Scientists have already found that rats seem to enjoy having their bellies tickled. They let out a noise too high-pitched for the human ear to detect and keep coming back for more. Now, adding to the idea that rats have their own version of a giggle, researchers at the University of Bern in Switzerland report in the journal PLOS ONE that they've also found the rat equivalent of a smile: lowered ears that get quite rosy. They call these "facial indicators of positive emotions" because, though it remains unclear whether rats are actually feeling happiness—the pink ears, for instance, could merely result from increased blood flow due to physical exertion—they do seem to exhibit a positive reaction in their return for more.
"While it’s likely that animals do feel emotions, it is unknown if they feel them the same way we do," one researcher tells National Geographic, which reports that rats can be "literally tickled pink," and that their ears droop because their ear muscles relax. The researcher calls himself a "rat-tickling master," and says he found that rats react to tickling differently; the shy and anxious ones, for instance, prefer less rigorous tickling. Researchers have already investigated negative emotions in rats (a "grimace" scale to measure pain exists) in an attempt to inflict less suffering whenever possible, but this is one of the first studies that attempts to sort out a lab rat's positive emotions. (The ability to tickle yourself appears linked to schizophrenia-like traits.)