Results of a nationwide census of the long-tailed macaque (Macaca fascicularis) population of Singapore

Abstract

Long-tailed macaques (Macaca fascicularis) are known for their ability to thrive in a wide variety of habitats, including urban areas. Singapore is an island city-state that has experienced rapid deforestation and urbanisation over the past several decades. These processes have led to the loss of most of Singapore's large mammalian species, but long-tailed macaques still live on the island. We conducted a census of long-tailed macaques between 2011 and 2012 to determine the current status of Singapore's macaque population. We surveyed forest edges, counted groups, and classified the age and sex of all individuals. We estimated the macaque population to be 1810–2166 individuals distributed among 92 groups, and a density averaging 6.86 individuals per km 2 (range: 0.89–33.63 across six zones) in areas with macaques. We found no evidence of overpopulation. Rather, the population characteristics mirror those of non-provisioned, wild long-tailed macaque populations. However, the interpretation of our results is limited by the fact that we did not have access to records documenting the number of macaques culled in Singapore before and during this census. Without accurate culling records, it is not possible to assess what shapes the current population structure, and thus research on the past effects of culling is needed.

Full text

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RAFFLES BULLETIN OF ZOOLOGY 2015
Results of a nationwide census of the long-tailed macaque (Macaca
fascicularis) population of Singapore
Crystal M. Riley1, 2*, Srikantan L. Jayasri2, 3 & Michael D. Gumert2
Abstract. Long-tailed macaques (Macaca fascicularis) are known for their ability to thrive in a wide variety
of habitats, including urban areas. Singapore is an island city-state that has experienced rapid deforestation and
urbanisation over the past several decades. These processes have led to the loss of most of Singapore’s large
mammalian species, but long-tailed macaques still live on the island. We conducted a census of long-tailed macaques
between 2011 and 2012 to determine the current status of Singapore’s macaque population. We surveyed forest
edges, counted groups, and classied the age and sex of all individuals. We estimated the macaque population to
be 1810–2166 individuals distributed among 92 groups, and a density averaging 6.86 individuals per km2 (range:
0.89–33.63 across six zones) in areas with macaques. We found no evidence of overpopulation. Rather, the
population characteristics mirror those of non-provisioned, wild long-tailed macaque populations. However, the
interpretation of our results is limited by the fact that we did not have access to records documenting the number
of macaques culled in Singapore before and during this census. Without accurate culling records, it is not possible
to assess what shapes the current population structure, and thus research on the past effects of culling is needed.
Key words. Singapore, long-tailed macaques, synanthropy, population census
RAFFLES BULLETIN OF ZOOLOGY 63: 503–515
Date of publication: 30 October 2015
http://zoobank.org/urn:lsid:zoobank.org:pub:02215AFD-D353-429B-8EBD-52E0D411793D
© National University of Singapore
ISSN 2345-7600 (electronic) | ISSN 0217-2445 (print)
1Department of Anthropology, Washington University in St. Louis, St. Louis, MO;
Email: crystalmriley@wustl.edu (*corresponding author)
2Division of Psychology, School of Humanities and Social Sciences, Nanyang
Technological University, Singapore; Email: gumert@ntu.edu.sg (MDG)
3Central Nature Reserves Branch, Conservation Division, National Parks Board,
Singapore
INTRODUCTION
Monitoring wildlife populations is important for successful
management. Management officials devise management
strategies based upon the results of studies examining the
status and distribution of species (Dice, 1938). Wildlife
management plans are particularly important when the
species of concern is one that shares a habitat with humans.
In these situations, the mitigation of human-wildlife conict
must also be considered. One species well known for such
conict is the long-tailed macaque (Macaca fascicularis).
Close interfaces between humans and long-tailed macaques
are driven in part by these macaques’ ability to thrive in
diverse habitats, particularly disturbed habitats such as forest
edges and human-inhabited areas (Gumert, 2011).
In Singapore, high human population density (7257 people
per km2 in 2012; SingStat, 2012) and limited space (land area
of 714 km2 as of 2012) bring people into close proximity
with macaques. Singapore has lost much of its original
biodiversity as a result of deforestation that began 170 years
ago, and eventually gave way to urbanisation that continues
today (Corlett, 1992). The highly urbanised island has patches
of protected forest, primarily in the Central Catchment
Nature Reserves (CCNR) and Bukit Timah Nature Reserve
(BTNR), which together comprise 3043 hectares, or 4.1%
of Singapore’s land area (National Parks Board, 2009).
Conict situations common elsewhere in SE Asia are absent
in Singapore. There are no monkey temples, crop raiding is
not a signicant issue, and feeding monkeys is illegal, which
discourages, but does not eliminate, people from feeding
macaques. However, macaques’ close contact with people
does lead to problems such as food theft, garbage raiding,
property damage, and, on rare occasions, macaques injuring
people (Sha et al., 2009a; Feng, 2011).
The National Parks Board of Singapore (NParks) received an
increase in macaque complaints in recent years, especially
from 2007 to 2011. Public complaints range from reports of
property damage and road kill incidents to people reporting
macaque sightings. As macaque complaints in Singapore
increased, the media has more frequently covered macaque
issues (Fuentes et al., 2008; Khew, 2014). For example,
Sha et al. (2009a) listed 47 macaque-related headlines from
Singaporean newspapers between 2004 and 2008. Wildlife
authorities have also increased attention to the rising
complaints. For example, in August 2012, the Agri-Food
and Veterinary Authority (AVA) opened a 24-hr hotline
for receiving wildlife complaints (Feng, 2013), which had
a major focus on macaques. Also in response to complaints,
NParks acted to better understand the trends in Singapore’s
macaque population, and initiated our study after having
received a 30% increase in macaque complaints in 2010
(611 in 2009 and 792 in 2010) (Gunasingham, 2011). The
goal was to examine the macaque population and whether
it had increased in size.
Conservation & Ecology

