In the 1950s, scientists discovered that rhesus macaques in Japan washed their food before they ate it. Recently, other studies have tried to determine if the monkeys really care if their food is dirty. Experimental methods included rolling sweet potato chunks in sand, and placing wheat grains or peanuts on real monkey poo, or fake poo. I won't give away the ending, you should read about it yourself!
The uncanny valley is that strange feeling you experience when you see something that is almost, but not quite, human-like, such as animation in a film or video game, or robotic movements that mimic our own. You may even experience this feeling when you see other primates. Some folks have suggested that "the valley" may be what caused us to kill off Neanderthals (although I'm still looking for a more reputable source on this topic)!
Long story short, turns out that a recent study suggests that monkeys are also disturbed by almost life-like images of other monkeys. Marc Bekoff gives some nice explanations for why animals might be hardwired to look for stimuli that match expectations of what other members of their species should look or act like (or as another scientist points out, these likenesses may not be visual at all, but could be sound or olfactory-based). Read it all here!