From the Forest

 From the Forest:

Chasing Orangutans in Hutan Lindung Wehea, East Kalimantan, Indonesia


Roberto A. Delgado Jr.

I first went looking for free-ranging orangutans in earnest at Wehea forest during the summer of 2007. I’d previously conducted field research on orangutan behavioral ecology in northern Sumatra and West Kalimantan for my dissertation before being lured to the easternmost extent of their geographic range by the promise of studying an endangered and relatively poorly-known subspecies, the Northeast Bornean orangutan (Pongo pygmaeus morio).

At the time, “morio” was thought to occur at moderately high population levels. This subspecies holds particular interest to some researchers such as myself because it is reported as having the smallest cranial capacity, the most robust jaw morphology, and an accelerated life history pattern. That is, although some Sumatran orangutan populations are known as having the longest inter-birth interval among primates, reaching up to nine years, females from this Bornean subspecies are thought to give birth approximately every six years. Such drastic variation within closely related species often implies meaningful selection pressures most likely based on important environmental factors. The availability of resources and, specifically, both the spatio-temporal distribution and relative abundance of food patches, tend to be the usual suspects.

There is now increasing evidence that the lowland forests of East Kalimantan, Indonesia, are of poorer quality and less productive than other habitats within the geographic range of orangutans. These findings further suggest that, compared to other orangutan populations, preferred foods are scarce and unpredictable in their availability; thus, this subspecies must cope with a different set of ecological challenges than their counterparts elsewhere. Accordingly, this subspecies smaller brains and larger teeth would appear consistent with a strategy that minimizes energy expenditure in a harsh, resource-scarce environment (see From the Forest, October 2011).

A reasonable scenario to explain such traits is one that, for this population, would speed up development and other life history variables in light of uncertain habitat productivity while constraining cranial capacity under limited energy and nutrient supplies, but at the same time, enhancing dental features to exploit available food resources. With the idea of examining these intriguing questions, and also falling within my interest in geographic variation in behavior, I ventured to the forests of eastern Borneo.

Wehea base camp

Before my arrival, a number of Indonesian and western colleagues had paved the way. Specifically, scientists from the Tropical Forest Research Center at Mulawarman University in Samarinda had identified several forest sites as having good orangutan habitat.  In addition, field teams from The Nature Conservancy (TNC) had estimated relatively high local population densities based on indirect nest surveys throughout a subset of forest tracts in East Kalimantan. In fact, two sites had been selected initially for further monitoring and biodiversity research, one of which was Wehea.

In the spirit of full disclosure, when I first assessed potential field localities with an eye for potential conservation research on orangutans, Wehea was not my initial choice. I felt it had two strikes against it; one was that it was located within a former logging concession, and the other was the site’s challenging topography.

I had reservations about working in a former logging concession because I’d conducted my previous research in national parks—Gunung Leuser in northern Sumatra and Gunung Palung in West Kalimantan—and I wanted to keep the disturbance levels to a minimum in an effort to be consistent in habitat features when making population comparisons.

As for Wehea’s topography, it is one of the most physically demanding field sites in which I have worked. Upon my initial visit, I confirmed that indeed it was “steep and very steep” hill forest and I felt that it would be impractical and a logistical nightmare to set up shop. Little did I know at the time that both the topography and former status as a logging concession would eventually be contributing factors leading me to return and establish one of my current research programs at Wehea.

Before commencing the orangutan project at Wehea, I spent the better part of three field seasons attempting to establish a research program at another nearby site known as Lesan. However, despite the tireless efforts of my team that included local Indonesians, TNC field staff, volunteer research assistants from North America, and a Master’s student from Hunter College, we were unsuccessful in encountering and identifying a viable study population of orangutans.

At least two reasons contributed to our failure. Although the forest at Lesan was relatively more pleasant than Wehea with regard to its landscape features, there was also a history of hunting pressure from indigenous peoples. As a consequence, orangutan (and overall primate) abundance was relatively low in those areas nearest to our base camps.  In addition, animals tended to be very elusive and skittish. Secondly, TNC scientists also later realized errors in their initial calculations for local orangutan population density, and the revised estimates were considerably lower than those previously reported. I should also add that several political and subsequent cultural restrictions prevented us from continuing our research and monitoring at Lesan, so it was ultimately more cost-effective to relocate.  Fortunately for us, we had a Plan B: Wehea.

