At last! An excuse for those of us who take our work-related stress out on our family. Dr. Øyvind Øverli of the University of Oslo has discovered that dominant rainbow trout reduce their stress levels by venting their frustration on socially subordinate animals. Presenting his results at the annual SEB meeting in Edinburgh (29th March – 2nd April 2004, session A15.8), Dr. Øverli suggests that this method of stress reduction may be an evolutionarily conserved strategy, which may increase our understanding of violent aggressive behaviour in humans.

“Rainbow trout are highly territorial animals” says Dr. Øverli “When a dominant fish harasses a subordinate, stress levels drop off quickly in the dominant animal but remain high in the subordinate. One possible explanation for this is that beating up a subordinate relieves stress.” To study the behavioural and endocrine effects of social stress, Dr. Øverli paired test fish with larger (dominant) and smaller (subordinate) fish. Test fish are first paired with a smaller, subordinate fish that they can beat up, then paired with a bigger fish that beat up the test fish, and finally put back with the subordinate fish (win – lose – win). A second group of test fish are paired with a subordinate, isolated, and then paired with the subordinate again (win – isolation – win), while a third group are subjected to a win-lose-isolation paradigm. “Test fish that suffer social defeat are far more aggressive when they are re-introduced to their subordinate partner” explains Dr. Øverli “In contrast, fish that are isolated between encounters with a subordinate show decreased levels of aggression when re-introduced to their subordinate partner.”

Interestingly, elevated forebrain serotonin and plasma cortisol levels in the test fish that are not re-introduced to their small partner (win-lose-isolation compared to win-lose-win) suggest that the presence of a socially subordinate fish inhibits the stress response of the test fish. These results suggest that central signalling systems controlling behavioural and endocrine stress responses may be highly evolutionarily conserved. Like humans and other primates, these territorial fish use displaced aggression as a stress coping mechanism.