Actually animals are very sensitive creatures and if you mistreat them they can take years to heal from the damage. I was a zoo keeper and I had to work with animals that had all range of mental health conditions right through to self mutilation from captivity related issues.
I'm not saying they don't experience long lasting trauma, and I realise they are very sensitive. I just think they don't seem to hold a grudge about it, or want revenge or any of that sort of thing that people do.
Did the animals you worked with seem stuck in the past, or unable to move forward emotionally? I guess it would be very hard to tell that, given that we don't actually know how animals experience emotions, or if they feel things the way we do?