Aristotle declared that humans are the only animal to laugh, but then, he never saw this video of Jaak Panksepp tickling rats. When you play it, you’ll hear the tickled rats chirping — an ultrasonic noise that’s audible thanks to the special equipment that enabled Dr. Panksepp and his colleagues to discover this phenomenon. Young rats make the same chirp when they chase and play with one another, and they like to hang out with other rats who chirp at this frequency (50 kHz). It seems to be a happy sound: rats will run mazes and press levers in order to be tickled, and they’ll emit the same chirp when the dopamine reward circuits in the brain are stimulated. Some researchers still aren’t sure these sounds qualify as animal laughter, but Dr. Panksepp, a neuroscientist at Washington State University, has been systematically gathering evidence of the parallels to human laughter.