The drone had already flown by in a practice run, at which time it caught the attention of several chimps in the enclosure. During that flyby, several chimps were seen grabbing twigs and climbing up towards where the drone was hovering.
And when it came back, they were ready: Two females sat where the drone was going to hover, holding long twigs. According to primatologists, their faces show exertion, but not fear -- indicating that they were acting deliberately, and not flailing around in terror at the foreign object.
"The use of the stick as a weapon in this context was a unique action," study author Jan van Hooff, a primatologist with life-long ties to the zoo, said in a statement. "It seemed deliberate, given the decision to collect it and carry it to a place where the drone might be attacked."
That tool use is even more impressive when you remember that no humans have taught the captive chimps how to use tools. The chimps have figured out how to use at least 13 unique tools by simply watching humans go about their business at the zoo -- and they're known to select the best tool for a particular job with ease.
The incident adds to growing evidence, the authors said, that chimps plan ahead when it comes to using tools.