I’m not quite following the previous conversation here, but your comment brings to mind that one theory of a possible “function” of depression, is as a “sickness behavior” to help isolate a sick animal from others to protect the group. A sheep or cow getting sick and going off on its own is a common thing.

I’m not sure if it has a technical name or if it’s been rigorously studied, but it’s a common observation which even I’ve seen (and reported to growers I work for).

A casual mention here: http://www.sheep101.info/201/behavior.html