It's true. Rats even laugh when they're tickled.
According to neuroscientist Dr. Jaak Panksepp, scientists were initially puzzled by the sounds they heard while observing rats at play. They suspected these sounds might be laughter, but weren't sure.
They realized that in order to understand exactly what they were hearing, they had to study the sounds in a register outside the range of human hearing, so they employed devices called bat detectors. The devices make high frequency sounds accessible to the human ear.
Scientists set out to test their theory about animal laughter by tickling the rats. They found that when tickled, the rats produced vocalizations (now audible to the scientists thanks to the bat detectors) that appeared to be laughter.
Panksepp notes that, once tickled, the rats began to seek out further interaction with the human hand. This is remarkable in that it suggests that they enjoyed the tickling, and thus were processing the interaction at a more sophisticated level than previously thought.
We all know our four-legged friends have minds of their own, but laughing rats now force more questions about just how complex the emotional lives of our nonhuman friends really are.