includes providing opportunities for them to control and predict environmental changes. Providing suitable opportunities for self-initiated activities (e.g., wood chips to permit foraging, and social companions with which to groom or play) allows animals some degree of control. When a routine is under human control (e.g., cleaning, feeding, and experimental manipulations), clear signals of the nature of the activity allow animals to anticipate and adapt to events. When a routine is essentially innocuous to the animal, clear signals that identify an environmental change as part of a familiar routine can be beneficial. On the other hand, pleasant surprises, such as treats and favored activities, need not be so routinized. Similarly, when procedures involve some level of discomfort to an animal, long anticipatory periods signaling the impending event can be a source of distress.
Individual animals can control some features of their environment through self-initiated activity if opportunities are provided. Even though an activity might be repetitive and the material familiar, animals can control some of the changes in their environment. This control, or "work," is a common feature of the activity of animals in natural settings and can take many forms (Reinhardt 1993, 1994a).
A degree of environmental control and challenge and the opportunities to engage in species-typical activities can be provided through enrichment techniques that provide opportunities for voluntary interaction. Such opportunities should be oriented to the animals' physical and cognitive capabilities, rather than aesthetic appearance. Enrichment devices should be carefully selected on the basis of the behavior that needs opportunities for expression. An enrichment program should be customized to the animals and the institution; what is successful in one facility might not work in another (Bayne and others 1993b). Enrichment devices can be used for both individually and socially housed primates; in the latter case, the devices should stimulate social interest and play, not competition that leads to fighting.
Environmental enrichment is a broad classification that encompasses various methods. For example, such cage furnishings as perches, shelves, and tunnels have been used to increase the comfort of animals by allowing them to get off the cage floor, assume a variety of physical postures, engage in various physical activities, and escape the attention of others when socially housed (Neveu and Deputte 1996). Methods to increase species-typical behaviors, such as foraging activities, include hiding food in wood shavings or wood wool spread on the enclosure floor (Chamove and others 1982), using foraging puzzles that require an animal to discover the presence of food in a container and implement a strategy to obtain it (Hayes 1990; Murchison 1995), spreading particulate food on foraging boards to increase the time spent in collecting it (Bayne and others 1992b), and placing food in different locations within the primary enclosure and so requiring an animal to move around its home to obtain food. The use of such devices can vary widely, and provision should be made to ensure that every