Young orang-utan at Perth Zoo. Credit: Ed Yong
Young orang-utan at Perth Zoo. Credit: Ed Yong

Apes Remember Specific Events From Years Ago

In the climax of the Pixar film Ratatouille, a mouthful of the eponymous dish whisks food critic Anton Ego back to his happier childhood days. The scene is reminiscent of one described by Marcel Proust in Remembrance of Things Past, when the taste of a petite madeleine dipped in lime tea brings back the memory of his aunt Leonie. Many of us can recount similar moments, when a peculiar combination of sensory cues—a smell or taste or sound—transported us back to a point in our past.

Do animals share similar experiences? Do they store deep autobiographies that can be triggered by the right set of cues? According to a new experiment by Gema Martin-Ordas, Dorthe Berntsen and Josep Call, the answer is yes, a least for two species—chimpanzees and orang-utans.

In 2009, the team ushered 12 of the apes into the middle of several connected cages. In full view, they hid two tools in different boxes within the adjacent rooms. Their job was to remember where these tools were—they would need them to reach an inaccessible piece of food in a later test. They had four shots at doing this.

Over the next three years, the apes went about their lives. They ate, slept, socialised, and took part in many more studies. Then, in 2012, eleven of them were led into the same set of rooms with tool-containing boxes in the same locations. And all of them, except for one, went straight to boxes and retrieved the tools. They remembered.

Call was surprised at “how quickly they retrieved the tools as soon as we opened the doors”. They all did this on their first attempt, without prompts or trial-and-error. They didn’t know this test was coming—in 2009, even the researchers hadn’t planned to repeat their experiment three years later. And by contrast, seven individuals that weren’t part of the original experiment didn’t head for the boxes; they just explored the rooms randomly.

Here’s the amazing thing: in the intervening years, the 11 apes had been into these rooms again. They’d worked with other scientists in other studies on tool use or cooperation. Martin-Ordas’s team had even tested them in the same rooms but on different tasks. But they had apparently bound the features of the original experiment—the researchers, the location and the task—into a cohesive memory. The combination of those features, reprised in 2012, took them back to 2009, just like ratatouille or madeleines made Anton Ego and Marcel Proust feel like kids again.

In a second experiment, the team tested 10 chimps and orang-utans (including many of the same animals) on another task. This time, tools had been hidden in trays at different placed within the same rooms. They only saw this set-up once, and two weeks later, they saw it again. As before, all but one of them went straight for the tools in the first half-minute. By contrast, eleven individuals, who had no experience with the experiment, took more than 5 minutes to find the tools.

Gaps of weeks and years might not seem that long, given that we can remember events that happened decades ago. But bear in mind that as recently as the 1970s, we didn’t even know if animals had these sorts of “episodic memories”—recollections  of what, where and when.

Certainly, Canadian psychologist Endel Tulving didn’t think so, and he coined the term “episodic memories” in the first place. He believed  that such memories relied upon our language skills, and were unique to humans. He was wrong; episodic memory is one of a long line of mental abilities that were once thought to be uniquely human but are actually shared by other animals.

Nicky Clayton from the University of Cambridge has done several seminal experiments with western scrub-jays, showing that they can remember the location of food that they had previously stored, and even which hoards were freshest. Other scientists have found similar examples among great apes, hummingbirds, rats, and perhaps even honeybees. But most of these studies used delays of a week at most. By contrast, Martin-Ordas, Berntsen and Call have shown that chimps and orang-utans can retain details of what, where and when for at least three years.

“We do not know about their upper limits,” says Call. “It’s conceivable that they retain memories for much longer, and some studies are planned to assess this.”

“I am not ready to say that apes possess autobiographical memories just like we do,” he says. For example, these memories form an important part of our social lives because we can share them with each other, as I did in the opening paragraph. We don’t know if other animals do the same. “I would say that we have documented some elements of human autobiographical memories that are also present in apes, and possibly in other animals as well,” says Call.

Reference: Martin-Ordas, Berntsen & Call. 2013. Memory for Distant Past Events in Chimpanzees and Orangutans. Current Biology.