Camera trap footage, taken with no humans present to cause a disturbance, shows one chimp after another pick up a rock and hurl it at the same tree. Rocks pile up at the foot of the tree, which starts to show signs of wear and tear. For some reason, the chimps have picked this particular tree for an accumulation of hurled rocks.

“It was unlike anything I had ever observed among wild chimpanzees,” said primatologist Ammie Kalan. Her team has discovered the behavior in four distinct populations, and it’s possible that more will turn up as they continue searching. What the rock piles mean is an open question, but the discovery of such a distinct and puzzling stone tool use is unquestionably exciting.

We’ve known for a while now that chimps use stone tools for more easily discernible purposes, like extracting food. They’ve also been observed throwing objects, like rocks and branches, haphazardly. What’s different about this behavior is the consistent location and the repetitive actions seen in multiple individuals, which is what has led the researchers to label it as tool use. “It’s definitely the first indication of something else going on other than stone tool use for food extraction or for throwing rocks… haphazardly,” Kalan told Ars.

At a loss for explanation

It’s not easy to work out why this is going on. Most of the individuals participating in this action are males, and they are also engaging in classic “display” behavior: big, attention-seeking movements, hair standing on end, and noisy calls. Display behaviors are “very showy, involve a lot of noise… [can be] used as a sort of aggressive call, as a way of showing off,” said Lewis Dean, a primatologist at the University of St Andrews who wasn’t involved in this research. You commonly see it among “males who are making a lot of noise about territory, females, and so on.”

It’s possible that researchers are seeing an element of display behavior that they haven’t observed before now or that has developed only in particular chimp communities. The tricky thing here is that it’s mostly adult males, but not entirely; an adult female and a juvenile chimp have also been seen doing it. The adult female uses the same pattern of vocalization that the males do, which “suggests the behavior is primarily associated with communicating with other individuals,” Kalan suggested. The juvenile, of course, is probably just mimicking adults.

The other possibility is that the chimps are intentionally creating rock piles, possibly to mark a particular spot for some reason, in much the same way that human societies have created cairns (rock piles) through the ages. “Whether it really represents ritualistic behavior is up for debate,” said Kalan. “Human ritual is already a contentious topic, so it’s hard to know what exactly we’re looking for when it comes to ‘rituals.’ We do think that the repeated, stereotyped behavior… by multiple individuals suggests there may be something more symbolic going on at these accumulative stone throwing sites.”

The study is “remarkable in its scope,” Dean told Ars—the researchers have data from 17 different sites, and they studied another 34 different chimp communities, a breadth that few studies manage. One element of the discovery that’s confusing, he added, is that it contradicts findings from experiments with captive chimps: studies have found that while humans are perfectly happy to copy actions when they can’t see the point of them (like ritual behaviors), chimps just want to get straight to the point. That makes it difficult to see how a ritualized behavior could develop in chimp society. “It’s a very interesting idea,” he said, but “I think ... there’d need to be more investigation before we could really say it was that kind of ritualized behavior." The “display” explanation, he added, is feasible.

Kalan pointed out that these experiments have tried to get chimps to copy human actions, which makes it difficult to judge whether chimps behave the same way when it comes to copying other chimps. The shifted social dynamics that emerge in captive chimps could also make a difference, she added. Despite this idea, she’s also hesitant about saying the behavior is ritualistic. It’s possible, she said, but “to date, we don’t have any evidence regarding the [ritual] hypothesis.”

What else have we missed?

One of the most exciting things about this study, Dean told Ars, is that we’ve missed such a “noisy, boisterous” behavior despite having studied chimps at long-term field sites for decades. “If they’re making a vast amount of noise and bashing around with big rocks and we’ve only just discovered this, there could be a whole load of behaviors that are really quite quiet and subtle that could be going on.”

“We are still going through lots of camera trap footage where we continue to find new and exciting behavioral variants,” said Kalan. Hear team is planning to collect more data and to conduct field experiments to try to figure out what’s going on. They are also running a citizen science project that allows the public to help with watching and coding the reams of footage.

It would be particularly interesting to see how other individuals in the group react to the behavior, said Dean. It’s tricky to investigate because it’s unethical to alter the environment of the chimps. But there’s urgency to the investigation, he added, saying, “Chimps are endangered. If we don’t find out about them soon, they will be gone.”

Part of this research was described in Nature Scientific Reports, 2015. DOI: 10.1038/srep22219 (About DOIs).