Journal of Threatened Taxa |
www.threatenedtaxa.org | 26 December 2021 | 13(14): 20302–20306
ISSN 0974-7907 (Online) | ISSN 0974-7893
(Print)
https://doi.org/10.11609/jott.7401.13.14.20302-20306
#7401 | Received 05 May 2021 | Final received
16 October 2021 | Finally accepted 01 December 2021
Crop and property damage caused
by Purple-faced Langurs Trachypithecus vetulus (Mammalia: Primates: Cercopithecidae)
Vincent Nijman
School of Social Sciences, Oxford
Brookes University, Gipsy Lane, Oxford OX3 0BP, UK.
Date of publication: 26 December 2021 (online & print)
Citation: Nijman, V. (2021). Crop and property damage caused
by Purple-faced Langurs Trachypithecus vetulus (Mammalia: Primates: Cercopithecidae).
Journal of Threatened Taxa 13(14): 20302–20306. https://doi.org/10.11609/jott.7401.13.14.20302-20306
Copyright: © Nijman 2021. 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.
Competing interests: The author
declares no competing interests.
Acknowledgements: Thank Lilia Bernede,
Alex Boulton, Richard Moore, Anna Nekaris, Will
Westwood, and Gehan da Silva Wijeyratne
for sharing their experiences with Sri Lanka’s primates. I thank the editor and
a reviewer for constructive comments and suggestions for improvement.
Sri Lanka is an excellent area to
study the relationships between Humans Homo sapiens and commensal
primates: the island is home to slender lorises, macaques and langurs and, for
the last 40,000 years or so, humans (Nekaris & de
Silva Wijeyeratne 2007; Nijman & Nekaris 2013). The extensive presence of home gardens,
mimicking the forest structure by adopting different layers, allows primates to
come into close contact with humans, and the predominant religions adopted by
Sri Lankans may facilitate a peaceful co-existence (cf. Rudran
2008; Rudran et al. 2021). Repeated crop-raiding and
conflict over space, however, may cause people to become increasingly
intolerant to primates.
Wijethilaka et al. (2021) lamented the small
number of reports on conflicts between Humans and Western Purple-faced Langur Trachypithecus vetulus nestor. In their introduction and discussion Wijethilaka et al. (2021) refer to a large number of
studies that report on Human-primate conflict and crop-raiding involving a wide
range of primate species, including Chimpanzees Pan troglodytes, Olive
Baboons Papio anubis,
Red-tailed Monkeys Cercopithecus ascanius, Vervet Monkeys Chlorocebus
pygerythrus, Rhesus Macaques Macaca
mulatta, Hanuman Langurs Semnopithecus
entellus, and Bonnet Macaques Macaca radiata.
What they overlooked were those studies that had been conducted on Western
Purple-faced Langurs. To rectify this and to provide the reader of the Journal
of Threatened Taxa with a more complete (chronological) overview I here
give brief summaries of the outcomes of these studies and how they concur or
contrast with that reported by Wijethilaka et al.
(2021).
Wijethilaka et al. (2021) evaluated the
intensity of Human-langur conflict through the identification of the crop and
property damages caused by these langurs. Their assessment was based on
interviews with 80 households in villages bordering Danawkanda
Forest in western Sri Lanka collected over a six-month period in 2014-2015.
They also quantified the loss (financial and otherwise) reportedly incurred to
these households. Over 90% of the interviewees indicated that the damage had
been done to commercially important plants (primarily Banana Musa paradisiaca, Papaya Carica
papaya, Rambutan Nephelium lappaceum and Mango Mangifera
indica). Three-quarter reported damage of roof
tiles and over 40% reported having had frightful encounters with the langurs. Wijethilaka et al. (2021) found a strong negative
correlation between the amount of money spent on firecrackers (a proxy for the
intensity of crop raiding and used by 99% of the interviewees) and the distance
between Danawkanda Forest and the interviewees’
residence or fields. The research I refer to that also reports on crop and
property damage caused by Western Purple-faced Langurs was based on interviews,
similar to Wijethilaka et al. (2021), workshops,
direct observations, and often a combination of these methods (Table 1).
