MAROON LEAF MONKEY

Presbytis rubicunda

Geographic Distribution and Habitat
The maroon leaf monkey, also called maroon langurs, maroon surilis, and red leaf monkeys, is endemic to the Island of Borneo, specifically Kalimantan, Indonesia, and Sabah and Sarawak, Malaysia. Maroon leaf monkeys are widespread and found high in the trees. They live in primary and secondary lowland forests. They are also found in swamp forests, especially on Karimata Island in Indonesia. Red leaf monkeys on Karimata Island also occasionally visit native gardens in search of food. These langurs adapt reasonably well in areas of regeneration and logged forests.

Populations are typically separated by large, lowland rivers. In upland areas, however, the rivers tend to narrow and are more easily crossed. Thus, the distinction between different subspecies may not be so clear in upland areas.

Maroon leaf monkey geographic range. Map: IUCN, 2020

Size, Weight, and Lifespan
The head and body length of maroon langurs can range anywhere between 16 and 22 in (40-58 cm). Their tails can be almost as long as their bodies, but often times are longer, ranging from 26 to 29 in (66-73 cm). The maximum weight for these monkeys is about 15 lb (7 kg), but they can be slightly lighter at about 13 lb (5.9 kg).

In the wild, maroon langurs can live up to 20 years. Captive maroon langurs can live over 25 years. Researchers believe that habitat destruction and hunting by humans has a large impact on their lifespan in the wild.

Appearance
Maroon leaf monkeys have short and broad bluish to black faces, with a deep jaw and wide dark eyes. Their upper lip is also dark blue or black, and their lower lip is a much lighter beige. The hindlimbs, compared to the forelimbs, are somewhat longer, which suggests leaping activities, although they mainly move quadrupedally through the forest. In addition, the thumb or pollex of maroon leaf monkeys is small or shortened compared to other primates. They have a shaggy auburn coat that can can range from red to reddish-orange with their stomachs being a lighter shade. The hands and tails of maroon leaf monkeys are often a darker shade.

Photo credit: Charles J Sharp/Creative Commons

Diet
Like all leaf monkeys, maroon leaf monkeys have sacculated stomachs that break down cellulose from their leafy diet. They prefer young leaves rather than mature leaves unless those food sources are scarce. They also consume fruits that have large seeds and little flesh, as well as piths and flowers. They avoid pulpy and ripe fruits since high levels of stomach acids can disrupt their stomach’s pH. For mineral nutrients, they consume termite clay or soil. Feeding on soil provides the monkeys with nutrients, such as magnesium, calcium, phosphorus, and potassium. They off the topsoil and then consume the freshly exposed soil beneath. Consuming soil may also aid in the relief of acidosis of the forestomach.

In Northern Borneo, from June to September, the diet of the maroon leaf monkey consists of 68 to 90 percent dry seeds. However, in general they tend to spend half the time eating vegetation and the other half eating seeds and flowers. Seeds have high levels of fiber and tannins. It is possible that tannins are important for aiding in protein digestibility by shielding them from their forestomachs’ microbes, which would most likely degrade them.

Maroon leaf monkeys combine young leaves with flowers to incorporate high levels of protein in their diet. Munching on fruits and seeds allows them to absorb carbohydrates and fats, which are stored for energy.

What Does It Mean?

Acidosis:
Overproduction of acid in the blood and body tissues. 

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

Canid:
A mammal of the dog family.

Diurnal:
Active during daylight hours.

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

Felid:
A mammal of the wild cat family.

Lowland forest:
An equatorial evergreen rainforest, commonly known as a tropical rainforest, which receives high rainfall (80 in; 2 m) throughout the year.

Pith:
The soft or spongy tissue of a plant or fruit, which is usually white or pale in color (i.e., the white part between the skin and fruit of an orange). 

Polygynous:
A mating system in which one male mates and lives with multiple females.

Quadrupedal:
Using four limbs to move about. This word comes from the Latin meaning “four feet.”

Sacculated:
Having a series of different sections. For example, langurs have sacculated stomachs (i.e., stomachs with three sections).

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.

Visit the Glossary for more definitions

Behavior and Lifestyle
Maroon langurs are diurnal and arboreal. They move through trees quadrupedally. They are very territorial and chase other groups out of their home range. Any intruders within their home range are challenged. Males emit a loud call to warn rivals away and to let other groups know this is their territory.

Most of the time, maroon leaf monkeys have a uni-male social system and a polygynous mating system. Groups consist of one adult male, one or more adult females, juveniles, and infants. Some believe that the reason there is only one male per group in maroon leaf monkeys is due to a reduced predation risk from eagles compared to colobus monkeys in Africa and howler monkeys in the Neotropics. Arboreal langurs are not exposed to monkey-eating eagles as much as other primates and thus do not need multiple males defending the group. Other predators such as canids, felids, and humans do go after maroon langurs. However, since they stay high in the trees it is rather inconvenient to pursue them.

