Zoos have to make accomodations for stick and poo flinging primates. At the St Louis Zoo, where I volunteered for 20 years, they actually had a bamboo screen with peepholes at an overlook where the gorillas regularly pelted visitors. I'm sure the primates weren't thrilled that the visitors had a shield.

Follow the link, as it tells of a remarkable old chimp, the dominant one in the group. The violent aspect of the chimp attacks isn't particularly a revelation. I had ongoing debates with a girl at my former job who kept saying she wanted to buy a chimp and have matching outfits with it. She obviously wasn't the sharpest knife in the drawer. I kept explaining how they'd grow up and really hurt her. This was shortly before the story of the chimp tearing off a woman's face. She conceded finally that maybe I had a point.