Gorillas (Gorilla gorilla) are apes and belong to Hominoidea. Hominoidea includes gibbons and siamangs (Hylobatidae), commonly referred to as the lesser apes, and orangutans (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).

The Miocene apes do not have derived morphology that is shared with the African apes and therefore none of the Miocene specimens can be categorized as ancestral to the African ape lineage. Additionally, no fossil specimens from more recent epochs exhibit derived traits that can be linked to the African apes. Therefore, although the last common ancestor between humans and African apes is believed to have existed between five and six million years ago, there is no consensus as to what this common ancestor would have looked like (Fleagle, 1999; Hartwig, 2002).

Gorillas are distributed throughout Sub-Saharan Africa and inhabit tropical forest habitats. Three subspecies of gorillas are recognized according to their geographic distribution. These include the western lowland gorilla, the eastern lowland gorilla, and the mountain gorilla. Mountain gorillas are almost entirely terrestrial and subsist mostly on leaves. While lowland gorillas are terrestrial, individuals frequently engage in arboreal behaviors and often eat and sleep in trees. Unlike the mountain gorillas, lowland gorillas include a substantial portion of fruit in their diet (Fleagle, 1999).

Notably, gorillas are the largest extant species of primate. In addition, gorillas exhibit one of the highest degrees of sexual dimorphism among primates. Females typically weight between 70 kg and 90 kg. Males are more than twice as large as females with weights of up to 200 kg (Fleagle, 1999).

Unlike many other species of primates, both males and females leave their natal groups and migrate to other social groupings. Mountain gorillas typically live in small groups of about 10 individuals. This grouping includes a dominant male, known as a silverback, several females, and their offspring. Young males that have not yet established themselves as dominant males will often form small groups together known as bachelor groups. Competition to become a dominant male is intense and infanticide is common when a new male takes over as the silverback. Lowland gorillas have been observed to have social groupings similar to mountain gorillas in addition to having fission-fusion groups in which small groups of individuals will fragment and regroup according to resource availability (Fleagle, 1999).

Gorillas use a unique form of quadrupedialism called knuckle-walking. Instead of contacting the ground with the palm or ventral side of the hand, gorillas curl their fingers into a fist and contact the ground with the dorsal side of their proximal phalanges. Chimpanzees are the only other species that exhibit knuckle-walking (Fleagle, 1999).

Additional Information on the Skull

Click on the thumbnails below for labeled images of the skull in standard anatomical views.

Dorsal view	Lateral view	Ventral view

This specimen, the skull of an adult male, was collected on Mount Viroke, Virounga Mountains, Rwanda by Diane Fossey on 3 May 1968. It was made available to The University of Texas High-Resolution X-ray CT Facility for scanning by Dr. John Kappelman of The University of Texas at Austin. Scanning was funded by an NSF grant to Dr. Kappelman (IIS-9816644). Funding for image processing was provided by a NSF Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

This specimen was scanned by Richard Ketcham and Matthew Colbert on 13 April 2001 along an oblique coronal axis for a total of 658 slices. Each slice is 0.5 mm thick, with an interslice spacing of 0.5 mm and a field of reconstruction of 181.0 mm. The specimen was rotated to orthogonal prior to image processing.

Literature

Fleagle, J. G. 1999. Primate Adaptation and Evolution. Academic Press, San Diego, CA. 596 pp.

Hartwig, W. C. 2002. The Primate Fossil Record. Cambridge Univeristy Press, Cambridge, U.K.

Links

Gorilla gorilla and other primates on the eSkeletons Project webpage (The University of Texas at Austin)

Gorilla gorilla beringei on the Animal Diversity Web (University of Michigan Museum of Zoology)

See more images of Gorilla from Last Refuge Ltd.