Ranging behaviour and seasonal movements of Sumatran orangutans (Pongo pygmaeus abelii) in swamp forests

The ranging behaviour of Sumatran orangutans was studied in a swamp forest within the Leuser Ecosystem, Sumatra, Indonesia. The use of line transect techniques for estimating orangutan densities from nests was also examined. The densities obtained, in conjunction with raw numbers of new nests along transects were then compared with estimates of forest productivity in an attempt to identify how orangutan movements are influenced by resource availability.

Using nests as an indicator of orangutan numbers produced serious underestimates of absolute densities for a variety of reasons. These were considered to stem mostly from the use of decay rates estimated from total time to nest disappearance, whilst under­detecting older nests in censuses, and from the difficulties of detecting nests on or above transects and accurately estimating distances. The number of new nests along a transect was still considered a reliable indicator of fluctuations in absolute densities.

Orangutan home ranges at Suaq Balimbing were found to be distinctly larger than previously reported elsewhere. Females appear to utilise core areas of at least 500 ha, whilst also using a peripheral excursion zone so that total home ranges probably lie between 900 ha and 1500 ha. Adult male and subadult male ranges were considered to be in excess of 3000 ha and perhaps as much as 10000 ha. There was no evidence that adult females or adult males need necessarily be transient or nomadic as previously proposed. Subadult males, however, may constitute a dispersal phase.

Orangutan movements were related to fruit availability, in that they will move to areas where fruit is abundant, but also into other areas when fruit is generally scarce. Thus there was evidence that orangutans shift diets during lean periods rather than travelling large distances. There was no evidence to support the occurrence of large-scale seasonal movements of orangutans.

There was evidence to suggest that dominant adult males behave differently to other, non-dominant males, in that they occupy smaller ranges, probably as a result of being able to restrict access by other males to receptive females. Non-dominant adult males tended to avoid the dominant male, whereas subadult males did not.

Evidence was also found to support the existence of clusters of females with similar ranges, who may be related, preferentially associate with each other, and are to some degree synchronised reproductively.