Phayre’s Leaf Monkey, Trachypithecus phayrei
PHAYRE'S LEAF MONKEY
Trachypithecus phayrei
Geographic Distribution and Habitat
Phayre’s leaf monkeys (Trachypithecus phayrei), also called Phayre’s langurs, are native to the South Asian countries of India, Bangladesh, and Myanmar. They make their homes in high-elevation tropical evergreen, semi-evergreen, and mixed moist deciduous forests. Though they prefer primary forests which have not been significantly impacted by humans, they also do well in secondary forests– younger forests that have grown back following a clear-cutting. They can also be found in bamboo groves and tea plantations. They usually live below an altitude of 2,600 feet (800 m) above sea level, though they can be found as high as 3,700 feet (1,125 m) above sea level. Sadly, the monkeys’ historic range has been reduced significantly in recent decades, and the habitat that is left is degraded and severely fragmented.
The Indochinese gray langur, T. crepusculus, was formally considered a subspecies of the Phayre’s leaf monkey. It is still debated whether another subspecies, T. p. shanicus, also known as the Shan State langur, represents its own species, though for now, the International Union for the Conservation of Nature classifies it as a subspecies of the Phayre’s leaf monkey.
Size, Weight, and Lifespan
Female Phayre’s leaf monkeys are slightly larger than males in terms of length but usually weigh less than males on average. Females measure between 26 to 34 inches (65-86 cm) from the top of their head to the base of their tail, while males measure 17 to 24 inches (44-61 cm). Their tails add another 26 to 34 inches (65-86 cm) on average. Females weigh about 13.7 pounds (6.2 kg) on average, while males are a bit heavier at 16 pounds (7.3 kg). In the wild, Phayre’s leaf monkeys live into their 20s. In captivity, they have lived up to the age of 28.
Appearance
Phayre’s leaf monkeys are easily identifiable thanks to their unique face markings. Their face is mostly black from the nose up, save for circles of white or pale pink around their eyes. Their lower face is light pink, contrasting with the black to give them the appearance of wearing a bandit’s mask. To add to this rather wild look, the hair on their head is long and ruffled (the iconic hair metal bands of the 1980s come to mind). This long hair tends to come together in a pointy tuft at the crown. They are mostly gray over their body, varying in shades from light to dark. Their belly is pale gray. Males and females exhibit slight sexual dimorphism or visual differences between the sexes. In addition to the females being slightly longer, they can also be distinguished by the rings around their eyes. In females, the rings bend in at the top of the nose bridge, giving the impression of a black triangle on the nose. In males, the eye rings are more circular, forming a black bar of even width down the bridge of the nose. Newborn Phayre’s leaf monkeys are born orange, and they lack the adults’ hair tuft. They begin to develop their adult coloration beginning at about three months of age.
Diet
As their name suggests, Phayre’s leaf monkeys are folivorous– leaf eaters. Leaves constitute about 50% of their diet, followed by other plant parts such as shoots, flowers and buds, fruit, and seeds. About a third of their diet comes from just two plant families: Fabaceae, the legume, pea, and bean family, and Moraceae, the fig and mulberry family. Their diet varies with the seasons. One study of a population in Bangladesh found that the monkeys consume more leaves during winter, more fruits and seeds during monsoon season, and more flowers during the summer. Bamboo shoots were found to be an important food source year-round.
Like other leaf monkeys, they possess a special multi-chambered stomach to ferment plant fibers and detoxify defensive chemicals, similar to the digestive systems of ruminants like cows, goats, and deer. This unique digestive system is an important evolutionary adaptation that affords them an otherwise inaccessible food source: mature leaves. Because few animals are able to effectively digest these leaves, leaf monkeys face little competition for this resource.
Behavior and Lifestyle
Phayre’s leaf monkeys are fully arboreal, meaning they spend nearly their entire lives in the trees. They are usually found 50-160 feet (15-50 m) above the ground. They seem to like being near water, and can often be found in bamboo groves near stream banks. They occasionally descend to the ground to feed on bamboo shoots or for a drink of water.