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Incomplete efforts to monitor Singapore’s macaques have
been made in the past. First, in 1986, the Malayan Nature
Society reported the total population on Singapore’s main
island was unlikely to exceed 1000 macaques (Lucas,
1995). However, the report did not include any methods,
and thus was unclear how the estimate was generated.
A few years later, Teo & Rajathurai (1997) conducted
a survey and estimated the CCNR population to be 850
macaques in 34 groups, even though they reported 1415
observations of macaques over four years. A census in 2004,
by Agoramoorthy & Hsu (2006), included more land area
beyond the CCNR, which included BTNR, Rie Range,
and Mandai area; however, it excluded other areas with
macaques such as offshore islands, Admiralty Park, and
Woodlands Waterfront. In this census, they estimated the
total population to be 649 individuals distributed among 32
social units, which was 24% less than the 1997 estimate;
however, similar to other censuses, their brief report provides
no explanation for how they came to the estimate. Taken
together, none of the early reports were complete nor
covered the entire country, and thus likely underestimated
total population size. Furthermore, none of these reports
provided population densities or distribution maps. These
problems make the utility of these past estimates limited for
understanding macaque population trends over time.
In 2007, Sha et al. (2009b) conducted the rst complete,
nationwide census of long-tailed macaques. Up to that date,
it was the most reliable measure of the macaque population
made in Singapore’s history. Their survey estimated the
macaque population was between 1218–1454 individuals in
71–112 groups. The population consisted of 53% immature
individuals (i.e., adolescents, juveniles, and infants) and
47% adults. There were 0.44 infants per female, and 0.63
adult males per adult female. Macaque density was 5.22
ind km−2 across all areas that contained macaques, and 28.2
ind km−2 in the nature reserve areas. Lastly, during 2007,
slightly over 200 macaques were recorded as culled, and
400–450 were recorded culled in total during the previous
5 years (2002–2006), indicating this population census was
conducted on a population heavily affected by culling (Sha
et al., 2009b).
Following Sha et al. (2009b) by ve years, we conducted
the second nationwide census for Singapore macaques in
2012. We investigated population characteristics of long-
tailed macaques in Singapore, including size, distribution,
density, and age-sex breakdown of the population. We
compared our results to populations of long-tailed macaques
in both provisioned and wild setting across their range to
assess whether Singapore’s population was overpopulated.
Furthermore, we made an attempt to identify trends in the
Singaporean macaque population over time by comparing
our results to the 2007 census. We end by discussing the
differences in the methods used and conditions between the
Sha et al. (2009b) census and our own. We also discuss the
lack of culling data in the time periods surrounding our study,
and how culling might affect census counts.
METHODS
Study area. Singapore is an island city-state at the southern
tip of the Malay Peninsula. The country of Singapore includes
a larger main island and more than 60 smaller offshore islands
(SingStat, 2012). Most of the smaller islands are uninhabited
and do not sustain large mammalian wildlife. Macaques have
been observed on the main island and some of the larger
offshore islands during previous censuses (Sha et al., 2009b).
For this census, we used the six survey regions used by Sha
et al. (2009b) (Fig. 1): 1) Bukit Timah, 2) MacRitchie, 3)
Old Upper Thompson, 4) Mandai, 5) Islands, and 6) Other.
The Other region encompassed any area not included in the
rst ve regions that contained macaques. MacRitchie and
Old Upper Thompson together made up the CCNR.
Selection of survey areas. To identify areas that might
contain macaques, we compiled information from the
previous census (Sha et al., 2009b), media reports (from The
Straits Times, The Sunday Times, The New Paper, and Asia
One), posts in online forums (e.g., Facebook and STOMP,
which is a Singaporean citizen-journalism website), and a
list of macaque complaints from NParks. The complaint
list contained general locations of macaque issues reported
by citizens between December 2010 and July 2012. We
also examined a map of Singapore and added to our list of
places to survey any forested areas not already identied.
We asked members of the public (see below) about when
and where they saw macaques, and if they mentioned places
that we did not already intend to survey, we added those
places to our list.
Once we compiled the list of survey locations, we began
visiting each location, with the exception of Ministry of
Defense (MINDEF) land, where the military conducts training
exercises, including live ring of weapons. We were not
granted permission to access or receive information about
those sensitive areas. Without being able to survey MINDEF
areas in the present survey, we just carried over the results
of the previous census (Sha et al., 2009b), during which the
government researchers were granted conditional access to
some MINDEF land or were provided macaque estimates
from MINDEF staff. Therefore, the estimates we used for
the MINDEF areas were the gures from the 2007 survey.
Census methods. Data were collected over 150 days in the
eld between October 2011 and November 2012. Researchers
had 535 contact hours with macaques, with additional time
spent searching for macaques. In standard primatological
censuses, each social unit is encountered and counted, and
then researchers move on (Ross & Reeve, 2003). In our case,
a pair of researchers (CR & SJ) each conducted their own
multiple counts of every social unit encountered to achieve
high accuracy. We spent an average of seven hours and 45
minutes with each social unit, typically over multiple days.
During follows, we obtained accurate individual counts,
assessed the age/sex distribution of individuals, and collected
scan samples on behavior for other studies. It was possible to
spend hours with social units because groups were sufciently
habituated to humans and did not ee; however, there were

