(CNN)Mice, unlike most people, cannot force a smile or disguise their disgust (as far as we know). Most of us may not have realized that their tiny, fuzzy faces can muster an emotional expression at all.
Mice make different faces depending on how they feel -- and that could impact how we treat mood disorders, a new study says
But a group of German neurobiologists have proven mice can, in fact, express emotions -- and they play out all over their petite faces. The researchers say that analyses of rodent brains in mid-emotional reaction could improve the ways we treat human patients with mood disorders.
"Being able to measure the emotion state of an animal can help us identify the 'how' and 'where' in the brain, and hopefully get hints at how emotions arise in humans, too," neuroscientist and study author Nadine Gogolla of the Max Planck Institute of Neurobiology told CNN.
Gogolla and her fellow researchers carried out a series of experiments with emotive mice and linked five emotional states -- pleasure, disgust, nausea, pain and fear -- to their facial expressions.
The results, published this week in the journal Science, could advance how we understand emotions -- and they're cute to boot.
It's certainly not easy.
First, the team stimulated mice to react in certain ways so they could observe how their faces changed. Drinking a sweet solution evoked pleasure, and drinking a bitter substance elicited disgust. A painful shock to the tail could incite fear, and a an injection of lithium chloride might have made them feel ill.
Not every mouse reacted the same way to the same stimuli, the researchers noted: A thirsty mouse expressed pleasure when drinking water more than a full mouse did. That's a fairly nuanced reaction for such a small animal.
Close-up footage of the mice showed subtle changes in their facial expressions: When a mouse experienced pain, their noses drooped and their ears flicked down. When a mouse felt fearful, their ears ticked up and their eyes widened.
Observations alone couldn't determine the intensity of those emotions, though. So the neurobiologists next built descriptors for what each facial expression would look like and tr