with which the animals can interact encourage investigation and activity and are generally acknowledged to enrich the environment (see Beaver, 1989).
Although attention is being focused on nonhuman primates, methods for environmental enrichment of other species should be evaluated. Concentration on enrichment of the environment by incorporating objects or devices within the cage should cause the extreme importance of social interaction, possibly the most important form of enrichment for most laboratory animals, to be ignored. The proper balance between conspecific and human social interaction, a cage and room environment developed with an understanding of the normal behaviors of the species, and caring personnel trained to handle and care for the species should be the goal.
The objectives of some experiments require the production of stress or even distress (e.g., through food and water deprivation, maternal deprivation, social isolation, etc.), and investigators should be sensitive to the ethical concerns raised by such objectives. Experiments should be justified, use the minimal number of animals consistent with an effective design and statistical analyses, and minimize the duration and magnitude of stress. Restriction of food intake to develop appropriate reward-motivated behaviors in behavioral studies, usually in rats, is common. In those experiments, rats are usually maintained at about 80% of their ad libitum feeding weight, which is considered neither unethical nor excessive deprivation. Although novel foods might be used as environmental enrichment (Chapter 3), the response to novel foods, in either an experimental or a husbandry context, can be stressful. Foods usually should not be changed in the course of an experiment.
Caging conditions (e.g., single housing of rodents) and restraint (e.g., of rodents and nonhuman primates) produce stress, which can be so extreme or prolonged that an animal is unable to adapt and becomes distressed and maladaptive. Those procedures often can be minimized by handling and appropriate adaptation procedures, respectively. Because the novelty of an experience increases an animal's emotional response to it, habituating laboratory animals to experimental procedures by regular handling and adaptation to potentially stress-producing procedures should be incorporated into experimental protocols. For example, stress is associated with the first experience of dogs introduced to the leash, monkeys restrained in a chair, or cage-reared rats removed from their cage. Whether the stress of those experiences manifests itself in maladaptive behaviors (e.g., twirling on the leash, self-mutilation in the chair, or freezing and immobility, respectively) and thus distress will likely depend on factors external to the procedures themselves, such as previous experiences of the animal that led to expectations of pleasure or stress, familiarity with people, development of coping strategies for other events, and even time of day. Likewise, the biologically adaptive