505
RAFFLES BULLETIN OF ZOOLOGY 2015
ve social units with which we spent less than an hour, due
to them being unhabituated and eeing from us.
Long-tailed macaques prefer forest edges (Gumert et al.,
2011); therefore, when surveying a location, we began
by walking edges in and around the area, except where
access was prohibited (i.e., restricted and private land).
Edges included designated outer boundaries of a park
(e.g., the boundary of BTNR), as well as ecological edges
within the region (e.g., trails through the forest, shorelines,
roads). Our method of walking edges to survey a primate
population differed from standard transect methods (Ross
& Reeve, 2003), and did not involve as much coverage of
the deep interior of the forest. However, Singapore’s small
forest patches and the presence of trails through the patches
provided adequate access for observing macaques. We also
did not use transects because it is a method for obtaining
sample counts to extrapolate a population size estimate to a
larger area. Our method attempted to count each individual
in the population directly, which was feasible given the small
population size and small area to cover. Similar full count
methods were used for surveying Karimunjawan long-tailed
macaques (M. f. karimondjiwae) on Karimunjawa Island, in
Indonesia (Afendi et al., 2011), common long-tailed macaques
(M. f. fascicularis) in parts of Tanjung Puting National Park
in Kalimantan, Indonesia (Gumert et al., 2012), and Burmese
long-tailed macaques (M. f. aurea) on Piak Nam Yai Island,
in Ranong, Thailand (Gumert et al., 2013).
If we did not nd macaques in an area we surveyed, we spoke
with people in the area about whether or not they ever spotted
monkeys. We wanted to speak with people familiar with the
area, so we sought out, in order of preference, NParks staff,
other employees (e.g., security guards, cleaners), residents,
and park visitors. We asked when, where, and how many
macaques they typically saw, and then we returned another
day to survey the area, preferring to return at the time and
place people reported seeing macaques (e.g., “early afternoon
near the bridge”). It took up to six trips to a location to verify
the presence of macaques. We were able to verify nine public
reports in locations where we did not initially see macaques
(e.g., Yishun Park; see Table 1). After a maximum of eight
trips, if we had not found macaques where locals reported
having seen them, we included the estimates in our records
as “public reports.” Public reports were helpful in identifying
places that required further investigation. However, most of
the people making reports, with the exception of NParks staff,
did not have experience counting wild animals. Therefore,
they were likely to provide overestimates, which have been
found to be a common bias for public reports of macaque
group sizes (Malaivijitnond et al., 2011). Due to possible
inaccuracy and bias, we treated public reports with less
condence than or own survey counts, and did not include
them into our minimum population estimate. We excluded
reports older than six months.
As we searched for macaques, a GPS device (Garmin
GPSMAP 62sc) recorded our location every 30 seconds
to mark our survey paths. When macaques were sighted,
we made a waypoint of our location, which served as the
starting point of our follow. From that starting point, our
30-second interval tracking record allowed us to map the
ranging pattern of the macaques we were following. If the
macaques stopped to sleep for the night, we marked the
location of their roost. If we lost sight of the macaques or
left them for any reason, we marked our location at that point
to denote the end of the recorded range. Ranging data was
used to ensure that we did not double-count a single group
in different parts of its home range.
Counting and classifying macaques: Upon rst encountering
a social unit, both researchers counted them and scored the
age class (i.e., adult, adolescent, juvenile, or infant) and
sex, if discernible, of each individual. We determined age
class by following rules based on descriptions from Fooden
(1995). For example, a female was classied as a full adult if
she had elongated nipples indicating that she had previously
given birth, and a young macaque was classied as an infant
if it retained black fur from its natal coat. We categorised
social units into two types: groups (two or more macaques)
and single animals. We further distinguished between multi-
male/multi-female groups and all-male groups.
We did counts repeatedly for the rst one to three hours
after encountering a social unit. After each researcher began
to achieve the same count consistently, we moved on to
counting every 30 minutes as we followed the social unit,
doing additional ad libitum counts if a good opportunity
presented itself (e.g., all macaques approached a human food
source). We entered count data into an Excel spreadsheet
on a Psion Workabout Pro handheld computer.
For each count, we rated our condence in the count as
high, medium, or low. Condence scores were a subjective
assessment of our certainty that all macaques in the social
unit were visible and counted, and were affected by the
amount of time spent with the unit and the conditions of
observation, including weather, forest coverage, and the
macaques’ activity. For instance, a count of a group that
we observed for 30 minutes in dense forest on a rainy day
would be low condence, whereas a group we observed for
10 hours in a variety of conditions would be high condence.
In addition to scored data, we used photographs to identify
individuals in each group. We photographed all adults,
and any immature macaques with unique identifying
characteristics (e.g., a distinct scar). Individual identication
was used in our study to assist counting and provided a
photographic count of individuals by which to compare our
eld count. We used this method to ensure that we did not
repeatedly count individuals, or repeatedly count a single
social unit encountered in more than one area.
We collected behavioral scan samples (Altmann, 1974)
every 30 minutes while with each social unit, using Noldus
Observer XT software on a Psion Workabout Pro. We
conrmed group counts by comparing the numbers and
age/sex classes obtained during scan samples with those
obtained during counts. Discrepancies indicated that we
needed to conduct additional counts to ensure accuracy.

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Scan samples also provided information on the amount of
human feeding, including both direct human feeding and
indirect access to human food (e.g., food foraged from trash
bins), for each social unit and for the population overall.
The results of our behavioral data collection are covered in
Riley et al. (in press).
Analyses: We attempted to account for error in our counting
by generating a range for our population estimate. First we
calculated the minimum estimate, which was the sum of
our physical counts in 2012, and the 2007 estimates from
MINDEF land. Secondly, we made a best, or most likely,
estimate by adding public reports to the minimum estimate.
Lastly, we calculated a maximum estimate, using corrections
discussed below.
Maximum estimates for primate populations are often
calculated by extrapolating a density estimate to an entire
forested area (Ross & Reeve, 2003). Although that method
can be practical for some forest-dwelling primates, it is
not appropriate for long-tailed macaques for two reasons.
First, long-tailed macaques prefer forest edges and are
not evenly distributed in a forest, so applying a density
estimate uniformly across the forest would lead to a major
overestimation of density. Second, the method of applying
a density estimate across a larger region is typically used to
compensate for not having surveyed an area. In our study,
we attempted to make a head-count of the entire population.
We therefore only needed to make estimations for the
potential error in our counting, but not for extrapolating to
un-surveyed regions.
We used our high-condence counts of group sizes as a
correction tool for estimating maximum population size.
To do this, we corrected all low and medium-condence
group counts to the mean from high-condence counts.
Two high-condence correction means were used because
group sizes differed between regions. We made a correction
mean for the core area groups (i.e., MacRitchie, BTNR,
Old Upper Thompson, and Mandai regions), and a second
correction mean for non-core area groups (e.g., Island and
Other regions). If a low or medium-condence group count
happened to be higher than the mean high-condence group
count for their region, we always corrected towards the
larger count. We did not replace low and medium-condence
estimates that were higher than the correction means because
we were correcting for under-counting not over-counting, the
latter being less likely. That is, it was much more likely to
overlook macaques that were not visible, than to mistakenly
count “phantom” macaques.
We calculated and examined variation in population density
and other population characteristics across the six regions
(Fig. 1), which contained a total area of 278.28 km2. We
Fig. 1. The four core zones dened for the census are circled in black, Mandai, Old Upper Thompson, Bukit Timah, and MacRitchie.
The Islands zone consisted of all the offshore islands that are labeled on the map, Ubin, Tekong, Sentosa, Coney, and Sisters. The Other
zone was any area not included in the other ve regions that contained macaques.

