Orangutans (Pongo pygmaeus) are apes and belong to Hominoidea. Homoinoidea includes gibbons and siamangs (Hylobatidae), commonly referred to as the lesser apes, and Porangutans (Ponginae), gorillas, chimpanzees, bonobos, and humans (Homininae), often referred to as the great apes (Hartwig, 2002). Apes are anatomically distinct from monkeys in several ways. First, apes typically have longer forelimbs than hindlimbs, the only exception to this trend being humans. Relatively long forelimbs are attributed to the highly suspensory behavior of all non-human apes. In addition to having long forelimbs, apes have a dorsally positioned scapula and globular humeral head. These anatomical features enable apes to have a wide range of overhead movement and aid in suspensory locomotion. Apes also have a broad, short thorax, as opposed to the long, narrow thorax found in monkeys. Finally, unlike all other primates, apes lack a tail (Fleagle, 1999).
Although there are only five extant ape genera, several ape genera have been identified in the fossil record. The first radiation of fossil apes, the proconsulids, occurred in Africa from the late Oligocene to the middle Miocene. Although the best record of proconsulids is found in Africa, specimens have also been found in Asia, suggesting that proconsulids migrated to Asia in the early Miocene (Fleagle 1999). Additionally, fossil apes have been discovered throughout Europe, distributed between the French and Spanish Pyrenees to the Republic of Georgia. The fossil record indicates that African homonoids arrived in Europe during the Miocene between 16 and 17 million years ago followed by a diversification of apes across Europe and Western Asia (Hartwig, 2002).
Unlike the African great apes, the ancestral lineage of the Orangutan is well documented. The orangutan lineage is traced back to Sivapithecus, a Miocene hominoid from Turkey dated to about 8.5-9.5 million years ago. Sivapithecus and Pongo share several derived cranial features including a high degree of prognathism, oval orbits, a narrow nasal aperture, and broad zygomatic arches (Fleagle, 1999).
Orangutans can be divided into two subspecies, with one subspecies inhabiting Borneo and the other inhabiting Sumatra. Orangutans subsist mostly off fruit and leaves, although males add termites to their diet when terrestrial. Orangutans differ from the African great apes most noticeably in their cranial anatomy. Orangutans have a high, rounded braincase, oval orbits, reduced brow ridges, and a high degree of prognathism (Fleagle, 1999).
Among primates, orangutans exhibit one of the most extreme examples of sexual dimorphism. Males, weighing about 120 kg, are about twice as large as females. Male orangutans are also unique in having two periods of maturation. The first period of maturation occurs between the ages of eight and fifteen. Males in this period are known as subadult and have the ability to reproduce although they have not reached full maturity. Fully adult males are larger in size than subadult males and also have distinct cheek pouches. There is not a specific age at which subadults mature into full adults, suggesting that social factors play a significant role in male maturation (Fleagle 1999).
Orangutans have a noyau social structure similar to that of nocturnal prosimian primates. In noyau groupings most individuals are solitary and have separate home ranges. Females have relatively small home ranges and are typically accompanied by their offspring. Males have larger home ranges that overlap those of several females.
Despite their incredibly large size, orangutans are highly specialized for suspensory behavior. Orangutans have very long forelimbs in relation to their hindlimbs, curved fingers, and a reduced pollex, or thumb. Females are almost exclusively arboreal while males often travel on the ground using terrestrial quadrupedalism. When traveling quadupedally, oranguatans use a unique type of locomotion called fist-walking. Due to their relatively long fingers, orangutans walk with their hands curved into fists. Orangutans are the only primate species that exhibits this type of locomotor behavior (Fleagle, 1999).
Additional Information on the Skull
Click on the thumbnails below for labeled images of the skull in standard anatomical views.
Fleagle, J. G. 1999. Primate Adaptation and Evolution. Academic Press, San Diego, C.A. 596 pp.
Hartwig, W. C. 2002. The Primate Fossil Record. Cambridge Univeristy Press, Cambridge, U.K.
Other primates on the eSkeletons Project webpage (Univ. of Texas)
Pongo pygmaeus page on the Animal Diversity Web (University of Michigan Museum of Zoology)
See more images of Pongo at primates.com