Travel adaptations of Bornean Agile Gibbons Hylobatesalbibarbis (Primates: Hylobatidae) in a
degraded secondary forest, Indonesia
Susan M. Cheyne1, Claire J.H. Thompson 2 & David J. Chivers 3
1,2 Orangutan Tropical Peatland Project, Centre for International Cooperation in
Sustainable Management of Tropical Peatland(CIMTROP), University of Palangka Raya, Indonesia
1 Wildlife Conservation
Research Unit, Department of Zoology, Tubney House,
Abingdon Road, Oxon, OX13 5QL, UK
2,3 Wildlife Research Group,
Anatomy School, University of Cambridge, UK
1 susan.cheyne@zoo.ox.ac.uk (corresponding author), 2 claire_t009@yahoo.co.uk,3 djc7@cam.ac.uk
doi: http://dx.doi.org/10.11609/JoTT.o3361.3963-8 | ZooBank: urn:lsid:zoobank.org:pub:D33B37B7-0E29-41DE-8662-8FAC77E71ED4
Editor: Mewa Singh,
University of Mysore, Mysore, India Dateof publication: 26 March 2013 (online & print)
Manuscript details: Ms #
o3361 | Received 23 September 2012 | Final received 11 February 2013 | Finally
accepted 07 March 2013
Citation: Cheyne,
S.M., C.J.H. Thompson & D.J. Chivers (2013). Travel adaptations of Bornean Agile Gibbons Hylobates albibarbis (Primates: Hylobatidae)
in a degraded secondary forest, Indonesia. Journal of Threatened Taxa5(5): 3963–3968; doi:10.11609/JoTT.o3361.3963-8.
Copyright: © Cheyneet al. 2013. Creative Commons Attribution 3.0 Unported License. JoTTallows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: Funding was provided to CJHT byRufford Small Grants for Nature Conservation.
Competing Interest: None.
Acknowledgements: This
work was carried out within the OuTrop-CIMTROP
multi-disciplinary research project in the northern Sabangauforest, Central Kalimantan, Indonesia. We gratefully acknowledge the
invaluable contribution of all the researchers and staff that assisted with the
project. We gratefully thank the Centre for the International Cooperation in
Management of Tropical Peatlands (CIMTROP) for
sponsoring our research and providing invaluable logistical support. We thank
the Indonesian Ministry of Science and Technology (RISTEK) and Director General
of Nature Conservation (PHKA) for permission to carry out research in
Indonesia. We are very grateful to Warren Brockelman,
Mark Leighton and an anonymous reviewer for comments on earlier drafts of this
manuscript.
Abstract: Data are presented on the
locomotion of Bornean Agile Gibbons (Hylobates albibarbis)
in a disturbed peat-swamp forest. Our results indicate that gibbons favourcontinuous-canopy forest, higher canopy heights and trees with a larger
diameter at breast height. Gibbons
select these trees despite the study site being dominated by broken-canopy
forest and small trees. Gibbons also change frequently between brachiation, climbing, clambering and bipedal walking in
this disturbed forest depending on the size of gap to be crossed. Gibbons are shown to be capable of
adapting to some human-induced disturbances in forest continuity and canopy
height, and to the presence of smaller trees, e.g., after selective
logging. Despite this, gibbons are
still limited to crossing gaps of ≤12m in a single movement, and more research
is needed to quantify levels of disturbance gibbons can tolerate.
Keywords: Brachiation, Gibbon, Hylobates albibarbis, locomotion,
peat-swamp forest.