507
RAFFLES BULLETIN OF ZOOLOGY 2015
Fig. 2. The distribution of macaques in Singapore in 2012, which includes the 2012 census observations and 2007 MINDEF survey.
also calculated density for the core area, which were the four
regions where the majority of the macaque population was
concentrated. Since group characteristics can vary depending
on habitat and level of human overlap (Aggimarangsee,
1992; Fooden, 1995), we examined group sizes and the
ratios of adult males to adult females, infants to adult
females, and immature to adult macaques to see how those
ratios compared to other populations, especially populations
living in environments less disturbed by people. We also
conducted a t-test to see whether groups that accessed human
food systematically differed in size from groups that did not
access human food. All statistical analyses were conducted
using IBM SPSS Statistics version 19.0, 2010, Armonk, NY.
Alpha was set at 0.05 unless otherwise noted.
RESULTS
Population size. We physically counted 1626 macaques in 69
social units on mainland Singapore and four offshore islands
(Table 1). The count excluded areas to which we did not
have access (i.e., MINDEF land). The macaques we counted
were distributed among 66 multi-male-multi-female groups
(95.7% of social units; 99.8% of individuals); one all-male
group (1.4% of social units; 0.1% of individuals); and two
lone males (2.9% of social units; 0.1% of individuals). We
calculated a mean (±SD) of 24.2 ± 9.85 macaques per group
from 1624 macaques in 67 groups.
In addition to our physical count, we had to account for
MINDEF and public reports. In MINDEF areas, we used
Sha et al.’s (2009b) count from 2007 of 184 macaques,
which was distributed among 13 groups (182 macaques)
and two single animals (Table 2). We excluded two groups
counted in 2007 at MINDEF (N = 23 and N = 15) because
we counted groups of comparable size (N = 21 and N =
14) in areas adjacent to MINDEF land where these groups
were in 2007. We thus considered them the same groups.
The mean (±SD) group size for the 13 MINDEF groups was
14.0 ± 8.78 macaques. Combining MINDEF groups and 2012
survey groups resulted in a mean group size of 22.58 ± 10.36
macaques, N = 80. All MINDEF social units were rated as
low condence, because we did not count them directly in
2012. Unveried public reports of macaques amounted to
an additional 99 macaques distributed among 12 groups (67
macaques) and 32 single animals (Table 3).
Using these counts, we calculated the best estimate of the
population to be 1909 individuals. This gure included our
physical count of 1626, plus the 2007 MINDEF count of
184, plus the 99 macaques from public reports. However,
since the public reports were not veriable and thus could
be inaccurate or biased, we also calculated a minimum count
of 1810, which excluded the public reports. This minimum
count was considered the lowest possible size the population
could be. Public reports were from our discussions with

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Table 1. Summary of the characteristics of all social units counted in 2012. Single animals italicised.
Region Social
Unit ID
2012
Count Adult ♂ Adult ♀ Adol/ Juv Infant Im-
mature
Adult
♂:♀
Adult:
Immature
Bukit Timah
BB01 38 6 12 16 4 20 0.50 0.90
BT01 47 7 16 19 5 24 0.44 0.96
BT02 29 4 11 9 5 14 0.36 1.07
BT03 43 6 14 12 11 23 0.43 0.87
BT04 20 4 6 8 2 10 0.67 1.00
BT05 38 5 12 17 4 21 0.42 0.81
BT06 18 2 5 11 0 11 0.40 0.64
BT07 3 0 1 2 0 2 0.00 0.50
CA01 15 2 7 6 0 6 0.29 1.50
CA02 26 4 5 17 0 17 0.80 0.53
DF01 26 3 6 15 2 17 0.50 0.53
RR01 21 2 7 8 4 12 0.29 0.75
RR02 32 4 10 15 3 18 0.40 0.78
RR03 40 5 13 14 8 22 0.38 0.82
RR04 35 4 11 13 7 20 0.36 0.75
RR05 31 6 10 14 1 15 0.60 1.07
ZP01 21 2 5 12 2 14 0.40 0.50
MacRitchie
BC01 27 3 7 16 1 17 0.43 0.59
IC01 28 7 9 10 2 12 0.78 1.33
IC02 22 3 9 8 2 10 0.33 1.20
IC03 26 5 4 15 2 17 1.25 0.53
IC04 40 6 10 24 0 24 0.60 0.67
MR01 26 4 7 12 3 15 0.57 0.73
MR02 48 7 11 28 2 30 0.64 0.60
MR03 26 5 8 11 2 13 0.63 1.00
MR04 28 5 5 16 2 18 1.00 0.56
MR05 26 6 6 12 2 14 1.00 0.86
MR06 22 4 6 10 2 12 0.67 0.83
MR07 16 2 6 5 3 8 0.33 1.00
MR08 23 3 4 12 4 16 0.75 0.44
MR09 22 2 8 10 2 12 0.25 0.83
MR10 11 2 4 4 1 4 0.50 1.50
MR11 18 1 2 14 1 15 0.50 0.20
MR12 16 2 4 9 1 10 0.50 0.60
MR13 8 2 3 3 0 3 0.67 1.67
MR14 20 6 4 8 2 10 1.50 1.00
MR15 15 2 5 6 2 8 0.40 0.88
SI01 33 4 9 15 5 20 0.44 0.65
Old Upper Thompson
IC05 26 3 10 5 8 13 0.30 1.00
LP01 25 3 6 14 2 16 0.50 0.56
LP02 27 4 6 14 3 17 0.67 0.59
LP03 19 4 5 8 2 10 0.80 0.90
LP04 25 3 7 13 2 15 0.43 0.67
OT01 22 4 4 13 1 14 1.00 0.57
OT02 25 4 9 10 2 12 0.44 1.08
OT03 32 7 9 14 2 16 0.78 1.00
OT04 35 3 12 16 4 20 0.25 0.75
UP01 27 6 9 8 4 12 0.67 1.25
UP02 24 5 10 8 1 9 0.50 1.67
Mandai
MD01 34 5 9 17 3 20 0.56 0.70
MD02 36 3 10 15 8 23 0.30 0.57
ML01 21 3 4 13 1 14 0.75 0.50
ML02 21 4 6 9 2 11 0.67 0.91
US01 31 3 12 10 6 16 0.25 0.94
US02 38 6 10 19 3 22 0.60 0.73

