Javan Fuscous Leaf Monkey, Presbytis fredericae
JAVAN FUSCOUS LEAF MONKEY
Presbytis fredericae
Geographic Distribution and Habitat
Javan fuscous leaf monkeys are native to Indonesia. As their name indicates, they live on the island of Java, which is surrounded by the Indian Ocean to the south and the Java Sea to the north. The island is almost completely covered in volcanoes—45 of which are still active.
Javan fuscous leaf monkeys inhabit the mangrove forests of Central Java, where temperatures average 64–82 °F (18–28 °C). Populations are found at Mt. Slamet (an active volcano and the highest mountain in Central Java), Mt. Merbabu (a dormant volcano), and the mountains in Dieng.
It is possible some populations may be present as well on Mt. Muria and Mt. Pembarisan, also in central Java; and possibly in Kuningan and Mt. Ciremay in the west of the island. The species used to be found on Mt. Lawu on the border of Central and East Java until 1995.
The Javan fuscous leaf monkey belongs in the Presbytis colobine genus of leaf-eating monkeys. They are also referred to as Javan fuscous langurs. The genus Presbytis has gone through many taxonomic revisions over the years.
Scientists don’t agree quite yet on whether leaf monkeys endemic to West Java (Javan surili, P. comata) and those found in Central Java (Javan fuscous langurs or leaf monkeys) should be classified as two separate species or not. Some studies indicate that the coat coloring and male loud calls are different enough to justify it. However, differences in mitochondrial DNA between the two groups are insufficient to justify a new classification at this stage.
Size, Weight, and Lifespan
Female and male Javan fuscous leaf monkeys are very similar in size. They weigh 14 pounds (6.5 kg) on average, and they can be 3.5 feet (108 cm) tall.
Their tail is not prehensile (meaning it cannot be used to grab things) and measures up to 43 inches (108 cm) in length. Javan fuscous leaf monkeys mostly use their tail to keep their balance while traveling from branch to branch in the canopy.
Their lifespan in the wild or in captivity is not documented, but other Javan leaf monkeys have been known to live up to 18 years in captivity. So this may be an indication of the Javan fuscous leaf monkey’s longevity until studies provide a definite answer.
Appearance
Javan fuscous leaf monkeys are as cute as can be. Their fur is thick and appears extremely fluffy and soft. Their tail, back, head, and the outside of their arms and legs are a medium to dark brown color. Their chest, throat, inner arms, and legs are a soft creamy color. Their tail is as long as their body.
Their big brown eyes are round and facing forward, giving them the appearance of being perpetually in a state of wonder. Their nose is thin and flat, with nostrils pointing downwards. Their chin and mouth are pinkish, and the skin on the rest of the face is a light, hairless brown. The face is framed by fur, as if they were wearing a hoodie. Their ears are pointy and sit high on each side of the face.
They have an opposable thumb and an opposable big toe, which come in handy to grab food and hold onto branches.
Diet
As their name indicates, these monkeys eat leaves (i.e, they are predominantly folivorous). They also consume unripe fruit, flowers, and seeds. Their diet varies depending on where they live; for instance, groups at Mt Dieng eat mostly fruit and seeds.
Leaves are not easy to digest but these primates have a sacculated stomach. This means their stomach is divided into several chambers, each with specific functions, similar to the stomach of a ruminant. Bacteria in the upper part of the stomach break down cellulose by fermentation and neutralize the toxins contained in the leaves. The breakdown of the cellulose enables better absorption of the nutrients contained in foliage. This is a great adaptation as it allows these monkeys to consume leaves from many species of trees and easily find food wherever they are.
However, some recent studies indicate that more and more invasive tree and plant species are growing on the island, thereby changing the natural habitat in which these primates live. Although they seem able to adapt to dietary changes, we won’t know what it means for the long term survival of the species until more studies are conducted.
Behavior and Lifestyle
Javan fuscous leaf monkeys are diurnal and arboreal. They dwell in the middle and upper layers of the canopy, mostly in secondary and lowland forests, hills, montane, and upper montane forests.
They spend a quarter of their time foraging during the day. Because their diet is mostly folivorous, they need a lot of energy to digest. This is why they spend half of their time resting.
Their lifestyle and behaviors are not well documented, but like other primates, they engage in grooming bouts which reinforce social bonds between members of the same group and are a necessity to keep their fur clean and free of parasites. At night, they retire to the tallest trees they can find to sleep.
Although there is no specific species survival plan for the Javan fuscous leaf monkey, the species benefits from efforts made to conserve the forests they share with other endangered species.
Javan fuscous leaf monkeys live in family groups that vary in size; some have up to 6 individuals, but on average, they include three to four with one or two adult males, females, and offspring.
The number of groups per square mile depends on the location—for instance, there are more groups at high altitudes than in the lowlands.
