NATUNA ISLAND SURILI

Presbytis natunae

Geographic Distribution and Habitat

The Natuna Island surili, or Natuna leaf monkey, lives on the Bunguran Island of the Natuna archipelago, some 140 miles (225 km) north of Borneo in South East Asia. The whole island is about 656 square miles (1,700 square km), which is about half the size of Rhode Island, the smallest state in the US. Limited distributions, like with the Natuna Island surili, happen when resources are few and competition is large—the surili had to develop adaptations to a specific niche that no other primate can occupy. On Bunguran Island, there are multiple tree-dwelling mammals that compete with the surili for space and food, including two other primate species: the Sunda slow loris (Nycticebus coucang) and the long-tailed macaque (Macaca fascicularis).

The Natuna Island surili occurs in varied forest types (natural, plantations, disturbed habitats) but thrives in the mountain forests native to the Natuna Islands. Logging activities have converted the natural habitat to secondary forests and rubber plantations, and it is unclear how the Natuna Island surili population is affected by these changes. Interestingly, we see fewer Natuna Island surilis as we travel up the mountain, and so for the survival of the species, it is important to have more forested areas in lowland regions, regardless of the type of forest (natural or otherwise).

TAXONOMY IN TRANSITION

Initially, in 1884, the Natuna Island surili was classified as Semnopithecus natunae because the surili outwardly resembled langurs found on the Indian subcontinent. As the entire primate group underwent reclassification, taxonomists put the Natuna Island surili into a more broad group of langurs called Presbytis. For many years, researchers considered the Natuna Island surili as a subspecies of the white-thighed surili (P. siamensis) because of how similar they looked. It took multiple genetic studies and redefinitions of classification names to conclude that the Natuna Island surili was distinct from other langurs and probably became a distinct species about 1 million years ago. As of 2001, researchers classify the Natuna Island surili as genus Presbytis and differentiated from langurs because they are almost completely arboreal and are not native to the Indian subcontinent.

Taxonomy and our understanding of how primates fit in an evolutionary tree are constantly being revised as we learn more through genetic studies. The process of classification is subject to intense debate and requires consensus among leading researchers.

Natuna Island surili range, IUCN 2020

Size, Weight, and Lifespan

Getting accurate data on body measurements of the rare Natuna Island surili is difficult because their population is so small that we cannot ethically allow trapping or collection. Therefore, we rely on collector records from the 1900s, when explorers and naturalists could trap or hunt these primates. From historical records of famous explorer and naturalist W.L. Abbott, we estimated their average total body length to be 42.5 inches (108 cm) and their average weight to be 11.2 pounds (5.2 kg).

Because of the small size of the Natuna Island surili, researchers think the species is a “probable dwarf.” In primatology, “dwarfing” means that a species (or group) evolved to have smaller-sized adults (especially smaller skulls) when compared to their evolutionary ancestral species.

Studies of wild Natuna Island surilis are rare, but anecdotes from people who have kept them as pets suggest low survival rates in captivity, particularly in juveniles that die less than a year into captivity. The juvenile stage is a dangerous time in any monkey’s life, when risky behaviors may lead to death. The stress of being caught by humans at that young age and being given non-native food could cause premature death. We do not have enough wild or captive studies of Natuna Island surlis to know what their typical lifespans are, but other better-studied species like Thomas’ leaf monkey (P. thomasi) can live up to 20 years—a reasonable assumption for Natuna Island surilis as well.

Appearance

The Natuna Island surili is a delicate-looking monkey with large eyes set in a small head, which is surrounded by dark fur that peaks to a tuft on the head; this makes them look similar to closely related tree-dwelling langurs and lutungs. Bushy white fur covers the cheek, chin, and front of the surili, which contrasts with the black fur on the back. This contrast creates a shade and light effect that helps the surili blend with the sunlight-dappled treetops.

The Natuna Island surili has long arms and legs to grab branches, and a long tail for balance as they navigate through the canopy of the forest. Male and female Natuna Island surilis do not look different from each other; there is no sexual dimorphism in the group.

What Does It Mean?

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Disturbed Habitat:
A temporary change in environmental conditions that causes a pronounced change in an ecosystem.

Endemic:
Native or restricted to a certain area or country.

Inbreeding:

The breeding of closely related individuals, especially over many generations.

Inbreeding depression:
A reduction in the overall health and reproductive fitness of a population due to inbreeding.

Prehensile:
Able to grasp or hold objects.

Secondary forest:
A forest that has regrown after a major disturbance, such as fire or timber harvest, but has not yet reached the mature state of primary forest.

Sexual dimorphism:
Distinct difference in size or appearance between the sexes of an animal in addition to difference between the reproductive organs themselves.

Visit the Glossary for more definitions

Photo credit: markindo/Creative Commons

Diet

Like other leaf monkeys, Natuna Island surilis are foliovores, so they specialize in eating and digesting leaves, which are low in nutrients when compared to seeds and fruits.

