The loutish behaviour of a stone-throwing chimpanzee at a zoo in Sweden has challenged scientists' beliefs about human beings.

Santino, a 31-year-old male at Furuvik zoo, may be the first animal to exhibit an unambiguous ability to plan for the future, a behaviour many scientists argue is unique to humans. Forward planning takes considerable cognitive skills, because it requires an animal to envisage future events it will have to deal with.

Santino would get agitated when the first groups of visitors arrived at his enclosure in the morning, and would start hurling stones at the spectators. When the zookeepers investigated, they found that, while the zoo was closed, Santino had been busy making piles of ammunition, and returned to them to resupply.

To catch the chimp in action, one zookeeper hid in a room overlooking the enclosure and observed the ape's behaviour before the zoo gates opened each morning. She saw Santino dragging stones from a protective moat that surrounded his island home, before placing them in piles. Further covert surveillance of the ape revealed he spent some time tapping areas of concrete floor with his fist. Occasionally, the animal would thump harder, releasing chunks of concrete that he broke into rough discs.

A survey of the enclosure showed that Santino made piles of ammunition only on the quarter of the island's shore that faced the visiting crowds.

Since becoming aware of the issue, zookeepers have removed hundreds of caches of stones from the island and have observed Santino gathering stones and putting them in piles at least 50 times. Santino's attempts to fashion concrete discs has been recorded 18 times, according to a report in Current Biology.

Staff at the zoo coped with Santino's antics by warning visitors when he was getting agitated, and erected a fence to try to contain the projectiles. Cognitive scientist Mathias Osvath, the author of the study, believes that such complex forward planning suggests Santino can anticipate future events and is able to devise ways of dealing with them. In this situation, he is trying to get the crowds to move on.

"Forward planning like this is supposed to be uniquely human; it implies a consciousness that is very special, that you can close your eyes you can see this inner world," he said. "Many apes throw objects, but the novelty with Santino is that he makes caches of these missiles while he is fully calm and only throws them much later on.

"We are not alone in the world within. There are other creatures who have this special consciousness that is said to be uniquely human."

Osvath interviewed zookeepers at Furuvik and examined records of the chimp's behaviour. He found that Santino only gathered rocks and made concrete missiles when the zoo was closed. He gave up the behaviour completely when the zoo was shut over the winter.

The zookeepers recently decided that an operation was the best way of controlling Santino's behaviour.

"They have castrated the poor guy. They hope that his hormone levels will decrease and that will make him less prone to throw stones. He's already getting fatter and he likes to play much more now than before. Being agitated isn't good for him," said Osvath.

• This article was amended on Wednesday 15 April 2009. We originally said that Furuvik was near the Arctic Circle. This has been corrected.