Intermale Interactions on NeutralTerritory and Subsequent Dynamics of Blood Corticosterone and TestosteroneLevels in Tame and Aggressive Norway Rats (Rattusnorvegicus)

Previously, stress responses in gray Norway rats (Rattus norvegicus) selected for the absence or enhancement of aggressive and defensive behaviors toward humans (tame and aggressive behaviors, respectively) were studied mainly to nonsocial factors, whereas data on the consequences of social stress induced, specifically, by interactions with conspecifics are scarce. As has already been shown, the above selection of Norway rats causes attenuation or enhancement of intraspecific intermale aggression. To find out whether the differences in aggressiveness are accompanied by hormonal alterations, we addressed the dynamics of corticosterone and testosterone blood levels after intermale aggression testing in tame and aggressive rats, and unselected rats bred in a vivarium for 7–8 generations as a reference. The goal of this work was to investigate the effect of selection toward humans on agonistic interactions under conditions of an unfamiliar cage or neutral territory and on the subsequent dynamics of blood corticosterone and testosterone levels in tame, aggressive, and unselected rats. In our experiments, tame males, as compared to their aggressive or unselected conspecifics, demonstrated a longer attack latency, as well as a shorter duration and smaller number of patterns of aggressive behavior, approximating zero values. When tested on neutral territory, aggressive male rats were inferior to their unselected conspecifics in the total time of confrontations. More pronounced manifestations of aggression in unselected males compared to aggressive or tame animals arose against the background of elevated basal corticosterone levels and enhanced stress responsiveness to interacting with an unfamiliar male. At the same time, reduced aggressiveness of tame rats in the neutral territory test, as compared to unselected or aggressive animals, correlated with the lower testosterone level.