It looks like monkeys can suffer from depression, too.
The researchers noted that some of the monkeys were lethargic, had higher heart rates and underwent hormone disruptions -- all considered definite or possible signs of depression in humans. The depressed monkeys were also more likely to occupy a subordinate role within the monkey community hierarchy, meaning they were often isolated and subject to aggression.
I bet they get really depressed when their house gets wrecked.
I suggest they spend more time looking at monkey pornography. Works for me. ;)