rors and video monitors. The list is limited only by the imagination of each person who has some responsibility for the welfare of animals. The type of enrichment selected is often based on whether an animal is singly housed or pair or group-housed. Toys have been used most commonly for singly housed animals, but might show promise for group-housed animals (Brent and Belik 1997; Novak and others 1993). Other variables are involved in the selection and evaluation of these devices, including the species for which they are used. Additional discussion of this topic is provided in Chapters 59.

Technologically more complex enrichment devices, such as joystick-operated computer games that challenge cognitive and motor capabilities, have proved of interest to chimpanzees and several species of monkeys for long periods. Unfortunately, they are relatively expensive and require considerable maintenance. Some single-housed primates will watch television that depicts activities of their own species (Brent and others 1989; Rumbaugh and others 1989) and a recent report suggests that rhesus monkeys watch television if the scenes change rapidly (Platt and Novak 1997).

In addition to the properties of an enriching device itself, other factors affect whether and to what extent an animal might find it "enriching." These factors include the mode of presentation, such as the spacing of puzzle feeders to avoid competition (Maki and others 1989); order of presentation and time of exposure to minimize declining interest (Cardinal and Kent 1998; Crockett and others 1989; Leming and Henderson 1996; Paquette and Prescott 1988); visibility; and ease of access. Other authors have discussed these issues as well (Bayne 1991; Fritz and Howell 1993a; Bloomsmith and others 1991).

Some foods provide both nutrition and opportunity for manipulation, such as scattering corn, popcorn, or other items in the bedding (Beirise and Reinhardt 1992; Grief and others 1992). Food treats can also be given in the form of unprocessed fruits and vegetables. Unpeeled bananas, artichokes, potatoes, and coconuts increase animals' processing time of the food and can provide entertaining moments for the animals and care staff (Bloomsmith 1989; Nadler and others 1992). Fruit juices, as liquid or frozen into cubes, are also enjoyed by many animals (Goodwin 1997). Even increasing the frequency of feeding seems beneficial (Nadler and others 1989; Taylor and others 1997b). There are many opportunities for creativity in the use of food treats and presentation of food as part of well-being programs. The diet should be nutritionally balanced and raw food treated to decrease the likelihood of infection (NRC 1996, p. 40).

Housing Design

Housing should permit the expression of species-typical postures and locomotion. Species, age, sex, and individual histories are important factors to consider when evaluating housing designs. Performance standards based on postural adjustments and locomotor activities preclude specification of dimensions

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