When visitors to the Furuvik Zoo in Sweden approach the pen of Santino, a cranky male chimpanzee, they often find themselves on the receiving end of flying rocks, which the 34-year-old ape keeps cleverly hidden in his enclosure. Now, Santino's increasingly tricky antics have drawn the attention of the scientific community. In a new study published in the journal PLoS One, researchers suggest that the chimp's deceptions hint at a deep level of thinking once only associated with humans. Here's what you should know about the supersmart ape:

How did the rock-flinging start?
One day in 2010, an apparently annoyed Santino was fed up with being ogled, so he scared visitors away by aggressively flinging stones. (No one was hit.) Later that same day, when a different group of people approached his pen, the chimp came up with a new idea. "Santino approached them holding two stones, but this time appearing nonaggressive and munching on an apple," says Eoin O'Carroll at The Christian Science Monitor. When he got close enough, the sly primate let the rocks fly, taking the humans by surprise.

Did his aggressive behavior stop there?
Not quite. After the coast was clear, "Santino went inside the enclosure and brought a good-sized heap of hay that he placed near the visitors section," and then he put stones under it, says study author Mathias Osvath. The mischievous ape plopped down by the hay, biding his time. "When visitors came back, he waited until they were close by, and without any preceding display, he threw stones at the crowd." Zookeepers, fed up with his behavior, removed the stones from his pen, but that didn't deter the chimp: Santino was caught breaking off pieces of the concrete floor, which he fashioned into disks he could throw.

Don't plenty of apes throw things at people?
It's true; dominant males have been known to fling missiles like sticks and feces at onlookers, but Santino is different for several reasons, says Osvath. For starters, "Santino doesn't have access to any good-sized sticks, and he really dislikes putting his fingers on gooey stuff, including feces."

So what does that mean?
The deliberate and complex nature of the attacks demonstrates a capacity for thinking nearly on par with humans. "What is interesting is that he made these preparations when the visitors were out of sight, and also incorporated innovations into the behavior," Osvath tells Discovery News. Santino's planning closely resembles something psychologists call "theory of mind," or the ability to understand that others possess thoughts, desires, and motivations different from one's own — a first for the ape world.

Why does Santino do it?
Michael Huffman of Kyoto University's Primate Research Institute, who has studied Santino, think the primate's stone-throwing "may serve to augment the effect of intimidation displays," which is a common practice for alpha males. Other scientists think the answer could be more primitive: Santino simply "finds it fun" to bug humans. 

Sources: The Christian Science Monitor, Digital JournalDiscovery News, Wired