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Region Social
Unit ID
2012
Count Adult ♂ Adult ♀ Adol/ Juv Infant Im-
mature
Adult
♂:♀
Adult:
Immature
Islands
CI01 17 2 3 12 0 12 0.67 0.42
PU01 18 4 5 7 2 9 0.80 1.00
PU02 17 5 2 10 0 10 2.50 0.70
PU03 15 4 4 5 2 7 1.00 1.14
SS01 17 4 3 10 0 10 1.33 0.70
ST01 7 2 1 4 0 4 2.00 0.75
Other
AP01 26 4 8 12 2 14 0.50 0.86
BB02 2 2 0 0 0 0 -- --
CA03 16 2 5 6 3 9 0.40 0.78
CA04 17 2 3 11 1 12 0.67 0.42
NU01 1 0 0 1 0 0 -- --
WT01 1 1 0 0 0 0 -- --
WW01 15 3 6 4 2 6 0.50 1.50
YP01 5 1 1 3 0 3 1.00 0.67
67 groups 1624 252 461 746 165 911 0.55 0.78
2 single
animals 2 1 0 1 0 1 -- --
Total 1626 253 461 747 165 912 0.55 0.78
Table 2. Estimates of macaques on MINDEF land that were carried over from Sha et al. (2009b). MINDEF provided Sha et al. (2009b)
a range estimate for these numbers. We have used the mean of the range provided. Single animals are italicised.
Location Count
Asrama 20
Khatib Bongsu 1
Lada Hitam 10
Lorong Kebasi 13
Mandai Track 15 Group1 10
Mandai Track 15 Group2 10
Pulau Tekong Reservoir 3
Pulau Tekong Highway 33
Pulau Tekong Grenade Range 22
Tagore 8*
Ulu Sembawang Forest 10
Western Catchment Poyan 15
Western Catchment Tengah 1
Western Catchment Track 13 25
Western Catchment Sarimbun 3*
13 groups 182
2 single animals 2
Total 184

510
Riley et al.: Population census of long-tailed macaques in Singapore
52 people in 25 locations. In 12 of those locations, people
reported no macaques in the past six months. In six locations,
people reported recent macaque sightings that were later
conrmed by us visually. In seven locations, we could not
conrm recent macaque sightings (Table 3).
We performed corrections to some group counts to account
for potentially uncounted animals and generated a maximum
estimate from these corrections. Of the 67 groups we
observed, 23 (34.3%) were judged as high confidence,
35 (52.3%) as medium condence, and 9 (13.4%) as low
condence. In addition, all 13 groups from the 2007 MINDEF
survey were consider low condence and were corrected.
We used the mean from high-condence counts to correct
all 35 medium and 22 low-condence group counts. We
found that the core area had a higher mean group size than
the rest of Singapore (independent samples t-test: t (65) =
−4.331, P < 0.001, two-tailed); therefore, we calculated a
high-condence mean for the core area (N = 19, M = 30.05
± 10.59), and a second high-condence mean, for everywhere
else (N = 4, M = 9.75 ± 7.365). We then raised all medium
and low-condence counts lower than this mean, to the
high-condence mean for the region of the group (i.e., core
or other). This correction resulted in adding 257 macaques
to our estimate of 1909, producing a maximum population
estimate of 2166 macaques, which we considered to be the
largest possible population size in 2012. From all counts and
estimates, we concluded the macaque population for mainland
Singapore and its offshore islands was between 1810 and
2166 individuals, with a best estimate of 1909 macaques.
Population characteristics. Data on age-sex composition
of groups were only available for social units directly
counted during the survey. Of 1626 macaques counted, 714
(43.9%) were adults (253 males; 461 females), 165 (10.2%)
were infants, 101 were adolescents (6.2%), and 646 were
juveniles (39.7%). Immature macaques accounted for 56%
of the population, with a ratio of 1.28 immature individuals
per adult. We did not nd a signicant relationship between
group size and the ratio of immature to adult individuals
(Pearson Correlation: r (65) = −0.088, P = 0.484). The ratio
of infants (N = 165) per adult female (N = 461) was 0.36.
There was a statistically signicant relationship between
group size and ratio of infants to adult females (Pearson
Correlation: r (66) = 0.350, P = 0.004). We calculated
a sex ratio of 1.85 adult females per adult male, or 0.54
adult males per adult female. The male to female sex ratio
of island groups was nearly two times higher (M = 1.38 ±
SD = 0.72, N = 6) than the ratio of non-island groups (M
= 0.56 ± SD = 0.26, N = 60), (independent samples t-test:
t (5.13) = −2.77, P = 0.038, two-tailed, equal variances not
assumed). There was a signicant correlation between the
size of the group and adult female to male sex ratio, r (66)
= −0.25, P = 0.041. Lastly, social units observed feeding
on human food during scan samples had signicantly larger
group sizes (M = 28.27 ± SD = 11.50) than social units not
observed feeding on human food (M = 21.36 ± SD = 9.28),
t (67) = 2.67, P = 0.010.
Population distribution. We calculated densities from the
minimum population estimate, which were the counts from
the 2012 survey and MINDEF areas carried over from 2007.
The density of macaques for all areas that contained macaques
was 0.46 social units per km2, and 6.86 ind km−2. Core area
density (i.e., zones one to four) was 0.97 social units km−2,
and 23.64 ind km−2. Across all six survey regions, we found
a range of 0.25–1.43 groups per km2, and 0.89–33.63 ind
km−2 (Table 4). Of 1810 macaques, 1636 (90.4%) were
found in the core area (Fig. 2). The mean group size was
signicantly larger for groups within the core area (N =
60, M = 25.17 ± 9.73) than for groups outside it (N = 9,
M = 12.89 ± 11.82), (independent samples t-test: t (67) =
−3.433, P = 0.001).
We calculated mean group sizes for each of the six survey
regions. Mean group sizes (±SD) were as follows: MacRitchie
region 23.86 ± 9.13; Bukit Timah region 28.41 ± 11.36; Old
Upper Thompson region 24.58 ± 6.68; Mandai region 23.10
± 11.07; Islands 16.56 ± 8.52; and Other region 11.46 ± 8.65.
A Kruskal-Wallis test showed that group sizes were not equal
across the six survey regions (H(5) = 26.30, P < 0.001). We
conducted Dunn-Bonferroni post-hoc pairwise comparisons,
which corrected for multiple comparisons. These tests showed
that the Other region (M =11.46 ± 8.65) was signicantly
lower than the Bukit Timah region (M = 28.41± 11.36), (t
= 39.15, P < 0.001), MacRitchie region (M = 23.86 ± 9.13),
(t = 29.86, P = 0.004), and Old Upper Thompson region (M
= 24.58 ± 6.68), (t = 33.39, P = 0.006), but did not differ
signicantly from the Mandai region (M = 23.10 ± 11.07),
(t = 27.57, P = .084) or Island region (M = 16.56 ± 8.52 ),
(t = 12.38, P = 1.000). None of the other surveyed regions
differed signicantly from one another across group size.
Comparison with 2007 census. In 2007, the population count
was 1218–1454 individuals in 71–112 social units (Sha et al.,
2009b), and in 2012 our estimate was 1810–2166 individuals
in 128 social units. The best estimate of the population in
2012 was 1909 and in 2007 the best estimate was 1454.
This is a difference of 455 individuals, and our survey had
a 24% higher count. Since the number of macaques per
group was higher in 2012, overall density of individuals
was also slightly higher in 2012 (6.86 ind km−2 in areas that
contained macaques) than 2007 (5.22 ind km−2 in areas that
contained macaques). The density of social units was very
similar between 2012 (0.46 social units per km2) and 2007
(0.40 social units per km2).
We also compared infant and sex ratios. In 2012, there were
fewer infants per adult female than in 2007. We found 0.36
infants per female, while in 2007 there were 0.44 infants per
female. The overall mean (±SD) adult male to adult female
sex ratio for 2012 was 0.55, while the mean sex ratio per
group was 0.63 ± 0.40, which is the same as the ratio of
0.63 ± 0.36 reported by Sha et al. (2009b).

