Tickling rats can tell us a lot about the ability of animals to laugh and joke. Right now, in a high-security research lab at Northwestern University’s Falk Center for Molecular Therapeutics in Illinois, scientists are tickling rats. Their goal? To develop a happiness pill. But their efforts might also produce some of the best evidence yet that humour isn’t something experienced exclusively by humans. Scientists believe human laughter evolved from the distinctive panting emitted by our great-ape relatives during rough and tumble play.

   That panting functions as a signal that the play is all in good fun and nobody is about to tear anybody else’s throat out. In a clever bit of scientific detective work, psychologist Marina Davila-Ross of the University of Portsmouth in the UK analyzed digital recordings of tickle-induced panting from chimps, bonobos, gorillas and orang utans, as well as human laughter.

   She....