Journal of Threatened Taxa |
www.threatenedtaxa.org | 26 June 2020 | 12(9): 15967–15974
ISSN 0974-7907 (Online) | ISSN 0974-7893
(Print)
doi: https://doi.org/10.11609/jott.5818.12.9.15967-15974
#5818 | Received 27 February 2020 | Final
received 08 April 2020 | Finally accepted 25 May 2020
Dusky Langurs Trachypithecus
obscurus (Reid, 1837) (Primates: Cercopithecidae)
in Singapore: potential origin and conflicts with native primate species
Andie Ang 1, Sabrina Jabbar 2 & Max Khoo 3
1,2 Raffles’ Banded Langur Working
Group, Wildlife Reserves Singapore Conservation Fund, 80 Mandai
Lake Road, 729826 Singapore.
3 Wildlife Management Research,
Wildlife Management Division, National Parks Board, 50 Dairy Farm Road, 679059
Singapore.
1 andie.ang@colorado.edu
(corresponding author), 2 macaque90@gmail.com, 3 max_khoo@nparks.gov.sg
Editor: Susan M. Cheyne, Borneo Nature Foundation
International, Palangka Raya, Indonesia. Date of publication: 26 June 2020
(online & print)
Citation: Ang,
A., S. Jabbar & M. Khoo (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. https://doi.org/10.11609/jott.5818.12.9.15967-15974
Copyright: © Ang et al. 2020. Creative
Commons Attribution 4.0 International License.
JoTT allows unrestricted use, reproduction,
and distribution of this article in any medium by providing adequate credit to
the author(s) and the source of publication.
Funding: The first two authors from
the Raffles’ Banded Langur Working
Group are funded by
Wildlife Reserves Singapore Conservation Fund.
Competing interests: The authors declare no competing interests.
Author details: Andie Ang is a research scientist with the Wildlife Reserves Singapore
Conservation Fund, managing a research and conservation project on the Raffles’
Banded Langurs in Singapore and Malaysia.
Sabrina Jabbar
is a research assistant with the Wildlife Reserves Singapore Conservation Fund,
working on the conservation of Raffles’ Banded Langurs. Max
Khoo is with the Wildlife Management Research team in the National Parks
Board, Singapore, and works on various terrestrial ecology research projects.
Author contribution: All authors contributed equally to the study design,
field work, data analysis and writing of article.
Acknowledgements: We thank Bryan Lim, Chanelle Lim, Ian Siah, Cindy Yeo, and Peter Chua for providing sighting
information of Dusky Langurs. We also thank citizen scientists Uriana Argeros, Alice Early,
Steve Early, and Katherine Kim for alerting authors to the sighting of Raffles’
Banded Langurs in Thomson Nature Park.
The first two authors from the Raffles’ Banded Langur Working Group are
funded by Wildlife Reserves Singapore Conservation Fund.
Abstract: The introduction of exotic
species can have detrimental effects on local populations via factors such as
resource competition and new threats from disease. Singapore has three native
species of non-human primates: Sunda Slow Loris Nycticebus coucang,
Long-tailed Macaque Macaca fascicularis, and Raffles’ Banded Langur Presbytis femoralis.
Over the past few months, several non-native Dusky Langurs Trachypithecus
obscurus were observed in Singapore. We document our observations, compile
reports from social media, and attempt to assess the potential impacts on local
primates. Whenever Dusky Langurs were encountered, we recorded the date, time,
GPS coordinates, group demographics, and behaviour,
including interactions with native primates. We also monitored sighting reports
of Dusky Langurs posted on local major Facebook groups from 30 December 2019 to
31 January 2020, and privately messaged the person(s) for more information. On
31 August 2019, three Dusky Langurs were seen near a residential area in the
northern part of Singapore, and two to three individuals were reported on 14
subsequent occasions. During one encounter on 18 January 2020, an adult male
Long-tailed Macaque chased a group of Dusky Langurs from a feeding tree. The
next day the same group of Dusky Langurs chased a group of 11 Banded Langurs
from another feeding tree. The Dusky Langurs appeared to be healthy and wild,
indicating that they may have swum across the Johor Strait and/or traveled on
the Johor-Singapore Causeway from Malaysia. Further monitoring of these Dusky
Langurs will be required to assess their impact on local primates.
Keywords: Colobine, dispersal, exotic,
impacts, macaque.
