Male house mice attack their familiar cagemates less than novel intrudersan effect often attributed to habituation of aggression toward odors emanating from the cagemate. This interpretation is overly simplistic in that the effects of familiarizing preexposure depend additionally upon two factors. One factor is the aggression-inhibiting odors emanating from the test male that are deposited onto the cagemate by cohabitation. Supporting evidence is that attack inhibition to the cagemate failed to generalize to noncohabiting same-strain intruders and that eliminating physical contact between subject and cagemate during preexposure prevented the usual postexposure decline in aggression. The second factor is nonolfactory social stimuli emanating from the cagemate during aggressive encounters. The same intruder odors that elicited aggression when placed on a socially active mouse elicited only investigation when placed on models. When subjects were preexposed to an intruder's odor while prevented from socially interacting with the intruder, this investigation subsequently declined while aggression paradoxically increased.

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