ORANGUTAN  DENSITY  AND  NESTING  PREFERENCE  IN  CENTRAL  KALIMANTAN,  

INDONESIAN  BORNEO  

 By:  Cynthia  Malone  

Advisor:  Dr.  Karen  B.  Strier    

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The extinction of orangutans has been predicted to occur by 2025 without a reversal of the extensive deforestation of Indonesia’s rainforests. Conservation efforts have begun to focus on measuring orangutan population densities and responses to change so that remaining habitat is adequately protected. Population densities are estimated using indirect surveys of orangutan nests, but our knowledge of nest construction is limited. I conducted a six-week study of orangutan density and nesting preferences in collaboration with the ‘Orang-utan Tropical Peatland Research Project’ (OuTrop). OuTrop is based in the Sabangau peat-swamp forest, home to the largest remaining population of orangutans in the world, Pongo pygmaeus wurmbii. Two sites of different disturbance levels, one a relic primary forest and the other a heavily degraded and fragmented forest, were compared in order to enhance our knowledge of how this subspecies copes with disturbance. In the degraded forest, density estimates were nearly twice that of those of the protected forest, potentially indicative of an over-crowding of individuals as the population is pushed into a smaller habitat patch fragmented by forest fires. Analysis of the environmental components of nest construction revealed preferences for particular tree species, which has been demonstrated previously in the protected forest, and a significant relationship between nest height and tree-tying, the use of multiple trees to build one nest. These results warrant further assessment of the degraded forest ecosystem and potential fire effects to better ascertain whether this population can persist without strong management efforts.

Introduction  The archipelago of Indonesia is considered a global hotspot for biodiversity (Brooks et al

2002), representing a richness in ecosystems and endemic plant, insect, bird, and mammal species

that is arguably unparalleled anywhere else in the world. For primates in particular, Indonesia is a

stronghold; nearly half of the over 40 species present are endemic (Gursky-Doyen and Supriatna

2010). Perhaps the most recognized of all of Indonesia’s primates is Asia’s only great ape and the

world’s largest arboreal mammal, the orangutan. This charismatic species serves as a vital catalyst to

protect Indonesia’s biodiverse tropical forests that are currently being severely degraded at rapid

rates. Extensive deforestation and the consequential destruction of massive expanses of carbon-

storing peat have earned Indonesia the title of the third largest contributor to global carbon emissions

(Schwarz 2010). The government has recently made efforts to curtail emissions by partnering with

the United Nation’s REDD (Reducing Emissions from Deforestation and Forest Degradation in

Developing Countries) Program, and also declaring a two year moratorium on logging all pristine

forests. However, a lack of legal enforcement allows for the continued establishment of plantations

in pristine forest and illegal logging (Nantha and Tisdell 2009), furthering the fragmentation of

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forests on the islands of Borneo and Sumatra, the only remaining habitat of the orangutan.

Orangutans were once widespread throughout continental Southeast Asia and mainland Asia,

including areas between Vietnam, northern India, and Southern China, until the late Pleistocene, or

40,000 years ago. Climatic changes combined with human expansion and selective hunting to limit

orangutans to their present day range of two Sunda-shelf islands in Southeast Asia, Borneo and

Sumatra (Rijkson and Meijaard 1999). Although all members of the genus Pongidae were once

considered members of the same species, Pongo pygmaeus, as they can successfully interbreed and

have fertile offspring, recognizing the extent of morphological and ecological variation across all

orangutan populations is vital to addressing their immediate and long-term threats. Bornean and

Sumatran populations are now formally classified into two distinct species. Pongo abelli refers to all

Sumatran populations, while Pongo pygmaeus refers to Bornean populations. The Bornean

orangutan, Pongo pygmaeus, is further classified into three subspecies (Warren et al 2001): P.

pygmaeus pygmaeus inhabit West Kalimantan, P. pygmaeus wurmbii inhabit Central Kalimantan,

and P. pygmaeus morio, inhabit East Kalimantan and Malaysian Borneo (Figure 1, Taylor 2006).

Figure 1. Current distribution of the genus Pongidae. Pongo abelli is endemic to the island of Sumatra (a) and Pongo pygmaeus is endemic to the island of Borneo (b). (Source: Taylor 2006)

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While the historical and current distribution of orangutans within tropical rainforests subject

to climatic variations and increasing anthropogenic pressure is enough to suggest vulnerability, they

are not the only primate species endemic to Southeast Asia and yet have suffered greater losses to

their populations. The biology of the genus Pongidae is one of extremes, allowing for intrinsic

sensitivity to disturbance. They are predominantly frugivorous and will subsist on fruit whenever it is

available. While orangutans have adapted to seasonal fluctuations in fruit availability, namely by

minimizing energetic social interactions and relying on fallback foods such as pith, bark and leaves

(Marshall 2009), their dependence on an unreliable resource is paired with a long inter-birth interval

and high parental investment. Females give birth once every seven to nine years and parents spend at

least six years with one offspring, as they must become equipped with a mental map of the location

of over 250 different tree species used for food (Delgado and van Schaik 2000). Orangutan

dependence on fruit in a habitat where fruit is not always reliable combined with their delayed

reproduction and long inter-birth intervals make recovery from population losses difficult.

Current Status

As of 2009, the IUCN lists Pongo pygmaeus, the Bornean species, as endangered and Pongo

abelli, the Sumatran species, as critically endangered. The population growth trend is decreasing, as

over 50% have been lost over the past 60 years. From a total remaining population of 60,000, an

estimated 5,000 are lost each year. In Sumatra, there are less than 7,300 individuals remaining while

in Borneo, there are an estimated 50,000 individuals separated by extensive fragmentation (Wich et

al 2008). Unfortunately, the prospect of population numbers increasing does not look promising.

With Indonesia’s economic development so tied to palm oil production and logging concessions, and

lack of sufficient government enforcement to prevent illegal logging even in protected forest, the

future of orangutan populations depends on effectively managing protected areas and preventing

more destruction in unprotected areas by establishing official protected status. Baseline

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quantification of how populations are impacted by destruction at multiple habitat scales is needed to

mitigate anthropogenic pressures.

