Recalling past events – what happened, when, and where – is a skill only humans were thought to possess.

This 'mental time travel' is called episodic memory, and some experts have long argued it sets us apart from other animals, in the way we can make decisions or strategize for the future based on what we've learned in the past.

Yet some animal behavior experts don't think humans are unique in this regard. For decades, they have argued that all sorts of animals have episodic-like memories.

Birds store food underground and dig it up hours or days later; bottlenose dolphins remember trivial information to pass an unanticipated memory task; and dogs and cuttlefish have shown similar abilities in experiments, too.

Now, a team of researchers from Indiana University led by neuroscientist Cassandra L. Sheridan has found evidence that rats can also replay past events in their minds to solve unexpected problems.

Unexpected, in that the animals weren't 'trained' through multiple practice rounds to learn the rules of a task, as happens in other animal behavior studies. Instead, the rats only had one go at each sniff test, that they didn't know was coming.

"What we wanted to test is a property of what people do in everyday life," explains senior author Jonathon Crystal, also a neuroscientist at Indiana University.

"We remember information even though it was seemingly unimportant when it was encountered. When we happen to need that information, we replay the stream of events to identify the information needed to solve our current problem."

First, the rats were randomly presented with a sequence of smells from a pool of possible 67 odors, then rewarded if they could identify the third-to-last odor they had smelt.

Next, the rats were placed in an eight-arm octopus-like maze where they foraged for food. On a single occasion, the rats encountered scented lids covering the food cups. Immediately, and unexpectedly, the animals were tested on the memory task again.

The experiments were designed in such a way that "the rats could not know that the odors in the radial maze were important or that they would be asked about the maze odors in the future," the researchers explain in their paper.

When tested immediately after exploring the maze, all nine rats picked out the third-to-last odor they had encountered while foraging for food. They also identified the correct odor after a 15-minute delay.

"These results are consistent with the hypothesis that rats can replay a stream of unique events that were not known to be important when the events were encountered and report this information when unexpectedly asked to search episodic memories," the researchers report.

While there's agreement that at least some traces of episodic memory appear to be present in mice, researchers remain cautious about its extent. University of Queensland psychologist Thomas Suddendorf argues that the 'what, when, where' test falls short of demonstrating animals can recall and process vivid memories like humans do.

Human mental time travel "involves more than storage of such traces" of episodic memory, Suddendorf said in 2019 when debating Crystal on the subject. "It depends on a host of other cognitive components that enable the (re)construction of mental scenarios and the embedding into larger narratives."

Felicity Muth, an animal cognition researcher at the University of Texas, also pointed out in 2011 that some studies have failed to find evidence of episodic memory in rats and other animals, like rhesus monkeys, when other researchers claim they do.

This new study may add some evidence in the rats' favor. But as Muth notes, we're relying on our interpretation of experiments that are designed to test an animal's memory, when we don't know if we're asking the right questions.

"Is it really the case that some animals can remember, whereas others just cannot?" she wrote in Scientific American. "What seems more likely is that the way in which we 'ask' the animals if they remember gives us different results."

The study has been published in Current Biology.