Details on communication in these monkeys are scant. However, we know that they use vocalizations to protect their territories from other members of the species. Like other primates, tactile communication (e.g. grooming, playing, mating, aggression) and visual communication (e.g. facial expressions and body postures) are probably both inmportant in these monkeys.
Communication Channels: visual ; tactile ; acoustic
Perception Channels: visual ; tactile ; acoustic ; chemical
There is very little information about the status of dusky leaf monkeys. I found no published estimates of how many monkeys currently exist.
Dusky leaf monkeys are confined within a relatively small area of southeast Asia. It is probable that these forested areas are under threat of development or logging. Therefore, there is reason to believe that the species is threatened to some extent because of habitat loss, but there are no studies to support this.
US Federal List: no special status
CITES: appendix ii
IUCN Red List of Threatened Species: near threatened
Dusky leaf monkeys are generally restricted to primary forests. Therefore, it is unlikely that they contribute to any serious agricultural (or other) problems that would adversely affect humans.
It is probable that dusky leaf monkeys are hunted for food by the human population, since a large number of primates are hunted as sources of food throughout Asia.
Positive Impacts: food
These monkeys are likely to be important predators of folliage. They may help to disperse seeds. To the extent that they serve as prey for other species, these monkeys may affect local food webs.
Ecosystem Impact: disperses seeds
The diet of these monkeys consists of young leaves, shoots, and seedlings. They feed from 87 different species of trees, ingesting both leaves and fruit. In general, a dusky leaf monkey eats up to 2 kg of food per day.
These monkeys can be maintained, in captivity, on sweet potato shoots, lettuce, cabbage, kangkong, grean beans, maize, carrots, and soft fruits. Meat was refused, but certain insects were occasionally accepted.
Plant Foods: leaves; seeds, grains, and nuts; fruit; flowers
Primary Diet: herbivore (Folivore )
Dusky leaf monkeys are found primarily on the Malay Peninsula, including southern Burma and parts of Thailand. They also inhabit the islands of Langkawi, Penang, and Perhentian Besar.
Biogeographic Regions: oriental (Native )
Dusky leaf monkeys can be found in a wide range of habitats. Being arboreal forest dwellers, they prefer dense forests with tall trees.
Terrestrial Biomes: taiga ; forest ; rainforest
Another monkey in the same genus, Trachypithecus cristatus, is reported to have lived over 31 years in captivity. It is likely that T. obscurus is similar. Lifespan in the wild is probably lower than in captivity.
Average lifespan
Status: captivity: 25.0 years.
Average lifespan
Sex: female
Status: captivity: 15.3 years.
Dusky leaf monkeys are widely variable in color. Their upper parts may be any shade of brown, grey, or black, whereas the under parts, hind legs, and tail are paler. The face is grey and is often marked with a patch of white fur located around the eyes and mouth.
The hands and feet are capable of grasping and closely resemble those of humans. The palms and soles are hairless and usually black. The fingers of dusky leaf monkeys are well developed, but are distinct because of their opposable thumb. The nonprehensile tail varies in length and fur coverage from short and hairless, to long and hairy.
Head and body length ranges from 42 to 61 cm, and tail length from 50 to 85 cm. There are no significant morphological differences between males and females except that males tend to be slightly larger and heavier than females. On average, a healthy adult male weighs 7.4 kg, whereas a healthy adult female weighs approximately 6.5 kg.
Newly born dusky leaf monkeys are bright yellow or orange in color, and have a pink face; the fur changes to a greyish color within six months.
Range mass: 5 to 9 kg.
Range length: 42 to 61 cm.
Other Physical Features: endothermic ; bilateral symmetry
Information on predation of these monkeys is not available. Because they are arboreal, it is likely that they do not have many predators. Possible predators are large carnivores, snakes, and raptors.
There is no information available regarding the mating system of dusky leaf monkeys. However, the social system typically involves groups with only one or two males. In other primate species, this social organization is typically associated with polygynous breeding. It is reasonable to assume that this species is like other similar primates in this respect.
Mating System: polygynous
Breeding in dusky leaf monkeys is intermittent and not always seasonal. Births usually take place during the months of January, February, and March, but have been documented to occur during the summer months as well. Typically one young is born. The gestation period is, on average, 145 days.
