PHAYRE'S LEAF MONKEY

Trachypithecus phayrei

Geographic Distribution and Habitat
Phayre’s leaf monkey, also known as Phayre’s langur, is an Old World monkey native to Southeast Asia. Its geographic distribution spans the countries of India (specifically, the northeastern states of Tripura, Mizoram, and Assam), Bangladesh, Myanmar (formerly known as Burma), China, Lao PDR (Laos), Thailand, and Vietnam.

Primary and secondary tropical forests of mixed evergreen/semi-evergreen trees and moist deciduous forests are the species’ preferred habitats, where the monkeys reside in trees 49–164 ft (15–50 m) above the ground.

Unfortunately, human disturbance to their environment has forced Phayre’s leaf monkeys to adapt to a variety of alternative habitats. Depending on the host country, these alternative habitats include thickets of bamboo with a preponderance of the evergreen shrub Macaranga denticulata and the ginger-family herb Alpinia allughas (in Assam, India), light woodlands, stream banks (in Myanmar), forests situated on limestone (in Laos), and areas just outside of tea plantations (in Tripura, India).

Phayer's leaf monkey range, IUCN 2020

Size, Weight, and Lifespan
Head-to-body length for Phayre’s leaf monkeys ranges from 17.3 to 24 in (44–61 cm). Females are typically a bit longer, or taller, than their male counterparts. Tail length adds another 2.1 to 2.8 ft (65–86 cm) to the body of these long-limbed primates; that’s nearly 70 percent of the monkey’s head-to-body length!

Males weigh an average of 16.3 lb (7.4 kg); females weigh an average of 13.7 lb (6.2 kg).

Lifespan for Phayre’s leaf monkeys has not been documented. Other species within the family Cercopithecidae, however, are known to have lifespans of 20 to 30 years in the wild.

Appearance
Nature created a beguiling creature when she made the Phayre’s leaf monkey. Infants are darling, cloaked in tangerine-colored fur coats. At about three months old, their coat changes to a silky, silvery-blue pelage with random brown flecks. The monkeys’ underside is white, tinted with light brown hues. Adults wear a fluffy, dark gray cap on the crown of their head, giving them a mild punk-rocker appearance. Their long, nonprehensile tail is the same shade of dark gray as the punk-rock cap they wear.

But it’s the face of Phayre’s leaf monkey that enchants and endears. Nature has powdered the skin around the monkey’s dark, inquisitive eyes with wide, white rings, contrasted by a bluish-black outline that also paints the bridge of the monkey’s nose. Using the same powdered compact that she used around the eyes, Nature dusted the area around the monkey’s mouth white. Then she attached a few whiskers to the chin to complete Phayre’s leaf monkey’s beguiling countenance.

Diet
As their name indicates, Phayre’s leaf monkeys eat lots of leaves, which makes them “folivorous.” Nature has equipped these monkeys with specialized, highly adapted, multi-sacculated stomachs that are able to digest plant cellulose, detoxify toxins found in the leafy materials they consume, and better absorb nutrients.

Phayre’s leaf monkeys consume the leaves of about 80 different species of trees; generally, young leaves are preferred over mature leaves. In Tripura, India, alone, the monkeys choose from a plethora of plant species that include ornamental, flowering, evergreen, and deciduous tropical plants.

Unripe and partly ripened fruits, tree bark, seeds, tree gum, and, occasionally, flowers and leaf stalks round out Phayre’s leaf monkeys’ diet. They also eat bamboo shoots when tree foliage is sparse. These complementary and alternative meal plans are indicative of both seasonable availability and sustainability (or lack thereof) of the monkeys’ habitat.

Behavior and Lifestyle
Phayre’s leaf monkeys are arboreal; that is, they spend most their time in trees. They descend to the ground only out of necessity, such as when wide gaps in the tree canopy prevent them from moving branch to branch; or when leaves are scarce and they must forage for food on the ground. They are active during daylight hours, making them diurnal.

