A groundbreaking scientific discovery was made in 1997 when a zoo chimpanzee began throwing stones at visitors

You see, Santino had developed a taste for throwing stones at visitors. Primates throwing things isn’t unusual, but Santino wasn’t just picking anything up during these episodes. Oh no, he had planned it.

Zoo animals get used to seeing humans walking around their enclosures. It’s the same reason why so many zoos have tried to make up for the sudden drop in attendance during COVID-19 lockdowns with chimpanzee-to-chimpanzee Zoom calls. That said, not all zoo animals are so fond of the stimulation of an audience, a category we can agree Santino fell into.

Every morning, when the zoo was closed and crowds were excluded, he walked the moat of his enclosure and collected stones. These rocks were left in small piles scattered around the enclosure, hidden in secret places.

That’s because Santino wasn’t putting them together for immediate use. No, he had something else planned.

“Throwing stones at a crowd has an instant and dramatic effect”

When visitors arrived, Santino was often agitated. The change in state would see his piles of rocks take on a new function, as he would begin tossing them into the air – sometimes with such frequency that Science reports that keepers referred to their fall as a “hailstorm”.

“Throwing stones at a crowd of people has an instantaneous and dramatic effect and was a way of eliciting reactions across the moat of water that surrounded the chimpanzee,” cognitive zoologist Mathias Osvath wrote in correspondence published in 2008 in Current Biology. “During the first three years that this male chimpanzee was in the dominant position, stone throwing was infrequent. This was probably because the outer island enclosure rarely contained stones that were immediately accessible during an exhibition.”

“But in early June 1997, rock throwing increased significantly, with several throwing per exposure. This prompted zoo staff to take precautionary measures.”

By 1998, Santino was starting to run out of stones, but then he developed another skill. He learned to mine the concrete in the center of the island he lived on by hitting it with a stone until he heard a hollow sound. This indicates the presence of a crack and would motivate Santino to start hitting harder until a splinter breaks off that he could add to his arsenal.

“The behavior described above is clearly identifiable as planning for a future state”

What makes this behavior so remarkable is the preparation that went into it. Santino was particularly careful with his missiles, stockpiling stones and fracturing fragile pieces of cement. What was also remarkable was that the preparation took place calmly, while the execution took place in a state of great agitation – a kind of pre-planning that was not really considered a characteristic of non-human animals in 1997.

“For a behavior to signal planning for a future state, the predominant mental state during planning must deviate from that experienced in the planned situation,” Osvath added. “The behavior described above is clearly identifiable as planning for a future state.”

A follow-up study conducted by Osvath also revealed Santino’s skills in deception. His stone throwing had become (in)famous, so to prevent visitors from having time to escape, he began hiding his stone caches under natural obstacles in his enclosure, or by collecting hay to cover them. A cunning one, this Santino.

In the decades since his antics, we’ve learned a lot about chimpanzee tool use, cultural diffusion, and even found evidence of human-like warfare tactics. Suffice it to say, they may be our close relatives, but you don’t want to mess with chimpanzees.