A single cue -- the taste of a madeleine, a small cake, dipped in lime tea -- was all Marcel Proust needed to be transported down memory lane. He had what scientists term an autobiographical memory of the events, a type of memory that many researchers consider unique to humans. Now, a new study argues that at least two species of great apes, chimpanzees and orangutans, have a similar ability; in zoo experiments, the animals drew on 3-year-old memories to solve a problem. Their findings are the first report of such a long-lasting memory in nonhuman animals. The work supports the idea that autobiographical memory may have evolved as a problem-solving aid, but researchers caution that the type of memory system the apes used remains an open question.
Elephants can remember, they say, but many scientists think that animals have a very different kind of memory than our own. Many can recall details about their environment and routes they've traveled. But having explicit autobiographical memories of things "I" did, or remembering events that occurred in the past, or imagining those in the future—so-called mental time travel—are considered by many psychologists to be uniquely human skills.
Until recently, scientists argued that animals are stuck in time, meaning that they have no sense of the past or future and that they aren't able to recall specific events from their lives—that is, they don't have episodic memories, the what-where-when of an event that happened.
Yet, several studies have shown that even jays have something like episodic memory, remembering when and where they've hidden food, and that rats recall their journeys through mazes, and use these to imagine future maze-travels. "There is good evidence challenging the idea that nonhuman animals are stuck in time," says Gema Martin-Ordas, a comparative psychologist at Aarhus University in Denmark and the lead author of the new study. But trying to show that apes also have a conscious recollection of autobiographical events is "the tricky part," Martin-Ordas admits.
To see if chimpanzees and orangutans have autobiographical memories that can later be triggered with a cue (as were Proust's by eating the pastry), Martin-Ordas and two other researchers devised a memorable event for the apes at the Leipzig Zoo. In 2009, eight chimps and four orangutans individually watched Martin-Ordas place a piece of a banana on a platform attached to the outside of a caged testing room. The apes could get the treat only by reaching through a slot with a long stick. The researcher then hid two sticks, only one of which was long enough to reach the banana. The animals watched as she hid each tool in a box in two different rooms. The chimp or orangutan observing her actions was then released into the area with the hidden tools. They had to find the correct tool, return to the room with the tempting banana, and use the tool to retrieve the treat.
Each ape took the test four times. "We set it up to see if cues—like Proust's madeleine—would trigger a memory event for them," Martin-Ordas says. But instead of using a single cue like a scent or a taste, the researchers offered the apes "a constellation of cues: me, the room, and the specific problem," Martin-Ordas says. They hoped that this combination would act as a trigger—that whenever the chimpanzees encountered this specific task with Ordas-Martin again, they would remember that they needed to search for the correct tool.
Over the next 3 years, the chimpanzees and orangutans took part in many other tasks with Martin-Ordas in the same room. Sometimes these tests required them to use a tool to reach for a banana, and sometimes they had to find a hidden tool. But they never experienced the same exact events as they had during the four tests—until one day in 2012.
Then, in a sort of déjà vu, they were faced with precisely the same setup with the researcher that they had encountered in 2009. Apparently, the combination of cues triggered something like a madeleine-moment for the apes because every ape, except for one orangutan, instantaneously remembered exactly what to do and solved the problem.
"I was really surprised that they could remember this event and they did it so fast," says Martin-Ordas, whose team reports its results today in Current Biology. The seven apes in a control group had not taken the original test and did not find the tool.
A second experiment established that the apes could also use a single unique cue—observing a seesaw—to remember 2 weeks later that they should use a wooden ball to get a frozen yogurt treat
Together, the experiments reveal that at least two species of great apes "can remember specific events and retrieve this memory to solve a particular problem," something never shown before in great apes, says Jonathon Crystal, a comparative psychologist at Indiana University, Bloomington. "Three years is a remarkably long time to draw on a memory—not just for animals, but for us. It's breathtaking," he says.
But Crystal and others are not convinced that this experiment demonstrates autobiographical memory. "Is there evidence here for 'cued recall,' as the authors argue? No," says Martin A. Conway, a memory researcher at City University London. Conway does think the apes have "moment-by-moment episodic memory, but they are not saying to themselves, 'I can't believe it. I'm back in this stupid lab with this stupid test.' Believe me, that's not what they're doing."
Perhaps not, but the experiments do "move us significantly closer to showing that chimpanzees and orangutans have humanlike episodic memory," agrees Michael Corballis, a cognitive psychologist at the University of Auckland in New Zealand, who helped invent the term mental time travel and used to be among those who argued that animals are stuck in time—a phrase that now seems increasingly out-of-date.
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