The cage design affects intermale aggression in small groups of male laboratory mice: strain specific consequences on social organization, and endocrine activations in two inbred strains (DBA/2J and CBA/J).

Effects of the enrichment of conventional laboratory housing cages with an additional labyrinth on intermale aggression, social organization, and activations of the pituitary-adrenocortical and sympathetic-adrenomedullary neuroendocrine subsystems are compared between adult male DBA/2J and CBA/J mice, differing genetically in their intermale aggression. Mice of both strains were kept three per cage for six weeks either in standard laboratory cages (SC) or in enriched cages (EC). Intermale aggression against a strange intruder and between cagemates was monitored once a week within each group. Dominance relations were derived from the distribution of attacks within a group. Plasma corticosterone titers (PCT) and activities of the tyrosinehydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) were determined for each mouse at the end of the study. The main findings were: 1. Intermale aggression increased in both strains in groups kept in EC. 2. In the more aggressive DBA/2J the pattern of social organization shifted from groups with a single permanent dominant mouse in SC to groups with a frequently changing dominant mouse in EC. 3. In CBA interchanges of the dominant mouse was prevailing and did not differ between the two housing conditions. 4. In DBA/2J mice PCT were significantly elevated in EC. 5. In CBA/J mice activities of TH and PNMT were significantly elevated in EC. 6. Body weight gain was significantly delayed in mice kept in EC in both strains. Findings revealed strainspecific environmental effects on both social organization and endocrine states. The PCT increase obtained only in DBA/2J indicate that alterations of the social relations between cagemates are more likely to induce states of stress than an increase in intermale aggression alone.