Group size fluctuates from 3 to 30 individuals (larger numbers are unusual, but are not unheard of), depending on habitat and country of residence. An alpha (dominant) male leads the group. A typical group includes 3 to 6 females, subadults, juveniles, and infants. Larger groups with a multiple male/multiple female membership might include two adult males; in such groups, the second male is subservient to the alpha male. Solitary males, though not the norm, have also been reported. Usually, these solitary males are searching for breeding females and for the opportunity to establish their own group to lead.

Females are often closely related to one other, but any dominancy hierarchy among them is unknown. They rarely leave their birth group.

Conversely, adolescent males are nudged out of their birth groups at about 3 years of age—prior to reaching sexual maturity. This forced, strategic departure prevents these soon-to-be randy young males from challenging the group’s alpha male for the right to mate with the group’s females.

Outcast males might eventually join other solitary males and form bachelor groups. They might even participate in a coup to depose an alpha male of an existing group.

Phayre’s leaf monkeys are typically quite shy and flee when threatened. However, this “lover not a fighter” mentality does not mean that an alpha male will simply hand over his harem to an intruder and alpha-male wannabe.

An under-siege alpha male sounds a loud, threatening roar in an attempt to dissuade an intruder male’s scurrilous intentions. While he’s doing his best to defend his status and his group, the females of the group move to quickly escape the machismo drama. Mothers clutch their infants as they flee, leaping from tree to tree with impressive agility, often transferring their infant to another female en route.

Should an intruder male be successful in ousting an existing group’s alpha male, the intruder male instills himself as the group’s leader and gains immediate breeding rights. In his early reign, he might kill the young offspring of the deposed alpha male, thereby unencumbering himself of his predecessor’s legacy and giving himself full status as the group’s leader.

Although they are highly territorial when it comes to defending their turf against intrusion by outsider Phayre’s leaf monkey groups, these monkeys peaceably share their territory with sympatric groups of other species. For example, in Bangladesh, Phayre’s leaf monkeys are sympatric with the capped langur (Trachypithecus pileatus), even feeding together on the same or adjacent trees.

Daily Life and Group Dynamics 
Individuals awaken just before dawn and get busy feeding. After a long morning feeding period, they rest or nap. Their treed sleeping sites are 26–95 ft (8–29 m) off the ground. The remainder of their day is spent traveling (they move through the forest quadrupedally), foraging, grooming, playing, or in other social activities, culminating in another feeding in the late afternoon.

Phayre’s leaf monkeys spend more time feeding during the winter months than they spend feeding during the summer months, or during monsoon season. During the winter months, researchers have observed individuals basking in the sun to warm themselves.

Home range for Phayre’s leaf monkeys ranges from 25 to 256 acres (10–100 ha), with little overlap between groups. In larger groups, particularly when food resources are limited, individuals sometimes break up into subgroups and spread out across their range to forage.

While home range is largely determined by group size, group size itself is strongly impacted by social stress. Researchers have identified three negative outcomes: lower birth rates (overstressed females have greater difficulty conceiving and may not carry their infants to term); compromised immune systems (overstressed monkeys are vulnerable to disease); and the departure of some individuals who leave their group because of social stress with other members, thereby leaving themselves open to harm as they try to go it alone.

What Does It Mean?

Anthropogenic:
Environmental disturbance or environmental pollution originating in human activity.

Anthropomorphic:
Describes a concept called anthropomorphism, the attribution of human traits, emotions, and other human characteristics to non-human beings, including animals and plants.

Countenance:
A person’s face or facial expression (visage); especially, the face as an indication of mood, emotion, or character.

Ecosystem:
A biological community of interacting organisms and their physical environment.

Family:
A taxonomic group of one or more genera, especially sharing a common attribute.

Genus (plural, genera):
A biological classification, or ranking, of living beings that includes a group(s) of species that are structurally similar or “related” to one another through evolution.

Gestation period:
The time of pregnancy from conception until birth.

Harem:
A group of females guarded by a male, who prevents other males from mating with them; an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, a second male is subservient to the dominant male.

Machismo:
Strong or aggressive masculine pride; an exaggerated masculinity.

Nonprehensile or non-prehensile:
Incapable of grasping or gripping (opposite of prehensile: capable of grasping).

Pelage: 
The fur, hair, or wool of a mammal.

Primary forest:
Also termed old-growth forest, virgin forest, or primeval forest—a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community, an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment.

