They suggest that both rats and primates use laughter to distinguish playful from threatening physical behaviour. A laughter expert from the University of Maryland, Baltimore, Robert Provine, believed many social mammals probably produced laughter-like sounds. Psychologists in the US claim to have the first credible evidence that animals other than humans and our close cousins are capable of laughter. For years, biologists have known that chimpanzees and even some monkeys produce a panting sound akin to human laughter, but evidence of mirth among other mammals has been vague and anecdotal. Jaak Panksepp and Jeffrey Burgdorf of Bowling Green State University in Ohio tried to rectify the situation - by tickling rats.
Rats respond to tickling by emitting ultrasonic whistles outside the range of human hearing.
These rodent chirpings have been detected before, but most researchers believe they signalled distress or agression, or were a prelude to sex. They also chirp more than their older relatives when they are tickled, which Panksepp says fits with the idea that children are more ticklish than adults. He suggests that rats and primates, especially juveniles, use laughter to distinguish play from threatening physical interactions. Panksepp expects many scientists to be sceptical about claims for human emotions in a lower species, but he points out that out laughter is triggered by evolutionarily ancient brain regions - its not exactly a cerebral activity. Robert Provine, who studies laughter at the University of Maryland, Baltimore County, believes many social mammals probably produce laughter-like sounds.