Introduction
Singapore is home to three species of non-human
primates—Sunda Slow Loris Nycticebus
coucang, Long-tailed Macaque Macaca
fascicularis fascicularis,
and Raffles’ Banded Langur Presbytis femoralis. While
the population size of the Sunda Slow Loris in
Singapore is unknown, it is considered nationally Critically Endangered (Lim et
al. 2008) and globally Vulnerable (Nekaris &
Streicher 2008). The Long-tailed Macaque
population has an estimated 1,800–2,200 individuals (Riley et al. 2015) and is
not facing immediate threats to survival (Ong & Richardson 2008). The Raffles’ Banded Langur is nationally
Critically Endangered (Lim et al. 2008) and is also considered Critically
Endangered globally given its small population size (60 individuals in
Singapore and 250-----–300 individuals in Malaysia), and a recent
recommendation to elevate it from subspecies to species (Ang et al. 2020).
In addition to these primate species, sightings of
others have been reported in Singapore over the years. Some sightings were attributable to pet
abandonment or illegal ownership of primate pets. For example in 2004 an African Vervet Monkey Chlorocebus
pygerythrus was repatriated to Zambia after being
kept as a pet in Singapore (ACRES 2004), and in 2018, five South American
marmosets were found abandoned in public areas in Singapore (Channel NewsAsia
2018). Other sightings may have been
arrivals from nearby areas. On 19
February 2008, a single adult Dusky Langur Trachypithecus
obscurus obscurus was observed at Kent Ridge
forest in southern part of Singapore, but its origin and subsequent fate were
unknown (Yeo & Lim 2013). In 2015, a
Southern Pig-tailed Macaque Macaca nemestrina was rescued from a car workshop and later
rehomed in Malaysia (AsiaOne 2015).
Many primates are able to swim, and there have been
instances of other animals reaching Singapore in this way; for example Malayan
Tapir likely swam across the straits from Malaysia into Singapore (Chew
2016). To the north of Singapore, the
Malaysian state of Johor is home to the non-human primates resident in
Singapore, as well as Dusky Langur, Southern Pig-tailed Macaque, and Malaysian
White-handed Gibbon Hylobates lar lar. To the
south of Singapore on the Indonesian islands of Bintan
and Batam are found Sunda
Slow Loris, Long-tailed Macaque, Bintan Pale-thighed
Langur P. siamensis rhionis,
and Silvered Langur T. cristatus cristatus.
Over the past few months several Dusky Langurs have
been observed in Singapore. Here we
document our observations, compile reports from social media, and attempt to
assess the potential impacts of Dusky Langurs on local primates.
Materials and Methods
At 10.30h on 31 August 2019, staff of the National
Parks Board (NParks) of Singapore were informed by a
member of public that a group of Dusky Langurs was seen at 891B Woodlands Drive
50 in the northern part of Singapore. NParks staff, including the last author, went to the
location at 12.35h to observe the Dusky Langurs until 19.15h that evening. The next day NParks
received further information about the location of what was presumably the same
group of langurs from members of the public.
On 30 December 2019, we were alerted to a photograph
of Dusky Langurs in Singapore which was posted in a Facebook group (Nature
Society Singapore). In order not to
alarm the public regarding the presence of a non-native primate species before
verifying the sighting, we did not publicly seek for observations on social
media platforms. Instead, we monitored
sighting reports of Dusky Langurs posted on local major Facebook groups (Birds,
Insects and Creatures of Asia; Nature Society Singapore; Wildlife of MacRitchie & Central Catchment) from 30 December 2019
to 31 January 2020, and privately messaged the person(s) for more information.
As part of an ongoing research project on Raffles’
Banded Langurs in Singapore, we conducted field observations on weekdays and
weekends within the Central Nature Reserve.
Whenever Dusky Langurs were encountered, we recorded the date, time, GPS
coordinates, group demographics (number of individuals, sex, and age group),
and behaviour, including interactions with native
primate species.
Results
A total of three Dusky Langurs with at least one male
(Image 1) were seen at 891B Woodlands Drive 50.
The Dusky Langurs were sighted on trees that were very close to
high-rise housing buildings, and spent most of their time between 12.35h and
19.15h on trees. On two occasions, the
Dusky Langurs used a building ledge, roof of a covered walkway (Image 2),
and/or the ground to get to another tree due to the lack of connectivity
between trees. On a separate occasion at
19.10h, one Dusky Langur descended from a tree to drink water from a drain
(Image 3). This langur was wary of its
surroundings and bolted back up the tree when humans approached, and only came
back down to continue drinking when people left. Throughout the entire duration of the
observation, the Dusky Langurs stayed within 150m of the location of the
initial sighting. On the next day, the
same group was sighted by members of the public at 556 Woodlands Drive 53 at
08.52h, 50 Woodlands Drive 16 at 10.20h, and 580 Woodlands Drive 16 at 14.17h
(Table 1). It appeared that the langurs
were travelling south-east from the initial sighting on 31 August 2019 (Image
4).
