As Bekoff points out, Darwin argued that the difference between human and animal intelligence is a matter of degree, not of kind. Or as Bekoff put it, "If we have a sense of humor, then nonhuman animals should have a sense of humor, too."
A similar sentiment inspired psychologist Jaak Panksepp to enter his lab at Bowling Green State University in Ohio one day in 1997 and tell undergrad Jeffrey Burgdorf, "Let's go tickle some rats." The lab had already discovered that its rats would emit unique ultrasonic chirps in the 50 kilohertz range when they were chasing one another and engaging in play fighting. Now the researchers wondered if they could prompt this chirping through tickling. Sure enough, when the researchers began poking at the bellies of the rats in their lab, their ultrasonic recording devices picked up the same 50 kilohertz sounds. The rats eagerly chased their fingers for more. Soon, as the news media trumpeted the existence of rat laughter, people the world over were opening up their rat cages and engaging Pinky and Mr. Pickles in full-scale tickle wars.
We met Burgdorf at his office at Northwestern's Falk Center, where as a biomedical engineering professor he's continued his rat-tickling efforts. He was cautious, however, about overselling what's happening with his rodents. "I don't necessarily call it laughter, I call it a signal of positive affect," Burgdorf told us. His careful choice of words makes sense. Not everyone was convinced he and Panksepp had uncovered real rat laughter when their rodent-tickling activities first went public. But whatever you want to call it, Burgdorf, a quick-witted guy with a boyish face and a sign on his office door that reads "Know It All," has been obsessed with that strange rat noise he first heard in 1997.