The apes are classified in the superfamily Hominoidea. The lesser apes (gibbons and siamangs) are placed in the family Hylobatidae. Great apes and humans are placed in Hominidae, but some would place the orangutan (Pongo), in a family by itself, Pongidae. Some recognize chimpanzees and gorillas, the two knuckle-walking African apes, as members of the same genus, Pan, but others prefer to keep the gorilla (Gorilla) separate to highlight the greater relationship of chimpanzees (Pan troglodytes) to bonobos or pygmy chimpanzees (Pan paniscus).
The hylobatids, or lesser apes, are all specialized for brachiation (arm swinging). The siamang is roughly the same height as the gibbon with slightly less elongated arms, but it weighs nearly twice as much. Gibbons are all of similar structure with greatly elongated arms, hands, and fingers and are divided into species on the basis of pelage, vocalizations, and variations in throat-sac adaptations. All hylobatids are native to the rain forests of southeast Asian and Indonesia. They live in monogamous groups and are territorial frugivores. Capable of long flying leaps, swinging from hand to hand suspended beneath branches, they are extraordinarily graceful to watch. They also "sing" duets, and the calls are loud but melodious. These calls are analogous to bird calls in that they often advertise a defended territory. Their arms are so long that if they are forced to walk on the ground and brachiation is not an option, they must walk bipedally. The gibbon's thumb is unusual in that the metacarpal is free; this allows the gibbon to use this "extra" joint to fold the thumb across the heel of the hand in locomotion.
Great apes are popular in zoological exhibits, and the chimpanzee is often the animal of choice in some research settings (NRC 1997b). The mountain gorilla, famous from numerous television shows about wild animals, has only rarely been seen in captivity; exhibitors almost always display the lowland variety. A third subspecies, the western highland gorilla (or eastern lowland gorilla), is also recognized in the wild. Orangutans all belong to a single species (Pongo pygmaeus ), but Bornean and Sumatran subspecies are recognized, and the orangutan species survival plan (SSP) insists that they be kept segregated. Hybrids are readily produced in captivity, but geographic separation prevents gene flow between the two subspecies in the wild. Although earlier systematists recognized four subspecies of chimpanzee, little effort has been made to segregate or even identify chimpanzee subspecies in captivity. (See also work by Morin and others 1992, 1994 on subspecies identification in Africa.) The bonobo is, however, a morphologically and behaviorally distinct species (Napier and Napier 1985; Susman 1984).
Adult gibbons and siamangs are not tolerant of other adults of the same sex for any long period, although large numbers of immatures can be safely housed