Geographic Distribution and Habitat
The Javan surili, also known as the grizzled leaf monkey, Java leaf monkey, and Javan grizzled langur, is a small endangered primate endemic to the island of Java, Indonesia. It inhabits the western and central regions of the island within fragmented pockets as far east as Mt. Lawu on the border with East Java. The Javan surili’s habitat overlaps with the Sundaland Biodiversity Hotspot, which is a species-rich region of Indonesia that is mostly dominated by Borneo and Sumatra. These large islands are well-known for their endangered species like the orangutan and the Sumatran rhino.
Javan surilis were historically found from sea level to 8,415 ft (2,565 m), but recently they have been restricted to fragmented mountain habitats. In some fragments, they can be found in primary and secondary forests, in lowland forests and on steep slopes, and in upper montane forests. They prefer middle and upper layers of the forest canopy and in some regions, they endure anywhere from 196 to 264 in (5,000–6,714 mm) of rainfall a year.
TAXONOMY IN TRANSITION
There is some debate about the taxonomy of the Javan surili. Some experts believe that the Javan surili is likely conspecific with the Javan fuscous langur (Presbytis comata fredericae), which has also been considered either a subspecies or a separate species. Other studies suggest that the Javan fuscous langur and the Javan surili are two distinct species. Those who believe the Javan surili is conspecific with its alleged subspecies suggest that there are representations of morphological gradation across the Javan surili’s range from gray to black, but that does not necessarily mean each morphological type indicates a distinct species.
Size, Weight, and Lifespan
The Javan surili weighs 13–18 lbs (6–8 kg). There is little else known or recorded about its size and lifespan.
The Javan surili’s pelage color ranges from whitish in some populations to dark gray or almost black in others. Their ventral fur—on the chest and abdominal region—is bright white from their chin to their long tails and the “grizzled” part of their name refers to the mix of colors in their thick, fluffy pelage. Like other langurs, their heads are relatively small compared to their body size. The Javan surili has long appendages, a small, pinched face with dark, round eyes, and little protruding ears. They are agile leapers with grasping fingers and a disinclination to leave the forest canopy.
The Javan surili consumes fruits, flowers, seeds, young leaves, and fungi. It is more folivorous than any other member of the genus Presbytis, as a study in Gede-Pangrango National Park recorded; the Javan surili has a diet of 63% young leaves, 17% flowers, 6% mature leaves, 6% fruits, 2% seeds, and 4% unidentified.
Their preferred leaves are ficus (Ficus pubinervis), grenadia (Passiflora ligularis), millaa millaa vine (Elaeagnus triflora), and a shrub known in the Javanese language as ki puyu (Schefflera aromatica). One of their preferred fruits comes from the sarangan (Castanopsis argentea).
The Javan surili, like all leaf monkeys, has a large, multi-chambered stomach with cellulose-digesting abilities that enables them to consume and digest plant fibers.
Behavior and Lifestyle
The Javan surili is arboreal and diurnal. From a study in Gede-Pangrango National Park, we know that their activity budget is 51% resting, 23% foraging and feeding, 21% travelling, and 5% engaging in social and other activities.
Further studies are required to learn more about the behavior and lifestyle of the Javan surili.
Daily Life and Group Dynamics
The Javan surili engages in three to four feeding and foraging bouts throughout the day. They travel among the upper canopy and they retire to high ridges and trees to sleep at night.
Groups consist of 2 to 14 individuals, with only one to two adult males. The Javan surili has been known to be sympatric with the East Javan langur (Trachypithecus auratus) and the West Javan ebony langur (Trachypithecus mauritius).
What Does It Mean?
A way to quantify animal behavior by observing an animal over an extended period and documenting activity. An activity budget demonstrates how much time an animal spends in various activities such as eating, resting, sleeping, and moving.
Physically adapted to living primarily or exclusively in trees.
A biogeographic region with significant levels of biodiversity that is under threat from humans.
Molecule found in plant matter that gives structure and strength to the cell walls of plants and provides dietary fiber.
Belonging to the same species.
The permanent cutting, clearing, and removal of trees to convert forest land for other use, such as pasture, cropland, or plantations.
Active during daylight hours.
Native or restricted to a certain area or country.
Having a diet that consists of leaves.
Having only one sexual partner.
The fur, hair, or wool of a mammal.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
Occurring or living in the same area; overlapping in distribution.
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There is very little known about the communication habits of the Javan surili, though it is likely they engage in allogrooming like the West Javan ebony langur. It is also rather likely that they have an audible alarm call, among other vocalizations.
Reproduction and Family
There is very little known about the reproductive habits of the Javan surili, though some groups have been reported as being monogamous. It is also possible that females reach sexual maturity around three to four years of age like the West Javan ebony langur and give birth once a year to one offspring at a time.
It is likely that the Javan surili acts as a seed disperser throughout their habitat due to their consumption of fruit.
Conservation Status and Threats
The Javan surili is listed as Endangered (EN) by the International Union for Conservation of Nature’s Red List of Threatened Species (IUCN, 2015). The population trend is declining due to extreme hunting, deforestation, and fragmentation; the population size is estimated to be lower than 1,400–1,500 mature individuals, with no more than 250 individuals per subpopulation. There has been some disagreement about the accuracy of the estimated population size, and the last survey was between 2006 and 2008 with an estimation of less than 6,000 individuals.
Threats to the Javan surili include hunting and habitat destruction and fragmentation due to illegal logging, tree felling (to obtain produce like orchids and pitcher plants), and firewood collection. In some areas, they are also persecuted for crop raiding, though some locals tolerate them as a nuisance and combat the problem by wrapping their bananas with plastic bags while still on the tree. Only 16.39% of original forest remains in Java due to their history of land conversion. Forest fragments, especially those in montane areas, consist of approximately 10% of the island.
The Javan surili is listed on the Convention on International Trade in Endangered Species (CITES) Appendix II and it occurs in many protected areas, especially the remaining populations in western Java that inhabit nature preserves like Gunung Tukung Gede Nature Reserve and Gunung Slamet Nature Reserve, and parks like Ujung Kulon National Park, Halimum National Park, and Gede-Pangrango National Park. It has been identified as a conservation priority by the Government of the Republic of Indonesia. The Javan surili does appear to tolerate road traffic and human activities quite well.
A recent study suggests that a balance needs to be met between the proportion of commercial tree species and food tree species in order to support a healthy population trend for the Javan surili and ensure the economic health of the local timber companies. Additionally, there need to be further studies about the distribution of the Javan surili.
Written by Rachel Heim, April 2021