Some people imagine primates as loving fruit, especially bananas. This image, which has become so ingrained in popular culture, assumes all primates have the same diet. The reality is far more complex. On the island of Java, there are several endemic primate groups that actually have different feeding strategies, such as the langur (Trachypithecus spp.) and surili (Presbytis comata).
These two protected primates are often considered fruit eaters. However, biologically, they are folivorous primates, or leaf eaters, with specialized digestive systems for processing high-fiber feed. Understanding this is crucial for meeting nutritional needs, which directly impact the animals' health and well-being. Therefore, understanding the biological characteristics of these primates is crucial for enhancing conservation efforts that focus not only on habitat protection but also on meeting physiological needs consistent with their evolutionary history.
Anatomy of the Compartmentalized Stomach
Taxonomically, langurs and surili are included in the family Cercopithecidae and subfamily ColobinaeColobines are known as primates that specialize in consuming leaves. Unlike other primate groups that consume a lot of fruit, this group evolved to utilize abundant food sources with relatively low nutritional quality, such as leaves (Hendrayana). et al. 2025).
This adaptation is evident in their eating behavior due to the anatomical and physiological structure of their digestive system, which differs from that of fruit-eating primates. One of the most prominent characteristics of the langur and surili is the presence of a compartmentalized stomach (sacculated foregut) (Liu et al. 2022).
While familiar primates like long-tailed macaques, siamangs, Javan gibbons, and even humans have a relatively simple stomach structure and anatomy, with digestion occurring largely through digestive enzymes, the stomach of langurs and surili consists of several chambers that house various symbiotic microorganisms, including various types of fermentative bacteria.
These microorganisms have the ability to break down the cellulose and hemicellulose contained in leaves. Cellulose is the main component of plant cell walls and is very difficult for most mammals to digest (Clayton et al. 2019). Without the help of microorganisms, the energy contained in plant fibers cannot be optimally utilized. Through the fermentation process, cellulose is converted into volatile fatty acids which are then absorbed and used as an energy source by the animal's body (Que et al. 2022).
This system is often compared to the digestive mechanisms of ruminant animals such as cows, goats, or deer. The basic physiological principles of the stomach appear similar, despite differences in anatomical structure and regurgitation. This system utilizes microbial fermentation to obtain energy from high-fiber feedstuffs. Colobines are sometimes referred to as the primate version of ruminants, possessing a unique digestive system among primate groups (Bauchop and Martucci, 1968).
This physiological adaptation explains why young leaves are a component of the diet of langurs and surili. Young leaves contain higher protein, lower fiber, and are more easily fermented than older leaves (Matsuda et al. 2017). They also consume plant shoots, flowers, certain seeds, and limited amounts of fruit. Therefore, fruit is not a primary food source but rather a supplement when available in their natural habitat.
Wrong Diet Increases Digestive Disorders
Feeding excessive amounts of fruit to langurs and surili monkeys can have serious health consequences. Cultivated fruits currently contain significantly higher sugars than wild fruits found in their natural habitat. Excessive consumption of high-sugar fruits by folivorous animals can disrupt the balance of fermentative microbes in the stomach (Sha, 2014).
Changes in microbial composition can cause fermentation disorders such as bloating, decreased digestive efficiency, and even chronic health problems. Individuals confiscated from illegal trade or in conflict with humans often experience changes in eating habits due to prolonged exposure to inappropriate food. Animals must undergo an adaptation process to recognize and utilize the natural food sources available in their habitat (Simoes et al. 2025).
Chronic feeding errors often result in anatomical damage to the stomach, leading to impaired metabolism. Mortality can even occur due to serious anatomical damage to the stomach, such as thinning of the lining or obstruction (Naito, 2024). Health problems such as diarrhea, metabolic disorders, obesity, and other degenerative diseases can occur. Therefore, modern nutritional management trends in the conservation of leaf-eating primates involve reducing the proportion of fruit and increasing the provision of fresh leaves, greens, and high-fiber feeds that more closely approximate natural conditions.
Differences in the Behavior of Folivores and Frugivores
The folivorous diet also influences the daily behavior of langurs and surili. Because leaves have a relatively low energy content compared to fruit, these primates allocate more time to rest. Energy obtained from fiber fermentation is not as readily available as energy from simple sugars, so their metabolism tends to be slower. Daily activities are dominated by eating, moving relatively short distances, resting, and social interaction. This behavior differs from frugivorous primates such as the Javan gibbon (Hylobates molochGibbons consume fruit as their primary energy source, requiring them to move more widely to find fruiting trees.
The high energy requirements also support the brachiation, or swinging from tree to tree, characteristic of this group of small apes. In contrast, langurs and surili often move quadrupedally on tree branches and conserve energy through longer rest periods. The presence of langurs and surili makes an important contribution to the dynamics of forest ecosystems. Leaf consumers help control the growth of certain vegetation and influence the structure of the forest canopy. They also play a role in seed dispersal, although not as extensively as fruit-eating primates.
The Irony of Natural Habitat Destruction
The presence of various primate species with distinct dietary patterns creates a shared ecological niche, allowing many species to coexist in a single habitat. Ironically, the existence of the langur and surili currently faces several serious threats. Deforestation, habitat fragmentation, infrastructure development, and agricultural expansion continue to reduce the size of their natural habitat. The surili is a highly stressed species, highly sensitive to environmental changes and highly dependent on intact forests. As habitat shrinks, its natural food sources, such as young leaves and forest vegetation, also diminish, increasing pressure on wild populations.
Education Increases Conservation Awareness
Public education is a conservation tool just as important as area protection. The public needs to understand that not all primates have the same nutritional needs. Giving fruit or human food to langurs and surili is not an act of caring. Instead, it is an act that has the potential to harm the animals' health. Respecting their natural behavior and preserving their habitat is a far more meaningful contribution. Understanding their uniqueness will undoubtedly enrich knowledge about Indonesia's biodiversity and provide a crucial foundation for science-based conservation efforts.
The more the public understands that langurs and surili are folivorous primates with unique nutritional needs, the greater the opportunity to care for the health, welfare, and future sustainability of these two endemic Javanese animals by prioritizing their natural lifestyles. Conservation means not only saving individuals or protecting forests, but also understanding the animal's way of life as nature has shaped it over millions of years of evolution.
Author: Azhar Burhanuddin (Veterinary Professional Education Student, FIKKIA)
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