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avoidance can increase before the first physical attack. Changes in established feeding orders and social relationships can also serve as warning signs. Excessive time hiding in physical structures—such as visual blinds, tunnels, barrels, and boxes—can also indicate that active intervention is required.
Social instability might be an indication that the environment lacks stimulation needed for species-specific behaviors and that enrichment is needed. Enriching the environment with perches (Bayne and others 1989, 1992a; Crockett and Bowden 1994; Shimoji and others 1993), visual breaks and hiding areas (if none exist), and foraging tasks (Bayne and others 1991; Boccia 1989; Chamove and Anderson 1979) might be successful in reducing hostility.
If combative behaviors continue after environmental complexity has been assessed and changed, various other techniques for restoring social stability should be considered. In some species, stability can be restored by removing the victim (Vermeer 1997); in other species, a new victim replaces the old. Likewise, removing an aggressor might restore harmony in one case and increase social instability in another. A victim that is removed and treated can be safely returned to some groups but not all. In some species, the longer the victim has been away, the riskier is the return. Clearly, no simple formula describes the most effective procedure for all species.
When animals repeatedly initiate biting attacks or when biting presents a potential for serious injury to personnel or other animals, dental modification should be considered. In many species, severe puncture and slashing injuries can be caused by elongated canine teeth. Extraction of canines is not advisable; these teeth are deeply rooted, and extraction places the animal at risk of structural damage to the maxillary sinuses, dental malocclusions, and periodontal disease. A better procedure is to blunt the canines (Carter and Houghton 1987; Coman and others, in press). In considering this procedure, it is well to remember that in Old World monkeys, the trailing edges of the upper canines are honed on the lower first bicuspid or premolar. Only when the canines are reduced so that they no longer project beyond the occlusal surface do they lose their potential to inflict slash and puncture injuries. The procedure is not without risk and might expose the tooth pulp chamber and result in an abscess. When required, a pulpectomy should be performed by a qualified professional and the tooth filled with dental amalgam or acrylic (Carter and Houghton 1987). That procedure, of course, does not preclude inflicting serious crushing injuries.
Aggression can be minimized by keeping social groups intact, but this is not always possible. If animals used in research protocols, or sick or injured animals, require removal for treatment, efforts should be made eventually to reintroduce them to their social group. Reintroductions often become riskier with the passage of time, although they are generally easier when an entire group is separated into single cages than when only one or two animals have been removed. All reintroductions, however, should be monitored continuously for the first hour and periodically thereafter. Introductions or reintroductions generally involve housing