Just because we can't tell doesn't mean it is or isn't. We can't tell if a dog has a migraine, or if they are so allergic to something inside or outside the house that their sinuses constantly bother them. I have always wondered how many pets go through things we humans would find intolerable.
Oh I think animals are definitely tougher than humans, but I do think animals can feel depression. To what degree, I don’t know.
Did you see this recent story about this goat that was depressed when he was separated from his love?:
http://www.freerepublic.com/focus/chat/3161195/posts