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1975), mixing two social groups by removing a partition between their cages (Zwirner et al., 1975), and requiring animals to compete for access to food by displacing one another from a tunnel (Miczek, 1974). Isolation of mice is not necessary to study aggression; pair-housing of a male with a female promotes consistent aggressive behavior when the male is tested in a resident-intruder situation (Fish et al., 2002). Animals may also simply be observed in their normal social groups, either in the laboratory or in the wild; this process is facilitated by the use of osmotic minipumps to deliver neuromodulators or hormones and radiotransmitters for remote collection of physiologic data.
Aggression modified by drugs. Using an “intruder” paradigm, it has been shown that drugs, such as alcohol and allopregnanolone (a positive modulator of the GABAA receptor) can increase the expression of aggressive behavior in mice (Fish et al., 2002). In contrast, other drugs, such as 5-HT1B agonists (for example, anpirtoline) will inhibit the expression of aggression (de Almeida and Miczek, 2002).
Predatory aggression. This involves introducing prey species to animals, especially introducing rodents to cats and mice to rats (the muricide model). If the object of the research is to understand or influence the full predatory sequence or if the sequence ensues so rapidly after initial attack that intervention is not possible, death of the prey animal is often the endpoint. Because pain and injury to both the prey animal and predator are significant welfare issues with these kinds of studies, methods to protect the prey animal from physical attack or modeling elements of the predation sequence should be considered (Novak et al., 1998b). It may not even be necessary to use live prey. The number of times an animal serves as prey should be limited. The use of wild caught animals may be preferred due to their potentially greater experience and skill in predatory avoidance (Novak et al., 1998b). In those instances where the prey animal dies, the study should be designed to expedite the predation sequence and to minimize the pain and distress experienced by the prey animal (Huntingford, 1992).
Any situation in which unfamiliar animals are mixed or established social groups are perturbed has the potential to result in aggression, whether or not aggression is central to the aims of a study. The effects of the aggression on the recipient animal will depend on the intensity, duration, and potential for injury associated with the aggression and hence on the species being studied, the ages and sexes of the animals, and their past social experiences. If aggression is incidental to the goal of the study, many methods can be used to reduce the potential for injury, including gradual introduction of animals, allowing partial contact (for example, visual, auditory, olfactory, or tactile) before mixing and providing refuge areas to which introduced animals can escape from aggressors (Bayne and Novak, 1998). Naturalistic approaches to inducing aggression or predation may not only minimize injury but also provide information that is more reflective of the range and types of behaviors shown by animals under more ecologically relevant circumstances (Kavaliers and Choleris, 2001; Mench and Shea-Moore, 1995).