In terms of why the chimp wants to bother human zoo visitors, Osvath said that's nothing new.
"A lot of great apes, especially dominant males, throw stuff at people at zoos," he said. "And I would think that this is something that comes naturally to them when performing their dominance displays. These are often aimed at making other apes move out of the way and, in effect, accept him as the boss."
"Humans at zoos don't move out of the way, unless they get thrown at," he continued. "Some apes throw sticks or feces, but Santino doesn't have access to any good sized sticks, and he really dislikes putting his fingers on gooey stuff, including feces."
After observing the chimp for days, the scientists also suspect that Santino just also "finds it fun" to bug humans. He even appears to target certain people that perhaps really get on his nerves. The attacks are all the more successful because Santino plays it cool, holding back on posturing before whipping out the stone or other projectile.
Michael Huffman of Kyoto University's Primate Research Institute has also studied chimp stone throwing, which he believes "may serve to augment the effect of intimidation displays." He further thinks that research on the behavior could shed light on the evolution of stone tool use in humans.