In their ‘Primates of Sri Lanka’,
Nekaris & de Silva Wijeyeratne
(2007), based on extended periods of fieldwork in western Sri Lanka, noted
specifically for the Western Purple-faced Langur that because the animals
eventually come into contact with people, conflict is inevitable. This conflict
mainly involved crop raiding (primarily of fruit trees) and using rooftops as
‘runways’ and thereby damaging the tiles. Interviews with local people showed
that conflict was greatest in areas with the largest populations (of langurs),
and in particular with those people who came into daily contact with the
langurs. They noted that conflict can be so severe that landowners may shoot
the langurs (Nekaris & de Silva Wijeyeratne 2007).
Parker et al. (2008) conducted
735 interviews throughout the range of the Western Purple-faced Langur. At the
village level, estimates of local langur population size (by means of counting
langurs directly as well as interviews) did not correlate with positive or
negative perceptions towards langurs but the proportion of interviewees that
held negative views towards the langurs was correlated with the time the
langurs spent in their gardens and farmland (more time equates to more negative
feelings). A quarter of all interviewees considered the langurs a nuisance,
primarily because, just as in Wijethilaka et al.’s
study, of the langurs raiding on Jackfruit Arctocarpus
heterophylus, Rambutan, Banana and Mango.
Destruction of roof tiles was reported as a problem. Several deterrents were
used by the interviewees to drive langurs from their land, including throwing
stones, shouting or setting dogs on them, and using firecrackers. Nijman & Nekaris (2008) noted that the Western Purple-faced Langur
appeared to be largely dependent on fruit grown in traditional home gardens,
making the langurs almost wholly dependent on the tolerance of local people.
With increasing urbanization, perceptions by local people of the langurs had
changed for the worse. Negative attitudes towards the langurs increased with
the amount of time langurs spend in close proximity to people. Besides the loss
of habitat, attacks on langurs by landowners’ and stray Dogs Canis familiaris
add another source of mortality, and the need for arboreal continuity has put
them into lethal contact with electric power lines in this urban domain.
Eschmann et al. (2008) and Moore
et al. (2010), over a seven-month period, studied five groups of Western
Purple-faced Langur and Humans in a suburban area in Talangama,
where the langur’s homeranges completely overlapped
with Human living space. Twelve of the thirteen trees most used for feeding by
the langurs were also used by Humans, with Jack Fruit, Mangos and Rambutans
topping the list. They noted that when langurs crossed rooftops of occupied
houses, the owners often chased them away or complained about damage the
langurs caused. The most common methods to chase the langurs out of feeding
trees or off rooftops were using firecrackers followed by throwing stones.
Eschmann et al. (2008) reported that to the meagre canopy continuity in Talangama exposed much of the langurs’ homeranges
to direct sunlight. While this in turn led to the langurs to start calling
earlier in the morning compared to groups living in forests, overall Eschmann
et al. (2008) found the langurs to be remarkably desensitised to the effects of
most Human activities.
Nijman & Nekaris
(2010) analysed data from 48 interviewees from villages with and without forest
in its vicinity and found that the intensity of conflict, and especially
reports of the killing of langurs, was higher in villages where the forest was
no longer present. The amount of forest and trees buffers against the need to
feed on Human-grown foods or the use of man-made structures as arboreal
pathways.
Of all the studies reported here,
the one by Dela (2011) is most closely aligned to
that of Wijethilaka et al. (2021) in terms of
methodology, results, and conclusions. In 1987, she conducted a questionnaire
survey with 112 participants in Panadura and Piliyandalawhere
where she also studied the langurs (Dela 2007). Roof
and crop damage were the most frequently cited damage (64% and 23% of
interviewees, respectively) and conflict was most prominent when the langurs
fed on Mango, Banana and Bread Fruit A. incisus.
Two-thirds of the interviewees resorted to making loud noises (firecrackers,
shouting) and one third to throwing stones or using slingshots or catapults to
deter crop raiding. When corrected for inflation to June 2021, the distribution
of household income in Dela’s (2011) study was
similar to that of Wijethilaka et al. (2021), with
~80% of people earning less than US$160–180 month-1. While the
desire to destroy langurs was higher in households with an income of over
US$160 month-1 compared to those that earned less (i.e., 12.5% vs
6.2%) the difference was not statistically significant (Dela
2011). Dela’s (2011) is one of the most comprehensive
study on how the perceptions and real day-to-day experiences of people have an
effect of on both the langur’s habitat and the langurs themselves and her
findings and conclusions, despite being partially based on a survey conducted
over three decades ago, ring true to this day.
