In behavior never before seen, chimpanzees in West Africa have been filmed picking up stones and throwing them at trees, all the while making “hooting” vocalizations.

The resulting accumulation of stones either next to or in the hollows of the trees has created a new puzzle for chimp researchers:

Scientists from the Max Planck Institute for Evolutionary Anthropology (MPI-EVA) made the discovery with the help of camera traps, placed after field teams noticed piles of stones next to trees at some of their dozens of research sites.

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The behavior was exhibited primarily by adult male chimps, but the scientists said some females and juveniles were also seen throwing the stones.

Data on these observations have just been published in the journal Scientific Reports.

“This study reports a new chimpanzee behavior not known,” said Christophe Boesch, a study co-author from the MPI-EVA, in a press release. “As the stone accumulation behavior does not seem to be linked to either the abundance of stones or the availability of suitable trees in an area, it is likely that it has some cultural elements.”

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Chimps are known to use tools when foraging for food, such as using sticks to poke around for termites or to extract honey or ants. They’ll also use stone or wooden “hammers” to smash nuts open.

But the researchers say the stone-throwing activity doesn’t seem to have anything to do with foraging.

“This represents the first record of repeated observations of individual chimpanzees exhibiting stone tool use for a purpose other than extractive foraging at what appear to be targeted trees,” the authors of the study wrote.

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Why chimps might throw stones in this manner is still an open question. But the researchers have some ideas.

Study co-author Laura Kehoe wrote in The Conversation that the scientists' current theories revolve around the rock-throwing being part of a male display; a sort of pathway or territory signpost; or perhaps even a marking of "sacred trees."

Of the latter, Kehoe wrote: "Indigenous West African people have stone collections at 'sacred' trees and such man-made stone collections are commonly observed across the world and look eerily similar to what we have discovered here."

Researchers on the study have been working under the aegis of the collaborative “Pan African Program: The Cultured Chimpanzee,” which has, since 2010, gathered data on chimpanzees across 39 research sites in Africa. Its public website Chimp&See; lets visitors pore over video gathered by the program.