They are also diurnal, meaning that, like humans, they sleep at night and are awake during the day. They usually start the day with a pre-dawn breakfast. Around noon, they take a nap in the shade to avoid the daytime heat. After this siesta, they resume eating in the afternoon. They spend about 40% of their time feeding and about a third resting. About 15% of their time is spent moving and about 7% is spent grooming. The remainder of their time is spent on various activities like playing, copulation, and aggressive interactions. On an average day, they travel more than 0.6 mi (1 km). Personality-wise, Phayre’s leaf monkeys have been described as very shy, and they usually flee when they encounter humans.
Phayre’s leaf monkey is named after British Indian Army officer Arthur Purves Phayre (1812-1885), a bit of a Renaissance man who was an amateur naturalist in his spare time. Phayre’s squirrel was also named for him, and he is commemorated in the Latin names of several other mammals, reptiles, and birds.
Groups are composed of one to five adult males and three to twelve adult females, and their offspring. There is a dominance hierarchy among males and females separately. Among females, older females are the most dominant, and younger females are ranked the lowest. The group’s home range can vary between 6.7 to 43.5 acres (2.7 to 17.6 ha) in size. Groups are highly territorial, though occasionally multiple groups may share the same territory. Despite being territorial, they are rarely aggressive with each other. Rather, they are more likely to simply push other groups out if they encroach on their territory. Groups often split up into smaller sub-groups when foraging, but they use contact calls to stay in close proximity to one another. This behavior is more common when food is scarce, as the group may need to spread out over a wider area for all the members to find food.
Phayre’s leaf monkeys use a variety of calls to communicate with one another. A loud “kahkahkah” or a softer “whoo” call is used as an alarm call, which alerts groupmates to predators. A call that sounds like “cheng-kong” is used by the dominant male to gather his group together. They also use a variety of facial expressions and body postures to communicate, and olfactory, or smell, communication may be used as well.
The mating system among Phayre’s leaf monkeys varies between polygynous (each adult mates with multiple females, while each female mates with just one male) and promiscuous (males and females both mate with multiple partners). Males typically begin to mate with adult females starting in their adolescence. Interestingly, males have been observed masturbating in the wild. While this may seem like a waste from an evolutionary standpoint, scientists have speculated that perhaps it serves the function of flushing out old, low-quality sperm and allowing the male to produce fresh sperm that has a better chance of fertilization. However, the exact reason remains uncertain.
The monkeys may signal their readiness to mate by shaking their head, presenting their hindquarters, or inspecting another’s hindquarters. Most breeding occurs between December and April, and babies are born about seven months later. Females have their first baby at an average age of five years old, waiting an average of two years between births. Babies are primarily cared for by their mothers, who feed, protect, and groom their young. They nurse for about a year and a half, a key factor in their survival. As they grow older, adolescents keep in contact with their mother, even after she has more babies. Males occasionally help with raising young. Additionally, they are known to intervene if a group member is harassing an infant.
Upon reaching maturity, females often leave their natal groups, the groups they were born into, to join another, while males are philopatric– they stay in their natal groups. This is the reverse of most primates, whose females usually remain in their groups while the males leave.
Phayre’s leaf monkeys have a close ecological relationship with the plants they consume, and they may play a small part in dispersing seeds in their droppings when they eat fruit. They are likely preyed upon by such animals as birds of prey and wild cats. Phayre’s leaf monkeys share their habitat with capped langurs (T. pileatus), living sympatrically in the same region, and even sometimes living together in mixed-species groups and hybridizing. Because hybridization can occur as a result of human interference– such as by habitat loss forcing different species to coexist and interbreed– scientists have called for research into whether this hybridization is a recent, and thus human-driven, phenomenon.
The International Union for Conservation of Nature lists the Phayre’s leaf monkey as Endangered (IUCN, 2021) appearing on the IUCN’s Red List of Threatened Species. This listing is based on a suspected 50% loss in population over the last 36 years, or about three generations. This substantial drop in population is mostly attributed to habitat loss and unsustainable hunting. There are believed to be approximately 1,200 individuals in India, and there have been 376 individuals recorded in Bangladesh. There is little information about the state of the species in Myanmar. Bangladesh has seen a particularly dramatic drop in Phayre’s leaf monkey population, with a suspected 80% drop over the last 20 years.
