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The male chimpanzee Santino displays a stone in his left hand at the at the Furuvik Zoo in Sweden. His forceful stride and hair on end are signs of agitation. ((Current Biology))

When Santino the chimpanzee began throwing rocks at zoo visitors in the summer of 1997, officials at the Swedish zoo had to wonder: where was he getting all of the ammo?

The answer, they discovered, was in a series of secret caches, where the chimpanzee had calmly collected — and in some cases manufactured — projectiles for later use.

According to Swedish researcher Mathias Osvath, it's "the first unambiguous evidence" of an animal other than humans making plans in one mental state for a future mental state, in this case, an agitated display of dominance from the lone male chimpanzee at the zoo.

"These observations convincingly show that our fellow apes do consider the future in a very complex way," said Osvath of Lund University, in a statement.

"When wild chimps collect stones or go out to war, they probably plan this in advance. I would guess that they plan much of their everyday behaviour," he said.

Writing in Tuesday's issue of the journal Current Biology, Osvath reported on years of observations of Santino at the Furuvik Zoo in Sweden.

Born in 1978, Santino became the dominant male at the zoo in 1994 and the only male a year later when the other male died. For his first three years of dominance the act of throwing stones across the moat separating the chimps from zoo visitors was infrequent.

However, in June 1997, zoo officials noted his stone throwing increased dramatically, with demonstrations involving the throwing of 10 or more projectiles if not curtailed — what one caretaker described as "hail storms."

This prompted an investigation of the chimpanzee island, where they discovered five caches containing three to eight stones each. Algae from the stones revealed the stones originated from the adjacent waterbed.

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A representative group of stones and concrete discs from the chimpanzee's cache, made into a pile by one of zoo's caretakers for illustration purposes. ((Current Biology))

A zoo worker placed herself in a blind to observe the chimpanzee's behaviour and found that, for five consecutive days before the opening of the zoo, Santino gathered stones from the water and placed them in the caches.

The following year, the chimpanzee added pieces of concrete to his ammunition, and was observed gently knocking on concrete rocks to break off smaller, disc-shaped pieces.

Since the initial finding, caretakers at the zoo have removed hundreds of caches, and the gathering of stones has been observed on at least 50 occasions, Osvath reported.

Osvath said while many apes have been observed collecting stones for nut cracking or other planning behaviour, it has been unclear whether the ape was doing the work to meet a current or future need: that is, is the ape looking to crack nuts because he is hungry now, or because he expects to be hungry?

Santino's stone-gathering however, is a clear case of planning for the future, he said, since the calm manner in which the chimpanzee collected the stones differed from the agitated state in which he later hurled them.

"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," he said.