Scientists tickled rats — and found laughter

A neuroscientist spent years tickling rats in a lab, and the rats kept coming back for more.

In the 1990s, Jaak Panksepp noticed that young rats at play made a particular chirp at around 50 kilohertz — far above human hearing. When he tickled them, especially on the nape and belly, they made the same sound. A lot of it. The rats would chase his hand to be tickled again, and they made the call most when they were relaxed and familiar with him. Stressed, startled or hungry rats went quiet.

The finding has been repeated by other labs since, and those 50 kHz calls are now a standard way of measuring positive emotional state in rats. Researchers are careful about the word laughter — it isn't a joke the rat is getting — but it does appear to be the sound of play and anticipated reward.

Why it matters at home: your rats are having conversations you can't hear, and the mood of those conversations depends on you.

Rats are intensely social and bore quickly. Keep them in same-sex pairs or groups, handle them daily in short, gentle sessions, and let them play rather than only be held. Many owners find rats enjoy a light fingertip tickle at the scruff and ribs — but go by the individual. A rat that moves towards your hand is asking for more. One that slips away is not.

Any rat that becomes suddenly quiet, hunched or unwilling to play needs a vet experienced with small exotic pets. Rats hide illness well, and respiratory disease is common.