Subsequently, there were no records of Dusky Langurs
until 30 December 2019, where a photo of a group appeared on social media
(Facebook). The sighting happened at
Thomson Nature Park in the central part of Singapore within the Central Nature
Reserve, but details such as the time and number of langurs could not be
obtained. Two days later, another
sighting was reported in Thomson Nature Park.
Subsequently, we encountered a pair of langurs, an adult male (Image 5a)
and a subadult male (Image 5b), five times in January 2020 (Table 1). Based on photographs, both belonged to the
group that was initially seen in Woodlands.
The absence of a third individual (Image 5c) after 1 September 2019
could mean it died or otherwise left the group.
On 18 January 2020 at 17.20h, a member of the public
reported a sighting of two Dusky Langurs at Upper Peirce Reservoir Park, which
is adjacent to Thomson Nature Park, feeding on Saga Adenanthera
pavonina leaves.
Sometime later, a group of at least three Long-tailed Macaques
approached, and an adult male began chasing the langurs away from the tree
(Image 6a). Both species acted
aggressively by baring teeth (Image 6b), but no loud calls were heard during
the encounter and the langurs moved away.
On the next day in Thomson Nature Park we observed a group of 11
Raffles’ Banded Langurs feeding on Saga seeds (Image 7). At 16.15h, two Dusky Langurs appeared and
chased after the banded langurs. Alarm
calls were exchanged and the Banded Langurs scattered out of sight. The Dusky Langurs then took over the Saga
tree, although feeding was not observed.
The following day at 08.12h, we observed the same pair of two Dusky
Langurs (facially identified using photographs) at Thomson Nature Park, 203m
(measured on Google Earth) from the site of encounter with the banded langurs a
day earlier. As the dusky langurs
sighted in both Woodlands and Thomson Nature Park have been identified to be
the same group, this would mean that the Dusky Langurs travelled from the
northern part to the central part of Singapore in approximately four months
between 31 August 2019 and 30 December 2019.
Discussion
Dusky Langurs, especially the orange-colored infants
are a popular pet in the illegal wildlife trade and are often sold within and
outside Malaysia (The Straits Times 2019).
These individuals often do not survive till adulthood, as they are kept
in poor conditions (e.g., cages) and are not fed their natural diet (Mariani Ramli pers. comm. 11 February 2020). The Dusky Langurs sighted at Woodlands and
Thomson Nature Park appeared to be healthy, wary of humans, and adapting well
to the local wild habitat. The pair who
remained in Thomson Nature Park were an adult and subadult. This indicates that these langurs were
unlikely pet releases or escapees, and may have entered Singapore by land over
the Johor-Singapore Causeway at Woodlands (960m), and/or by water by swimming
across the Johor Strait from Malaysia to Singapore (750m). Many primates, including Dusky Langurs are
known to be able to swim (Zainol et al. 2019). Silvered Langur Trachypithecus
cristatus, a closely-related species to the Dusky
Langur, have been observed swimming in the sea next to Indonesia’s Bintan Island (Supplementary Video). Like most other quadruped mammals, langurs
use the dog-paddle style to swim, i.e., with each diagonal pair of limbs moving
alternately through the water directly beneath their body (Dagg
& Windsor 1972; Gabbatiss 2017). Moreover, Asian colobine primates like Dusky
Langurs often exhibit male dispersal when they reach subadult-hood. Given this, along with the initial sighting
of the Dusky Langurs at Woodlands being very close to Johor, it is plausible
that the Dusky Langurs may have come from neighbouring
Malaysia.
Dusky Langurs and Raffles’ Banded Langurs are known to
be sympatric in a number of forests in Peninsular Malaysia (states of Johor and
Pahang) (see Roos et al. 2014; Ang & Baker 2019) but Dusky Langurs are not
known to be native in Singapore (Raffles 1821).
While the two species have evolved sympatrically
and partitioned resources in the same habitat in Peninsular Malaysia during the
Pleistocene (Ang et al. 2020), the sympatric occurrence of Dusky Langurs and
Raffles’ Banded Langurs in Singapore is novel.
