Stockholm ? A canny chimpanzee who calmly collected a stash of rocks and then hurled them at zoo visitors in fits of rage has confirmed that apes can plan ahead just like humans, a Swedish study said Monday.

Santino the chimpanzee’s anti-social behavior stunned both visitors and keepers at the Furuvik Zoo but fascinated researchers because it was so carefully prepared.

According to a report in the journal Current Biology, the 31-year-old alpha male started building his weapons cache in the morning before the zoo opened, collecting rocks and knocking out disks from concrete boulders inside his enclosure. He waited until around midday before he unleashed a “hailstorm” of rocks against visitors, the study said.

“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.”

Osvath’s findings were based on his own observations of Santino and interviews with three senior caretakers who had followed the chimpanzee’s behavior for 10 years at the zoo in Furuvik, about 93 miles north of Stockholm.

Seemingly at ease with his position as leader of the group, Santino didn’t attack the other chimpanzees, Osvath told The Associated Press. The attacks were only directed at humans viewing the apes across the moat surrounding the island compound where they were held.