East Sumatran Banded Langur, Presbytis percura
EAST SUMATRAN BANDED LANGUR
Presbytis percura
Geographic Distribution and Habitat
East Sumatran banded langurs (or surilis) are only found in the Riau province of east Sumatra. The Rokan River marks the northern limit for the species. The southern limit has not been determined. They typically reside in mangrove forests and along riverbanks, however, with the high level of agricultural development in the Riau province, many langur groups live near plantations and urban areas. Because of the high rate of deforestation in the region, which is causing severe forest fragmentation, many groups of East Sumatran banded langurs are isolated from each other.
Originally described as a unique species in the early 1900s, the East Sumatran banded langur, along with the Robinson’s banded langur, was considered a subspecies of Raffles’ banded langur. In 2020, genetic analysis confirmed that the original description of the East Sumatran banded langur as a distinct species was correct. While only recently being recognized as a separate species has limited the amount of research available on the East Sumatran banded langur, because the three species do not overlap in range, we can recognize previous studies of Raffles’ banded langur as actually being the East Sumatran banded langur based on location.
Size, Weight, and Lifespan
East Sumatran banded langurs are slender, medium-sized monkeys, with a head-body length of 18.11-20.28 inches (46–51.38 cm) and a weight range of about 12-16 pounds (5.44 kg). Like many tree-dwelling monkeys, their tails are longer than their body, ranging from 26-31 inches (65–78.51 cm), to help them balance on slender tree branches. They are not known to be sexually dimorphic, meaning males and females fall within the same size range.
Their specific lifespan is unclear but is likely similar to other langurs that typically live 15-18 years in the wild.
Appearance
Like other langur species, the East Sumatran banded langur has a long lanky build that’s perfect for life in the trees. Their appearance is very similar to Robinson’s and Raffles’ banded langurs, lending to the confusion in their taxonomy, but there are a few characteristics that can help differentiate East Sumatran banded langurs from the other banded langurs. Like the other banded langurs, their fur is very dark in color with white fur on the inner thighs that, like Raffles’ banded langur, spreads onto the belly, chest, and under the chin. Their hair comes to a point on the top of their heads, and they have two white markings framing their eyes, almost like a set of parentheses. Unlike the other two banded langurs, East Sumatran banded langurs also have a short stripe of pale fur on the underside of their tails, although this is sometimes also present in Robinson’s banded langur.
Diet
East Sumatran banded langurs are frugivore-folivores, meaning they like to eat fruits and leaves. They aren’t picky eaters. They have been recorded eating 136 different plant species. Some groups have also started feeding on the leaves and seeds of the rubber trees farmed in their region, which is not a native plant to Sumatra.
Leaves are known for being difficult for animals to digest, so herbivores have a variety of adaptations that allow them to get the most out of their diet. One adaptation langurs have developed is a multi-chambered stomach that allows them to ferment the large amount of leaves they eat with the help of some friendly bacteria and specialized stomach acids. This process is referred to as “ruminant-like” by scientists, as it is very similar to the digestive process of ruminant animals like cows. These similarities between langur and cow digestion are an example of convergent evolution, where two animals evolve similar adaptations independently of each other.
Behavior and Lifestyle
The East Sumatran banded langur is arboreal, and spends most of their time in the trees. They are experts at leaping and climbing between the trees and will walk quadrupedally (on all fours) along branches and across the ground when necessary. They are most active during the day and sleep during the night high up in the trees. While studies on the species are limited, a previous study recorded the home range of the species as 0.88 square miles (23 ha), and on average a group will have a daily range of 0.58 miles (936m). With the recent decline in forest habitat available to East Sumatran banded langurs, more recent studies are needed to determine if these numbers have changed.
The average group size for East Sumatran banded langurs is about 11 individuals, with typically only one adult male present. Occasionally, the adult male will tolerate the presence of a second adult male. When this happens, tension will typically increase between the two males over time until the group splits into two, with the females deciding which male to follow. Other than feeding together and reproductive behaviors, males are not often observed socializing with the females in their group and grooming mostly takes place between females. Once mature, males will leave the group they were born in and find their own group to join. Female immigration to a new group has also been observed in this species, but this is based on only a single instance.
To communicate with each other while scattered amongst the trees, East Sumatran banded langurs mostly rely on vocalizations. They have a variety of alarm calls to let other group members know when they see a potential threat, and where that threat is located: for example, the alarm call for a threat from the sky (such as an eagle) is different from the alarm call for a threat from the ground (like a tiger). They also vocalize when socializing, and respond to each other with a “kok kok kok” sound (described as similar to a clucking hen) during affiliative interactions, or to show submission. When trying to locate another troop member, such as an infant searching for their mother, they make a nasally snorting sound.
When near each other, body language or facial expressions can also be used to communicate. Submissive individuals will avert their eyes from dominant individuals or crouch when they are near. During aggressive interactions, individuals will stare at each other and even lunge or bite at the air as a threat or to show dominance.
As a newly defined species, many of the specifics on the reproductive biology of East Sumatran banded langurs are still unclear. They likely have a similar gestation length to other langurs, which is around 6-7 months. The few observations of East Sumatran banded langur groups suggest females will typically initiate sexual encounters by crouching in front of the male.
