Rats show happiness by lowering their ears, according to scientists who found that tickling the animals made their ears more droopy and pink. 
Researchers from the University of Bern in Switzerland selected 15 male Lister hooded lab rats and subjected them to two types of environmental experiences they labelled as either positive or negative. 
The positive experiences consisted of sessions of tickling by one of the researchers. For negative experiences the rats were put in a room and subjected to random blasts of white noise. 
During both types of experiments, microphones were used to record vocalisations by the rats and observations were made of physical features, particularly changes to faces. 
Researchers found that the rats undergoing tickling vocalised more and their ears became more droopy and pink. 
They exhibited no discernible reactions during the negative experiences, ‘Phys.org’ reported. 
Researchers attributed the lowered ears to the rats being in a more relaxed state, but they were not clear on the reason for the pink ears. 
They suggested it could have been a sign of happiness or simply increased blood flow due to the small degree of physical exertion that went along with laughing while being tickled. 
The study appears in the journal PLOS ONE.