Habitat disturbance presents
a major problem for arboreal primates that travel exclusively in the canopy
(Cannon & Leighton 1994; Cannon et al. 1994), where efficient travel
requires the animal to take the most direct route available. Natural (e.g., tree-falls) and unnatural
canopy gaps (e.g., from logging, clearing for hunting fruit bats and fire
damage) may pose a problem if the canopy becomes highly uneven, producing less
direct travel paths than were originally available (Estrada & CoatesEstrada 1996; Kakati 2000; Onderdonk & Chapman 2000; Estrada et al. 2002; Baranga 2004; Anderson et al. 2007; Chapman et al. 2007;
Cristobal-Azkarate & Arroyo-Rodriguez 2007). Indonesia retains about 52.1%, or about
94,432,000ha, of original forest according to FAO (Food and Agriculture
Organization of the United Nations http://www.fao.org/forestry/en/). Of this 3.8% (3,549,000ha) is classified
as primary forest, the most biodiverse and
carbon-dense form of forest. These
alarming data on type of forest type remaining indicate that the majority of
remaining forest, and thus remaining gibbon habitat, will have experienced some
form of disturbance. Gibbons are
obligate canopy dwellers, who require intact canopy
structure for all aspects of their behaviouralecology (Carpenter 1972; Andrews & Groves 1976; Gittins1979; Bleisch & Chen 1990; Chivers1990; Feeroz & Islam 1992; Asquith 1995; Cannon
& Leighton 1996; Campbell et al. 2008; Cheyne2010; Hamard et al. 2010; Kakati2000; Marshall 2010; Oka et al. 2000). Highly territorial animals such as gibbons may remain within their
former ranges even following intensive forest clearance or fires which destroy
a high proportion of trees (Marsh & Wilson 1981; Marsh et al. 1987).
Logging (legal and illegal)
creates large patches of fragmented forest through (1) fragmentation at ground
level (roads and skid trails) and (2) creating gaps in the canopy, making
movement more difficult for arboreal species (Meijaard et al. 2005). Selective-logging
has been seen as the long-term ‘compromise’ for both humans and animals, but
areas that are set to be logged selectively are often
over-exploited by the timber industry. In this study we provide insight into the actual level of disturbance which gibbons can tolerate by documenting gibbon
preferences in a disturbed forest and the long-term implications of forest
degradation on gibbon behaviour. The future of gibbon conservation and
management depends on understanding how well they can adapt to these altered
canopy conditions.
Materials and Methods
The study was carried out in
the Natural Laboratory for Peat-swamp Forest in the northeastern corner of the Sabangau Forest (2019’S & 113054’E,
Fig. 1) over a period of nine months from September 2005 to June 2006. The area is operated
by the Centre for International Cooperation in Management of Tropical Peatlands (CIMTROP). Comparisons between wet (flooded) and dry seasons can provide insight
into felid movements in response to a potentially spatially mobile prey
base. Tropical peatlandsare one of the largest near-surface reserves of terrestrial organic carbon, and
hence their stability has important implications for climate change (Page et
al. 2002). Burning peatland in Indonesia may release 13–40 % of the mean
annual global carbon emissions from fossil fuels (Page et al. 2002; Aldhous 2004; Rieley et al.
2004). It is the largest area of
contiguous lowland rainforest remaining in Kalimantan and is recognised as one of the most important conservation areas
in Borneo for a variety of reasons including carbon storage, regulation of
water supplies and conservation of flora and fauna (Aldhous2004). The area has been subjected
to long-term legal logging, illegal logging, fire and drainage from logging
canals, but is now the focus of concerted protection and restoration efforts (Morrogh-Bernard et al. 2003; Cheyne2010).
Study Gibbons
Gibbons in this study site
are breeding on average every 2.5 years (Cheyne in
prep.) and their density is estimated at 3.92 groups/km2 (Cheyne et al. 2007; Hamard et al.
2010).
Data were collected on 24
individuals from six groups and a total of 1,212 data points were recorded by
means of continuous, consecutive behavioural sampling
on focal individuals for the duration of one follow from morning to evening
sleeping trees. The sampling unit was a complete segment of locomotion when
movement was initiated from a resting position and ended when the focal
individual returned to a resting state, following the definitions of (Cannon
& Leighton 1994). Only full
locomotion sequences, where both start and end positions could be observed, were
included in the analysis. All
age/sex classes were included to look for differences between the effects of
body size and the presence of infants ventrally. Data were collected for climbing,
leaping, brachiation and bipedal walking and merged
into two main locomotor modes—brachiating and
leaping for comparison. It was
decided to focus only on the two most recognisableand distinguishable forms of locomotion to reduce inter-observer error:
leaping, defined as discontinuous progression where the hindlimbsprovide all the propulsion, and brachiation (arm
swinging), defined as discontinuous progression in which the forearms are used
in a suspended posture (Fleagle 1976; Cant 1986;
Cannon & Leighton 1994). The
authors recognise the importance of studying several
forms of locomotion and we recommend that in such cases data
are collected by only one or two researchers to ensure that there is no
confusion between locomotion types.