Nijman and Nekaris
(2013) advocated the adoption an ethnographic perspective as this would allow
for mitigation policies to be defined with regard to a local cultural context
in which traditions and religious parameters often exist for the preservation
or to the detriment of wildlife. Based on 182 days of observations of langurs
(and Toque Macaques M. sinica) and an
interview survey of 1,036 people they assessed levels of conflict between
Humans and primates both in an agricultural and urban context. With respect to
the Western Purple-faced Langur they found generally high levels of tolerance
in both settings, but high levels of crop-raiding along the forest edge and
frequent use of roofs as part of arboreal pathways caused discontent. They concluded
that a heavy reliance on fruit by otherwise folivorous
langurs may compromise their dietary needs and parasite loads of commensal
primates were suspected to be unhealthily high.
Weerakkody et al. (2018) found that in
urban and semi-urban landscapes the homerange size of
Western Purple-faced Langurs is both determined by the resource base (e.g., the
number of fruiting trees) and impassable boundaries (rice fields, houses);
these anthropogenetic factors hinder the movement of
the arboreal langurs across the landscape, and may explain the proximate causes
of Human-langur conflict.
Rudran et al. (2020b) addressed in
detail and with great consideration how to deal the issue of engaging local
communities in the conservation of Western Purple-faced Langurs at a time when
the people are facing difficulties caused by the conflicts with the langurs.
While they did not present new data on crop or property damage caused by the
langurs, the solutions they offer are relevant to the challenges highlighted by
Wijethilaka et al. (2021). Finally, for completeness
I will discuss Rudran et al. (2021) who reported on a
mitigating Human-primate conflict study by means of interviewing people from 11
districts in Sri Lanka. This paper appeared after the publication of Wijethilaka et al. (2021) but data collection for this
study had commenced only one year after Wijethilaka
et al. (2021) had finished collecting theirs (viz., it ran from January 2016 to
June 2018). While Rudran et al. (2021)’s study
covered all three diurnal primate species, some of their data can be attributed
to Western Purple-faced Langurs as it specifies data for Colombo District. Of
the 147 people interviewed in Colombo, 70% were Buddhist and 8% Hindu. In
contrast to reports by Wijethilaka et al. (2021) levels
of tolerance towards Western Purple-faced Langurs was high (ranked 4.3 on a
scale from 1 to 6) something that Rudran et al.
(2021) attributed to the majority of interviewees practising Buddhism which
preaches compassion to all living beings. The reported monthly cost, corrected
for inflation to June 2021, of damage due to Western Purple-faced Langur and
Toque Macaques in Colombo ranged from less than US$2.79 (19% of respondents),
US$2.80-5.59 (36%), US$5.60-27.96 (28) to more than US$27.97 (16%). This is
within the same range as the monthly average of US$10.05-18.10 reported by Wijethilaka et al. (2021).
The overview presented here
demonstrate that a great deal is known conflict between Humans and Western
Purple-faced Langurs. This is not surprising as arguably the Purple-faced
Langur is among the most thoroughly studied colobine monkeys globally (Table
1). It is also clear that over time there has been a shift in research focus
from purely ecology and behaviour of the langurs in the 1970s to 1990s to a mixture
of ecology and behaviour, and ethnoprimatology during
the last two decades (Table 1). There is a great level of concordance between
the data from Danawkanda and that from Panadura, Piliyandala and Talangama, and
indeed other sites. What remains unanswered are the possible underlying factors
for the apparent differences. For instance, Wijethilaka
et al. (2021) make no mention of langurs being electrocuted or killed by dogs,
while this did come up prominently in the studies of Parker et al. (2008), Nijman
& Nekaris (2008), Moore et al. (2010), Dela (2011) and indeed Rudran
(2008), and Nahallage et al. (2008). It is possible
that there are no dogs in Danawkanda (or no dog large
enough to do harm to Purple-faced Langurs), and perhaps there are still enough
natural arboreal pathways for the langurs to use so they can avoid powerlines
(it is even possible that there are no powerlines), but a proper comparison by Wijethilaka et al. (2021) of their data with that collected
by others could have made that clear. Shooting at Purple-faced Langurs with
air-rifles or pellet guns is also commonly mentioned –Wijethilaka
et al. (2021) pooled this with the use of catapults and reported that 18% of
household in Danawkanda used this method– but there
seems to be significant variations in its frequency. In 2011 the Ministry of
Coconut Development and State Plantations as well as local Directors of
Planning, endorsed the idea of issuing air rifles to farmers and planters to
ward off primates and Common Giant Squirrel Ratufa
macroura from damaging their crops (Rodrigo 2011; Sugathapala
& Wijeweera 2011; Dittus
et al. 2019), but at present it is unclear what the uptake of this has been and
the effects it has on Purple-faced Langurs.
