Individual Housing. There are both advantages and disadvantages to individual caging. The physical health of a primate might be best protected when it is maintained in a cage that can be completely sanitized. Proper sanitation has virtually eliminated many endemic diseases, such as shigellosis and salmonellosis. Individual caging also minimizes wounding due to fighting. However, some animals that previously exhibited normal behavior in social settings develop atypical patterns of activity, including self-wounding, when kept in individual cages for an extended period (Bryant and others 1988), although this period has not been defined. Similarly, some physiological measures appear to be altered in individually housed primates (Coelho and others 1991; Gonzalez and others 1982; Mendoza and Mason 1994; Mendoza and others 1991; Saltzman and others 1991; Shively and others 1989). Reinhardt and others (1991) found no differences in cortisol levels with single vs. social housing.
The ability to see, hear, and smell other primates and even touch them to a limited degree by reaching through the cage walls does not preclude the development of abnormal patterns. Extensive tactile contact with conspecifics, at times of the animals' choosing, seems to contribute substantially to psychological well-being. Other factors associated with single caging—such as reduced mobility, restricted visual field, inability to get out of sight of a nearby animal, low environmental diversity, and minimal control over or predictability of a given environment—also might influence an animal's psychological well-being. Variation by species, age, and sex and even between individual animals of the same age-sex classification in the same environment has also been reported (Suomi and Novak 1991).
Most primates are social creatures and should not be housed in a room alone except for short periods. Whenever possible, social species should be socially housed. Even in individual cages, however, nonhuman primates interact with one another, so cages should be arranged to ensure that animals within visual range are compatible. Primates that continually threaten each other should be moved out of direct visual contact. Some individual primates also appear to experience stress if they are housed close to the animal-room door or a window that exposes them to human traffic. Such animals can be moved to the back of the room away from doors and windows. Husbandry practices can also be a source of stress and should be conducted in a smooth, predictable manner that minimizes disruption and decreases extraneous noise. Cages should be cleaned in a manner that does not wet the animals (NRC 1996, pp. 42–43).
Where social opportunities are limited, environmental enrichment can take on increased importance. Environmental enrichment programs attempt to increase environmental diversity by providing manipulable objects (Bayne 1989, 1991; Brent and Belik 1997; Cardinal and Kent 1998; Line and Morgan 1991); social stimulation through interaction with known humans (Bayne and others 1993a; Wolfle 1985, 1987) through the use of mirrors (Collinge 1989; Eglash and Snowdon 1983; O'Neill-Wagner and others 1997; Platt and Thompson 1985) or