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video monitors (Brent and others 1989; Rumbaugh and others 1989); increased visual stimulation (Fritz and others 1997; Reinhardt 1997c) and auditory stimulation (Morgan and others 1998); and additional foraging opportunities (Bayne and others 1992b; Murchison 1994; Reinhardt 1994a). But not all stimuli elicit interest on the part of all species or ages of animals (Line and others 1991), and some stimuli might evoke negative reactions in some species or individuals; e.g., mirrors elicit aggressive behavior in some animals, such as chimpanzees (Lambeth and Bloomsmith 1992). Furthermore, some primates rapidly habituate to many kinds of stimulation. To complicate matters, rapid habituation to manipulable objects has been noted in individually housed rhesus monkeys, whereas socially housed animals continued to manipulate objects for months after initial exposure (Novak and others 1993). Thus, environmental enrichment, although of greater importance to singly caged animals, might be more difficult to achieve in these circumstances.
Aggression directed toward the physical environment or toward the aggressor's own body is greater in small single cages than in enriched cages or social settings (Bryant and others 1988; Chamove and others 1984; Line and others 1990b; Reinhardt 1990b.) Such aggression is apparently rare in free-ranging animals, although the use of branches and other objects in aggressive displays occurs in some species of free-ranging New World and Old World monkeys and in apes. In captivity, these forms of aggression are more common in some species than others. They might be examples of what has been called redirection, which is characterized by the direction of an act toward a different target from the one that elicited it. In a singly caged animal, of course, the eliciting stimulus is generally out of reach.
Chronic self-injurious behavior that causes tissue damage is particularly troubling. Although this behavior has long been labeled "self-directed aggression," the association between aggression and self-directed biting is probably not absolute (Novak and others, in press). We therefore prefer to call it self-injurious behavior or self-directed biting. It has been most frequently reported among adult male macaque monkeys housed individually (Bayne and others 1995; Chamove and others 1984; Gilbert and Wrenshall 1989; Line and others 1990b); if it is a firmly established pattern, it is resistant to treatment. Although the causes of severe self-directed biting are poorly understood (Pond and Rush 1983), prolonged individual housing is probably an influential contributing factor. The handling of self-directed biting is an example of how the overall program or plan for the psychological well-being of an institution's nonhuman primates relates to the procedures adopted for intervention in a specific situation. As in the institution's occupational health and safety, veterinary care, and sanitation programs, there need to be standard operating procedures (SOPs) for each aspect of the program. When single housing is required and an animal exhibits self-directed biting, the SOP should detail the steps to be taken. These steps need to be based on current scientific information but accommodate flexibility in adapta-