When Santino the chimpanzee started pelting zoo visitors with stones, his keepers were mystified.

Not that they were surprised by his displays of aggression — the 31-year-old chimp is, after all, a dominant male. But there was no obvious source of stones in his enclosure; so where was he finding all the missiles?

All became clear when they carried out a search and found his stockpiles of rocks. Santino had been fishing stones from the moat surrounding his enclosure - and, even more impressively, he had been shaping odd pieces of concrete into aerodynamic disc-shaped missiles. Then he had been stashing them away for future use.

Fortunately for visitors to Furuvik Zoo, in Sweden, Santino wasn't very good at throwing, preferring the underarm method, so no one was badly hurt.

But his behaviour has led scientists to conclude that premeditation is not a uniquely human trait.

It seems chimpanzees, too, are capable of forward planning, and have a sophisticated understanding of past and future. The chimp is also furtive about his behaviour, almost as if he knows he is being sneaky.

“I've never been able to get a good picture of him collecting the stones,” said Mathias Osvath, of Lund University, who has made a study of Santino. “As soon as he sees you he stops.”

Dr Osvath said the chimp's behaviour was the first unambiguous evidence of spontaneous forward planning in a non-human animal.

While chimpanzees had been shown to plan ahead in an experimental setting before, previous observations in either wild or captive circumstances had never been able to exclude other explanations for apparent premeditation, Dr Osvath said.

“Previous cases that could be explained in terms of planning have been anecdotal, but this rests on a decade of firm observation,” Dr Osvath said.

“Santino is noticeably calm when he collects his stones or makes his discs, and it is always at least several hours before he uses them in his dominance displays, when he is very agitated.”

He said the findings, published in the journal Current Biology, suggested that chimpanzees had a highly developed consciousness, through which they could conceive of the past and the future, and plan ahead to prepare for actions they intended to undertake later.

“These observations convincingly show that our fellow apes do consider the future in a very complex way,” Dr Osvath said.

“It implies that they have a highly developed consciousness, including life-like mental simulations of potential events.

“They most probably have an 'inner world' like we have when reviewing past episodes of our lives or thinking of days to come.”

Something similar almost certainly happened in the wild, he added: “I think that wild chimpanzees might be even better at planning as they probably rely on it for their daily survival. The environment in a zoo is far less complex than in a forest.

“Zoo chimps never have to encounter the dangers in the forest or live through periods of scarce food. Planning would prove its value in ‘real life' much more than in a zoo.”

Santino was born in Munich Zoo in Germany in 1978 and was moved to Furuvik Zoo at the age of 5. He became the dominant male in his group at the age of 16.

He is now 31, which is the prime of life for a male chimp. The animals can live to about 50 in the wild and to 60 in captivity.

So there's plenty of time to perfect his overarm technique.

Times Recommends

• 6 reasons not to give gifts this Christmas
• Remembrance Day: All the lonely heroes