511
RAFFLES BULLETIN OF ZOOLOGY 2015
Table 3. Reports of macaques that were unveried by researchers. We gathered reports from complaints to NParks from
November 2011 to November 2012 from conversations with the public, and from media coverage. Groups are bolded.
Regions are: M = MacRitchie; O = Old Upper Thompson; Md = Mandai; I = Islands, Ot = Other.
Location Region Reported # of macaques
Source: Report to NParks
Ang Mo Kio Town Garden West Ot 3
Bishan Park O 1
Circuit Road Ot 1
Clementi Woods Park Ot 2
Duchess Ave Ot 1
Greenleaf Lane Ot 1
Heartland Mall Ot 1
Holland Grove View Ot 1
Jln Peminpin M 1
Jln Pintau M 1
Jurong West St 64 Ot 1
Lentor Road O 1
Lorong Ah Soo Ot 1
Lower Seletar Reservoir Park Ot 1
Mayower Crescent O 4
Oxford Road Ot 1
Pasir Ris Ot 1
Paya Lebar Way Ot 1
Pearl’s Hill Ot 3
Potong Pasir Ave 1 Ot 1
Segar Road Ot 3
Sembawang Ot 2
Singapore Botanic Garden Ot 1
Spottiswoode Park Ot 1
Tavistock Ave Ot 1
Third Avenue Ot 1
Watten Rise Ot 3
West Coast Park Ot 1
Whampoa Drive Ot 1
Yio Chu Kang Gardens O 1
Thomson Green O 2
Toh Guan Road Ot 1
Ulu Pandan Park Connector Ot 1
Upper Wilkie Road Ot 1
Source: Report to Researchers
Bedok Reservoir Park Ot 1
Chinese Garden Ot 1
Mount Pleasant M 10
Sentosa Ropes Course I 20
Southern Ridges Ot 5
Springleaf Md 10
Sungei Buloh Ot 1
Source: Media Report
Dover Ot 1
Khatib MRT Ot 1
Singapore Flyer Ot 1
12 groups 67
32 single animals 32
Total 99