On one occasion in Thomson Nature Park, although there were only two
Dusky Langurs, they displayed aggressive territorial behaviour
toward the native and Critically Endangered Raffles’ Banded Langurs and
displaced them from a food resource. As
the Raffles’ Banded Langurs are naturally more shy and are smaller in body size
than the Dusky Langurs, displacement of Raffles’ Banded Langurs from within
their home range by Dusky Langurs may occur given that the two langur species
share similar food resources as primarily folivores. As such, further monitoring is required to
ascertain if the presence of Dusky Langurs is negatively impacting the Raffles’
Banded Langur in Singapore in the short term and long term.
The Long-tailed Macaques and Dusky Langurs are also
sympatric in forests of Peninsular Malaysia (Bernstein 1967; Johns 1986; Ruslin et al. 2019).
The diet of both species overlaps to a lesser extent as compared to the
Dusky Langur and Raffles’ Banded Langur, with the Long-tailed Macaque being
primarily frugivorous, and the Dusky Langur being primarily folivorous
but with a large part of their diet being fruits and seeds as well (Johns 1986;
Sha & Hanya 2013). It has been hypothesized that Dusky Langurs
can switch to feeding on leaves to avoid competition, while Long-tailed
Macaques could shift to eating anthropogenic foods (Ruslin
et al. 2019). Along with Long-tailed
Macaques being comparably more assertive in defending their resources, the
impacts of Dusky Langurs on Long-tailed Macaques can be expected to be lower as
compared with the impacts on Raffles’ Banded Langurs. The observation of the adult Long-tailed Macaque
that chased away the two Dusky Langurs at Upper Peirce Reservoir Park on the
feeding tree further illustrates the ability of Long-tailed Macaques to defend
food resources.
Non-native species can present risks to native
biodiversity through the transmission of diseases (Hatcher et al. 2012). The introduction of parasites and pathogens
can have serious implications at a population level due to a lack of immunity
in the new hosts (Peeler et al. 2011).
Islands are also highly vulnerable to the impacts of non-native species
as the native biodiversity have often evolved in isolation without diseases
that are found on continents (IUCN 2018).
Additionally, introduced species can present detrimental effects to
local biodiversity if it is able to hybridize with closely-related local
species. In primates, hybridization at
both the genus and species levels have resulted in speciation at the population
level (Zinner et al. 2011) but hybrids may outcompete
native species at a local level (see Oliveira & Grelle
2012). In the subfamily Colobinae, to which the Dusky Langur and Raffles’ Banded
Langur belong, there has been evidence of multiple hybridization events such as
with the genera Trachypithecus and Semnopithecus (Roos et al. 2011). More recently, there have been two separate
hybridization events of the Francois’ Langur (T. francoisi)
and White-headed Langur (T. leucocephalus)
(Liu et al. 2013). As for the Dusky
Langur and the Raffles’ Banded Langur, there have been no records of
hybridization in Peninsular Malaysia where both species are sympatric. As such,
hybridization may not be an issue between these two species. It is unlikely for
the Dusky Langur and the Long-tailed Macaque to hybridize given that both
species belong to different subfamilies.
Non-native species introduced into an existing
ecosystem can disrupt local biodiversity and environment in the form of
resource competition, disease transmission, hybridization, among others. Even though the two Dusky Langur males will
not be able to establish a population in Singapore in the long-term (unless
further dispersals happen), it is nonetheless vitally important to continue
monitoring efforts on the Dusky Langurs and the native biodiversity so as to
assess their impacts and to devise measures to minimize the risks. If it is assessed to be necessary to
translocate the Dusky Langurs out of Singapore and back to their native
habitat, proper and ethical protocols must be followed (e.g., the IUCN
Guidelines for Reintroductions and Other Conservation Translocations; IUCN SSC
2013). Among other requirements, it is
important to determine the most likely source of the Dusky Langurs by doing
genetic analyses, to ensure that the health and safety of the Dusky Langurs are
not compromised during the translocation process, and to select a suitable destination
habitat that is under legal protection for the long-term survival of the
langurs.
Table 1. Sightings of Dusky Langurs in Singapore.