The majority of research regarding the effects of disturbance on orangutan populations

pertains to the alteration of their density and distribution, as conservation efforts attempt to discover

how many remain in the wild, why some populations seem to be more successful than others, and

how perturbations can be mitigated. While orangutan distribution across Borneo and Sumatra is

fairly well known, the same cannot be said for orangutan density. Because orangutans are a semi-

solitary and arboreal species that naturally occur in low densities with clumped spatial distribution

(Galdikas 1988), obtaining direct counts of orangutan individuals is notoriously challenging and

expensive, requiring intense tracking of an elusive arboreal species on difficult terrain. As a result,

accurate density estimates are difficult to obtain (Spehar et al 2010).

Recent efforts have relied almost exclusively on indirect measurements. The most widely

used non-encounter survey method utilizes the nests orangutans construct daily. Orangutans, the

other great apes, and prosimians are unique among the Primate order in their construction of nests

(Frothman and Hughman 1996). Unlike prosimian nests that are built instinctively and for a variety

of purposes, such as concealing young or breeding, the nests of great apes are flat platforms used for

sleeping. As nests are typically built daily, rarely re-used, and can remain visible for weeks or

sometimes years, they can act as reliable indirect markers of individuals within a given space.

Counting nests using line-transect techniques and a variety of habitat and population-specific

parameters is a method that is widely used for the estimations of chimpanzees, gorillas, and

orangutan densities (Husson et al 2009). For orangutans, the nest count method is less sensitive to

changes in densities over the short-term that tend to be associated with direct observations. It is an

efficient and cost-effective method to rapidly capture orangutan density at the population level.

Because of the parameters involved in calculating nest density, such as the time it takes a nest

to degrade, numbers cannot accurately be extrapolated over different habitat types and are restricted

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to the population level. This is ideal for conservation, as management efforts considering orangutan

density can be effectively tailored to a given area. However, orangutan density surveys utilizing the

nest count method have in the past concentrated on areas that are already protected, and only recently

have degraded areas become a topic of research and recognized as having conservation value

(Meijaard et al 2012).

Husson and colleagues (2009) shed light on the effects of disturbance on density across sites

in Borneo and Sumatra. Their analyses demonstrate that in heavily to moderately logged forests,

orangutans are present at lower densities than they are in unlogged forests of similar habitat, largely

due to the reduction in fruit availability. However, they also noted that disturbance can shift the

distribution of a population from a site of high disturbance to more suitable habitat if accessible,

which can lead to an overcrowding of individuals. Such overcrowding has been reported in sites

across Borneo, namely in West Kalimantan where nest densities were moderately high in sub-

optimal, previously logged forest adjacent to an area undergoing active disturbance (Russon et al

2001).

In Borneo, an estimated 75% of orangutans live outside of protected areas in plantations and

former logging concessions. Meijaard and colleagues (2010) demonstrated that an acacia plantation

in East Kalimantan with a history of illegal logging and forest fires contains as high as 3-4% of the

total Bornean orangutan population. Currently, there are no laws in place to prevent degraded areas

from undergoing further logging and anthropogenic disturbance. Density estimates can act as

justification for mitigating destruction of degraded forests, while knowledge of how orangutans cope

in such areas can serve to inform management decisions. .

Orangutan nests have the potential to reflect important aspects of orangutan behavior that can

inform conservation. As nests are currently widely used as the standard method for on-going density

surveys, and likely will continue to in the future, considering other aspects of nest construction in

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greater detail will enhance our understanding of orangutan habitat preferences and ecological

responses to disturbance in a cost-effective manner.

Nest Construction and Habitat Preferences

A variety of explanations have been proposed for the demonstrated time and effort

orangutans devote to nest construction. These include avoidance of predators, reducing heat loss, and

guarding against parasites (those that would either disturb apes on the ground or potentially be

transmitted if nests were reused often) and comfort while sleeping (Prasetyo et al 2009). Given that

all of these factors may play a role, it is unsurprising that orangutans start practicing before they are

one year old, and often build a number of nests in a day to hone their skills. At three years old, they

are able to build suitable nests (van Schaik et al 2005).

Nest construction has been recognized as a process that requires an advanced level of

cognitive ability comparable to tool use (van Casteren et al 2012). Observations of nest construction

in the wild demonstrate the significant effort and time an orangutan takes to construct a nest; the

process can sometimes take up to 30 minutes. In order to construct a nest, orangutans situate

themselves on a solid base, typically a single, strong lateral branch, and build a sturdy platform by

bending and weaving branches inward. A lining is created by breaking surrounding, typically leafy,

branches and placing these on top of the structure. Embellishments such as pillows, made from leafy

twigs, or blankets, in the form of detached leafy branches placed atop of a reclined individual, are

often added. Although materials are typically taken from the tree in which the nest was built, “leaf-

carrying,” where an orangutan will select branches from one or a few species to be used in

constructing a nest at a different site (Russon et al 2007).

The demonstrated effort and selectivity involved in nest construction is indicative of its

importance to the habitat preferences of orangutans. Analyzing the placement and structure of

orangutan nests within trees promises to reveal important factors involved in selection. A study in

Tuanan revealed that a tree’s structural characteristics, including the type of branching, were

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important to choice. Nests are typically built close to the tip of a tree in one of recognized four basic

patterns that reflect variations in the positioning of the platform at a particular height (Prasetyo et al

2009). While the first three patterns utilize one to two branches from a single tree as a base, the

fourth pattern is distinguished in its utilization of multiple trees to build a sleeping platform.

Orangutans bend and lock the branches of these trees together in a variety of ways to make the base

for their nest, a process referred to as tree-tying.

Tree-tying was once thought to be restricted to chimpanzees (Marchant 1996), but studies of

nest-building in orangutans document nests that employ over six trees for one nest. Patterns one to

three, those using just one tree, are most common in sites across Borneo and Sumatra, while the

fourth pattern is rare and most common in Central Kalimantan. Prasetyo et al (2009) suggest this

may be a result of selective logging and the absence of larger trees, since those that employ tree-

tying tend to be smaller, although it may also represent a subspecies preference related more to

geography than habitat loss.

