Was it thyme or coffee? Rats, I can’t remember FLPA/REX/Shutterstock

Much like students doing a test, rats tend to skip questions when they have forgotten the answer. A series of smelly experiments suggests rats are aware of what they remember, and behave differently when they can’t recall something.

Victoria Templer at Providence College, Rhode Island, and her team trained rats to dig through sand to sniff samples of cinnamon, thyme, paprika or coffee, and then go to a dish smelling of the matching scent. If the rats picked the correct dish, they got a piece of cereal.

But there was a twist. Although rats that chose a dish with the wrong scent got no reward, rats that positioned themselves next to a fifth, unscented dish received a quarter-piece of the cereal. This meant that when rats forgot what they had smelled in the sand, their best bet was to pick the unscented dish – provided they could tell that they had forgotten the relevant smell.


Nine rats were each tested many times across multiple experiments. In some of these, the unscented dish was not there, forcing the rats to choose a scent even if they couldn’t remember it.

Read more: Building blocks of memories seen in brains for the first time

Without the unscented option, the rats picked the wrong dish 48 per cent of the time. But when it was available, they chose the unscented option 20 per cent of the time, and in those cases where they did choose a scented dish, the rate of picking the wrong one fell to 39 per cent – a drop that wouldn’t be expected by chance alone.

Because the rats’ performance improved when the unscented dish was available as an opt-out, Templer says this shows the rats weren’t simply choosing the unscented dish for no reason. They seem to have known that declaring that they had forgotten would still earn them a small reward.

Read more: Rewinding memories may help us recall where the good stuff is

In further experiments, the team found that letting the rats sniff the sample twice before choosing a match reduced how often they chose the unscented dish. But making the rats wait longer between sniffing the sample and choosing a match pushed up the number of times they opted out.

“When they made good memories, the rats declined [to choose a scent] less often. When they made bad memories, the rats declined more often,” says Benjamin Basile of the US National Institutes of Health, who was not involved in the study.

People and some primates have “metamemory” – the ability to know what they remember – but whether rats have it too had been unclear, and this latest study suggests they might. People with Alzheimer’s disease or dementia often don’t realise when they have forgotten something, so a test for metamemory in rats could be useful for rodent studies of these conditions.

Journal reference: Animal Cognition, DOI: 10.1007/s10071-017-1109-3