Infants will typically ween off of their mother’s milk by 9-10 months of age. Alloparenting has also been observed in this species, where females besides the baby’s mother will help care for the baby. Researchers think that this is why some langur species, including East Sumatran banded langurs, have infants that are fully white when first born. The bright white color helps attract other group members’ attention, and protection, to the infant.
The relationship between East Sumatran banded langurs and their habitat is another area of their biology lacking in research, but we can find some hints from what we do know about them. Based on their diet, they are likely seed-dispersers for the trees they consume fruits and seeds from and contribute to maintaining the overall biodiversity of the forests they are found in. As predominant leaf-eaters, East Sumatran banded langurs may also act as ecosystem engineers for their environment. “Ecosystem engineer” is a term scientists use for animals that modify their habitat in some way that has an impact on the overall structure of the ecosystem, for example how beavers will modify waterways by building dams. Researchers have speculated that folivorous primates, such as langurs, may impact the growth of trees in their ranges, either by controlling the growth of certain species or by stimulating some species to generate more leaves in response to herbivory. While the specifics of the East Sumatran banded langur’s role in their ecosystem aren’t fully known, the consequences of losing them should not be taken for granted.
The East Sumatran banded langur is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.
As an understudied species, the exact population size is unknown, however, the East Sumatran banded langur is believed to have experienced significant population decline over the last several years, and this trajectory is expected to continue. The species’ habitat is severely fragmented due to agricultural development, primarily for oil palm, paper pulp, and rubber. The Riau provenance, where the East Sumatran banded langur is only found, has experienced the highest rate of deforestation in Sumatra. Researchers found that between the years 2000 – 2012, 82% of their forest habitat was lost. Forests are often burned in order to be cleared for farmland, and with the hotter and dryer conditions resulting from climate change, fires often spread farther than intended. Due to their forced proximity to farmland and the destruction of wild trees that they rely on for food, East Sumatran banded langurs have been known to crop raid, making them a target for retaliation from farmers who see them as pests. They may also be targeted in the pet trade, however, the data is lacking in this regard.
The East Sumatran banded langur 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. While facing extreme deforestation, the species is found within the protected Giam Siak Kecil-Bukit Batu Biosphere Reserve, providing them some amount of protection.
While specific conservation efforts are still being established for the East Sumatran banded langur, the recognition of them as a separate species from Raffles’ banded langur is already a huge step forward. Genetics plays an essential role in conservation in many ways, including identifying separate species from others with similar morphology. This allows conservationists to recognize what efforts are needed to conserve biodiversity and focus those efforts on the species most in need. Scientists are currently calling for further research on the species to establish effective conservation strategies.
- Ang, A., Roesma, D. I., Nijman, V., Meier, R., Srivathsan, A., & Rizaldi. (2020). Faecal DNA to the rescue: Shotgun sequencing of non-invasive samples reveals two subspecies of Southeast Asian primates to be Critically Endangered species. Scientific Reports, 10(1), 9396.
- Ang, A., & Jabbar, S. (2021). Raffles’ Banded Langur: The Elusive Monkey of Singapore and Malaysia. World Scientific.
- Bauchop, T., & Martucci, R. W. (1968). Ruminant-like digestion of the langur monkey. Science, 161(3842), 698-700.
- Chapman, C. A., Bonnell, T. R., Gogarten, J. F., Lambert, J. E., Omeja, P. A., Twinomugisha, D., … & Rothman, J. M. (2013). Are primates ecosystem engineers?. International Journal of Primatology, 34, 1-14.
- Hrdy, S. B. (1976). Care and exploitation of nonhuman primate infants by conspecifics other than the mother. In Advances in the Study of Behavior (Vol. 6, pp. 101-158). Academic Press.
- Najmuddin, M. F., Haris, H., Norazlimi, N., Ruslin, F., Matsuda, I., Md-Zain, B. M., & Abdul-Latiff, M. A. B. (2021). Dietary habits of free-ranging banded langur (Presbytis femoralis) in a secondary-human modified forest in Johor, Malaysia. Zoological Studies, 60.
- Kardos, M. (2021). Conservation genetics. Current Biology, 31(19), R1185-R1190.
- Kirkpatrik, R. C. (2011). The Asian Colobines Diversity Among Leaf-Eating Monkeys. In Primates in Perspective (second, pp. 189–202). Oxford University.
- Lyon Jr, M. W. (1908). Mammals collected in eastern Sumatra by Dr. WL Abbott during 1903, 1906, and 1907, with descriptions of new species and subspecies. Proceedings of the United States National Museum.
- Megantara, E. N. (1989). Ecology, behavior and sociality of Presbytis femoralis in Eastcentral Sumatra. Comparative primatology monographs, 2, 171-301.
- Rizaldi, Ilham, K., Prasetio, I., Lee, Z. H., Jabbar, S., & Ang, A. (2019). PRELIMINARY STUDY ON THE DISTRIBUTION AND CONSERVATION STATUS OF THE EAST SUMATRAN BANDED LANGUR Presbytis femoralis percura IN RIAU PROVINCE, SUMATRA, INDONESIA. Asian Primates Journal, 8(1).
- Supriatna, J., Dwiyahreni, A. A., Winarni, N., Mariati, S., & Margules, C. (2017). Deforestation of primate habitat on Sumatra and adjacent islands, Indonesia. Primate Conservation, 31(71-82).
- https://www.iucnredlist.org/species/39805/205841390
- https://cites.org/eng/app/appendices.php
Written by Lina Rademacher, July 2024