Females have a menstual cycle lasting approximately three weeks. Oestrus is often accompanied by a swelling of the genitalia.
The normal interbirth interval is about 2 years.
Sexual maturity is reached between 3 and 4 years of age.
Breeding interval: These monkeys reproduce every two years.
Breeding season: These monkeys breed intermittantly throughout the year.
Range number of offspring: 1 to 2.
Average number of offspring: 1.
Average gestation period: 145 days.
Range age at sexual or reproductive maturity (female): 3 to 4 years.
Range age at sexual or reproductive maturity (male): 3 to 4 years.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; viviparous
Average birth mass: 300 g.
Average gestation period: 145 days.
Average number of offspring: 1.
Research on the parental behavior of these monkeys is lacking. However, we may assume that they are like other primates in that the mother provides the bulk of the parental care. She grooms, protects, and feeds the newborn. The role of the father in parental care is not known.
There is no information available regarding the weaning age of dusky leaf monkeys. It is known, however, that the newborn monkey is fully furred and active.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female); extended period of juvenile learning
The dusky leaf monkey (Trachypithecus obscurus), also known as the spectacled langur, or the spectacled leaf monkey is a species of primate in the family Cercopithecidae. It is found in Peninsular Malaysia, Myanmar and Thailand,[2] and can occasionally be found in Singapore.[3] During the day, these small, folivorous primates divide in sub-groups and forage for vegetation and fruit throughout the tropical forests.[4][5] According to the IUCN, the dusky leaf monkey's population is declining due to habitat loss, poaching, and anthropogenic land use, which prompted the IUCN to classify the species as endangered in 2015.[2]
Within the Cercopithecidae family, the dusky leaf monkey is a member of the lutung taxonomic group. Although most researchers agree with this classification, others believe that this primate should be categorized under the genus Presbytis.[6][7] Researchers have also disputed the number of subspecies that exist, with some claiming that there are 11,[8] while others claim there are seven. Currently, the following seven subspecies are officially recognized:[4][9][10]
The dusky leaf monkey can appear in different colour variations, depending on the sub-species. While dusky leaf monkeys are born with an orange coat, the colour of their fur changes as they mature into adulthood.[3] Along the abdominal area, their fur is often several shades lighter than that of their dorsal area, which can develop in shades of brown, black, or grey. Surrounding their eyes are circular patterns of bright white fur.[8][11][12]
The dusky leaf monkey is a rather small primate. In a study conducted by G. J Burton, the average weight of the 25 male dusky leaf monkeys that were sampled was 7.39 kilograms, while that of the 44 female dusky leaf monkeys that were sampled was 6.47 kilograms. These results suggest that dusky leaf monkeys are sexually dimorphic species, as the females were found to be, on average, 12% smaller than the males sampled. Burton's study also revealed that, among the 25 male primates, there was a direct correlation with the weight of one testicle with the weight of the other, suggesting that one testicle may be used to estimate the weight of the other. However, the female ovaries were found to not be statistically significant enough to predict the weight of one ovary with the other.[13]
The dusky leaf monkey can be found across Malaysia, Myanmar, and Thailand, where they inhabit tropical forests.[2] However, these primates have occasionally been spotted in Singapore. Research conducted within the country between 2019 and 2020 revealed that three dusky leaf monkey individuals were located in August 2019 within the northern region of the island, near the body of water that borders Singapore and Malaysia. While dusky leaf monkeys are not native to the region, researchers speculate that they are able to swim to the country. Researchers are concerned about these migratory patterns because these foreign primates can increase competition for resources, such as vegetation, and transfer diseases and pathogens to other native species that are not equipped to recover from them.[3]
