Figure 1. The experimental design.

The experiment included three experimental groups, Naïve, Subordinate+Saline (Sub+Sal), and Subordinate+Clomipramine (Sub+Clo), and four phases. Phase 1 consisted of a 7 day stress-free period during which the animals adapted to the experimental environment. Phase 2 was the social defeat (SD) phase during which animal pairs in the Sub+Sal and Sub+Clo groups housed separately in connected cages were allowed direct access to fight for social dominance. In contrast, Naïve group tree shrews remained undisturbed. Phase 3 was the drug administration plus SD phase lasting 28 days during which the subordinate (Sub) tree shrews were exposed to daily social defeat stress as before but also treated daily with oral clomipramine (50 mg/kg/day) or saline (1 ml/kg/day). The final recovery phase consisted of a 7 day period with neither social defeat nor drug treatment.