Based on current knowledge available about leaf monkeys in general, we can surmise that adult males leave their natal group around the time of puberty and may form bachelor groups until they’re ready to start a family of their own.
These monkeys share their habitat with, among others, Javan gibbons, grizzled leaf monkeys, ebony leaf monkeys, and Javan leopards.
All primates use vocalizations and body posture to communicate—this is no different for Javan fuscous leaf monkeys. The calls they use have different meanings and can be very loud. Some calls are used to rally the group members, others to warn about danger, and others yet to tell intruders to keep their distance.
Grooming is a way to communicate affection and reinforce bonds between members of the group.
Not much is known about the reproduction and family life of Javan fuscous leaf monkeys. What is known is that there is no specific breeding season. Females give birth to one baby after a gestation of about five months. Babies are born with a medium to dark grey or black coat.
Mothers carry their babies on their bellies, nurse them, and take care of them for one year. Once they reach the ripe age of one, offspring are independent and can feed themselves. They become mature and ready to breed at four or five years old.
By eating leaves, these primates help keep the forest healthy. They clear foliage, break branches, and, in essence, act as gardeners. Indeed, pruning lets sunlight reach the lower levels of the forest and promotes new plant growth. The seeds they spread through their feces also allow various plants to travel and thrive throughout the forest.
The International Union for Conservation of Nature lists the Javan fuscous leaf monkey as Vulnerable (IUCN, 2022), appearing on the IUCN Red List of Threatened Species.
As of 2022, the population was estimated at between 5,000 and 17,000 individuals. Deforestation, habitat fragmentation, and illegal hunting are impacting the species, and the population is expected to decline by 30% in the next 30 years. The status of the species will very likely change in the coming years.
The human population is very dense, and over 55% of Indonesians live on Java island. Forest exploitation dates back to the eighteenth century when the Dutch started harvesting and exporting teak. In the first half of the nineteenth century, mountain forests were lush and inhabited by tigers, banteng, and rhinoceroses. In the second half of the nineteenth century, the lower slopes were cleared and converted to government coffee estates. Those estates were returned to the Forest Service around 1925, when coffee plantations became less lucrative. Since the 1970s, the government has made it a priority to plant more trees, however, original forests are not restored but replaced with single tree species, and invasive plants have made their way into mangroves. Ecosystems are changing and disrupting the natural habitats of animals that are native to the region.
Mangrove forests of Central Java are also deteriorated due to plastic waste that covers roots and sediment layers, thereby depriving trees of oxygen and killing fish, mollusks, crabs, and other creatures native to that ecosystem.
The deterioration of the upper canopy, which creates gaps between forest patches, is also an important factor to consider when thinking about the survival of species, as it prevents dispersal. Where Javan fuscous leaf monkeys cannot cross over from one patch of forest to another, genetic diversity will suffer, and food may not be abundant enough to sustain a large population.
Climate change is another threat, with more frequent, drastic downpours that displace both humans and non-human inhabitants.
Finally, these monkeys may also be hunted for food or captured for the domestic pet trade.
The Javan fuscous leaf monkey is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Although the species is not granted any specific protection, most of the forests it inhabits are either managed by the state forestry department or fall under watershed protection.
Indeed, habitat protection is urgently needed, especially since leaf monkeys or langurs in the Presbytis genus are difficult to keep in captivity and very often captive animals are the lifeline of conservation as they offer an opportunity to build up a viable gene pool and the possibility of releasing offspring to repopulate endangered species in the wild. More studies are also needed to confirm the taxon identity and the geographical origin of these primates.
- IUCN Red List 2022 – Presbytis fredericae, Javan Fuscous Leaf Monkey
- World Agroforestry Center – Java; Less Forest, More Trees Outside Forest, Persistent Poverty
- Forest ecology on Java: human impact and vegetation of montane forest – Alfred C. Smiet
- Java’s mangroves pay a high price for stopping plastic flowing out to sea – Julia John
- Conservation and ecology of Javan fuscous leaf monkey, in Mt. Slamet, Central Java Indonesia – Arif Setiawan
- A faunal survey of the Dieng Mountains, Central Java, Indonesia: distribution and conservation of endemic primate taxa – Vincent Norman and S. (Bas) Van Balen
- Mitochondrial phylogeny of leaf monkeys (genus Presbytis, Eschscholtz, 1821) with implications for taxonomy and conservation – Dirk Meyer, Ir. Dones Rinaldi, Hatta Ramlee, Dyah Perwitasari-Farajallah. Keith Hodges, Christian Roos
- Overstory community analysis of the habitat of fuscous Javan Langur in Mount Merbabu with implication for the management plan for the Langur’s conservation strategy – Alanindra Saputra, Nurmiyati, Umroh Fudolla, et al.
- Forest (and) primates: Conservation and ecology of the endemic primates of Java and Borneo – Vincent Norman
Written by Sylvie Abrams, April 2025