Numerous activity budgets have been conducted across Malaysia to study the behavioural patterns of the dusky leaf monkey. For instance, one activity budget conducted in 2016 with 12,480 primates in Teluk Bahang, Malaysia, found that, within the eight-month period, dusky leaf monkeys spent 40% of their time positioning, 33% of their time feeding, and 20% of their day moving. The sampled primates also allocated their time allo-grooming (4%), playing (2%), and foraging (1%).[14] Ten years prior, another activity budget with 18 dusky leaf monkeys was conducted at the Penang Botanical Garden, which is also located in Malaysia. Researchers at the University of Kebangsaan concluded that, within 18 days, the dusky leaf monkeys that were sampled spent 40% of their day eating, almost 24% of their day resting, and approximately 22.5% of their time moving. The dusky leaf monkeys also allocated time for autogrooming (6.52%) and allo-grooming (1.33%), among other behaviours. However, a more recent study conducted in Malaysia from November 2017 to February 2018 in Bukit Soga Perdana, a forested area that is heavily influenced by industrial activity, concluded that the dusky leaf monkeys sampled spent the majority of their days moving, although these results were skewed due to unfavourable weather conditions that occurred during observations.[8]
Dusky leaf monkeys live in troops that separate into sub-groups while scavenging for food. They tend to consume leaves growing at higher elevations in the forest canopy. They often rest at these high elevations to feed on their foraged vegetation.[15][8] Fruit availability is season-dependent, and although research suggests that the dusky leaf monkey consumes more vegetation that fruit, it prefers to eat fruit when it is available.[5]
Research suggests that hostile behaviour within troops does not occur often. A study conducted in 1996 by researchers at the University of St. Andrews revealed that, after a group of dusky leaf monkeys engage in aggressive behaviour, conflict resolution occurs more frequently and much faster afterwards than within other primate species. In fact, reconciliation among dusky leaf monkeys can occur as quickly as 60 seconds after fighting, and can be instigated by either the aggressor or the receiver through gestures such as embracing.[16]
In another study conducted by Burton, 44 male dusky leaf-monkeys were sampled and the seminiferous tubules within each gonadal were observed. Burton concluded that the seminiferous tubules within each gonadal remained the same size throughout the year, and that there was no evidence of seasonal variation in testicular weight. This reveals that the male dusky leaf monkey does not have a seasonal mating period and are able to reproduce throughout the year. Burton suggests that this may be the case due to the continuous, warm climates experienced within Southeast Asia, as the mating period for many seasonal breeders occurs during warming climates.[17]
As of 2015, the dusky leaf monkey is listed as an endangered species in the IUCN Red List. Notably, anthropogenic land-use within Southeast Asian forests have resulted in the endangerment of the dusky leaf monkey's natural habitat, resulting in noticeable population declines.[18][8] However, the dusky leaf monkey also faces other threats brought on through anthropogenic activities and the introduction of predators within their habitat. Notably, domesticated dog packs have been reported to hunt and kill the dusky leaf monkey.[19] The construction of roads within tropical forests has led to a large increase in road mortality among dusky leaf monkeys, and is seen to be one of the more prominent threats to the dusky leafy monkey's survival, along with habitat loss, habitat degradation, and poaching.[20][18] While the dusky leaf monkey is often killed for their meat,[2] there are other incentives to poach them as well, as they are considered agricultural pests across Southeast Asia.[4] In the 1960s, this prompted Malaysian business owners to shoot them as they were feeding on leaves within rubber plantations.[13][17] Infant dusky leaf monkeys are also hunted and sold as pets internationally. However, when owners attempt to domesticate these primates, they are often not properly cared for and die as a result.[3]
Collective action is being taken to help conserve the dusky leaf monkey. In 1977, the dusky leaf-monkey had been introduced and listed under appendix II in the CITES legislation.[21] Species under appendix II are considered to be near threatened or endangered. The trade of appendix II species is heavily regulated, and often illegal.[22]
Dusky leaf monkey - Kaeng Krachan National Park
Lost dusky leaf monkey calling - Kaeng Krachan National Park.
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Dusky leaf monkey - Kaeng Krachan National Park
Adult male spectacled langur
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Dusky leaf monkey - Ko Tarutao
The dusky leaf monkey (Trachypithecus obscurus), also known as the spectacled langur, or the spectacled leaf monkey is a species of primate in the family Cercopithecidae. It is found in Peninsular Malaysia, Myanmar and Thailand, and can occasionally be found in Singapore. During the day, these small, folivorous primates divide in sub-groups and forage for vegetation and fruit throughout the tropical forests. According to the IUCN, the dusky leaf monkey's population is declining due to habitat loss, poaching, and anthropogenic land use, which prompted the IUCN to classify the species as endangered in 2015.
