Siberut Langur, Presbytis siberu
SIBERUT LANGUR
Presbytis siberu
Geographic Distribution and Habitat
The Siberut langur (Presbytis siberu), also called the sombre-bellied Mentawai Island langur, is endemic to the island of Siberut, the northernmost of the Mentawai Islands off the coast of Sumatra in Indonesia. Being endemic means that they are only found on that single island, and are not found naturally anywhere else in the world. They inhabit the wet rainforests of the island, in primary and secondary forest. Primary forests are those that have been largely undisturbed by humans, and the ecosystems may be, quite literally, millions of years old. Secondary forests are much younger, as they have been disturbed by humans before growing back. They often do not offer the same habitat value as older forests. Occasionally, Siberut langurs may find themselves living among mangroves and swamp forests and sometimes wander into the gardens of the local people.
Siberut langurs were formerly considered to be subspecies of Mentawai langurs (P. potenziani), but were elevated to full species status after genetic studies found them to be sufficiently distinct from Mentawai langurs. While the new classification was first proposed in 2011, it was only widely adopted in recent years, and many sources still list Siberut langurs as P. potenziani siberu. Because of this relatively recent taxonomic change, research on the Siberut langur as a species is, unfortunately, severely lacking.
Size, Weight, and Lifespan
Siberut langurs are about 19–20 inches (48–50 cm) in length from nose to rump, with their tail adding another 23–25 inches (58–64 cm). Females may be slightly smaller than males, ranging in weight from 9–13 pounds (4–6 kg), compared to the males’ 11–13 pounds (5–6 kg). There is little information available about the monkeys’ lifespan, but it is likely around 20 years, based on related species.
Appearance
Dark gray to black over most of their body, the most prominent physical feature of the Siberut langur is the mane of white hair that frames their face. This sometimes extends down to their shoulders as well, and they also have a patch of white hair in their pubic region. Their hair color changes to a reddish-brown on their abdomen and inner thighs. They have what is called ischial callosities on their rumps, which are hard calloused pads on their buttocks. The skin on their face is bare and black, though it lightens towards their mouths. While they look very similar to Mentawai langurs, they are slightly darker on their underparts and their white pubic patch is more distinct. Sexual dimorphism—i.e., differences in the size or appearance between the sexes in addition to differences in their reproductive organs—among Siberut langurs is very subtle. In addition to females being, on average, slightly smaller, males have ischial callosities that are connected, while in females, the calluses are separated.
Diet
The diet of Siberut langurs consists mostly of fruit supplemented with leaves. They may eat the occasional flower or other food item as well. Their diet is unusually fruit-heavy for monkeys in the Colobine subfamily, who are often referred to collectively as leaf-eating monkeys.
Behavior and Lifestyle
Siberut langurs are diurnal (they are awake during the day) and arboreal, meaning they live in trees. They use their long arms and legs to move about quadrupedally—on all fours. They spend most of their active time in the mid-canopy and they sleep in the upper canopy. At night, they each sleep in specific trees, which are usually tall and towards the center of their home range. They wake up very early, earlier than most of the other primate species that they share habitat with. This may be to decrease competition for food among the species. They spend most of their day resting and feeding, and a small amount of it traveling and foraging. They spend relatively little time socializing.
Although they spend very little time on social behaviors such as grooming one another, especially as compared to other primates, Siberut langurs like to stay close to each other. They are usually within 100 feet (30 m) of other group members while eating, and within 30 feet (10 m) while sleeping.
It was previously thought that Siberut langurs always live in one-male one-female family groups, but groups composed of a single male with up to three females have also been observed. Their home ranges are typically between 37 to 74 acres (15 to 30 hectares) in size, and may overlap with the ranges of other groups. Intrusion into other groups’ territories usually results in aggressive behaviors like loud vocalizations and threatening displays. Aggression within groups is infrequent. Females and males co-lead groups, with equal responsibility shared among the sexes for responsibilities like leading movement.
Vocal calls include alarm calls, duetting, and contact calls. Males and females are known to duet together, and they give loud alarm calls to warn others of dangers, such as pythons or humans. Males let out loud calls very early in the morning, often between 3 am to 5 am. Siberut langurs also communicate visually, through body language, but the specifics are not well documented.