Prasetyo and colleagues (2009) demonstrated that in study sites across Borneo and Sumatra,

tree species were not nested in proportion to their availability, or abundance, in the forest. Although

a wide variety of tree species are used, orangutans do not typically build their sleeping platforms in

active fruiting trees, where they are subject to disturbance by sympatric primate species or predators.

For those species that are commonly used within a population and across sites, further research is

required to determine what factors result in these apparent preferences.

To explore whether nests reflect important ecological responses to particular habitat types

across all orangutan populations, a comparative framework of the ecological components of

orangutan nesting and its relation to habitat viability is needed. This study was undertaken to explore

the effects of habitat disturbance on orangutan density and the potential of nest construction as an

indicator of important habitat preferences. Density and nesting preferences are compared between a

protected, secondary growth forest and an unprotected, degraded forest within a shared peatland

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ecosystem as part of ongoing research of the southern Bornean orangutan, P.p. wurmbii by the

Orangutan Tropical Peatland Research Project (OuTrop). Density in the protected area was expected

to remain similar to previous estimates, as the area has not undergone recent environmental or

anthropogenic disturbance. Orangutan density in the degraded forest was expected to be higher than

in the protected site as a result of overcrowding of individuals seeking refuge from degradation at the

forest edge as a result of periodic fires.

OuTrop’s research in the protected site demonstrates that orangutans prefer particular tree

species and locations for nests. Tree species preferences are also expected for the degraded site, but

the tree species and variables that explain their preference may be different as a result of the

ecological effects of disturbance. Alternatively, the unprotected site may not support nesting

preferences, as orangutans may not have the flexibility of choosing particular tree species if overall

the diversity and abundance of trees is low.

Study Site and Methods

OuTrop is a non-governmental organization and research camp based in the lowland peat-

swamp forest of the Sabangau basin in Central Kalimantan, Borneo. Along with being the largest

remaining lowland forest in Borneo, the Sabangau basin is home to the largest remaining continuous

population of Bornean orangutans, an estimated 6,900 individuals (Singleton et al 2004). The

distribution of P. p. wurmbii, is extensively fragmented (Husson et al 2009) and areas outside of the

protected national park are still subject to the threats of deforestation. OuTrop has conducted

extensive research on the population status, distribution, and behavioral ecology of P.p. wurmbii, and

the peatland ecosystem in which they inhabit.

Bisected by the Sabangau River, the study area of OuTrop is divided into two regions, the

Sebangau National Park and the National Laboratory of Peat-swamp Forest (NLPSF) and to the west

of the river and the unprotected Block C of the former Mega-Rice Project to the east (Figure 2). This

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study will compare parts of both these catchments, as they historically represent the same peatland

ecosystem supported by the Sabangau basin but have been altered to different degrees by disturbance.

Sabangau

Sabangau – National Laboratory of Peat-swamp Forest

The Sebangau National Forest is a 5780 km2 protected area situated between the Sabangau

River and the Katingan River. Although legal logging concessions ceased in 1996 upon its official

designation as a national park and research site, the area subsequently suffered from intense illegal

logging. The result is a habitat mosaic in different successional stages with patches of varied habitat

quality and secondary growth. The NLPSF is categorized into three main habitat sub-types: the

Figure 2: Map of the Sabangau basin, Central Kalimantan. OuTrop is based at the NLPSF, located in the Sabangau forest, the western catchment. Block C of the Mega-Rice Project is located in the eastern catchment, east of the Sabangau River,

Source: The Orangutan Tropical Peatland Research Project

Block C of the Mega Rice Project

NLPSF

Block C

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mixed-swamp forest, low pole interior forest, and the tall interior forest. This study is based within

the mixed-swamp forest, an area that stretches from the edge of the forest to six kilometers into the

interior characterized by a diversity of tree species up to 35 meters in height. The last reported

orangutan density estimate from this forest was 1.93 individuals/km2. There is minimal

anthropogenic disturbance in this portion of the forest and fires are infrequent.

Research of the orangutan nests in Sabangau provides interesting insights into nesting

behavior. A study of nest construction (Rowland 2008) identified 23% of the total tree species used

for nests as significantly preferred for nesting and 8% of tree species as avoided, making a total of

31% of nest trees used at numbers disproportionate to their abundance in the forest. Rowland

measured variables potentially related to the comfort and stability of a nest, such as the size and

number of leaves and branch strength, to determine which most explained the preference of these

particular tree species. Sap volume, viscosity, and stickiness were found to be the most important

factors in explaining the level of selection for a particular tree species.

Gibson (2005) revealed that overall, orangutans prefer to nest in tall trees with complete

canopy coverage and actively avoided patches of shorter trees with incomplete canopy. However,

significant differences between age-sex classes were reported. Flanged males nested in trees that

were significantly shorter in height and smaller in DBH (diameter at breast height) than sub-adult

males, females, and all adolescents. Adolescents nested in trees that were often twice the height of all

other age classes, and generally larger in DBH. Flanged males employed tree-tying in 58% of all

observations. Considering that flanged males were also shown to build the largest nests and

adolescents the smallest, there is a clear relationship between nest size and tree size, height, and the

number utilized in construction. This is suggestive of an age-sex class relationship to tree-tying.

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Block C - Kalampangan

The smaller and fragmented eastern catchment of the Sabangau basin is situated between the

Sabangau River and the Kahayan River. This area is Block C, one of five formerly forested blocks of

Indonesia’s former Mega-Rice Project, a failed attempt to alleviate a national food shortage in 1996

by converting one million hectares of peatland into rice padi. Aside from deforesting much of the

region by fire clearance, over 4000 km of drainage and irrigation channels were constructed (Boehm

and Siegert 1999). The combination of these canals and the severe drought imposed by the 1997 El

Nino Southern Oscillation has resulted in extensive destruction of the peat’s hydrology. Logging has

exacerbated the problem and 83% of forest cover has been lost since the initiation of the project in

1996. The region is prone to extensive fires annually during Indonesia’s dry season. From 2000 to

2007, 45% of Block C’s peatland had been lost, in addition to substantial vegetation loss and soil

drying from remaining forested areas (Cattau 2010).