Habitat loss is unfortunately rampant in the monkeys’ range. Over the last 36 years, there has been a 10-27% loss in forest cover. Forests are cut down to make room for rubber and timber plantations, tea gardens, livestock ranching, charcoal production, human settlements, and a type of agriculture known as “jhum” or “shifting” agriculture. Shifting agriculture is an agricultural system in which a plot is farmed until its soil nutrients are depleted, at which point it is allowed to revert back to its natural vegetation, and a new plot is cleared and farmed. This system of farming was developed to compensate for the rather poor soil quality in the region, although it unfortunately has significant environmental impacts. Shifting agriculture depletes soil nutrients, and though it is left to revert back to a natural state, it takes many years for the soil to be restored. Additionally, because a new plot must be cleared when the previous one is depleted, it requires heavy land use and clearing of natural vegetation. Habitat loss results not only in an overall reduced population of Phayre’s leaf monkeys but also causes the populations that remain to be small and isolated.
Phayre’s leaf monkeys are sadly also impacted by humans in other ways, such as collisions with cars and power lines, health impacts from pollution, and they are sometimes captured for the pet trade. Phayre’s leaf monkeys are sometimes hunted for their gallstones, which are used in traditional medicine. They may also be hunted for meat by local people.
Phayre’s leaf monkeys are 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. They are also federally protected in all three countries in which they reside.
Phayre’s leaf monkeys reside in numerous protected areas throughout Bangladesh and India, such as Rema-Kalenga Wildlife Sanctuary and Satchari National Park in Bangladesh, and Dampa Tiger Reserve and Murlen National Park in India. Some experts have called for stricter implementation of the rules and regulations of protected areas, to limit encroachment, poaching, and illegal logging within their boundaries. Additionally, conservation experts have called for working with the local communities within the monkeys’ range to increase awareness of the species. Alternative livelihood development would also allow the local people to earn money while reducing their impact on the surrounding forests and, by extension, the species within.
In addition to working within the monkeys’ habitat, ex-situ conservation also takes place. Ex situ conservation refers to actions that take place outside of the species’ home range. In the case of Phayre’s leaf monkeys, they have been identified by India’s Central Zoo Authority as the subject of conservation breeding programs in Indian zoos. The captive populations are carefully managed to ensure long-term genetic diversity.
- Ahmed, T. et al. 2024. Mixed-species groups and genetically confirmed hybridization between sympatric Phayre’s langur (Trachypithecus phayrei) and capped langur (T. pileatus) in northeast Bangladesh. International Journal of Primatology
- Al-Razi, H. and H. Naher. 2021. Status of Phayre’s langur Trachypithecus phayrei in Satchari National Park, Bangladesh. Asian Primates Journal 9(1):10-19.
- Naher, H., et al. 2022. Activity budget and diet of Phayre’s langur (Trachypithecus phayrei) in Satchari National Park, Bangladesh. Primate Conservation 36:173-189.
- Roos, et al. 2020. Mitogenomic phylogeny of the Asian colobine genus Trachypithecus with special focus on Trachypithecus phayrei (Blyth, 1847) and description of a new species. Zoological Research 41(6): 656-669.
- Shalauddin, M., M. Islam, T. Ahmed. 2021. Masturbation in a male Phayre’s langur, Trachypithecus phayrei. Primate Biology 8:43-45.
- Timmins, R., et al. 2013. The conservation status of Phayre’s lead monkey Trachypithecus phayrei in Lao PDR. Primate Conservation 26(1):89-101
- https://www.iucnredlist.org/species/175862145/175862149
- https://cza.nic.in/uploads/documents/studbooks/english/Phayre’s%20leaf%20monkey%20(Trachypithecus%20phayrei)%20-final%20(25%20July).pdf
- https://www.britannica.com/topic/shifting-agriculture
Written by K. Clare Quinlan, November 2024