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Dusky leaf monkey

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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]

Dusky leaf monkey[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Cercopithecidae
Genus: Trachypithecus
Species group: Trachypithecus obscurus group
Species:
T. obscurus
Binomial name
Trachypithecus obscurus
(Reid, 1837)
Dusky leaf monkey range

Taxonomy

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Within the Cercopithecidae family, the dusky leaf monkey is a member of the genus Trachypithecus. Although most researchers agree with this classification, others believe that this primate should be categorized under the genus Presbytis.[6][7]

Subspecies

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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]

  • T. o. obscurus
  • T. o. flavicauda
  • T. o. halonifer
  • T. o. carbo
  • T. o. styx
  • T. o. seimundi
  • T. o. sanctorum[4][9][10]

Description

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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]

Distribution

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While the dusky leaf monkey's fur can develop into shades of grey, brown, or black, they are born with an orange coat.

The dusky leaf monkey can be found across Malaysia, Myanmar, and Thailand, where they inhabit tropical forests.[2]

Singapore population

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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 considered native to the region, researchers speculate that they are able to swim to the country. Researchers are concerned about these migratory patterns because these 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] Despite this, the species is not considered an invasive species, as their colonization of Singapore is natural and not assisted by humans.

Behaviour and diet

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Dusky leaf monkeys feed in the emergent and canopy layers.

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.[citation needed] 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]

Mating

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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]

Conservation

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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]