Within Block C, OuTrop has conducted research from a small base camp in the Kalampangan

site, the northwestern portion of the segment. LANDSAT images reveal that it is isolated from other

forested areas within Block C and fragmentation has increased since 2000. Research of the

orangutans here is limited; this analysis of nesting preferences is the first study of this unhabituated

population’s ecology. In 2004, researchers (OuTrop, personal communication) observed orangutans

travelling on the ground along the banks of a 10 km long canal that runs 100 m from the base camp.

As orangutans are primarily arboreal, this is unusual and potentially indicative of overcrowding

pushing individuals out of the forest, and also may suggest habitat in the interior is unsuitable. The

last survey of orangutans reported a density of 1.6 individuals/km2 (Cattau 2009). Forest fires have

occurred at Kalampangan since this survey, though the extent of their effects has yet to be measured

(OuTrop, personal communication).

Field Methods

In order to estimate orangutan density in both study sites, previously established transects

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were surveyed for orangutan nests. For each nest encountered, the following variables were recorded

in accordance with standard protocol: distance and position (left or right) along transect,

perpendicular distance to nest, nest size in 0.5 meter increments, and the nest decay stage. Nest decay

stage was identified to one of the following categories: A: New, leaves still green, B: Older, nest still

in original shape, with brown leaves attached, C: Old, holes in the nest, or D: Very old, nest no

longer in original shape, but some twigs and branches still present (Morrogh-Bernard et al 2003).

While not employed in this study, nest decay is important in estimating the nest degradation rate of a

site and may potentially aid in future studies.

Nest Tree Species Preference in Kalampangan

The variables that may affect the preference of a tree species in nest construction were

recorded from the same transects surveyed for nest density. For every nest encountered along a

transect, the tree species was identified with the help of experienced OuTrop field assistants. In order

to determine whether orangutans are selecting specific tree species as nest sites, only data from nests

constructed in just one tree species were considered in this part of the analysis to avoid potential

confounding effects.

To investigate whether orangutans are using tree species at a level not predicted by the

presence of that species in the study area as a whole, vegetation data collected from six 100 x 5 meter

plots, a combined area of 0.3 hectares, in Kalampangan was used to calculate the proportions of each

nest tree species in the forest as represented by the plots. If a nest tree species was present in this

sample but not in the plots, it was assigned an observation of 0.5 stems/ha, as the minimum number of

stems in the forest is one, but the true abundance per hectare can be expected to be much less than this.

It is important that those tree species that are not represented in the plots but present in the sample are

included in the analysis, as they potentially represent species that are highly preferred for nests. These

species are present in the forest, but at such low abundance they were not observed in the vegetation

plots.

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In order to assess what factors may determine the selection of a tree species in Kalampangan,

the variables investigated in a previous study of nest construction at the Sabangau site (Rowland 2008)

thought to be associated with stability and comfort were considered here. Leaf, branch, and sap data

were considered to be intrinsic properties of each tree species, and data collected from the Sabangau

study that utilizes the vegetation plot data from this forest was used here in addition to data obtained

from this study of tree species in Kalampangan.

Sap measurements were taken by cutting into a section of the tree at breast height. Sap

viscosity was determined by the distance the sap traveled alongside a metric ruler. Measurements

were distributed into 5 class estimates, 0 representing no sap emitted, and 4 representing sap that

traveled the farthest distance. Sap stickiness was also estimated into 5 class estimates: 0 representing

no sap emitted and 4 representing a very sticky sap. Two branches thought to be typical of the kind an

orangutan might use for a nest were cut with the help of an OuTrop field assistant. For each branch,

the length was measured in meters and the number of secondary branches, branches perpendicular to

the main branch line, counted. The total number of leaves was recorded and the length and width of

the first 50 recorded. The area of a leaf was calculated utilizing the formula:

. As 50 leaves were not recorded in all cases, the average total leaf area

was weighted by the number of leaves recorded.

Tree-tying and Habitat Quality Surrounding Nests

In addition to variables affecting the selection of a tree species, the following variables were

collected to assess habitat quality surrounding a nest and their potential effects on tree-tying. For

each tree encountered with a nest, both tree height and nest height were measured in meters with a

clinometer. Circumference at breast height (CBH) was recorded 1.3 meters above the base of the

tree with measuring tape in centimeters, then converted to diameter at breast height (DBH) by

dividing this value by Π. Basal diameter was measured directly above the roots of tree in

centimeters.

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Variables related to habitat quality were only measured in the Kalampangan site. To maintain

consistency and avoid inter-observer bias, the following data were always recorded by me. Canopy

cover was measured using a hand-constructed “density-ometer”. The top of a bottle was cut and

plastic film with a 6 x 6 one inch square drawn grid was placed over the broadest part on the opposite

end of the mouthpiece, which acted as an eye piece. Canopy health was determined utilizing an index

developed by OuTrop, which includes five categories of health based on percentage of leaf cover.

Trees that have full coverage of leaves were assigned a score of 0 and those with little to no leaf

coverage were assigned a score of 5 (Figure 3).

Figure 3. Canopy health, based on an index developed by OuTrop (Gibson 2005). There are 5 class estimates: 0 represents 80-100% leaf cover, 1 represents 60-80% and 5 represents 0-20% leaf cover.

Interconnectivity, the level of connectedness between the tree observed with a nest and the

rest of a canopy, was also estimated into five classes. A score of 0 was given when a tree was not

connected to the rest of the canopy and a score of 5 was given when a tree was 100% connected to

the canopy on all sides.

