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READY, AIM, FIRE: Santino carries a stone in his left hand. His forceful upright walking and piloerection (hair on end) indicate his agitation.
MATHIAS OSVATH, CURRENT BIOLOGY, MARCH 9, 2009

Think people are the only ones who can plan for the future? You may change your mind when you hear the story of Santino the chimpanzee, whose premeditated attacks on zoo visitors are described today in Current Biology.

When Santino was first transferred to Sweden's Furuvik Zoo in 1983 at the age of five, he was relatively calm and passive, lead study author Mathias Osvath, a postdoctoral student in cognitive sciences at Lund University in Sweden, tells ScientificAmerican.com. But by the time the primate reached sexual maturity at age 17, he had become so aggressive that he killed the only other male chimp at the zoo. (Oddly, Santino had saved the life of his comrade just five years earlier by untangling a play rope that had wrapped around his neck.)

It was shortly after this fatal attack that zookeepers began to notice that Santino had developed a habit of throwing stones at zoo visitors, who were safely situated behind a five-foot- (1.5-meter-) high fence. The fence was located some 30 feet (nine meters) from the enclosure's chimp island, which is surrounded by a water moat that prevents the chimps (which over the years have ranged between four and seven individuals of mixed ages and genders) from approaching them.

Every day around 11:00 A.M., Osvath says, Santino would put on a show of dominance (typical of male chimps), which entailed yelling and running around dragging branches. His macho display elicited laughter from zoo-goers, except, that is, when they became targets of Santino's rock missiles. Every so often, the chimp would pick up a stone and chuck it toward the crowd, causing it to disperse. Initially, Santino would only lob one rock at a time. But zoo workers in 1997 reported that the chimp had begun hurling "hailstorms" of stones.

Workers wondered where Santino was getting his ammunition (there weren't many stones just laying around the island) and decided to investigate. One morning, they discovered several piles of three to five stones carefully placed along the waterfront. They suspected that Santino must have been stockpiling them in anticipation of throwing them – and that they came from the water in the moat, because they were covered with algae. To confirm their suspicions, a zoo worker began watching the chimps from a building overlooking the island every morning, and discovered that  Santino –the only one of the bunch that threw stones -- would routinely reach into the moat and scoop out rocks, placing them in piles along the waterfront. He would always scavenge before the zoo opened at 10:00 A.M., so that his arsenal was ready for his 11 A.M. show.

Over the years, Santino's  operation has become increasingly sophisticated, Osvath says, progressing from simple gathering to fabrication. He has been observed chipping away at the concrete rocks on the island with his hands to sculpt dessert plate–size discs to launch at zoo visitors. In the past decade, zoo workers have witnessed him throwing stones on about 50 separate occasions. Santino has managed to hit a handful of gawkers during his assaults but, happily, none have been injured.