The premeditated attacks on zoo visitors made by Santino the chimpanzee were described Monday in Current Biology. The 30-year old chimpanzee, which has lived in a Swedish zoo most of his life, often gets agitated when zoo visitors begin to gather on the other side of the moat that surrounds his enclosure, where he is the dominant and the only male in a group that includes a half-dozen females.

Therefore for years was in the habit of spending two hours before the zoo opened collecting rocks and other "ammunition" to hurl at the human gawkers who started gathering outside his enclosure around mid-morning. That’s how he often suceeded in making his displeasure by flinging stones or bits of concrete at the human intruders, but finding a suitable weapon on the spur of the moment perhaps isn't so easy.

"These observations convincingly show that our fellow apes do consider the future in a very complex way," said the author of the report, Lund University Ph.D. student Mathias Osvath. "It implies that they have a highly developed consciousness, including lifelike mental simulations of potential events."

The explanation is made all the more compelling by the fact that the chimp was in an entirely different state of mind during the planning phase of his endeavor (calm) than during the implementation phase (agitated), said the study author.

Over the years, Santino’s operation has become increasingly sophisticated, Osvath says, progressing from simple gathering to
fabrication. He has been observed chipping away at the concrete rocks on the island with his hands to sculpt dessert plate–size discs to launch at zoo visitors. In the past decade, zoo workers have witnessed him throwing stones on about 50 separate occasions.