Do rats laugh when you tickle them?
50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables
Neuroscientists tickled rats and recorded happy ultrasonic chirps that might be rat laughter.
Jaak Panksepp and Jeffrey Burgdorf gently tickled young rats with their hands and listened with special microphones for high-pitched 50-kHz chirps, far too high for human ears.
Tickled rats chirped loads and quickly learned to chase the tickling hand for more. The scientists think the chirps may be an early form of laughter, which means somewhere in a lab, a rat is giggling.
Laughter: older than people, and higher-pitched.