Very little is known about the reproductive lives of Siberut langurs. Based on their group compositions, it is likely that they are either monogamous or polygynous, meaning that each male mates with multiple females. Based on related species, the gestation time of pregnancy is likely between five to seven months. Females give birth to a single offspring every other year between July and August. Babies are born with white hair and pale skin, and develop their adult coloring by about 3 to 4 months of age. It is unclear how much of a role each parent plays in rearing offspring, although the mother naturally nurses her baby, likely for about a year. It is unknown whether, upon maturity, both genders of offspring disperse from the group they were born into to become members of unrelated groups.
Siberut langurs live alongside the Siberut macaque (Macaca siberu), the pig-tailed langur (Simias concolor), and Kloss’s gibbon (Hylobates klossii). As frugivores, they likely play a role in seed dispersal. While specific information is not well known about their predators, it is likely that they are preyed upon by animals such as birds of prey and predatory mammals.
The International Union for Conservation of Nature lists the Siberut langur as Endangered (IUCN, 2015) appearing on the IUCN’s Red List of Threatened Species. This assessment is based on a historic population decline of more than 50% over the past 40 years. Unfortunately, this population decline is expected to continue without immediate and significant conservation action. In the 1980s, the Siberut langur population was estimated to be about 46,000 individuals. Studies throughout the 2000s and 2010s have estimated the population to be somewhere between 11,000 and 27,000 individuals within Siberut National Park, which covers 40% of the island and contains most of the suitable habitat for the species. There is an urgent need for population surveys that cover the unprotected areas of the island, so we can get a better idea of the total population size—and decline—of the species.
Their decline is largely due to habitat loss and hunting. Forest clearing is usually a result of logging and clearing for oil palm plantations. Siberut langurs are hunted heavily and the rate of hunting has been increasing in recent years. Oftentimes when an area opens up to logging, hunting increases in that area as well since it is then more accessible to people. Hunting has also increased due to the adoption of air rifles replacing bows and arrows, and because of the gradual loss of local taboos against hunting monkeys. This is thought to be due to missionary and government influences on the local people. One study estimated that at least 17% of the Siberut langur population is hunted annually. Interviews with local people on Siberut found that 24% of respondents hunted actively and of those, more than two-thirds target Siberut langurs. Each one hunted an average of 9.2 langurs each year.
Siberut langurs are listed under their former taxonomic name, P. potenziani siberu, in Appendix I 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. They are also protected under that name by Indonesian law.
Siberut langurs are protected by just one protected area, Siberut National Park, although this area covers a sizable portion of the island. Siberut langurs also live in relatively high densities in Peleonan Forest in the northern area of the island. There is no formal protected area here, but there are loose local agreements to avoid logging this area. Conservation actions that researchers have suggested include increasing the enforcement of protection for Siberut National Park, establishing a protected area in the Peleonan Forest, promoting conservation education around hunting, developing economic alternatives to decrease the likelihood of local people selling their land to loggers, improving agricultural practices on the island to decrease the reliance on bushmeat for food, and engagement with the local communities on conservation projects.
- Hadi, S., Ziegler, T., Waltert, M. et al. 2012. Habitat Use and Trophic Niche Overlap of Two Sympatric Colobines, Presbytis potenziani and Simias concolor, on Siberut Island, Indonesia. Int J Primatol 33, 218–232. doi:10.1007/s10764-011-9567-y
- Meyer, D., et al. 2011. Mitochondrial phylogeny of leaf monkeys (genus Presbytis, Eschscholtz, 1821) with implications for taxonomy and conservation. Molecular Phylogenetics and Evolution 59(2), 311-19. doi:10.1016/j.ympev.2011.02.015
- Quinten, M., et al. 2014. Knowledge, attitudes and practices of local people on Siberut Island (West-Sumatra, Indonesia) towards primate hunting and conservation. Journal of Threatened Taxa 6(11). doi:10.11609/jott.2785.8.7.8953-8969
- Quinten, M., Nopiansyah, F., & Hodges, J. 2016. First estimates of primate density and abundance in Siberut National Park, Mentawai Islands, Indonesia. Oryx 50(2), 364-367. doi:10.1017/S0030605314001185
- Waltert, M., Abegg, C., Ziegler, T., Hadi, S., Priata, D., & Hodges, K. 2008. Abundance and community structure of Mentawai primates in the Peleonan forest, north Siberut, Indonesia. Oryx 42(3), 375-379. doi:10.1017/S0030605308000793
- https://www.iucnredlist.org/species/39816/17986963
- https://www.gbif.org/species/7976011
- https://animaldiversity.org/accounts/Presbytis_melalophos/
Written by K. Clare Quinlan, August 2023