512
Riley et al.: Population census of long-tailed macaques in Singapore
Table 4. Density and count data in the six regions surveyed during the 2012 survey. Density calculations include MINDEF estimates
carried over from the 2007 census, but do not include public reports from 2012.
Zone Area
(km2)# Social Units
Social Unit
Density
(per km2)
Individuals
Individual
Density
(per km2)
Bukit Timah 16.00 17 1.06 483 30.19
Bukit Timah, Dairy Farm,
Zhenghua, Bukit Batok
MacRitchie 20.90 22 1.00 511 24.45
MacRitchie, Bukit Brown, Sime
Road
Old Upper Thompson 9.07 13 1.32 305 33.63
Old Upper Thompson Rd, Lower
& Upper Peirce
Mandai 19.17 11 0.52 241 12.57
Upper Seletar, Mandai Rd
Islands 40.12 10 0.25 169 4.21
Sister’s, Sentosa, Coney, Ubin,
Tekong
Other 173.02 55 0.30 200 0.89
Admiralty Park, Yishun Park,
other areas not within the dened
regions
Total 278.28 128 0.46 1909 6.86
DISCUSSION
As we will discuss below, the Singapore macaque population
is comparable to wild long-tailed macaque populations
in terms of density, group size, and age-sex distribution,
despite the Singapore population having access to human
food resources and being heavily affected by human
inuence. One major human impact is that Singapore has
been culling macaques at least since the 1970s, when all
of the macaques in Singapore’s Botanic Gardens were
culled following complaints (Sha et al., 2009a). Culling has
continued off and on since that time and culling was ongoing
by AVA while we conducted our census. Unfortunately,
our requests to review culling records were not permitted.
Consequently, we do not know the number and locations of
macaques culled. We therefore cannot assess how culling
may have impacted our results, and what role culling might
play in inuencing the Singaporean macaques’ population
parameters. In 2013, the year following our study, the media
reported that approximately 570 macaques were culled by
AVA’s program (Khew, 2014). That is a removal of about
a third of the population in one year, which is a signicant
source of mortality, and the largest annual cull ever reported
to date in Singapore. Despite regular culling there have been
no reported studies conducted in Singapore to investigate
the impact culling has on the structure of the population.
Without studying the culling records in detail, we can only
draw limited conclusions on which factors impact the size
and characteristics of Singapore’s macaque population.
Our estimate of Singapore’s macaque population is larger
than all previous estimates of the population (e.g., Teo &
Rajathurai, 1997; Agoramoorthy & Hsu, 2006; Sha et al.,
2009b); however, prior to the 2007 survey, all censuses
were incomplete. Moreover, even at this count, Singapore’s
macaques do not exhibit any evidence of overpopulation.
Long-tailed macaque population densities have a wide range
of variation in natural conditions, ranging from 0.6–7.7
groups km−2, and averaging around 55 individuals km−2
(Fooden, 1995). Densities of macaques in Singapore are well
within that range, and even the highest density at Old Upper
Thompson zone (33.63 ind km−2) is only at a moderate level.
If we compare Singaporean macaques to other synanthropic
macaques (i.e., those living with and beneting from humans,
primarily from human food sources), we nd Singapore’s
density is much less, even though provisioned. Typical
synanthropic macaque densities are regularly higher than
100 ind km-2 (Fooden, 1995), and have been reported to be
as high as 1600 ind km−2 in one case in Bali (Wheatley,
1989). Singaporean long-tailed macaques are nowhere near
such extreme densities, and thus the Singaporean macaque
population density is distinctly not comparable to other
synanthropic populations. Rather, they compare better to
wild populations.
We also did not find any symptoms of overpopulation.
Malaivijitnond & Hamada (2008) have pointed out that
where macaques are overpopulated in Thailand, they have
shifted from natural forests to becoming more concentrated
in human environments, such as temples. During our study,

513
RAFFLES BULLETIN OF ZOOLOGY 2015
Singapore’s population was concentrated primarily in the
core reserves or other forested areas, and we did not nd
macaques living entirely in human environments. That
being said, the reserves and forested areas are not pristine
wilderness and include residences, recreation parks, and
country clubs, and thus do have some degree of human
disturbance. However, these areas are still mostly forested
and macaques have not moved much from these areas. If
the macaque population were overcrowded, we would have
seen more growth outside of this core area. Whether due to
culling or the natural state of the population, Singaporean
macaques did not show population-level symptoms of being
overpopulated during our census.
Our ndings also showed group sizes more comparable to un-
provisioned populations of long-tailed macaques, rather than
provisioned populations. The mean group size in Singapore
is comparable to average group sizes of long-tailed macaques
elsewhere in SE Asia (van Schaik et al., 1983; van Schaik
& van Noordwijk, 1985; Fooden, 1995). Our mean group
size of 24.2 is within the mean group size range of 12–25
reported in Fooden (1995), and is comparable to average
group sizes reported for long-tailed macaques in wild and
non-urban habitats. For instance, Wheatley (1982) reported
a mean group size of 30 for common long-tailed macaques
in the swamp forests of Southern Kalimantan, Indonesia, and
van Schaik & van Noordwijk (1985) reported a mean group
size of 27 for common long-tailed macaques and Simeulue
long-tailed macaques (M. f. fusca) in forests in Sumatra
and Simeulue Island, Indonesia, respectively. In contrast,
synanthropic macaques in other parts of SE Asia can attain
group sizes over 100 individuals (Fooden, 1995; Gumert
et al., 2011). Provisioned long-tailed macaque groups in
Thailand, primarily living around temples, had an average
group size of 77 (Aggimarangsee, 1992), which is more than
three times the mean in our study. Group sizes in Mauritius,
where long-tailed macaques are an invasive species and
capitalise on farmlands, also exceeded 70 (Sussman et al.,
2011). Once again we can see that the Singaporean long-
tailed macaques resemble natural populations, more so than
human-disturbed populations.
Other characteristics of the Singapore macaque population
also indicated a normal population structure. Among
numerous groups reviewed by Fooden (1995), the range
of within-group adult sex ratios was 0.14–1.67 males per
female. In our study, all groups except for two had sex
ratios within that range. The two groups that were outside
of this range were ST01 (ratio = 2.00), and PU02 (ratio =
2.50). Both groups were on offshore islands (i.e., Sentosa
and Ubin islands, respectively), which could have affected
male migration patterns due to insular isolation. The nding
that the male to female sex ratio of island groups was
nearly two times higher than that of non-island groups is
consistent with this assertion. Also at the population level,
the adult male to female ratio in the Singaporean long-tailed
macaque population was 0.54, which is within the range
of population-level adult male to female ratios (0.22–0.90)
reported in a meta-analysis by Fooden. Thus, as with other
population parameters, here we can see no clear deviations
from typical long-tailed macaque populations, other than
the island dwelling groups in Singapore.
There were three groups that were exceptionally large in
Singapore compared to other groups we observed. All three
of these groups (N = 43 and N = 47 in BTNR; N = 48 in
MacRitchie, in the CCNR) were all observed regularly
feeding on human food, which is probably an important factor
in their attaining these large sizes. Previous studies found a
positive correlation between birth rates and food availability
(Suzuki et al., 1998; van Noordwijk & van Schaik, 1999).
However, it could also be that larger groups out-compete
smaller groups for access to human food resources.
We could not always verify public reports of macaques and
thus we have concerns of their validity, or our inability to
nd reported groups. The high frequency of single animals
reported by the public (N = 32), as compared to low numbers
counted by researchers (N = 2), could have been because
a few single animals were seen repeatedly, perhaps being
males migrating. It could also have been due to over-reporting
by the public. Some public reports of groups were also
difcult to verify. For instance, macaques were reported in
the Southern Ridges, but we were unable to locate there.
Park ofcials reported seeing them, however, they were
possibly wary of humans because they were being culled
at that time (Lee, B., pers. comm.) due to a biting incident
(Feng, 2011). Additionally, the group was also very small,
making it difcult to locate.
Our primary nding was that we counted a higher number
of macaques in 2012 (range: 1810–2166) than was counted
in 2007 (range: 1218–1454) (Sha et al., 2009b). It is easy to
draw a conclusion that the Singaporean macaque population
increased by several hundred in ve years. However, this
result requires some consideration. First, the two studies
differed in their methods, making direct comparisons
difcult. Consequently, it is not possible to fully rule out
methodological differences as one source of variation in
counting. One methodological difference is that we took
photographs of all individuals to disambiguate groups and
conrm counts. Using photographs resulted in revisions to
counts obtained in the eld, which improved and increased
counts. Secondly, conditions were better. We had two
permanent researchers on the project. We also had the
advantage of having Sha et al.’s (2009b) study, as well as
numerous other accumulated reports, which provided an easy
guide on where to search for macaques. This likely increased
our efciency to document known macaques, and nd more.
Aside from possible methodological inconsistencies, we also
do not know the effects of culling on these counts. Culling is
an important variable for understanding population changes
in Singapore’s macaques because culling occurs regularly
and at signicant levels. We must consider that the 2007
population was small, and around 600–650 macaques had
been culled in the years up to that census. Sha et al. (2009b)
found many groups of less than 10 individuals, which is
much smaller than typical group sizes. He also observed
numerous single animal sightings. This could have been