Statistical Analysis

The program DISTANCE (Thomas et al 2009) is a statistical distance-sampling software that

utilizes the length of a transect (meters), the numbers of nests along a transect, and the perpendicular

distance of each of these nests to the transect to estimate nest density for a given survey area. It

employs robust semi-parametric models to determine the probability of detection as a function of the

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distance of the observed nest from transect, and uses this detection function to estimate an effective

strip width. The effective strip width of the area is accepted from the model that provides the lowest

Akaikes Information Criterion (AIC) and input into the following formula to estimate nest density: N

=n / L * 2w, in which N is nest density, n is the total number of nests observed subtracted by nests

beyond the effective strip width; L is the sum of transect lengths in km; and w is the effective

transect width in km.

Nest density was then used to estimate orangutan densities with following formula: D = N

/ (p * r * t), in which D is orangutan density (individuals/km2), N is nest density, p is the proportion

of nest builders in the population, r is the rate at which nests are produced (n/day/individual), and t is

the decay rate of nests, or time during which a nest remains viable (Morrogh-Bernard et al 2003).

The values of p, r, and t are parameters subject to variability across field sites, but require intensive

surveys if measured directly for each study. This study uses values previously obtained in Sabangau,

(Husson et al 2009), where p = 1.17, r = 0.89, and t=365. The final estimates were then multiplied by

correction factors to account for variability in experimental design (Husson et al 2009). A correction

factor of 1.25 accounts for the line-transect survey design employed, which typically produces lower

estimates than density surveys based on a plot design. A correction factor of 1.18 accounts for single

transect surveys such as this, where transects are only walked once.

A chi-squared test was performed to determine which of the tree species observed with nests

varied significantly from the vegetation plot abundance of each respective species, with the null

hypothesis that there is no difference between the nest sample proportion and the control, plot sample

proportion. An electivity index, “Chesson’s α,”was employed to measure the degree of preference

for tree species within this sample that differed significantly from vegetation plot proportions.

“Chesson’s α” index (Chesson, 1978) is a measurement of the selectivity of a given item that is

robust to changes in relative abundances, which allows for comparability across studies. The

expected value for random nest tree species selection (i.e. use is directly proportional to abundance

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in the environment) within this index is a function of number of nest tree species: 1/n where n is the

number of nest tree species in the sample. The value alpha, α, was calculated for each tree species

with the formula: α i = W i = ( ri / pi )/∑I (ri / pi), where r = the proportion of the tree species

encountered with nests and p = the proportion of tree species in the forest as measured by the

vegetation plots. Alpha values above 1/n indicate preference and those below 1/n indicate avoidance.

Values that are equal to 1/n indicate that the tree species is neither preferred nor avoided and the

proportions of these species would not vary significantly from the plot proportions.

A principal components analysis test, a multivariate statistical technique for analyzing the

source of variation within a data set, was used to determine which variables contribute to the

selection of a tree species. For tree-tying, a generalized linear model, in the form of a probit analysis,

was used to assess the contributions of each relevant variable and functional relationship to the

binary response of whether more than one tree was utilized or not.

Results

Orangutan Density

In Sabangau, a total of 124 nests was recorded over 11.63 kilometers divided into six transects.

The hazard + polynomial model provided the lowest AIC and best histogram fit, and an effective strip

width of 19.42 meters was used to calculate nest density. The orangutan density was estimated to be

0.88 individuals/km2 before the correction factor. The final estimate with the correction factor is 0.944

individuals/km2. In Kalampangan, a total of 158 nests were recorded over 10.63 kilometers surveyed

divided into 5 transects to the interior and 5 transects towards the forest edge. The uniform + cosine

model provided the lowest AIC and best histogram fit. An effective strip width of 12.14 meters was

used to estimate nest density. The orangutan density of Kalampangan was estimated to be 1.2

individuals/km2 before the correction factor. The final estimate with the correction factor is 1.77

individuals/km2.

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Nest Tree Species Preference

A total of 33 tree species were recorded for all nests that were constructed with branches from one

tree (Table S1). When comparing the proportions of each tree species observed with a nest from transect

surveys with the proportion of these tree species observed in the vegetation plots, seven tree species, or 21

% of the total tree species encountered were determined to be significantly preferred by orangutans for

nesting and five species, or 15 % of the total tree species encountered, were determined to be significantly

avoided (Figure 4).

The tree species variables relating to the sap, leaves, and branch characteristics of these 13 tree

species were used in the Principal Components Analysis. The model was first run to determine the

number of important components as visually displayed in a scree plot. The first three principal

Figure 4: Chesson’s α Index Values for significantly preferred and avoided tree species (critical value = 3.85, df=1, p < 0.05). The horizontal line marks the value expected if there were no preference, 1/n, calculated for this study as 0.03 (1/33 tree species). Tree species with Chesson’s values below this are considered avoided and tree species with values above are preferred.

18

     

components provided Eigen-values over one and contributed the most to the variation in the data.

Principal Component 1 accounts for 46% of the variation in the data, while Principal Component 2

accounts for 20% of this variation. The loadings scores and plot (Figure S1) show that variables

relating to tree security and comfort, as represented by average total leaf area and leaf area to branch

ratio, explain the majority of the variation between species that are preferred and avoided. Secondary

branches may explain around 20% of the variation.

In Kalampangan, 109 observations, or 69% of the total, were single tree nests and 49 observations,

or 31% of the total were built using multiple trees. Before analysis, six rows with missing values were

removed. Each measured variable was first compared between the two forms of nest construction

independently of all other variables with two sample t-tests. There was no significant difference in nest

size between single nest trees and those constructed with tree-tying (p = 0.118). Tree size (Figures 5)

measured by DBH (DBH for single tree nests: mean = 21.14 cm, SD = 8.80; DBH for tree tied nests:

mean = 12.81, SD = 6.37, two sample t test t: 5.74, df=150, p <0.05) and basal area (basal area for

single tree nests: mean = 24.34, SD = 9.62; basal area for tree tied nests: mean = 15.06, SD =7.29, two

sample t test t – 5.80 , p<0.05) were both significantly greater for single tree nests. Nest height (Figure

6) and tree height were also significantly greater for single tree nests (nest height for single tree nests:

mean = 16.33, SD = 6.66; nest height for tree tied nests: mean = 8.97, SD = 3.98, two sample t test t:

6.71, df=150, p<0.05).