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References

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  1. ^ Groves, C. P. (2005). Wilson, D. E.; Reeder, D. M. (eds.). Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Baltimore: Johns Hopkins University Press. p. 177. ISBN 0-801-88221-4. OCLC 62265494.
  2. ^ a b c d e Boonratana, R.; Ang, A.; Traeholt, C.; Thant, N.M.L. (2020). "Trachypithecus obscurus". IUCN Red List of Threatened Species. 2020: e.T22039A17960562. doi:10.2305/IUCN.UK.2020-2.RLTS.T22039A17960562.en. Retrieved 15 November 2021.
  3. ^ a b c d Ang, A.; Jabbar, S.; Khoo, M. (2020). "Dusky Langurs Trachypithecus obscurus (Reid, 1837) (Primates: Cercopithecidae) in Singapore: potential origin and conflicts with native primate species". Journal of Threatened Taxa. 12 (9): 15967–15974. doi:10.11609/jott.5818.12.9.15967-15974. S2CID 225650126.
  4. ^ a b c d Nor Rahman Aifat; Muhammad Abu Bakar Abdul-Latiff; Christian Roos; Badrul Munir Md-Zain (2020). "Taxonomic Revision and Evolutionary Phylogeography of Dusky Langur (Trachypithecus obscurus) in Peninsular Malaysia". Zoological Studies. 59 (59): 2–11. doi:10.6620/ZS.2020.59-64. PMC 8181166. PMID 34140981.
  5. ^ a b Ruslin, Farhani; Matsuda, Ikki; Md-Zain, Badrul Munir (2019-01-01). "The feeding ecology and dietary overlap in two sympatric primate species, the long-tailed macaque (Macaca fascicularis) and dusky langur (Trachypithecus obscurus obscurus), in Malaysia". Primates. 60 (1): 41–50. doi:10.1007/s10329-018-00705-w. ISSN 1610-7365. PMID 30564972. S2CID 56170185.
  6. ^ Napier, John Russell; Napier, P. H. (1967). A handbook of living primates: morphology, ecology and behaviour of nonhuman primates. London: Academic. ISBN 0-12-513850-4. OCLC 255860.
  7. ^ Thongnetr, Weera; Sangpakdee, Wiwat; Tanomtong, Alongklod; Nie, Wenhui; Raso, Sayam; Pinthong, Krit (2021-11-27). "The chromosomal homology between dusky langur (Trachypithecus obscurus Ried, 1837) and human (Homo sapiens) revealed by chromosome painting". The Nucleus. 65 (2): 233–237. doi:10.1007/s13237-021-00381-0. ISSN 0976-7975. S2CID 244740122.
  8. ^ a b c d e Siti-Kauthar, M.; Najmuddin, M. F.; Md-Zain, B. M.; Abdul-Latiff, M. A. B. (2019). "PrimaTourism: preliminary study on activity budget of dusky leaf monkey Trachypithecus obscurus obscurus in Bukit Soga Perdana, Batu Pahat, Johor". IOP Conference Series: Earth and Environmental Science. 269 (1): 012045. Bibcode:2019E&ES..269a2045S. doi:10.1088/1755-1315/269/1/012045. S2CID 201211018.
  9. ^ a b Don E. Wilson; DeeAnn M. Reeder (2005). Mammal species of the world : a taxonomic and geographic reference (3rd ed.). Baltimore: Johns Hopkins University Press. ISBN 0-8018-8221-4. OCLC 57557352.
  10. ^ a b "Trachypithecus obscurus". Integrated Taxonomic Information System. Retrieved 2 November 2021.
  11. ^ Groves, Colin (2001). Primate Taxonomy. Washington, DC: Smithsonian Books. ISBN 978-1560988724.
  12. ^ Roos C, Boonratana R, Supriatna J, Fellowes JR, Groves CP, Nash SD, Rylands AB, Mittermeier RA (2014). "An updated taxonomy and conservation status review of Asian primates". Asian Primates Journal. 4.
  13. ^ a b Burton, G.J. (1981). "The Relationship Between Body and Gonadal Weights of the Dusky Leaf Monkey (Presbytis obscura)". International Journal of Primatology. 2 (4): 351–368. doi:10.1007/BF02693484. S2CID 42972066.
  14. ^ Yap, J.; Ruppert, N.; Fadzly, N. (2016). Activity patterns and diet of a group of wild dusky leaf monkeys (Trachypithecus obscurus) in Penang, Malaysia. Seminar Ekologi Malaysia.
  15. ^ G. Gene Montgomery (1978). The ecology of arboreal folivores : a symposium held at the Conservation and Research Center, National Zoological Park, Smithsonian Institution, May 29-31, 1975. Washington, D.C.: Smithsonian Institution Press. ISBN 0-87474-646-9. OCLC 4004986.
  16. ^ Arnold, K.; Barton, R. (2000). "Postconflict Behavior of Spectacled Leaf Monkeys (Trachypithecus obscurus). I. Reconciliation". International Journal of Primatology. 22 (2): 243–266. doi:10.1023/A:1005623631652. S2CID 7025717 – via ResearchGate.
  17. ^ a b Burton, G. J. (1984). "Testicular histology of the dusky leaf monkey (presbytis obscura) as it relates to birth pattern in Peninsular Malaysia". International Journal of Primatology. 5 (2): 183–195. doi:10.1007/BF02735740. ISSN 0164-0291. S2CID 41458567.
  18. ^ a b Boonratana, Ramesh. "Of road kills and dead langurs. Malaysian Naturalist". CAB Reviews Perspectives in Agriculture Veterinary Science Nutrition and Natural Resources.
  19. ^ MOHD FAUDZIR, N.; Haris, H.; Atiqah Norazlimi, N.; Munir Md Zain, B. "PREDATION OF DOMESTIC DOGS (Canis lupus familiaris) ON SCHLEGEL'S BANDED LANGUR (Presbytis neglectus) AND CRESTED HAWK-EAGLE(Nisaetus cirrhatus) ON DUSKY LEAF MONKEY (Trachypithecus obscurus)IN MALAYSIA". Journal of Sustainability Science and Management. 14 (6): 43.
  20. ^ Lappan, Susan. "Primate Research And Conservation In Malaysia" (PDF). CAB Reviews Perspectives in Agriculture Veterinary Science Nutrition and Natural Resources: 5.
  21. ^ "Species+". speciesplus.net. Retrieved 2022-04-11.
  22. ^ "Convention on International Trade in Endangered Species of Wild Fauna and Flora | CITES". cites.org. Retrieved 2022-04-11.
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