Next time you eat a leaf, such as kale, pay attention to how much you have to chew it. It is a real workout for your teeth! Leaves have a lot of fiber and that is because a plant’s cell wall is made of a stiff material called cellulose. This cellulose helps plant cells keep their shape, helps to keep plants cells from losing too much water, and protects the plants from damage. All this means that cellulose is hard to digest. Surilis need a specialized stomach and helpful bacteria to break down the cellulose and get to the good stuff (sugars) that is stored inside the plant cells. You may have heard that cows have many stomach chambers to help them digest grass. Surilis also have a chambered or sacculated stomach. The fiber-rich food (leaves) are fermented (broken-down/digested) by bacteria in these chambers. Digesting plant material this way is a slow process, and the surilis need to eat a lot of leaves to get enough nutrition to survive. One side effect of eating so many leaves and fermentation digestion is the formation of gases, which can cause surili stomachs to look bloated.

Surilis especially like eating young leaves that are more tender and have less cellulose. Like most animals, the diet of the Natuna Island surili most likely changes depending on the season and availability of nutrient-rich food. So, they could feed on seeds and fruits; however, there are not enough field studies to give us a good idea of their complete feeding habits in the wild.

Behavior and Lifestyle

Details of the Natuna Island surili’s behavior and lifestyle have not been documented. There are three main reasons for this gap in information. One, the Natuna Island surili population is sparse, so they are inherently difficult to find. Two, they live in dense forests, which makes watching them for long periods difficult. And three, human activity (such as deforestation and trapping animals) has made the surilis shy and they flee when they encounter humans, even if the humans are researchers that want to help the monkeys.

Daily Life and Group Dynamics

Natuna Island surilis live in family groups made up of one male and multiple females. Group size is usually between 2 and 4 individuals, which is smaller than other members of the Presbystis genus. Larger group sizes means there are more individuals to look out for predators. Large group sizes also mean that individuals will have to share resources like food and space, which may not be abundant. One theory to explain the small group sizes of Natuna Island surilis is that the Bunguran region does not have feline predators like the clouded leopard that are excellent at hunting in tree canopies.

You know how eating candy or something sugary can give you a burst of energy? Well, imagine if all you could digest were leaves, which are low in nutrients. There is no sugar-rush for the leaf-eating surilis and they have to spend most of their day foraging for as many leaves as they can eat. Their digestive system ferments the leaves, which is a slow and energy-costly process. Surilis have to balance their time between eating and conserving their energy to deal with dangerous situations like fighting for dominance or escaping threats. So in the wild, surilis spend most of their mornings  stuffing their faces with leaves, and then later in the afternoon they lazily chew leaves while their bulbous stomachs digest their food. In tropical countries like Indonesia, the midday can get extremely hot, so animals lose energy through sweating or moving about in the heat. To conserve energy, Natuna Island surilis are most active in the early mornings (5–11 am), when it is cooler.

Male and females forage together and, whenever you live in a group, there is bound to be some argument about who ate the best leaves or crossed into someone else’s space. Females spend most of their time feeding and taking care of their young offspring. The resident male has the additional job of guarding the troop and so he is on the lookout for intruders that may want to displace his position as the dominant male. 

In one-male, multi-female troops, the resident dominant male has exclusive reproductive access to the adult females in his troop. This form of territoriality ensures that the resident male will successfully pass on his genes to the offspring. He will defend the females in his troop from other males and, if he loses, the new male will father the next generation of offspring. This system introduces new genetic material into the population, which makes for a stronger population. We do not know how often resident male Natuna Island surilis gets replaced by a new dominant male, but in more thoroughly studied langur species (with similar social structure and diet as surili) the resident males hold on to his position for about 27 months.

Fun Facts

Natuna Island surilis are considered a “probable dwarf” species because of their small size.

They are endemic to the small Bunguran region of the Natuna Island.

Natuna Island surilis prefer lower altitude forests.

Communication

Natuna Island surilis are chatter-boxes! The lack of predators and human hunters mean these monkeys need not be silent to avoid danger. Instead, they forage for food and stay in contact with fellow family members by making constant chattering noises, which is typical for surilis. In forests—where you cannot see long distances because of all the trees—sound becomes an important method of communicating, because you do not have to see your family member to tell them something. Natuna Island surilis are most vocal during the early hours of the day, between 4 and 10 am.

When the Natuna Island surili sees predators or threats, they make alarm calls. When the surili flees, he or she vocalizes and moves loudly through the canopy. The echoing sounds and shaking leaves serve as a distraction. A predator only knows the rough whereabouts of the potential prey but cannot pinpoint the exact location; those extra seconds of confusion are usually all it takes for the monkey to escape.

Reproduction and Family

We know little about the specifics of Natuna Island surili reproduction or family dynamics. However, generally in a one-male/multi-female society—like the Natuna Island surili—the same male will father almost all the offspring. Females in the group may be close relations (mothers, daughters, aunts), and the theory is that male offspring will leave the family (or natal troop) when he matures. Other primate species use this strategy to maintain a healthy population because it limits inbreeding between closely related males and females and prevents inbreeding depression. Researchers have also suggested that female offspring could also leave the natal troop if there are too many closely related females (mothers and daughters), which could also cause inbreeding. This would be important if a rival does not replace the resident dominant male, and there is a possibility of a male mating with an offspring.

