One of the most cited studies on this interesting animal behavior comes from Dr. Jaak Panksepp, professor of veterinary and comparative anatomy at Washington State University in Pullman. A video of him playing with rats shows his findings (see above).
When Dr. Panksepp tickles rats' bellies, the animals appear to laugh, and their giggly behavior suggests that humans aren't the only mammals with emotional lives, he told The Huffington Post.
"It has long been clear that deep subcortical brain regions are evolutionarily related (homologous) in all mammals, and since rat-laughter is strictly a lower brain process (as in humans), this indicates that we share a form of social-joy with other animals," he wrote in an email.
Rat "laughter" consists of high-frequency chirps that are beyond the range of human hearing. But for the video, Dr. Panksepp used special sound equipment to hear the rats' responses, according to The New York Times.
It does seem evident in the video that the little creatures have a case of the giggles–and the rats enjoyed the tickling, Dr. Panksepp said. But how does Dr. Panksepp know the rats are laughing–and not complaining? In part, it's because he tied the chirps to activity in "reward centers" of the rat brain. And in the video, which was recorded in 1998, the rats seem to chase Dr. Panksepp's hand for more when he stops tickling them.
And a good time for rodents may not be the only benefit of research like Dr. Pansepps.
Laughter has been studied in apes, Dr. Panksepp told The Huffington Post. "However, more neuro-scientific experimental work has been done on rat-laughter than of any other species, including humans," he said.