Over the course of a decade, researcher Mathias Osvath said he watched Santino, a male chimpanzee, calmly collect stones and fashion concrete discs before visitors arrived each day at the Furuvik animal park in Skutskär outside of Gävle on Sweden’s east coast.
Later, the chimp would hurl the rocks across a moat toward onlookers.
According to correspondence published in Monday's edition of Current Biology, zoo minders uncovered hundreds of stone caches in Santino's enclosure.
They decided to stake out the enclosure to get a better idea of the chimp's behavior.
"On five consecutive days, before the zoo opened, the chimpanzee gathered stones from the water and placed them in caches," wrote Osvath, a Lund University researcher.
"Later on each of these days, the stones were used as ammunition."
Apparently dissatisfied with the ammunition on hand, Santino then began to craft his own.
"The chimpanzee was observed to gently knock on the concrete rocks, from time to time delivering harder blows to break off the detached surface section in discoidal pieces.
"These manufactured missiles were often transported to the caches at the shoreline."
Osvath said that Santino was "without exception" calm when gathering or manufacturing his ammunition, "in contrast to the typically aroused state" during displays.
"These observations convincingly show that our fellow apes do consider the future in a very complex way," he 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," Osvath added.
"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 behavior."