In conclusion, despite the
langur’s small area of occupancy (2,000 km2) and its small
population size (Rudran et al. 2020b) I find that
over the last decades a number of studies have been published on the
interactions between Humans and Western Purple-faced Langurs (Table 1), the
perceptions that people have towards the langurs, especially in
(semi-)urbanised areas, and how to best mitigate any negative effects of having
langurs and people living in close proximity. For primate conservation to work
in Sri Lanka sharing space with monkeys and ensuring levels of Human tolerance
towards primates remain high is paramount (in addition to having enough areas
set aside specifically for primates and other wildlife: Dittus
et al. 2021). The findings by Wijethilaka et al.
(2021) in terms of the proportion of people that report crop damage or damage
to roofs is high in comparison with earlier studies, which may reflect a
decrease in tolerance towards the langurs, differences in methodology or it may
reflect random geographical differences.
Table 1. Non-exhaustive overview of research conducted
on Purple-faced Langurs (sometimes in addition to other primates or
vertebrates). Researcher(s) only list one of the main persons involved, but it
is acknowledged that the team often comprised more people. Dates in Italics
indicate periodic monitoring or intermittent studies with gaps of up to two
years. Ethnoprimatology includes studies on
crop-raiding and human-wildlife conflict. Methods for ethnoprimatological
studies: A—direct observation of langurs and people | B—interviews and
questionnaires | C—workshops.
|
Period |
Researcher(s) |
Location |
Topics studied |
Methods |
Example reference |
|
ii 1968–iv 1970 |
R. Rudran |
Horton Plains, Polonnaruwa |
Ecology, behaviour |
|
Rudran 1973 |
|
iii 1969–ii 1970 |
C.M. Hladik |
Polonnaruwa |
Ecology |
|
Hladik 1977 |
|
ii 1985–ii 1987 |
J.D.S. Dela |
Piliyanadala, Panadura |
Ecology, behaviour |
|
Dela 2007 |
|
iii 1987–xii 2006 |
J.D.S. Dela |
Piliyanadala, Panadura |
Ecology, behaviour, ethnoprimatology |
A, B |
Dela 2011 |
|
1998–2018 |
W.P.J. Dittus |
Polonnaruwa |
Ethnoprimatology |
A, B, C |
Dittus et al. 2019 |
|
vi 2000–xii 2001 |
P. Jayasekara |
Sinharaja |
Behaviour |
|
Jayasekara et al. 2007 |
|
v 2001–vii 2010 |
K.A.I. Nekaris |
Island-wide |
Ethnoprimatology |
A, B, C |
Nekaris et al. 2013 |
|
? 2004–ii 2007 |
C.A.D. Nahallage |
Island-wide |
Ethnoprimatology |
A, B |
Nahallage et al. 2008 |
|
vi 2004-vii 2008 |
R.S. Moore |
Talangama, Masmullah |
Ecology, behaviour, ethnoprimatology |
A, B |
Douglas et al. 2008 |
|
vi–vii 2007 |
R. Rudran |
West |
Ethnoprimatology |
A, B |
Rudran 2008 |
|
vii 2008–xi 2009 |
R.P. Vandercone |
Kaludiyapokuna |
Ecology, behaviour |
|
Vandercone et al. 2012 |
|
vi 2009–xii 2010 |
R. Rudran |
Waga |
Ecology, behaviour |
|
Rudran et al. 2008 |
|
vii 2014–i
2015 |
S. Wijethilaka |
Danawkanda |
Ethnoprimatology |
B |
Wijethilaka et al. 2021 |
|
i 2015–ix 2016 |
C.A.D. Nahallage |
Mihintale |
Ecology, behaviour, ethnoprimatology |
B |
Kumara et al. 2019 |
|
ii 2015–iii 2019 |
R. Rudran |
Island-wide |
Ethnoprimatology |
B, C |
Cabral et al. 2018 |
|
iv–xi 2016 |
W.A.D.S.N. Weerakkody |
Delkanda, Homagama |
Ecology |
|
Weerakkody et al. 2018 |
|
v–vii 2018 |
C.A.D. Nahallage |
Mihintale |
Ethnoprimatology |
B |
Westwood 2018 |
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