Habitat | Behaviour | Development
Orangutans live in primary and secondary tropical rainforests
of South East Asia on the islands of Sumatra and Borneo. Annual rainfall
varies on average from 3000 mm (Sumatra), to 4300 mm (Borneo). Temperatures
on both islands are quite comparable (Sumatra: 17°C - 34.2°C;
Borneo: 18°C - 37.5°C) as is the humidity (in both islands
is near 100 % year-round).
The orangutans live in lowland swampy areas as well as mountainous
areas, from peat swamps to dipterocarp forests.
Peat swamp forests
are located immediately behind the coastline and extend inland along
the lower reaches of the main river systems. Dipterocarp forests occur
on dry land just above sea level to an altitude of about 900 metre.
They are characterized by the fact that most of the largest trees
forests belong to one plant family known as Dipterocarpaceae. These
trees are highly valued timber resources, as are also the Ramin trees
bancanus) in peat swamp forests.
The highest densities of orangutans are found in areas which have a
mixed habitat that provide high quantities of food throughout the year,
as lowland swamp forests. Peat swamps support medium-densities of orangutans,
and dipterocarp forests, with their typical extreme variability in
the abundance of fruit from season to season and year to year, only
low densities of orangutans.
Sumatra is comprised of lowland swamps and wide, mountainous regions,
with elevations up to 1500m. The orangutans here are found primarily
in the lowland dipterocarp, freshwater and peat swamp forests. Higher
elevations have lower population densities of orangutans, as tree diversity
is much lower. The dipterocarp forests of Sumatra are especially famous
for their mast fruiting, which occurs every 2 to 10 years. Mast fruiting
is a phenomenon in which a large numbers of trees fruit simultaneously,
despite the absence of any seasonal change in temperature or rainfall.
During this time, orangutans greatly exceed their daily caloric intake
requirements and put on additional fat stores. This propensity to overeat
and store fat reserves may be why captive orangutans often struggle
with obesity. When mast fruiting does not occur during a year, there
an annual fruit peak.
Borneo consists largely of peat swamp forests and dipterocarp forests.
Orangutans on Borneo are not found in forests at elevations higher
than 1000 m. Nevertheless they still occupy the remaining
Sumatran orangutans are subject to predation by tigers, clouded leopards,
pythons and crocodiles, but tigers constitute probably the major predatory
threat. Clouded leopards are capable of killing orangutan adolescents
and small adult
females, but have not been known to kill adult males. The presence of predators
is probably the reason that Sumatran orangutans are rarely seen venturing onto
Bornean orangutans, have a smaller risk of being predated, which can happen
leopards or occasionally large pythons, and are seen more frequently
on the ground than their Sumatran relatives.