The majority of observations
were recorded during follows typically lasting up to six hours (SD
1.3–8.4). Training consisted
of independently testing individuals’ ability to estimate heights tested
against known standards (where tree heights were measured using clinometers and
range finders). Observers were
sampled by SMC to measure inter-individual variability in estimating heights,
canopy condition and locomotion as part of the long-term data collection (Cheyne 2010).
Tree variables recorded were
(1) height of tree in which the gibbon started and finished the locomotion
bout; (2) average surrounding canopy height measured at the location where the
locomotion started; (3) distance of the travel bout in metres(between the two trees) was estimated using methods already in place for
estimating the distance a gibbon moves by extrapolating from the distance on
the ground. These methods are also subjected to rigorous training and regular
evaluation (Cheyne 2010); (4) forest type at the
start location was estimated as follows; ‘continuous canopy’—trees of
roughly the same heights, not much undergrowth; ‘continuous with emergents’—similar to ‘continuous canopy’ but with
more tall, emergent trees and slightly more undergrowth; ‘broken
canopy’—the commonest type found in the study area; uneven canopy and
thick undergrowth and ‘gaps’—areas that had been subjected to some
disturbance following the descriptions in Fig. 2. All heights were measured using visual
estimation following extensive training. Height categories used were 1–5 m, 6–10 m, 11–15 m,
16–20 m, 21–25 m, 26–30 m, 31–35 m, 36–40 m and
>40m (Cheyne 2010).
Habitat Data Collection
Following, and adapting,
methods from (Cannon & Leighton 1994), four plots were set up randomly in
each of the six groups territories using random number generation based on GPS
coordinates. Twenty-four 50m
transects (four/group) were constructed within the territories of each of the
six groups. This is half the length
of transects used by (Cannon & Leighton 1994), due to the frequent changes
in habitat type at the study area. Cannon & Leighton (1994) have described the small dbh size of trees to be a good indicator of poor-quality
forest. Habitat data were collected
in order to compare the frequency of use of habitat structure to its frequency
of availability. At 25m intervals
along the transect (3 points/transect, 72 points in total) the distance to the
nearest tree with a diameter at breast height (dbh)
larger than 6cm was measured by means of the point-centered quarter method
(Mueller-Dombois & Ellenberg1977) and tree height and average canopy height were measured using methods
already described.
Additionally, a 20m line was
laid at right angles to the transect on alternating
sides. At 5m intervals along this
line (5 points/line, 360 total) the forest type and canopy height were
sampled. The
transects were always moved at least 25m away from any original tracks
used for walking, so as to give a fair representation of the majority of actual
forest structure. The location of
each transect was determined in a stratified random fashion with the proviso
that transects had to remain entirely within the same habitat type and within
the home ranges of the study groups. Preference for different structural
features was measured using Jacob’s D value (Jacobs 1974):
D = (r-p)/(r+p-2rp)
where r is the relative frequency
of use and p is the relative value of availability. Jacob’s D value is
delimited between -1 and 1, and is symmetrical around 0 which indicates
neutrality i.e. neither disproportionate avoidance nor selection. Statistical tests
were performed with SPSS v16.0.
Results
General travel: Brachiation was the most common form of
locomotion (66% of observations, n=800) followed by leaping (34%, n=412:
χ²=11.59, d.f=1, n=1269, P<0.001). Leaping was employed significantly more
for travelling shorter distances (mean: 3.96m, range 1–4 m) and brachiation for longer distances (mean: 6.38m, range
5–9 m), one-way ANOVA: F=61.329, d.f=1, n=1268,
P<0.001, Fig. 2.