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

Randy:
Sexually aroused or excited; lustful, lecherous.

​Sacculated:
Having a series of different sections (e.g., 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.

Slash and burn agriculture:
A method of unsustainable farming where farmers clear land by cutting and burning flora in order to create an empty field—a “swidden”—for cultivation. Such practices are detrimental to local ecosystems.

Species:
A group of living organisms (animals) consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus.

Subadult: 
An animal who is not fully adult.

Sympatric: 
Occurring or living in the same area; overlapping in distribution. 

​Taxonomic ranking:
In biological classification, the relative level of a group of organisms (a taxon) in a taxonomic hierarchy. Examples of taxonomic ranking are family, genus, species, and subspecies. 

Visit the Glossary for more definitions

Communication
Phayre’s leaf monkeys emit several vocalizations, and each conveys a specific message or sentiment.

Adult males emit a loud “kah kah kah” call when alarmed. They emit a softer “whoo” call to warn group members of a predator in the area.

An alpha male emits a high-pitched roar, known as a loud call, when defending his territory from intruders. He uses a two-part “cheng-kong” honking call to call members of his group together.

Young Phayre’s leaf monkeys emit a distress call when they fall from the treetops. (In these occurrences, the mother or an older sibling descends to the ground and rescues the fallen monkey.)

Females emit a “lost call” to locate lost or misplaced newborns. They are also known to emit this call when reacting to deceased newborns (where the call is indicative, perhaps, of a deeper, more primal loss).

Death of a group member can elicit more than a mournful vocalization. According to a researcher’s account, after discovering the corpse of a fellow member, Phayre’s leaf monkeys touched and caressed the deceased and remained by his side. Researchers speculate that by remaining close to the body, the stress experienced by the surviving members of the group was lessened. (We might risk being called anthropomorphic and suggest that, by remaining by the deceased, the monkeys were paying their respects and demonstrating reverence for the departed.)

As with most nonhuman primates, social grooming is an important tactile activity for Phayre’s leaf monkeys that strengthens social bonds between individuals.

Fun Facts

The species name commemorates the late Sir Arthur Purves Phayre, a lieutenant general in the British Indian Army and commissioner for the entire province of what was once British Burma. As a naturalist and explorer, Phayre is credited with “discovering” several animal species, including Phayre’s leaf monkeys.

The Phayre’s leaf monkey is a species of lutung, a group of Old World monkeys who make up the entire genus Trachypithecus (derived from Greek words trachýs, meaning “rough,” and píthekos, meaning “monkey”). The word “lutung” is derived from the Sundanese language and means “blackness.”

Other
Phayre’s leaf monkey has three subspecies:

  • Trachypithecus phayrei phayrei
  • Trachypithecus phayrei crepusculus
  • Trachypithecus phayrei shanicus

Each of these subspecies is distinguished by its pelage coloring, and each inhabits a different geographic range.

Reproduction and Family
Phayre’s leaf monkeys breed intermittently throughout the year. Theirs is a polygynous society: a group’s alpha males gets to copulate with each breeding female in his harem. Males are capable of siring offspring by the time they are 4 years old. Females are just over 5 years old when they first give birth. They give birth to a single infant after a gestation period just shy of 7 months.

Most births occur in March and April. Mothers nurse their young for a full year, a relatively long weaning period for nonhuman primates, which wildlife biologists believe increases infants’ chances for survival. Phayre’s leaf monkeys are considered independent (though not sexually mature) at 1 year old. Despite their biological status of independence, adolescents maintain some physical contact with their mothers—even after she gives birth to additional offspring.

Mothers are the main caregivers; they nurse, protect, and groom newborns. Older female siblings of a newborn might lend a helping hand if their mother is away foraging.

Ecological Role
Thanks to the fruits they consume, which complement their mostly folivorous diet, Phayre’s leaf monkeys disperse seeds, via their feces, to help regenerate their habitat. Researchers further believe that the monkeys’ penchant for leaf eating may influence the plant composition in the local ecosystems where the monkeys reside.

Conservation Status and Threats
Phayre’s leaf monkey is listed as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. The population is in serious decline due to anthropogenic activity, foremost habitat loss and poaching.