Ecological Role

Natuna Island surilis can eat a lot of leaves and this puts some serious pressure on trees. In tropical forests, trees compete for sunlight, which is needed for photosynthesis (the process by which leaves make food for the tree). So taller trees can access more sunshine, make more food, and out-grow smaller trees. If this continues, no other plants under the canopy of leaves will grow. By eating leaves, surilis create gaps in the canopy where sunlight can reach the plant species below. We can argue that tropical forests would not be as diverse in plant life if monkeys did not limit the growth of tall trees. This complex interaction between the surili and its environment is a beautiful balance with all the other species on the island. The forest could not sustain itself without the Natuna Island surili, and vice versa.

As with other primates, when the Natuna Island surili eats fruits and seeds, it can help disperse seeds through the forest and assist with forest regeneration.

Conservation Status and Threats

The International Union for Conservation of Nature lists the Natuna Island surili as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.

We know very little of the direct threats or ecological factors that affect their survival. Researchers estimate that there are only two subpopulations of the Natuna Island surili left, with fewer than 10,000 individuals on the entire island. The population is decreasing primarily because of loss of habitat. Human developments are encroaching into forests and forests are being cut down and converted to plantations.

Though they are rarely hunted for food, people sometimes trap and capture Natuna Island surilis to keep as pets.

Conservation Efforts

The Natuna Island surili 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.

The listing of Natuna Island surilis in Appendix II shows that wildlife trade and trafficking is not a concern for the survival of the species. Therefore, wildlife trade for the Natuna Island surilis is not regulated at an international level. However, local government can put restrictions on capturing these monkeys.

The government has not yet established protected areas or national parks that will protect habitat, which is the best chance to conserve the Natuna Island surili. 

References:

  • Borries, C., Lu, A., Ossi‐Lupo, K., Larney, E., & Koenig, A.(2011). Primate life histories and dietary adaptations: a comparison of Asian colobines and macaques. American Journal of Physical Anthropology, 144(2), 286-299.
  • Fleagle, John G. (2013). Chapter 6 – Old World Monkeys. Primate adaptation and evolution. Academic press.
  • Frazier, B. C. (2011). The cranial morphology of dwarf primate species. PhD thesis, The Pennsylvania State University, State College, PA. https://etda.libraries.psu.edu/files/final_submissions/634. Accessed April 14, 2022.
  • Lambert, J. E. (2012) Primates in Communities: The Ecology of Competitive, Predatory, Parasitic, and Mutualistic Interactions Between Primates and Other Species. Nature Education Knowledge 3(10):85. https://www.nature.com/scitable/knowledge/library/primates-in-communities-the-ecology-of-competitive-59119961/. Accessed April 26, 2022.
  • Laurie R., Godfrey (2005). Chapter 3 – General Anatomy. The laboratory primate. Wolfe-Coote, S. (Eds.) Elsevier.29-45. https://www.sciencedirect.com/science/article/pii/B9780120802616500039.
  • Lawrence, J. M. & Cords, M. Old World Monkeys. Nature Education Knowledge 3(7):13. https://www.nature.com/scitable/knowledge/library/old-world-monkeys-83033815. Accessed April 14, 2022.
  • Mitani, J. C., Call, J., Kappeler, P. M., Palombit, R. A., & Silk, J. B. (Eds.). (2012). The evolution of primate societies. University of Chicago Press.
  • Nashimura, Abigail (preparer and uploader). Primate Measurements- Presbytis natunae. National Museum of Natural History.Division of Mammals, Department of Vertebrate Zoology, Smithsonion Institute. Accessed April 14, 2022. https://collections.nmnh.si.edu/search/mammals/.
  • Nijman V, Nekaris K, A. (2012). Loud Calls, Startle Beh (viour, Social Organisation and Predator Avoidance in Arboreal Langurs (Cercopithecidae: Presbytis). Folia Primatol 2012,83:274-287. doi: 10.1159/000339647.
  • Osterholz, M., Walter, L. & Roos, C. (2008). Phylogenetic position of the langur genera Semnopithecus and Trachypithecus among Asian colobines, and genus affiliations of their species groups. BMC Evolutionary Biology 8, 58. https://doi.org/10.1186/1471-2148-8-58.
  • Setiawan, A., Cheyne, S. & Traeholt, C. (2020). Presbytis natunae. The IUCN Red List of Threatened Species 2020: e.T136500A17955492. https://dx.doi.org/10.2305/IUCN.UK.2020-2.RLTS.T136500A17955492.en. Accessed April 14, 2022.
  • Shelmidine, N., Mcaloose, D., Mccann, C. (2013). Survival Patterns and Mortality in the North American Population of Silvered Leaf Monkeys (Trachypithecus cristatus). Zoo Biology 32, 177–188. doi:10.1002/zoo.21055.

Written by Acima Cherian, May 2022