It has been known for many years that animals are actively curious about their surroundings (Berlyne, 1960). Depending on the species, they will explore novel settings; manipulate puzzles, levers, chains, and ropes; look at photographs; and in various other ways voluntarily expose themselves to fresh and unusual objects and stimuli. They will also take advantage of the opportunity to control environmental events by such actions as turning lights and sounds off and on and performing actions that cause food to be delivered, even when adequate amounts of the same food are freely available.
That animals engage in those activities does not necessarily mean that they have a strong need to do so. Many domesticated and wild animals spend long periods during the waking day lying down, sleeping, or sitting quietly. Nevertheless, by analogy with modern society's view of human nature, it might seem that a stimulating and responsive environment is a common requirement for well-being. Whether that is generally the case for captive animals has not been established, and opinions vary (Wemelsfelder, 1990).
The assumption of a true need for varied activities and stimulation is the basis for current efforts at environmental enrichment. Enrichment can take various forms. One of the most common approaches is to provide toys, puzzles, or mechanical devices that animals can use if they choose, without forcing them to do so. Food is also used as a reward or incentive to encourage the use of enrichment devices (Markowitz, 1978, 1982; Line, 1987; Beaver, 1989; Fajzi et al., 1989; Bayne et al., 1991).
The systematic study of environmental enrichment is just beginning, and many questions need to be investigated. Although it seems plausible that boredom is a serious problem for some captive animals and can cause stress or distress, there is little objective supporting evidence. Most likely, reactions to monotony will vary with species or strain of animal, with age, and with previous experience, but these possibilities have not been thoroughly investigated. The design and assessment of enrichment devices are also in their early stages. Designs are based mainly on intuition and guesswork, and there is no way to know in advance that a particular device will be used as expected or whether, if it is used, it will make a positive contribution to well-being. As with other cage amenities, the benefits of enrichment devices need to be established and weighed against the attendant costs and risks.
Several generalizations about the effects of captive environments on stress and distress can be made. It is well established that the species or strain of an animal (its genetic makeup) is critical. Temperament and responsiveness differ widely, even among closely related species. Species that have departed only slightly from the wild state are likely to react more strongly to stressful conditions and to find more conditions stressful than species that have been selectively bred for adaptiveness to captive environments (Price, 1984). A wealth of scientific information indicates that an animal's early experience can have profound and lasting effects on the kinds