Both species differ in morphological features such as length and color of the hair, shape of the male's cheek pads, the size and shape of the throat pouch and body build. However, the classification into two species is still debated given the high degree of variability of morphological features but also social characteristics within each species.
Orangutans are characterized by a strong sexual dimorphism. With a weight between 60—80 kg, males are twice as heavy as females, (Rowe 1996; Schulz, 1956). In addition, there are two different sexually mature male morphs (which is sometimes referred to as bimaturism, Bennett, 1998) that are distinguished by both morphological features and behavioral traits (Delgado & van Schaik, 2000). Fully developed or flanged males develop a number of secondary sexual characteristics, such as wide cheek pads, long hair and a large laryngeal sac (Knott, 1999; Rodman, 1988). Unflanged males are about the same size as adult females (Galdikas, 1985) and do not show these secondary sexual features, but they are sexually fully mature and able to sire offspring (Rijksen & Meijaard, 1999).
Current population estimates refer to 23,500 orangutans in Borneo and 3,500 in Sumatra.
Distribution and ecology
The orangutan density varies between 2-7 individuals per km2 close to rivers and less than one individual per km2 in higher elevations/altitudes. Orangutans are a frugivorous species feeding mainly on a wide range of different species of fruits, supplemented by leaves and animal protein, such as insects or eggs.
They spend most of their daily activity with feeding (45.9%) and resting (39.2%). Beside traveling (11.1%) and nest building (1%), only less than 2% are spent with social activities, such as mating, vocalizing or socializing.
Social structure and behaviour
The size of female ranges varies between 0.6 and 1 km2, whereas adult males can cover ranges larger then 10 km2. However, residents represent the minority of an orangutan population. The majority are commuters that are characterized by a nomadic existence and are regularly present in a certain area for several weeks or months each year over several years.
In addition, there are few wanderers that are only seen infrequently or once. Since the ranges of different individuals can overlap, different social units can aggregate to form temporary associations or social groups during feeding in the same fruit tree or traveling. Whereas adult flanged males inhabit a certain range, unflanged males travel widely to enforce copulations with females. Their later ascent to a higher socio-sexual status appears to be associated with the establishment of long-lasting sexual relationships.