Santino, a male chimpanzee at the Furuvik Zoo in Gävle, Sweden, started throwing rocks at the zoo visitors after he became the only male in the group, which was completed by four females. He started gathering rocks and concrete when he was calm in the morning and while the zoo was closed. Apparently he was preparing for the time of the day when he became agitated by visitors. Primatologist Mathias Osvath of Lund University in Sweden thinks that his behavior is clearly identifiable as planning for a future mental state.
To read more about Santino and the implications of his actions, check out the Science Now article.