For variables thought to be related to habitat quality, canopy coverage did not vary significantly

(p=0.946) between the two forms of nest construction, while canopy health and interconnectivity did.

Canopy health scores were significantly higher for trees constructed with tree-tying (which according

to the scale denotes less leave coverage and poor health (Mann-Whitney U Test, p-value = 0.0009).

Canopy connectivity scores were significantly lower for single nest trees than for those constructed

with tree-tying (Mann-Whitney U Test, p-value = 0.0014).

19

     

Trees TiedSingle Tree

50

40

30

20

10

0

size

in c

m

Diameter at Breast Height (DBH) in Kalampangan

Figure 5. Box plot comparison of DBH for single tree nests and those constructed with tree-tying in Kalampangan

Trees TiedSingle Tree

40

30

20

10

0

Met

ers

Nest Height

Figure 6. Boxplot comparison of nest heights for single tree nests and those constructed with tree-tying.

20

     

In order to fully examine the interactions and relative contributions of these variables to tree-

tying, a general linear model incorporating all variables was conducted, with tree-tying treated as a

binary response (0 denoting single nest trees and 1 denoting tree-tying). The full, initial model only

showed nest height to be a significant factor in tree-tying. The negative point estimate demonstrates

that the probability of tree-tying decreases with increasing nest height. As many of these variables

were thought to be related and potentially confounding, a stepwise AIC model reduction was formed

to reduce the Akaikes Information Criterion and increase the predictive power of the model. Table 1

displays the final model, with nest size and tree height as significant factors in tree-tying and nest

height as highly significant. The reduced AIC model was a better fit than the initial model and was

further determined to be a good fit by comparing model predicted verses actual values of tree-tying

for each composite point estimate.

Table 1. Reduced general linear model for tree-tying analysis. The probability that a variable was greater than or equal to z represents a significant relationship to tree-tying.

Estimate Std. Error z value Pr ( > I z I ) Intercept 0.20452 0.54255 0.377 0.7062 Nest Size 0.78450 0.39348 1.994 0.0462 Nest Height -0.27469 0.06341 -4.332 1.48 e -05 Tree Height 0.08700 0.04158 2.092 0.0364 Canopy Health 0.22320 0.12482 1.788 0.0737

Nest height and tree height explain a large proportion of the data and seem to co-vary with all

other measured variables. Basal diameter was the last variable to be removed in the step AIC process,

and is likely captured by the predictive power of nest height and tree height. Figure 7 demonstrates

how nests placed high in tall trees with large basal diameters were more often built in just one tree

and did not employ tree-tying, while tree-tying was more often seen in shorter trees with smaller

basal diameters.

21

     

In Sabangau, 67 nests, or 54% of the total, were single tree nests and 57 nests, or 46% of the

total, were constructed with tree-tying. CBH (CBH for single tree nests: mean =20.36 cm, SD = 7.83;

CBH for tree tied nests: mean = 12.68 cm, SD = 3.71, two sample t test t: 6.69, df=120, p <0.05;

Figure 8), nest height (nest height for single tree nests: mean = 16.12, SD = 4.90; nest height for tree

tied nests: mean = 12.97, SD = 4.41, two sample t test t: 3.68, df=119, p<0.05), and tree height (tree

height for single tree nests: mean = 18.95, SD = 5.07; tree height for tree tied nests: mean = 15.31,

SD = 3.41, two sample t test t: 4.51, df=119, p<0.05) were all significantly greater in single tree nests

than in those with trees tied. Nest size did not vary between single tree nests and those with trees tied

(p-value = 0.79).

Figure 7: Nest height and tree height as a function of tree-tying, where 0, represented by circles, denotes single tree nests and 1, represented by squares, denotes tree-tying. Each point reflects basal diameter with size.

 

22

     

Trees TiedSingle Tree

40

30

20

10

0

size

in c

m

Diameter at Breast Height (DBH) in Sabangau

Figure 8. Box plot comparison of DBH for single tree nests and those constructed with tree-tying in Sabangau.

Nest height, tree height, and CBH did not vary significantly between Sabangau and Kalampangan (in

two sample t-tests performed with all trees, single nest trees alone, and nests constructed with tree-

tying).

Discussion

Contrary to our prediction for Sabangau, the density estimate, 0.94 individuals/km2, has

decreased from the last reported estimate in 2009 of 1.93 individuals/km2. As there have been no

major environmental perturbations or anthropogenic disturbances within the national park, it is

possible that these numbers are a reflection of either sampling error or the natural fluctuation known

to occur in orangutan density over a period of time (Galdikas 1988). Additional analysis of these

estimates and potential explanations are in progress.

In Kalampangan, the density estimate of 1.77 individuals/km2 is similar to the last reported

23

     

estimate of 1.6 individuals/km2 in 2009. While it was assumed that the density would have decreased

as a result of fires diminishing fruit availability and survival, these numbers may reflect a time lag in

the effects of disturbance. Such a time lag has been reported in a study in Sabangau, (Husson,

unpublished) where orangutan distribution was altered following logging activities, but a substantial

change in orangutan density was not seen until four years after the disturbance, when a 30% decline

was measured over just one year (Husson et al 2009).

The estimates obtained from this study suggest that the number of orangutans in

Kalampangan is currently almost twice that of a similar measurable area in Sabangau. This provides

support for our prediction of overcrowding, as individuals are pushed into the interior of a shrinking

habitat patch that is becoming more fragmented due to forest fires. In addition to the aforementioned

evidence for Bornean orangutans, this has been documented in other primate species. In West

Malaysia, Johns (1986) noted significant changes to the ranging and activity patterns of several

primate species prior to logging events. Gibbons (Hylobates lar) either restricted their activity to

areas not yet logged or moved beyond the core areas of disturbance. As they have no means of

dispersal to other forested patches, orangutans in Kalampangan may be altering their distribution to

the forest interior, where fruit availability and the structural integrity of the canopy is less disturbed

by fire. Additional research of the typical ranging patterns of P.p. wurmbii is warranted to better

assess if and how distributions are modified in Kalampangan.

