How can you tell when a rat is happy? The answer may be to look at its ears. A recent study has found that in a similar way to how humans smile when expressing pleasure, the rodents relax their ears, which then flush pink.

There have been plenty of studies looking into how rats behave when they are upset or distressed, mainly to allow those using them as biological models to understand if they are being negatively impacted. Depressed rats, for example, isolate themselves from a group and don’t interact, or fail to try and escape when held up by their tail. Scientists have even devised a “rat grimace scale” to assess how much pain a rodent is suffering from by looking at their facial expressions.

But not much has been done to look for positive emotions in rats.

A happy rat smiles with his ears. PLOS ONE (2016). DOI: 10.1371/journal.pone.0166446

It has recently been found that when tickled, rats seem to act in a happy way, returning for another bout of tickling and even vocalizing – albeit in a frequency too high for humans to hear – in a manner some have compared to laughing. Building on this, the researchers of this latest study decided to use belly tickling to test how rat’s facial expressions change when they are happy.

The 15 test subjects were treated to stints of being tickled by the team, who then photographed their faces to see if there had been any noticeable change. They found that the rodents effectively “smiled” with their ears. When the rats were happy, their ears relaxed and became all droopy, while they also flushed red. When the animals were not as happy, after being blasted with white noise, their ears faced more forward.

How to make a rat laugh. PLOS ONE (2016). DOI: 10.1371/journal.pone.0166446

Why they do this is a little harder to explain. Their ears probably droop due to the relaxed nature of the rodents, but the changing in color of the ears is more difficult to decode. They turn pink because there is an increase in blood flow to their sound holes, but whether this is conclusively because they are happier, or because of the increase in physical activity when their bellies are tickled, is not known.

How the rats felt when laughing and drooping is even harder to understand. You can’t exactly ask the furry rodents. But the scientists can look at brain scans, and, considering their brains are made up of the same connections as our own, including all the same chemicals and functions, it is not too farfetched to suggest they feel similar things to us.