MAROON LEAF MONKEY
Geographic Distribution and Habitat
The Maroon leaf monkey (Presbytis rubicunda) is an arboreal primate that is found on the island of Borneo, as well as the smaller nearby island, Karimata. Within Borneo, they can be found in Danum Valley living at altitudes of 6,563 feet (2,000 m) or less. Borneo is an island that is shared by Malaysia, Indonesia, and Brunei.
Maroon leaf monkeys inhabit primary forests, or forests that have not had human disturbances; as well as secondary forests, areas that have had regrowth after natural or human disturbances. According to some studies, maroon leaf monkeys can adapt relatively well to regenerated forests, where disturbances such as logging have taken place in the past.
These primary and secondary forests are a part of a rainforest in Borneo, and the monkeys rely heavily on this natural environment. Maroon leaf monkeys can also be found in swamp forests and lowland forests. Their home range is larger than that of many other primates because the trees they rely heavily on are widely dispersed, thus making travel across greater distances necessary. They have also been known to venture into native gardens in order to obtain more food resources.
The maroon leaf monkey (Presbytis rubicunda) is a part of the subfamily Colobinae, which includes colobus monkeys of varying kinds. They are a part of a group of monkeys referred to as langurs; however, they are more commonly referred to as leaf monkeys. They are an Asian monkey that is endemic to Borneo, where they share the island with 12 other primate species, including 4 different leaf monkey species. They are also known by other common names, including red langur, maroon langur, maroon surili, and red leaf monkey.
Size, Weight, and Lifespan
On average, the maroon leaf monkey weighs about 12–14 pounds (5.8–6.7 kg). They measure 16–22 inches in length (40.2–55.8 cm), which is notably smaller than other members of their subfamily Colobinae; however, it is about average for other langurs. Their tails are about the length of their bodies, but they can be even longer in some cases! In the wild, they are believed to live up to 20 years, and in captivity, they can live over 25 years. Some researchers believe that their life expectancy in the wild is declining because of the deforestation and hunting occurring in Borneo.
As their name might suggest, the maroon leaf monkey has a reddish pelage that covers the entire body. Males and females are the same color, and there is variation in hue between individuals and populations, from a light orange to a darker, almost maroon color. Infants can be born with an orange-cream to a sandy orange coloring, and their hair begins to darken to red as they age. The maroon leaf monkey’s face is bare and they have gray (almost blue sometimes) skin. Their faces are small, and their jaws are large—this helps them when chewing the large amount of leaves and other vegetation in their diet! These monkeys have long and thin non-prehensile tails, which they use for balance while leaping from tree to tree. Some Asian monkeys use brachiation, that is swinging between trees using only their arms. It is unknown from the literature if maroon leaf monkeys brachiate—however, if they do it is rare.
The maroon leaf monkey’s diet, as suggested by its name, is primarily folivorous. They prefer the younger parts of the plant over more mature leaves, although during the wet season, young leaves become scarce, so they eat more mature leaves during this time. As with many other species, their diet varies throughout the year depending on food availability. They are also known to eat seeds, fruit, and occasionally flowers.
The fruiting season in Borneo occurs from June to September, which is when their fruit intake is the highest. The sugars found in ripe food tend to disrupt the balance of their stomachs, so during the fruiting season, they prefer young, unripe fruit. The rest of the year they can be found munching on various leaves and plants. A particular favorite of the maroon leaf monkey is foliage that grows from lianas, a type of woody vine, which makes up about 32% of their diet. Like other leaf monkeys, the maroon leaf monkey has a multi-chambered stomach that allows for the breakdown of cellulose that is in abundance in the plants they eat.
Behavior and Lifestyle
The maroon leaf monkey spends most of its life in the trees, rarely coming down to the forest floor. They are a diurnal species, meaning they are awake and forage during the day and sleep at night. The females and infants spend most of their day eating, and males spend their time eating as well, but they are also preoccupied with keeping their territory safe. These leaf monkeys are polygynous, meaning that one male mates with multiple females—these groups are often referred to as harems. Males can be highly territorial and will emit loud vocalizations in order to secure and enforce their territory.
These monkeys are known to practice geophagy as well, which is the purposeful ingestion of soil in order to add minerals and other compounds to their diet, with calcium and magnesium being the most abundant in this soil. Soil consumption for the maroon leaf monkey is always from arboreal termite mounds. Many animals exhibit geophagy and it is believed that it helps alleviate the effects of tannins in high plant-based diets; however, it is seen in these monkeys regardless of the season and primary food intake. This suggests that while soil consumption might help with tannin absorption, it most likely has other purposes as well.
Maroon leaf monkeys are sometimes referred to as ‘the distant cousin to orangutans!’ While they are indeed related, they are many times removed. Nonetheless, their likeness cannot be denied!
