Even when aggression is a desired outcome of a study, attention should be given to minimizing injury and distress (Anonymous, 2002; Bayne and Novak, 1998; Ellwood, 1991; Huntingford, 1984). Ways of doing that include minimizing the numbers of animals used; decreasing the length of an encounter to the shortest time necessary to collect the required information, which may involve continuous observation with intervention to stop aggression at predetermined points; using artificial “model” animals rather than real animals as the recipients of aggression or the initiators of predatory encounters; placing introduced animals behind protective screens (for example, Habib et al., 2000) or barriers (Perrigo et al., 1989); and allowing the introduced or subordinate animal to control the intensity of aggression by providing refuge areas. Each of those strategies has limitations, and their usefulness will depend on the species being studied and the purpose of the study. Animals that are severely injured during an encounter should be removed as soon as possible and treated or euthanized. The use of specific animals as targets of prolonged aggression should be well justified.

Environmental Deprivation

Animals may be exposed to nonsocial behavioral stressors to determine their effects on neural and neuroendocrine function. For example, animals may be restrained for brief or for sustained periods by being held, tethered, chaired, or immobilized by other restraint devices or placed in small enclosures or wrappings that restrict movement. Restraint may be repeated at intervals to cause intermittent stress. The animal-welfare issues associated with restraint are discussed in Chapter 3 (“Physical Restraint”).

In other studies, the behavior of animals is restricted by placing them in barren environments that provide few opportunities for normal behaviors or by restricting sensory input. One or more sensory modalities (touch, audition, vision, and olfaction) may be restricted, or animals may even be kept in complete sensory isolation. The goal of such studies is generally to determine the effects of restricted environmental input on neural development. Restricted sensory or behavioral input often leads to the development of severely abnormal behaviors. Whether these effects are reversible depends on the species, the duration of restriction, and the age at which the animals are restricted. Consideration should be given to the impact of this type of research using long-lived animals due to the protracted and resilient behavior changes invoked.

Environmental Stimulation

Stress can be induced by exposing animals to novel or extremely complex environments. The emphasis is usually on neural development, generally with a focus on fear and exploratory behaviors. Fear and exploration may be assessed with a standard battery of tests, some of which are described earlier in this

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