals, and any of these might prey on Javanese flying squirrels. ("List of mammal species present in Singapore", 2009; "Mammals of Thailand", 1977)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Little is known about how Javanese flying squirrels impact their ecosystem. Because they are probably herbivorous and frugivorous, they likely benefit their ecosystem by helping to disperse seeds.

Economic Importance for Humans: Positive

As probable herbivores and frugivores, Javanese flying squirrels likely serve in seed dispersal.

Economic Importance for Humans: Negative

Javanese flying squirrels can be pests on orchard plantations. ("Iomys horsfieldii", 2010; "The Terrestrial Mammals of Pulau Tioman, Peninsular Malaysia, with a Catalogue of Specimens at the Raffles Museum, National University of Singapore", 1999)

  • Negative Impacts
  • crop pest

Conservation Status

Iomys horsfieldii is listed by the IUCN as a species of least concern because it is widespread and common in Indo-Malaysia, and is also very adaptable. Still, these flying squirrels have become rare to extinct in Singapore. This probably reflects a skyrocketing human population and the deforestation of much of the island. Fortunately, this is not a major threat to the species in most of its range and populations outside of Singapore are stable. Iomys horsfieldii is currently a protected species in Indonesia and has additional protection in several other areas within its range. ("Iomys horsfieldii", 2010; "The ecological transformation of Singapore, 1819-1990", 1992; Thorington and Hoffman, 2005)


Shannon Cruz (author), Michigan State University, Barbara Lundrigan (editor), Michigan State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.



uses sound to communicate


living in landscapes dominated by human agriculture.


Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.


found in the oriental region of the world. In other words, India and southeast Asia.

World Map


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scrub forest

scrub forests develop in areas that experience dry seasons.


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


Boonsong Lekagul. 1977. Mammals of Thailand. Kurusapha Ladprao Press: Nai Kamthon Sathirakul.

1992. Mammals of the Indomalayan Region. Oxford, New York: Oxford University Press.

2010. "Iomys horsfieldii (Waterhouse, 1838)" (On-line). ITIS Report. Accessed February 25, 2010 at

2010. "Iomys horsfieldii" (On-line). The IUCN Red List of Threatened Species. Accessed February 25, 2010 at

2009. "List of mammal species present in Singapore" (On-line). National Parks Singapore. Accessed March 16, 2010 at

2002. Phylogenies of Flying Squirrels (Pteromyinae). Journal of Mammalian Evolution, Vol. 9, No. 1/2: 1573-7055. Accessed March 13, 2010 at

National University of Singapore. 1999. The Terrestrial Mammals of Pulau Tioman, Peninsular Malaysia, with a Catalogue of Specimens at the Raffles Museum, National University of Singapore. The Raffles Bulletin of Zoology, Supplement No. 6: 101-123. Accessed March 13, 2010 at

1992. The ecological transformation of Singapore, 1819-1990. Journal of Biogeography, Vol. 19, No. 4: 411-420. Accessed March 16, 2010 at

Farelli, 2008. "Iomys horsfieldii, Javanese Flying Squirrel" (On-line). National Parks Indonesia. Accessed March 16, 2010 at,%20J,%20K,%20L/pages/iomys-horsfieldii.htm.

Thorington, R., R. Hoffman. 2005. Family Sciuridae. Pp. 754-818 in D Wilson, D Reeder, eds. Mammal Species of the World: A Taxonomic and Geographic Reference. Baltimore, Maryland: The Johns Hopkins University Press.