In 2009, I began collaborating with Dr. Stephanie Spehar, from the University of Wisconsin, Oshkosh. We set out initially to address questions regarding niche overlap at Wehea among orangutans and two other sympatric species of non-human primates: the maroon leaf monkey (Presybytis rubicunda), or langur, and Mueller’s gibbon (Hylobates muelleri). In particular, we were interested in comparing their foraging ecology, demographic and population parameters, as well as their grouping and association patterns.

In the face of the scarcity and unpredictable availability of fruit in a forest such as at Wehea, our objectives were to understand the extent of behavioral flexibility among these three non-human primate species.

The fact that Wehea was within a former logging concession actually fit well into our comparative approach of examining how the gibbons, langurs, and orangutans responded to changes in their environment.  For us, that history of disturbance served as a natural experiment because we were familiar with how these species behaved in pristine habitats. The rugged terrain also turned out to be beneficial in that is has helped to preserve the structure of the forest at Wehea. Large, towering old-growth trees are difficult to extract from steep slopes and are more likely to stay in the forest than end up at the nearest timber mill.

Field staff member by old growth tree

Unfortunately for us, however, the orangutans at Wehea were also initially overestimated, and we have striven to gain more accurate and reliable data for calculating their local population density. By our concerted efforts these past few years, we are finding that the orangutans occur at local densities of approximately one individual per square kilometer throughout the main study area. Consequently, habituation and data collection have been slow-going, with most of our behavioral observations of orangutans being opportunistic. Nonetheless, we continue to monitor the Wehea orangutan population and record direct measures of resource availability, via phenological plots, and indirect measures of habitat use, based on the distribution of nests.

At this time, we can report that orangutan populations have remained stable while we have maintained a research presence. We’ve also made inroads in habituating groups of the other primate species, in particular the maroon langur, which we encounter more frequently and predictably. We now have reliable information on their group composition and highly accurate GPS data on ranging patterns and daily travel distance that we’ll be presenting at the annual meeting of the American Association of Physical Anthropologists in Portland later this spring.

Admittedly, like many field researchers, we are struggling to acquire funding and maintain our research program. We owe much of our success to the efforts of Indonesian field staff and volunteer research assistants from North America as well as sponsoring organizations like Mulawarman University and the Indonesian Ministry of Research and Technology. Support from local individuals, including members of the Dayak communities nearest Wehea, and international organizations have also been a boon to our project, but orangutan conservation in Indonesia is becoming more and more fraught with conflicting political and economic motives.

Although a local management body administers the daily operation of the field site at Wehea, there is also input from other stakeholders ranging from nearby village elders and governmental agencies to western scientists and international non-governmental organizations.

Orangutan nest

Presently, orangutan research at Wehea remains in its early stages. While we are increasingly gaining a firmer handle in understanding the ecology of the site, we still have some ways to go before we will obtain sufficient reliable observational data from enough different individuals to reach preliminary conclusions about the behavioral repertoire of this endangered orangutan subspecies. However, despite all of the obstacles to successful fieldwork in the tropics, we remain dedicated to maintaining our research and monitoring presence, working together with the local community, governmental agencies, and NGOs, and promoting our activities and findings to insure that the wildlife and biodiversity at Wehea remain protected and well managed into the future.

By Roberto A. Delgado Jr./edit by Tom Mills Orangutan Conservancy

Roberto A. Delgado Jr. is an Assistant Professor of Anthropology & Biological Sciences
Section in Human and Evolutionary Biology
Dornsife College of Letters, Arts and Sciences
University of Southern California

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4 Responses

  1. Excellent article, thank you! This was so interesting – I will look forward to learning more about future conservation efforts and the research results regarding this unusual subspecies!

  2. Great article. I certainly am learning a lot about Orangutans and their struggle for survival. The photographs
    are really a great help. Glad you are including them in your articles.

  3. I did see this subspecies of orangutan on one of my visits to Borneo. Most people do not know about the differences between the populations. I find it fascinating.

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