Have you ever heard a rat laugh? Jaak Panksepp has, and he finds nothing unusual about it. Panksepp, Professor Emeritus of Psychology at Bowling Green State University, tickles rats in his lab to elucidate the fundamentals of laughter.

Scientists have long known that humans are not the only species capable of laughing. In fact, most mammals, from chimpanzees to dogs, can laugh as well. Similar to other abilities that are shared among many species, some believe that there must be a reason the ability to laugh at a good joke, from tickling, or some other source is shared among so many different species. Given its prevalence and importance in social interactions for all of these species, scientists seek to learn more about the origins and purpose of laughter.

Panksepp is at the forefront of such research, and his work on rat laughter has led to some interesting and unexpected observations. First, Panksepp clarifies that rat laughter is slightly different from that of humans. Rat laughter comes in the form of high frequency 50-kilohertz ultrasonic calls, or “chirps,” that are distinct from other vocal emissions in rats. In other words, one cannot hear rat laughter; they are actually high-pitched chirps that must be measured using sensitive and specialized equipment.

In addition to differences in frequency, rats also laugh in different situations than most humans do. While rats laugh when tickled in sensitive areas such as the nape of their neck, young rats also laugh when they anticipate rewards or enter new environments. Rats also laugh when they are nervous and when trying to diffuse aggressive situations. These observations have led Panksepp to hypothesize that by laughing, rats display emotional health and engage in social bonding with other fellow rats. Therefore, rats that laugh more frequently might have a higher social standing within a group because they attract other rat, somewhat like the class clown in elementary school.

Laughter among children during boisterous play is similar to young rats laughing when they are tumbling together. According to Panksepp, laughter among human children and young rats is actually quite similar. The main difference in humans, he notes, is that humans activate “higher order structures” like the frontal cortex when laughing at jokes, leading to laughter in response to multiple kinds of stimuli. On the other hand, adult rats do not necessarily have the cognitive mechanisms to understand verbal jokes and sarcasm. “The use of language-based jokes is clearly unique to humans,” says Robin Dunbar, a professor of evolutionary psychology at the University of Oxford. Dunbar also claims “laughter predates the appearance of language in human evolution and was used as a mechanism to allow bonding between a large number of individuals.”

Laughter in humans releases endorphins, which produce the feeling of well-being in the brain. Releasing endorphins allows for bonding among individuals in a group, which is beneficial to the hyper-social societies humans live in. Sharing of laughter is likely to help people bond and facilitate closer connections. Beyond this, however, behavioral neuroscience has yet to clearly link how these tiny chemical changes add up to cause something to seem funny to us — or rats.