For the past 12 years, visitors to Sweden's Furuvik Zoo have boldly approached the chimpanzee enclosure only to beat a hurried retreat after being rudely pelted with stones by one of the apes.
Now scientists have claimed that the culprit, Santino, who collects arsenals of weapons ready to throw at visitors, is living proof that animals are able to plan for the future, a skill previously thought to be restricted to humans.
Since the first recorded stoning in 1997, staff at the zoo, north of Stockholm, have had to step in to protect members of the public, who regularly found themselves caught in a hail of stones and bits of concrete unleashed by the irate ape.
The attacks were initially thought to be spontaneous, but scientists became fascinated in Santino's behaviour after he was observed fishing stones out of the water-filled moat that encircled the Chimpanzee Island attraction, before stockpiling them in carefully selected ammunition dumps with unimpeded views of the crowded viewing areas.
To top up his weapons cache, the ape would also fashion his own missiles by breaking off pieces of concrete from larger blocks in the centre of the enclosure. The ammunition gathering would take place in the early hours of the morning before the zoo opened, so the stones were ready to be rained down on the first unsuspecting tour.
Some visitors have been bombarded by 10 or more missiles hurled over the moat. One keeper described the most intense attacks as "hail storms".
After Santino refused to respond to verbal commands to cease his barrages, staff were eventually forced to remove his ammunition and keep him indoors.