It is also important to consider that orangutans naturally occur at different densities across

sites (Galdikas 1988), and as we do not have density estimates for Kalampangan prior to its

extensive fragmentation, we cannot rule out the possibility that these estimates are merely a

reflection of an intrinsic difference in densities between Sabangau and Kalampangan. While it may

not be possible to ascertain whether such an intrinsic difference exists, an assessment of habitat

characteristics known to impact the demography, health, and density of orangutan populations, such

as fruit availability and canopy structure, should be conducted to determine whether Kalampangan

24

     

can continue to support orangutans at this density if fires and the resulting loss of vegetation and

expanses of peatland in Block C continues at its current rate.

This study has demonstrated that orangutans prefer particular tree species for nest

construction in Kalampangan. While none of the variables measured here are significantly associated

with such preferences, the Principal Components Analysis revealed that tree characteristics relating

to the security of a nest, such as branch strength and the leaf area to branch ratio may play an

important role in tree species preference. Considering habitat quality between the two study sites

could shed light on these results. More tree species are avoided for nesting in Kalampangan than in

Sabangau. What would explain the higher proportion of avoided species? If comparing the usage of

tree species for nests with their abundance in the forest is an accurate way to measure preference,

there are two possible explanations.

First, due to time restrictions, only one measurement of each tree species characteristic was

considered. Such a small sample size may not be entirely reflective of the typical sap, branch, and

leaf characteristics of these tree species. The second explanation is related to this study’s assumption

that selected tree species characteristics did not vary between Sabangau and Kalampangan.

Measurements were analyzed interchangeably, which would not account for potential differences in

tree species and their characteristics between each site. A study in East Kalimantan (van Nieuwstadt

and Sheil 2004) demonstrated the severe effects of fire and associated water-deficient soils on trees,

which ultimately lead to an exhaustion of stem energy reserves, loss of leaves, and potential

mortality for many tree species. Given the diminished soil hydrology documented within Block C

(Cattau 2010), it is possible that sap quantity, branch lengths, and leaf number and size have been

affected by fire events, and therefore the same tree species would not be comparable between sites.

The Principal Components Analysis of the measured variables could only explain a

maximum of 46% of the variation between tree species that were preferred and avoided for nesting at

Kalampangan. One possible explanation for this result aside from small sample size is that

25

     

orangutans prefer tree species for characteristics different to the ones measured in this study. Habitat

disturbance is known to affect the distribution of tree species within the forest – from dispersal of

their seeds to regrowth prior to fire (van Nieuwstadt and Sheil 2004). The tree species orangutans

prefer may be ones that occur in areas of the forest that have more complete canopy coverage or tend

to occur in the interior of the forest rather than the edge. Future research of nest tree species

preference should aim for larger sample sizes and incorporate an understanding of tree species

phenology and any potential effects of disturbance on the distribution of trees and measured

characteristics.

Analysis of variables related to tree-tying allow for a deeper understanding of nest

construction and reveal more than tree species preferences can alone, especially when comparing

between sites. There is likely something more than merely an association between nest size and age-

sex class at work. These results demonstrate a relationship between tree height and habitat quality, as

canopy health is significantly lower for nests constructed with tree-tying in Kalampangan. The lower

canopy interconnectivity for single tree nests may be related to the canopy structure of this forest.

Incomplete canopy coverage and frequent gaps within the canopy of Kalampangan may leave tall

trees disconnected from the rest of the forest. There may be a compromise between the stability of

using one, large tree for a nest that may not have complete coverage, to trees that are smaller in size,

shorter in height, but more connected to the rest of the canopy.

In Sabangau, the patterns observed for tree-tying were similar to those in Kalampangan.

Nests constructed with just one tree were on average greater in DBH, nest height, and tree height. As

single tree nests are seen more often in both sites, the structural stability given by larger, healthier

trees seems to be an important factor in nest construction for P.p. wurmbii. Tree DBH distributions

from vegetation plots in Sabangau and Kalampangan demonstrate that the majority of trees fall

between 0 and 10 cm in DBH in both sites. This is indicative of a potential preference for larger trees

than are available - in both sites the average DBH for single tree nests was around 20 cm, and even

26

     

for tree-tying is around 12 cm. Given that tree-tying is rare outside of Central Kalimantan, these

results suggest that tree-tying may be a compromise when better quality nest sites (with increased

structural stability in tree height and size, and greater canopy health) are not available.

Conclusions

This study has shown that orangutans in the degraded forest of Kalampangan can occur in

higher numbers than the protected forest of Sabangau. While further research is needed to establish

whether this is a result of overcrowding, the presence of orangutans is enough to warrant this site’s

protection from further logging and subsequent management efforts. If the habitat is deemed

unsuitable and numbers continue to increase, corridors should be established to facilitate the

dispersal of orangutans within the Kalampangan site to another forest patch.

Orangutans at both of these sites in the Sabangau basin of Central Kalimantan have

demonstrated preferences for specific tree species in nest construction. While further research is

required to provide more robust conclusions for which tree species attributes underlie the

demonstrated tree species preferences, forest characteristics such as tree size and height play an

important role in the nest construction process. Management efforts may want to ensure that trees of

a certain size and height remain in the forest for use in nesting. A comparative framework of nest

construction should be established from sites across Borneo and Sumatra to examine how the

preferences observed for P. p. wurmbii compare with other populations.

Acknowledgements

I would like to give a special thanks to my advisor, Dr. Karen B. Strier for her unrelenting

assistance and guidance from the beginnings of this project. Funding was crucial in allowing me to

have this great opportunity to conduct research abroad, and I am thankful to the Holstrom

Environmental Scholarship committee for their generous recognition of this project. I appreciate the

University of Wisconsin-Madison, particularly the Letters and Science Honors Program and Dr.

27

     

Chuck Snowdon, for their encouragement of undergraduate research. Thanks to Benjamin Stenhaug

for his help with multivariate statistics.

