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What These Chimps Were Filmed Doing Gave Researcher Goose Bumps

Chimpanzees are known to use stone tools for certain foraging tasks, like cracking open nuts or slicing into fruit. But scientists recently uncovered a group of chimps who have found another purpose for stones altogether.

And the reasoning behind it is far more mysterious.

Researchers from the Planck Institute for Evolutionary Anthropology were working in the West African jungle when they made a curious discovery. At the base of trees in at least four locations, they found piles of rocks which seemed to have been placed there intentionally.

Hoping to determine who was responsible, the team positioned motion-sensing camera traps near the trees and waited. What they found amazed them.

"Nothing like this had been seen before and it gave me goose bumps," wrote researcher Laura Kehoe, on The Conversation.

Kühl et al (2016)

The camera traps captured numerous incidents of chimps picking up stones and hurling them at the trees while making hooting vocalizations - never-before-seen behavior that defies easy explanation.

"Occasionally, chimps throw rocks in displays of strength to establish their position in a community," Kehoe wrote. "But what we discovered during our now-published study wasn't a random, one-off event, it was a repeated activity with no clear link to gaining food or status - it could be a ritual."

Kühl et al (2016)

In its study, published in the journal Scientific Reports, the team notes that both male and female chimps, and even juveniles, were observed partaking in the stone-throwing, adding that it's "likely that it has some cultural elements."

Kehoe theorizes that the behavior might be a sort of showy display of power, or the rock piles could serve as a geographical signpost.

Most interesting, however, is the possibility that the behavior may indicate that the trees themselves are of deeper significance to the chimps - a notion, Kehoe points out, humans are quite familiar with:

"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."

Watch footage of the chimpanzee behavior in its entirety below.