together in the presence of an adult or adult pair (Bernstein and Schusterman 1964). Perhaps as a function of the large cage area necessary for their brachiating locomotion, relatively few gibbons or siamangs are found in laboratory settings, although exhibitions often feature these animals in settings that permit display of their graceful locomotion. Cages provided with horizontal supports that permit suspensory locomotion are well suited for these animals, and they will normally remain in high locations, descending to the floor only occasionally. They are not physically powerfully animals, and cages can be made of lighter material than required for Old World monkeys of comparable size.

The great apes, in contrast, are extremely powerful animals with enormous hand strength. All three genera of great apes are commonly found in exhibits, but only the chimpanzee is found in laboratory settings in substantial numbers (Byrd 1977; Fritz and others in press; NRC 1997b). Exhibits generally maintain great apes in small groups in outdoor compounds provided with heated indoor shelters. The orangutan is generally regarded as semisolitary in the wild, and, although it will do well in small social groups in captivity, it might be wise to have only a single adult male in such a group. Adult male chimpanzees can be housed together (Alford and others 1995), and male gorillas that grow up together can be tolerant of one another. A few laboratories maintain chimpanzees in individual cages under biocontainment conditions. Even under these conditions, however, chimpanzees will benefit from visual and auditory contact with others (NRC 1997b; see also discussion below).

Although chimpanzees and gorillas locomote primarily as quadrupeds, supporting their weight on the soles of their feet and knuckles of their hands, both are active climbers and often suspend themselves by their hands. Accordingly, cages should be tall enough to permit any ape to hang by the fingers without touching the floor. Among the great apes, orangutans are least suited to terrestrial locomotion, and they are generally quadrumanous, cautious, slow climbers in which diagonal limbs (e.g., right arm and left leg) move in synchrony as in quadrupedal walking (Hunt 1991). Although generally slow, orangutans can move very rapidly when alerted or during acts of aggression. They probably will profit the most from platforms, ropes, and hanging structures, although all apes will make good use of such furnishings (Fritz and others in press).

All great apes construct sleeping nests in the wild and will use straw, cloth, and any other suitable materials to make nests in captivity. Young apes appear to need to practice this skill lest they fail to make nests as adults (Bernstein 1962, 1967). Young apes, and especially orangutans, enjoy crawling inside cloth bags, barrels, and similar containers. Such items stimulate much play, but as with all such objects, care should be taken to avoid items that could be dangerous to the animals through ingestion or entanglement.

When adequately adapted, apes can tolerate temperature extremes, as can Old World monkeys, but shelter from direct sun, wind, rain, and temperature extremes should be available, as recommended in the Guide (NRC 1996) and

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