Iomys horsfieldii
Javanese flying squirrel

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)

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)

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)

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)

The home range of Javanese flying squirrels is unknown.

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)

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)

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.

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

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)

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)