Date |
Time |
No. of individuals |
Location |
Observer(s) |
31.viii.2019 |
10.30h |
3 |
891B Woodlands Drive 50 |
Member of public |
31.viii.2019 |
12.35–19.15 h |
3 |
891B Woodlands Drive 50 |
Bryan Lim, and authors |
01.ix.2019 |
- |
3 |
10 Woodlands Drive 50 |
Member of public |
01.ix.2019 |
08.52h |
3 |
556 Woodlands Drive 3 |
Member of public |
01.ix.2019 |
10.20h |
3 |
50 Woodlands Drive 16 |
Member of public |
01.ix.2019 |
14.17h |
3 |
580 Woodlands Drive 16 |
Member of public |
01.ix.2019 |
Afternoon |
3 |
576 Woodlands Drive 16 |
Member of public |
30.xii.2019 |
- |
- |
Thomson Nature Park |
Gerald Lim (Facebook) |
01.i.2020 |
17.45h |
2 |
Thomson Nature Park |
Ian Siah, Ethan Siah, Raphael Siah, Cheryl Goh
(Facebook) |
03.i.2020 |
08.00h |
2 |
Thomson Nature Park |
Authors |
07.i.2020 |
08.36h |
2 |
Thomson Nature Park |
Authors |
18.i.2020 |
17.20h |
2 |
Upper Peirce Reservoir Park |
Cindy Yeo (Facebook) |
19.i.2020 |
16.15h |
2 |
Thomson Nature Park |
Authors, Rahayu Oktaviani, Alice Early, Steve Early, Katherine Kim, Uriana Argeros |
20.i.2020 |
08.12h |
2 |
Thomson Nature Park |
Authors |
22.i.2020 |
17.40h |
2 |
Old Upper Thomson Road |
Authors |
References
ACRES (2004). First ever repatriation of a confiscated pet primate.
http://acres.org.sg/core/wp-content/uploads/2015/05/4_may_04_first_ever_repatriation_of_a_confiscated_pet_primate.pdf Accessed 2 February 2020.
Ang, A. & N. Baker (2019). Raffles’ banded langur Presbytis
femoralis femoralis in
southern Pahang, Peninsular Malaysia. Southeast Asia Vertebrate Records
2019: 52–53.
Ang, A., D.I. Roesma, V. Nijman,
R. Meier, A. Srivathsan & Rizaldi
(2019). Faecal DNA to the rescue: Shotgun sequencing of non-invasive
samples reveals two subspecies of Southeast Asian primates to be Critically
Endangered species. Scientific Reports 10: 9396. https://doi.org/10.1038/s41598-020-66007-8
AsiaOne (2015).
Endangered Pig-tailed Macaque rehomed in Malaysia. AsiaOne,
24 August 2015. https://www.asiaone.com/singapore/endangered-pig-tailed-macaque-rehomed-malaysia
Accessed 2 February 2020.
Bernstein, I.S. (1967). Intertaxa interactions in a
Malayan primate community. Folia Primatologica
7: 198–207. https://doi.org/10.1159/000155119
Channel NewsAsia (2018). Wildlife group rescues abandoned marmoset seen
‘appearing lost’ at Punggol HDB. Channel NewsAsia, 17 December 2018. https://www.channelnewsasia.com/news/singapore/abandoned-monkey-marmoset-punggol-lost-rescued-acres-11039274
Accessed 2 February 2020.
Chew, H.M. (2016). Rare sighting of endangered Malayan tapir at Changi.
The Straits Times, 25 June 2016. https://www.straitstimes.com/singapore/rare-sighting-of-endangered-malayan-tapir-at-changi
Accessed 2 February 2020.
Dagg, A.I. & D.E. Windsor (1972). Swimming in northern terrestrial mammals. Canadian
Journal of Zoology 50: 117–130. https://doi.org/10.1139/z72-019
Gabbatiss, J. (2017). The strange experiments that revealed most mammals
can swim. BBC Earth, 21 March 2017. http://www.bbc.com/earth/story/20170320-the-cruel-experiments-that-revealed-most-mammals-can-swim
Accessed 8 April 2020.
Hatcher, M.J., J.T.A. Dick & A.M. Dunn (2012). Disease emergence and invasions. Functional
Ecology 26: 1275–1287. https://doi.org/10.1111/j.1365-2435.2012.02031.x
IUCN (2018). Guidelines for invasive species planning and
management on islands. Cambridge, UK and Gland, Switzerland, 40pp.
IUCN SSC (2013). Guidelines for reintroduction and other conservation
translocations. Version 1.0. Gland, Switzerland, 57pp.
