Effects of environmental enrichment on aggressive behavior, dominance hierarchies, and endocrine states in male DBA/2J mice.

Adult male mice (DBA/2J) siblings were housed three per cage for 6 wk, either in standard cages (SC) or in enriched cages (EC). Both attacks among group members and attacks against strange intruders were monitored once a week within each of 22 experimental groups. According to its attacking behavior, each mouse was categorized into one of three dominance categories: dominant, subdominant active, subdominant passive. Aggressive behavior and social organization were compared between the two types of housing conditions, and the effects of housing condition and dominance category on endocrinological and some organometrical parameters were analysed. The main findings were: (a) Mice in EC attacked intruders significantly more frequently compared to mice in SC; (b) In EC groups the position of the dominant male was less stable than in SC groups; (c) Plasma corticosterone titers (PCT) were significantly elevated in EC. Activities of tyrosinehydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT) did not differ significantly between the housing conditions; (d) TH and PNMT activity were significantly enhanced in the dominant males of SC groups compared to subdominant passive males. Intermediate activities for both enzymes were determined for subdominant active males; and (e) PCT were significantly elevated in dominant males of the EC groups compared to subdominant active and subdominant passive males and also compared to the dominant males in the SC groups. Findings suggest that keeping adult male mice in structured cages can result in increased aggression towards intruders, a change in the social organization, and altered endocrine states, depending on the individual dominance position.