I understand, but in this particular case it's a stylised experiment... I am just trying to understand the possible evolutionary rationale. I suppose what I am trying to say is that I am having trouble with a concept of "chronic stress" for an animal. Isn't it possible that even short-term stress for, say, a rodent is most often associated with a bad outcome (e.g. getting eaten; a rodent's survival probability conditional on short-term stress is very low, I imagine)? Which might suggest that "chronic stress" is just stress, period. So the response which is being observed is actually just a generic 'panic"-style reaction? Unfortunately, I can't access the full text of the article, so not sure whether they may have addressed this.