Maroon leaf monkeys are social, and their troops generally consist of 2 to 14 individuals. They spend their days foraging, and will often forage in smaller groups. These smaller groups will forage for the morning and later converge with the rest of their group for midday naps. Males are highly territorial, and are ready to defend their home range and harem whenever an issue might arise.
Their primary defense is emitting loud calls in order to intimidate other males. Vocalizations as a defense mechanism are common in many primate species, as it is much more energy efficient than fighting. Maroon leaf monkey males will engage in physical altercations with others if they are forced, though. When faced with a predator, such as a leopard, these monkeys have been known to “predator mob,” in which the monkeys will all emit loud calls and cause an uproar in order to overwhelm and chase the predator away.
As with other primates, the primary mode of communication for the maroon leaf monkey is through vocal calls. They use grunts and calls to express emotions to other members of the group, or warn them of danger. Males will use a booming call to warn other members of the group of danger or intimidate rival groups. These calls can be heard several hundred feet away (over 100 m). These calls are usually a short series of “ka-ka-ka” sounds and, depending on vocal inflection, they can mean different things. Their vocalizations are a testament to their lineage, as many leaf monkeys or members of the genus Presbytis sound incredibly similar.
The maroon leaf monkey’s tail is an important communication tool as well. Depending on the movement and positioning of their tail they can convey excitement, annoyance, alarm, fear, and aggression.
The role of allogrooming is vital in many primate groups, and the maroon leaf monkey is no exception. Grooming one another is an important form of communication within the group, conveying friendship or alliance, or strengthening familial ties. The absence of grooming can indicate tension between individuals. It is also important for their health, as parasites are removed during the grooming process.
When a maroon leaf monkey is born, the infant is wholly dependent on their mothers for about 2 years. They nurse for the first year, which elevates their health and long-term odds for survival. An adolescent female will reach sexual maturity at around three to four years old. Males, on the other hand, reach maturity at four years old or a little later.
These monkeys live in a polygynous mating system, in which one male has multiple females he is able to mate with. These harems usually consist of one sexually mature male, multiple sexually mature females, infants, and adolescents.
Female maroon leaf monkeys generally give birth to singlets—that is, to one infant. Maternal investment is high in this species, as the mothers will groom, feed, and defend the infant until they are over the age of two. There is evidence to suggest that adult females will sometimes maintain a relationship with their mothers, which is not always the case in primate species.
During their seed and fruit-eating season, the maroon leaf monkey plays a vital role in their environment as a seed disperser. Upon eating fruit, and sometimes ingesting seeds as a byproduct of leaf-eating, the seeds run down their digestive system. The seeds are then expelled through feces and distributed all around the forest. This helps different tree species propagate in a variety of locations, as well as extending the plants’ distribution. Leaf monkeys are essential for the regeneration and maintenance of the forest. Seed-eating by maroon leaf monkeys is highest in August, and lowest in February. When they do eat fruit, they love Actinidia chinensis, better known as golden kiwifruit.
The International Union for Conservation lists maroon leaf monkeys as Vulnerable (IUCN, 2015) and their population is decreasing. There is no current estimate for their overall population size right now. The primary threats to the maroon leaf monkey are habitat loss due to deforestation, as well as hunting for meat, and use for traditional medicine.
Deforestation in Borneo is primarily caused by the ever-increasing palm oil trade. Forests are being cut down to accommodate new palm oil plantations. Additionally, deforestation is the result of a few other crops, most commonly wood and pulp plantations, and non-timber crops. These monkeys are the target of hunting and trapping, sometimes for bushmeat and other times for unknown reasons.
Increasing forest fires are currently an issue as well, and are projected to increase in occurrences and severity. Maroon leaf monkeys are considered to be adaptive, and there is evidence to support their ability to live in secondary environments. Scientists believe that maroon leaf monkeys populations have decreased up to 50% in the past 30 years. In just a 10-year period (2000-2010) this species lost around 10% of their habitat due to outside threats.
The maroon leaf monkey is protected in two regions of Malaysia, Sarawak, and Sabah. They live in 10 protected areas; however, there are populations that live outside of these protected areas. They are listed in Appendix II. These protected areas are Betung Kerihun National Park, Bukit Baka Bukit Raya National Park, Gunung Palung National Park, Kayan Mentarang National Park, Kutai National Park, Pleihari Martapuri Nature Reserve, Tanjung Puting National Park, and the Sungai Wain Protection Forest. For the populations that live outside of these protected areas, they are often found in lowland wetlands, which are often cleared for palm plantations. Legislation surrounding these ecological areas is needed to further ensure the safety and status of the maroon leaf monkey.
Written by Robyn Scott, August 2023