it looks intimidating it happens every couple of years. >> it's rare the gorillas actually break the glass, but occasionally the glass will break op its own, but they have broke didn't before in the past, and it's usually more an engineering thing than it is a gorilla thing. >> reporter: these windows are designed to stay in tact to keep both animals and the public safe. >> it's a triple layer laminate. there's actually three panes of glass on top of one another. if one is broken the other two are in place. >> reporter: the animals continue to pound on the glass to date. calling it normal behavior for three males trying to establish dominance. >> whatever makes them look like the biggest, baddest guy around, that's what they do to actually avoid a physical confrontation. >> that was our affiliate kmtv. thanks for joining us. i'm paula newton. for viewers in the unite, "new