Forestland has been razed through slash and burn (a practice locally known as jhum) and transformed into paper mills along with rubber, timber, and tea plantations. Animal agriculture, charcoal production, and firewood collection have also taken over former Phayre’s leaf monkey habitat. These enterprises have brought toxins and pollutants to the environment, leaving the monkeys susceptible to related illnesses. And the practice of jhum delays forest regrowth, negatively impacting the availability of food sources necessary to Phayre’s leaf monkeys’ survival.

Additionally, human settlements are increasingly displacing the monkeys. Phayre’s leaf monkeys have been forced into fragmented, isolated populations throughout their range.

Poaching is the other dire threat against the species. Humans are the primary predators of Phayre’s leaf monkeys. (The monkeys are at far lesser risk of predation by leopards, their natural predator or predation by large snakes and raptors who might, researchers suggest, feed on newborns.) Native tribes hunt and kill the monkeys for their meat. The monkeys are also killed and slaughtered for their gall stones, which are used in medicinal remedies.

The presence of humans has brought another threat to Phayre’s leaf monkey populations: domestic dogs. In studying a Phayre’s leaf monkey group in the Cachar district of Assam, India, researchers noted that the monkeys were being harassed by dogs belonging to the human settlers. Male individuals in the Phayre’s leaf monkey group appeared to protect the weaker members of their group.

Conservation Efforts
Phayre’s leaf monkey 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 species is also included in specific wildlife protection laws in several of the countries where it resides. In Myanmar, the species has been given complete protection status since 1994.

In-situ (on site) conservation efforts are helping to decrease the practices of jhum and poaching, the two major threats to Phayre’s leaf monkeys’ survival. But conservationists call for more stringent enforcement of protection laws, the creation of educational programs to raise awareness and appreciation of the species and their natural habitat, and the creation/designation of additional protected areas where Phayre’s leaf monkeys can live unmolested by humans.

Those Phayre’s leaf monkeys residing within the boundaries of nature reserves and national parks are afforded a level of protection. Cúc Phương National Park in Vietnam, located in the Red River Delta area of Ninh Binh province, is one such area. Cúc Phương is also Vietnam’s first national park, the country’s largest nature reserve, and an important biodiversity site.

In January 2018, an amateur photographer and an ecological protection volunteer visiting Yunnan province in southwest China was greeted by the sight of nearly 200 Phayre’s leaf monkeys (an extraordinarily large number). Zheng Shanhe shared video of his amazing encounter: https://news.cgtn.com/news/794d544f78677a6333566d54/share_p/

References:

  • https://www.iucnredlist.org/species/22040/9349717
  • https://acta.mendelu.cz/media/pdf/actaun_2015063041105.pdf
  • http://cza.nic.in/uploads/documents/studbooks/english/nsPhayre’s%20Leaf%20Monkey.pdf
  • http://www.xinhuanet.com/english/2018-07/18/c_137333254.htm
  • https://news.cgtn.com/news/794d544f78677a6333566d54/share_p/  https://animaldiversity.org/accounts/Trachypithecus_phayrei
  • https://en.wikipedia.org/wiki/Phayre%27s_leaf_monkey
  • https://en.wikipedia.org/wiki/Lutung
  • http://www.conservationindia.org/gallery/phayres-leaf-monkey-tripura
  • http://www.theprimata.com/trachypithecus_phayrei/
  • http://genomics.senescence.info/species/entry.php?species=Trachypithecus_phayrei
  • http://earthsendangered.com/profile.asp?gr=M&sp=4297
  • https://www.speciesconservation.org/case-studies-projects/phayres-leaf-monkey-phayres-langur/2766
  • https://www.researchgate.net/publication/241713426_Utilization_of_forest_flora_by_Phayre%27s_Leaf-Monkey_Trachypithecus_phayrei_Primates_Cercopithecidae_in_semi-evergreen_forests_of_Bangladesh
  • https://www.google.com/search?q=phayre%27s+leaf+monkey+baby&sa=X&ved=2ahUKEwj2qp-456ThAhWyiOAKHfEQCV4Q1QIoAXoECAsQAg&biw=1093&bih=524

Written by Kathleen Downey, April 2019