Johns, A.D. (1986). Effects of selective logging on the behavioral
ecology of west Malaysian primates. Ecology 67(3): 684–694. https://doi.org/10.2307/1937692
Lim, K.K.P., R. Subaraj,
S.H. Yeo, N. Lim, D. Lane & B.Y.H. Lee (2008). Mammals, pp.190–207. In: Davison, G.W.H., P.K.L. Ng
& H.C. Ho (eds.). The Singapore Red Data Book. Threatened Plants &
Animals of Singapore. 2nd Edition. The Nature Society
(Singapore), Singapore, 285pp.
Liu, Z., B. Wang, T. Nadler, G. Liu, T. Sun, C. Huang,
Q. Zhou, J. Zhou, T. Que, Z. Wang & C. Roos (2013). Relatively recent evolution of pelage coloration in Colobinae: phylogeny and phylogeography
of three closely related langur species. PLoS
One 8(4): e61659. https://doi.org/10.1371/journal.pone.0061659
Nekaris, A. & U. Streicher (2008). Nycticebus coucang. In: IUCN 2008. 2008 IUCN Red List of
Threatened Species. Downloaded on 1 February 2020.
Oliveira, L.C. & C.E.V. Grelle
(2012). Introduced primate species of an
Atlantic Forest region in Brazil: present and future implications for the
native fauna. Tropical Conservation Science 5(1): 112–120. https://doi.org/10.1177/194008291200500110
Ong, P. & M. Richardson (2008). Macaca fascicularis fascicularis. In:
IUCN 2008. 2008 IUCN Red List of Threatened Species. Downloaded on 1 February
2020.
Peeler, E.J., B.C. Oidtmann,
P.J. Midtlyng, L. Miossec
& R.E. Gozlan (2011). Non-native aquatic animals introductions have driven
disease emergence in Europe. Biological Invasions 13(6): 1291–1303. https://doi.org/10.1007/s10530-010-9890-9
Raffles, S.T. (1821). Descriptive catalogue of a zoological collection made
in the island of Sumatra and its vicinity. Transactions of the Linnean Society of London 13: 246.
Riley, C.M., S.L. Jayasri
& M.D. Gumert (2015). Results of a nationwide census of the long-tailed
macaque (Macaca fascicularis)
population of Singapore. Raffles Bulletin of Zoology 63: 503–515.
Roos, C., D. Zinner, L.S. Kubatko, C. Schwarz, M. Yang, D. Meyer, S.D. Nash, J. Xing,
A.M. Batzer, M. Brameier,
F.H. Leendertz, T. Ziegler, D. Perwitasari-Farajallah,
T. Nadler, L. Walter & M. Osterholz (2011). Nuclear versus mitochondrial DNA: evidence for hybridization
in colobine monkeys. BMC Evolutionary Biology 11(1): 77. https://doi.org/10.1186/1471-2148-11-77
Roos, C., R. Boonratana, J. Supriatna, J.R. Fellowes, C.P. Groves, S.D. Nash, A.B.
Rylands & R.A. Mittermeier (2014). An updated taxonomy and conservation status review of
Asian primates. Asian Primates Journal 4(1): 2–38.
Ruslin, F., I. Matsuda & B.M. Md-Zain (2019). 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. https://doi.org/10.1007/s10329-018-00705-w
Sha, J.C.M. & G. Hanya
(2013). Diet, activity, habitat use, and
ranging of two neighboring groups of food-enhanced long-tailed macaques (Macaca fascicularis).
American Journal of Primatology 75(6): 581–592. https://doi.org/10.1002/ajp.22137
The Straits Times (2019). Celebs and influencers in Malaysia sharing shots of
exotic pets, fueling concern for illegal wildlife trade. The Straits Times, 22
July 2019. https://www.straitstimes.com/asia/se-asia/celebs-and-influencers-in-malaysia-sharing-shots-of-exotic-pets-fuelling-concern-of
Assessed 3 February 2020.
Yeo, H.H.T. & K.K.P. Lim (2013). Dusky langur at Kent Ridge. Singapore Biodiversity
Records 2013: 125.
Zainol, N., A.N. Roshidi, S.M. Husin, A. Md-Shukor, R. Ilias & S. Md-Nor (2019). Impact of forest isolation on mammals diversity and
distribution due to impoundment of Hulu Terengganu hydroelectric project,
Terengganu, Peninsular Malaysia. International Conference on Dam Safety
Management and Engineering 2019: 468–478.
Zinner, D., M.L. Arnold & C. Roos (2011). The strange blood: natural hybridization in primates.
Evolutionary Anthropology: Issues, News, and Reviews 20(3): 96–103. https://doi.org/10.1002/evan.20301