Social defeat is an important event in the life of many animals, and forms part of the process of social control. Adapting to social defeat is thus an intrinsic part of social "homeostasis", and mal-adaptation may have pathological sequelae. Experimental models of social defeat (e.g. inter-male aggression) have existed for many years. However, very few studies have investigated the changes in brain activity in male animals exposed to the social stress of being defeated by another conspecific male, and in all these studies the expression of the immediate-early gene c-fos has been used as the marker of neuronal activity. In general, the results obtained inform that many areas of the brain, especially those involved in the general stress response, increase their activity when animals are exposed to an acute defeat. However, when animals are defeated repeatedly over many consecutive days, the level of activation of the brain shows different patterns of adaptation depending on the brain areas (varying from complete habituation to persistent activation). Discrepancies between studies may be due to differences in the experimental procedure. On the other hand, further research has to be conducted in order to understand what these changes in the brain activity mean in relation to the other stress responses to social defeat. Furthermore, knowing that the corresponding protein products of many immediate-early genes are transcription factors that can promote or inhibit the expression of target genes, research following this approach is also necessary.

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This study assessed whether the differences observed in a previous experiment [Martinez M, Salvador A, Simon VM (1994): Aggressive Behavior 20:441–451] in the behavior of isolated male mice over several agonistic encounters in which they confronted either an “anosmic” or an “intact” non‐aggressive “standard opponent” were due to the differences in the behavior of the opponents themselves. Ethologically‐inspired analysis was used to assess the behavior of the opponents during the first agonistic encounter. Anosmic opponents spent less time in social investigation and defense and more time in immobility than intact ones. These results suggest that the differences in the behavior of the opponents could be one of the causes of the differences observed in the behavior of the aggressive males confronting them. However, more research is needed to study if other variables, such as the production of odors, are also affected by the state (deprived or intact) of the olfactory sense. These findings provide additional support for the view that the type of opponent used in studies on intermale aggression is of paramount importance.Aggr. Behav. 23:179–181, 1997.© 1997 Wiley‐Liss, Inc.