This project would not have been possible without the wonderful opportunity the Orangutan

Tropical Peatland Project has given me to participate in their research. I would like to thank all of the

OuTrop researchers for their inspiring work in the Sabangau forest and help with this project, in

particular Dr. Mark Harrison, for his continued assistance in the field and with data analysis. Thanks

also to Karen Jeffers for her persistence in getting me the necessary permits to work in Indonesia. I

am so grateful for all of OuTrop’s field assistants, in particular, Ari Purwanto and Franciscus

Harsanto - their expertise in nest spotting and intimate knowledge of the Sabangau forest made every

day in the field an amazing learning experience. Thanks to Joana Aragay, Lilia Kapsali, Nick Boyd,

Victoria Hatton, Cassie Freund and Megan Cattau for their assistance with data collection and great

companionship in the field.

References

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Boehm, H. D. V., and Siegert, F. (1999) Application of Remote Sensing and GIS to survey and evaluate tropical peat. Kalteng Consultants. International Conference and Workshop on Tropical Peat Swamps, Safeguarding a Global Natural Resource. 27-29 July 1999. Penang, Malaysia.

Cattau (2010) Land cover change in Sabangau, Central Kalimantan from 2000-2007: Implications for the Bornean orangutan (Pongo pygmaeus). MSc Project, Duke University.

Galdikas, B. M. F. (1988). Orang-utan diet, range and activity at Tanjung Puting, Central Borneo. International Journal of Primatology, 9:1-35. Husson, S.; Wich, S.A.; Marshall, A.J.; Dennis, R.D.; Ancrenaz, M.; Brassey, R.; Gumal, M.; Hearn, A.J.; Meijaard, E.; Simorangkir,T.; and Singleton,S. (2009) Chapter 6 – Orang-utan distribution, density, abundance and impacts of disturbance. In: Wich, S.; Atmoko, S.; Setia, T.M; and van Schaik, C.P. Orang-utans: Geographic Variation in Behavioral Ecology and Conservation. Oxford University Press, USA. Johns, A. (1986) Effects of Selective Logging on the Behavioral Ecology of West Malaysian Primates. Ecology, 67 (3): 684-694.

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Marshall, A. J., Boyko, C. M., Feilen, K. L., Boyko, R. H. and Leighton, M. (2009), Defining fallback foods and assessing their importance in primate ecology and evolution. Am. J. Phys. Anthropol., 140: 603–614. Meijaard, E, Guillaume A, Nardiyono, Rayadin Y, Ancrenaz M, and Spehar S. (2010) Unexpected ecological resilience in Bornean orangutans and implications for pulp and paper management. PLoS ONE, 5 (9): 1-7. Meijaard, E., Wich, S., Ancrenaz, M. and Marshall, A. J. (2012), Not by science alone: why orangutan conservationists must think outside the box. Annals of the New York Academy of Sciences, 1249: 29–44. Morrogh-Bernard, H; Husson, S; Page, S.E; and Rieley, J.O. (2003) Population status of the Bornean orang-utan (Pongo pygmaeus) in the Sebangau peat swamp forest, Central Kalimantan, Indonesia. Biological Conservation, 110:141-152. Nantha and Tinsdall (2009) The orang-utan–oil palm conflict: economic constraints and opportunities for conservation. Biodivers Conserv, 18: 487-502.

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van Casteran, A.; Sellers, W.I.; Thorpe, S.K.;Coward, S.; Crompton, R.H.; Myatt, J.P.; and Ennos, R.A. (2012) Nest-building orangutans demonstrate engineering know-how to produce safe, comfortable beds. PNAS, 109 (18): 6873-6877.

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Warren, K.S.; Verschoor, E.J.; Langenhuijzen, S; Heriyanto, Swan, R.A.; Vigilant, L.; and Heeney, J.L.(2001) Speciation and Intrasubspecific Variation of Bornean Orangutans, Pongo pygmaeus pygmaeus. Molecular Biology and Evolution, 18(4): 472-480.

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Supplementary Material

Table S1 – All Tree Species Encountered with a Nest

Tree Species Local Indonesian Name Shorea belangeran Belangaran/Kahui

BintanTristaniopsis sp.4 Blawan punai

Palaquium leiocarpum HangkangSyzygium sp./sp. 1 cf garcinifolia Jambu burung

Xylopia fusca Jangkang kuningCalophylum hosei JinjitCalophylum sp. Kapurnaga laut

Koompasia malaccensis KempasBlumeodendron elateriospermum/tokbrai Kenari

cf. Anisoptera (Dipterocarp) Keruing sambunNephellium maingayi Kelumun bohis

Eugenia spictata Kayu Lalas Diospyros bantamensis Malam malamElaeocarpus mastersii Mangkinang/Blueberi

Gymnacranthera farquhariania Mendarahan daun kecilShorea teysmanniana Meranti semut/Meranti

Dactylocladus stenostachys MertibuPalaquium ridleyii/cf. xanthochymum Nyatoh burung

Palaquium cochlearifolium Nyatoh gagasLithocarpus rassa Pampaning

Lithocarpus conocarpus Pampaning bayangLithocarpus sp.1 cf. dasystachys Pampaning bitik

Sandoricum beccanarium PapongMezzetia leptopoda/parviflora Pisang pisang besar

Tetramerista glabra Ponak Neoscortechinia kingii Pupu palandukTetractomia tetrandra Rambangun

Nephellium lappaceum RambutanGonoystylus bancanus Ramin

Stemonorus cf. scorpiodes Tabaras yang tdk punya akar tinggi Mesua sp. 1 Tabaras akar tinggi

Campnosperma coriaceum Terontang

31

     

0.500.250.00-0.25-0.50

0.50

0.25

0.00

-0.25

-0.50

-0.75

First Component

Seco

nd C

ompo

nent

No. Secondary Branches

Total Leaves

Leaf Area:Branch

Avg Leaf Area

Strength

Sap Stickiness

Sap Viscosity

Loading Plot of Tree Variables

Figure S1. Loadings plot of variables relating to tree species attributes from Principal Components Analysis.