Canopy height: The availability of canopy
height for travel is dominated by 11–15 m and 16–20 m (total 61%,
Fig. 3). Jacob’s D values for canopy height were 0–10 m, D=-0.9; 11–20, D=0 and 21–30 m, D=0.3. There is a significant preference for
canopy height of 21–30 m or main canopy (χ2=12.19, d.f=2, P>0.005) and a significant avoidance of trees
1–10 m tall (χ2=9.9, d.f=2,
P>0.005).
Habitat type: Broken canopy was by far the
most available forest type in the study area (59%) with gaps representing 24%,
continuous forest with emergents 14% and continuous
canopy only 3%. The Jacob’s D value
of forest type could only be calculated if the expected values were ≥1.0, so
‘continuous canopy’ and ‘continuous with emergents’
were combined for analysis, as the frequency of availability for ‘continuous
canopy’ was constantly low (hereafter known as continuous canopy). Jacob’s D
values for canopy type are continuous and continuous emergent combined D=0.4;
broken canopy D + 0 and gap D=-0.5. There is a significant preference for continuous canopy (χ2=13.9,d.f=1, P>0.001) and a significant avoidance of
gaps and broken canopy (χ2=10.1, d.f=2,
P>0.005).
Discussion and Conclusions
The frequency of observed brachiation exceeded that recorded by Fleagle(1976) for Siamang (Symphalangus syndactylus 50%) and by Cannon & Leighton
(1994) for Bornean Agile Gibbons (H. albibarbis, 48%). Data reported in this study (66%) are closer to those reported by
(Andrews & Groves 1976a) for Lar Gibbons (Hylobates lar, 80%). Despite this, gibbons are limited in the
distances they can cross with each locomotion mode, with the maximum distance
seen crossed by brachiation being 12m and 6m by
leaping.
Gibbons are actively
selecting bigger, taller trees with a more uniform canopy than is predominantly
available. The
amount of time spent in ‘broken canopy’ far outweighs the others. It must be noted that the
selection of larger and taller trees by the gibbons in Sabangauis probably due to the extensive damage and lack of continuity in much of the
canopy but could also be a behavioural adaptation to
increased food availability in larger trees (Cheyne2008) or predator avoidance (Cheyne et al.
2012). This demonstrates that
selective logging has affected the gibbons’ ability to move through the canopy
though gibbons in highly disturbed areas have been known to travel on the
ground (S.M. Cheyne pers. obs. 2003 & 2006).
Uneven canopy and canopy gaps
pose a crucial problem for arboreal primates, as they either present a very
large break in the canopy or a succession of smaller breaks (uneven
canopy). Efficient, cost-effective
travel through the canopy, in terms of reducing distance (and time) of direct
travel between two points, is heavily constrained by the presence of gaps
(Cannon & Leighton 1994). Gibbons may be hypothesised to select
continuous forest types over discontinuous types and higher canopies over
low. During travel, gibbons tend to
follow established routes through the trees, referred to as ‘arboreal highways’
(Chivers 1974). These routes minimise their chance of encountering
gaps and also provides support for the theory that they appear to be selecting
actively certain structures for travel.
The key findings of this
study are: (1) gibbons can adapt in their locomotorecology to the effects of selective logging, i.e., reduce the level of travel
by brachiation and increase other modes of travel;
(2) the gibbons are choosing a ‘limited’ resource, the continuous tall canopy,
but there is evidence of a level of disturbance to which they cannot adapt.
Loss of trees between 6–15 m and/or 7–17 cm dbhwould be severely detrimental to this gibbon population given the limited
availability of larger trees following the selective logging. The exact
percentage loss of these trees which gibbons could tolerateneeds more work; (3) gibbons clearly prefer continuous canopy. We did not
observe crossings of gaps larger than 12m, which may be a constraint of the
gibbons’ physical abilities rather than a direct response to the presence of
gaps, i.e., gibbons cannot cross gaps >12m in one movement. There were no parts of the forest which were completely avoided. Daily path length for Sabangau gibbons ranges from 1–5 km depending on
season (Cheyne 2010), considerably more than that
reported for lar gibbons in Khao Yai (Bartlett 2009), thus, because the gibbons must
be more selective in their use of the habitat they may be having to travel much
further though this requires more testing.
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