required by Animal Welfare Regulations (9 CFR Subchapter A). Because their black bodies absorb heat from direct sun rays, there is potential for chimpanzees and gorillas to suffer from heat stroke at relatively mild temperatures, although chimpanzees are routinely maintained outdoors in the southern United States with temperatures in the shade routinely above 38°C (100°F). Chimpanzees tolerate cold quite well for short periods but bonobos and lesser apes require shelter even at air temperatures of 10°C (50°F) and above. All animals should have access to indoor ''night quarters."

Chimpanzees often engage in charging and drumming displays, which intimidate cagemates and visitors and can weaken cage structure. Inanimate objects and other group members are sometimes hit during these displays. Under these conditions, a door for retreat into a separate area is particularly helpful. Barrels are provided to enable this species-typical behavior in some facilities and constitute an effective and less-expensive option than repairing damage to caging or handling injuries to other animals. Proper design might be more important than space itself in the design of ape quarters (Fritz and others in press; NRC 1996, 1997b). Chimpanzees in social groups will make use of separate areas. The opportunity to join and leave a social group at will during times of tension is beneficial for animals that might be picked on during noisy displays. Chimpanzees have well-developed mechanisms to restore social harmony (de Waal 1989), but the opportunity for brief voluntary separation is helpful.

Ideally, housing permits the full range of locomotor expression, and socially housed chimpanzees enjoy space to play. Areas that are divided by a large obstacle provide a circular arena for such games as chase. Vertical space is also very important to apes, and one or two areas a few meters (9–15 ft) high, permitting a broad view of the surroundings, are favored locations (Traylor-Holzer and Fritz 1985). If possible, outdoor areas should have other than concrete floors, preferably natural grasses or bedding. A deep straw, pine– or cedar-bark, or woodchip cover 25–30 cm (10–12 in) deep has worked well in outdoor enclosures (Brent 1992; Rumbaugh and others 1989) and decomposes into a medium not conducive to insects. The ground cover stays clean as long as all fecal and urine-soaked material is removed daily.

Most agree that the best enrichment for a chimpanzee is another chimpanzee. Even in social groupings, however, attention should be given to the inanimate part of their environment. Attempts by zoological and research institutions have demonstrated the value of providing a wide assortment of furnishings for chimpanzees' use and amusement, including providing them with large uprooted trees (Maki and Bloomsmith 1989); "termite" feeders (Maki and others 1988); television or live-action video of other chimpanzees (Rumbaugh and others 1989; see also Platt and Novak 1997 for use in macaques); puzzle feeders (Maki and others 1989); traffic cones (Fritz and Howell 1993a); nesting materials, such as woodchips (Brent 1992), straw, cloth, or shredded paper (Fritz and others in press); and novel methods of providing food treats (see also Fritz and Howell



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