Many animals may have their own forms of laughter, says a US researcher writing in the magazine Science.
Laughter may not be unique to humans
Professor Jaak Panksepp says that animals other than humans exhibit play sounds that resemble human laughs.
These include the panting sounds made by chimps and dogs when they play and chirping sounds observed in rats.
This suggests that the capacity for laughter may be a very ancient emotional response that predates the evolution of humankind, says Panksepp.
Research suggests the capacity for human laughter preceded the capacity for speech.
Professor Panksepp, of Bowling Green State University in Ohio, US, explains that neural circuits for laughter exist in "ancient" parts of our brain, whose general structure is shared amongst many animals.
Young chimps "play pant" as they mischievously chase and tickle each other.
And when rats play, they make chirps which some scientists associate with positive emotional feelings.
Rats emit chirps when they are at play
When rats are tickled in a playful way, they become socially bonded to humans and are rapidly conditioned to seek tickles, the US neuroscientist explains in Science.
The chirping sounds could be provoked by nerve circuitry in the brain which releases the neurotransmitter dopamine. These dopamine circuits also light up in the human brain during human amusement.
"Such knowledge may help to reveal how joking and horsing around emerged in our expansive higher brain regions," Professor Panksepp writes.
"Although no one has investigated the possibility of rat humour, if it exists, it is likely to be heavily laced with slapstick."
Other researchers prefer to view laughter and joy as uniquely human traits.