Shortly before adolescence, males (and sometimes females) leave their prenatal group. Males either travel solitarily or in all-male groups, before finding a uni-male group. Males that travel alone may challenge a local group male for control over the group and sometimes he may take females with him to form his own group. Females, on the other hand, do not display dominance in terms of access to food resources.

Fun Facts

Maroon leaf monkeys share the forest with orangutans, gibbons, crab-eating macaques, proboscis monkeys, and occasionally pig-tailed macaques. 

Maroon leaf monkeys sometimes form coalitions with Hose’s langurs. 

Daily Life and Group Dynamics 
Maroon langurs generally live in groups of 2 to 13 individuals. In Indonesia, groups of maroon langurs have been found to consist of 1 to 18 individuals, such as those in Kalimantan Tengah. Groups usually split into subgroups at dawn to forage for food. They forage all morning and afternoon, reuniting in the late afternoon. After many feeding rounds, they take a long midday rest. When a group feels threatened, an individual (usually a male) makes an alarm call and moves the group away from that area. 

Different groups of maroon leaf monkeys have home ranges that tend to overlap; however, each group has their own central area where their sleeping site is located. The home ranges of groups are correlated with the size of the group—smaller groups have smaller home ranges, and larger groups have larger home ranges.

Communication
Only a few forms of communication have been observed in the maroon leaf monkey so far. Vocal communication consists of a loud call, which is used by males to establish the group’s territory. Another call, known as the alarm call, is also emitted by an adult male. This call is used when an intruder is spotted nearby. The male sounds this call while the rest of the group is fleeing to safety.

Another form of communication is tactile. This social grooming is used to reinforce bonds between individuals of a group.

Reproduction and Family
Maroon langurs have a polygynous mating system. That is, one male, usually the dominant male, reproduces with more than one female. Adolescent males often leave the troop before adulthood and will likely not challenge the dominant male within their natal group.

Between the ages of three and four, female langurs reach sexual maturity. Males reach sexual maturity at around four years old. Female maroon langurs usually give birth to one offspring. Infants nurse for about a year, which greatly increases their chances of survival. The infant stays with his or her mother for at least two years. Mothers invest a considerable amount of time taking care of infants, as they feed them, protect them, and groom them. It is not unlikely that adolescent females provide some care to infants when the mother is not present. Some biologists have found that adolescent leaf monkeys tend to keep some contact with their mothers, even after additional births.

Photo credit: Mike Prince/Creative Commons

Ecological Role
Due to their diet, maroon leaf monkeys help regenerate their habitat by dispersing seeds they eat from fruits around the forest.

Conservation Status and Threats
The International Union for Conservation of Nature’s Red List of Threatened Species lists the maroon langur as Vulnerable, since they are fairly common, have a wide range distribution, and are able to adapt to secondary habitats (IUCN, 2015). The forests of Borneo are undergoing some of the world’s highest deforestation rates, largely as the result of the expansion of oil palm plantations. Habitat loss due to deforestation is a huge threat to maroon leaf monkeys. Much of their habitat is being destroyed for palm oil plantations. Development of oil palm plantations have severe negative effects on the forests and native peoples, such as poor water quality, soil erosion, and air pollution, among many others. Reducing environmental damage and stopping the destruction of critical habitat for many threatened and endangered species are serious concerns that are often ignored when large companies are in pursuit of plantation development. Palm oil is the main agricultural export of Malaysia and Indonesia. As a result, more than 5.5 million hectares of forest were lost to oil palm plantations between 1990 and 2008, and this is increasing each year. Although Indonesia has taken several steps to make palm oil more sustainable, indigenous people who have inhabited and protected the forests for centuries are brutally driven out from their land. Therefore, palm oil cultivation comes at a heavy environmental and social cost. Efforts to make it more sustainable still have a long way to go.

This species is also threatened by hunting from humans. Humans often hunt maroon leaf monkeys for their meat and traditional medicine purposes.

Conservation Efforts
The maroon langur is protected in Sarawak and Sabah, Malaysia. The species is listed in the Convention on International Trade in Endangered Species (CITES) Appendix II. According to the IUCN Red List of Threatened Species, maroon langurs are found in at least ten protected areas, such as Betung Kerihun National Park, Bukit Baka Bukit Raya National Park, Ganung Palung National Park, Kayan Mentarang National Park, Kutai National Park, and Pleihari Martapuri Nature Reserve to name a few found throughout Indonesia.

References:

  • https://www.inaturalist.org/taxa/43502-Presbytis-rubicunda
  • http://www.iucnredlist.org/details/18131/0
  • ftp://169.158.189.34/pub/Biotropica/1980s/1988/20-3/Biotropica-1988-20-3-p252.pdf
  • http://www.theprimata.com/presbytis_rubicunda/
  • https://www.ecologyandsociety.org/vol17/iss1/art25/
  • https://www.ecologyasia.com/verts/mammals/red-leaf-monkey.htm
  • van Schaik, C.P. and Horstermann, M. 1994. Predation risk and the number of adult males in a primate group: A comparative test. Behavorial Ecology and Sociobiology.

Written by Tara Covert, October 2018