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Like humans, chimps and orangutans remember their past

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Sometimes a particular smell, flavor or other sensory cue can trigger a flood of distant memories. New research shows chimpanzees and orangutans have similar capacities for this kind of episodic memory — they're able to accurately recall an event that happened three years prior. The results suggest, once again, that we're just not as unique as we think we are.

Over the years, a lot of work has gone into studying the remarkable memory of our primate relatives. For example, a recent video shown at the 179th meeting of the American Association for the Advancement of Science demonstrated that chimpanzees have a far greater working memory — which is used to temporarily hold and manipulate information — than we do. In the video, chimps are able to recall the exact sequence and location of numbers that flash on a screen.


But these animals are no slouches when it comes to long-term memory, either. There are two types of long-term memory, explains Gema Martin-Ordas, a primatologist at Aarhus University in Denmark. Semantic memory essentially boils down to knowledge and facts about the world. "We know that chimpanzees can remember concepts and things, such as the meaning of symbols, that they learned 30 years ago," Martin-Ordas tells io9.


The other type of long-term memory is episodic in nature — basically remembering past events that occurred at a particular time and place. These memories are often easier to access if they're tied with a certain cue, such as a smell. Previous studies have shown that chimps can remember events for at least a week, which is a far cry from our ability to remember childhood events years down the road. So Martin-Ordas and her colleagues wondered if that's really the extent of other primates' memories. Could they remember events that happened years ago, and could these events be triggered by relevant cues?

To find out, the researchers set up an experiment involving a tool-finding task with 15 chimpanzees and four orangutans. At the beginning of a trial, an ape would start off in a caged room with two doors leading to adjacent rooms. At the other side of the cage is a food reward, which could only be reached with a tool (a long stick). While the apes watched, an experimenter would then hide two tools — one long stick and one short stick — in two boxes, one in each adjacent room.


To get the reward after the doors opened, the ape had to remember which box the experimenter hid the long stick. Each chimp and orangutan went through four trials, with the correct tool's location changing with each trial.

The researchers tested the apes again three years later — they put the apes in the same room as before, with the same experimenter, who had them complete the same task. This time, however, the tools were hidden beforehand. "We were just looking to see if they knew what to do," says Martin-Ordas.

Turns out, they did (all except for one ape): The primates started searching for the tool within a mere 5 seconds after the doors opened. And if the ape found the incorrect tool first (because this time they didn't see where each was hidden), they immediately rejected it and began looking for the long stick. By comparison, the seven control apes were clueless when the doors opened, even after 5 minutes.


It's important to note that the apes are quite familiar with the arena because multiple scientists use it on a daily basis for various primate studies, such as research on cooperation and tool-use. Martin-Ordas says that the unique combination of features — the rooms where apes were tested, the experimental setup and the experimenter — provided a distinctive cue that activated the apes' relevant memory of the past event. And changing just one feature affects how long the apes take to realize what to do. "We have one experiment where we are testing that," she says. "When we change the cues, the latency increases."


From the first experiment, it's clear the apes can recall a general event that happened years before. In a second, similar experiment, Martin-Ordas and her team showed that chimpanzees and orangutans could recall a different, unique tool-finding event (which required a different task to get the food reward) that happened only one time two weeks prior.

The researchers are now interested in further testing the apes' long-term memory. "We could test if they can recall memories when they have no cues," Martin-Ordas says. "It would be like strategic recall, where you are talking to someone and consciously trying to retrieve a memory."


Overall, the main take-home message is that we're not the only primates with remarkable memories, Martin-Ordas says. "What this study shows is that this episodic memory that appears to be unique to human is not that unique," she says.

The researchers detailed their work recently in the journal Current Biology.

Top image via Jim Epler/Flickr.