514
Riley et al.: Population census of long-tailed macaques in Singapore
the result of undercounting, and thus methodological. Or,
perhaps some groups were genuinely that small. Long-tailed
macaques tend to have small group sizes where they have
been hunted (Wheatley et al., 1999), and thus atypically
small group sizes could have been a symptom of heavy
culling. The count difference between 2007 and 2012 may
have been a population rebounding from culling. Indeed,
the difference in counts between 2007 and 2012 was around
450–700 individuals, which is comparable to the 600–650
macaques culled between 2002 and 2007. However, this
comparison is limited by not knowing the levels of culling
between 2008 and 2012. These gures do indicate though, that
research on culling records is a necessary factor to include
when assessing Singaporean macaque population trends.
To assess if the population was growing during our census,
we can look for markers of population growth, such as age
ratios in order to assess changes in birth rates. Singapore
does not have immigration from other macaque populations,
thus any population growth would be from reproduction. If
the population was rapidly growing, we would expect a large
amount of the population to be below ve or six years of
age; however, we found that the age ratio did not indicate
an atypically high percentage of immature individuals. The
ratio of young to mature was not higher in 2012 than 2007.
Moreover, in our study, 56% of the population was immature,
which is comparable to the composition of most natural
long-tailed macaque populations (Fooden, 1995). Looking
at infants, we found the infant to adult female ratio (1:2.81)
in 2012 was lower than the ratio (1:2.3) in 2007 (Sha et al.,
2009b), which does not support a faster reproductive rate
in 2012 than in 2007. Better information is needed on rates
of reproduction across years. Additionally, we must also
consider what effects culling might have had, since young
are disproportionately caught during culls.
In moving forward, we also need better assessment of the
causal factors behind complaints. NParks initiated this study
in response to a surge in macaque complaints. Without
research on the reasons for increasing complaints, we cannot
draw conclusions about the factors driving human-macaque
conict in Singapore. Since complaining drives culling,
this is important. It is generally assumed that rising animal
populations drive complaints regarding conict; however, it
is quite possible the reverse is true. Rather than an abnormal
number of macaques, it may simply be that the increasing
human population of Singapore (from 4.6 million in 2007
to 5.3 million in 2012; SingStat, 2012) is the primary driver
of human-wildlife conict. Indeed there has been extensive
development of housing complexes, particularly near forested
areas, in Singapore during the time periods assessed in our
study, and during Sha et al. (2009b). Another potential
inuence on complaint increases was the opening of the
AVA hotline in 2012 (Feng, 2013), which preceded the
large 2013 cull.
Our census showed that Singapore has a macaque population
exhibiting parameters within the range of wild non-
provisioned populations of long-tailed macaques throughout
SE Asia, and does not appear overpopulated or atypical like
some other synanthropic populations of macaques. Ideally,
regular monitoring of this population should continue into
the future in order to document demographic patterns across
time. Furthermore, future monitoring of the population should
attempt to standardise census methodology, to ensure more
accurate analyses of population trends over time. Finally,
it is necessary to note that monitoring is incomplete and
limited in its usefulness unless culling records are also
assessed. In-depth investigation into the effects of culling
on the health and structure of the macaque population is
critical for understanding population trends in Singaporean
long-tailed macaques.
ACKNOWLEDGEMENTS
The National Parks Board funded this research, with
supplemental support from the Ministry of Education,
Singapore, Tier 1 Grant: RG 95/07. Washington University
in St. Louis provided funds for report preparation. Nanyang
Technological University provided administrative support.
We would like to thank several NParks staff who helped
to set up the project or otherwise assisted us; Tuan Wah
Wong, James Gan, Claire Su Ping Ng, William Ng, and
Ping Ting Chew. We also thank Agustín Fuentes, Crickette
Sanz, Joanna Setchell, and two anonymous reviewers for
comments on earlier versions of this manuscript, Bryan
Koenig for assistance with statistics, and John Sha for
answering questions about the 2007 census. Lastly, we thank
Sophie Borthwick, Amy Klegarth, Julia Kohn, and Anne
Kwiatt for assistance with surveys.
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