Having just concluded perhaps the first formal (i.e., well-controlled) ethological analysis of rough-and-tumble play in the human species in the late 1990s, where laughter was an abundant response, I had the “insight” (perhaps delusion) that our 50-kHz chirping response in playing rats might have some ancestral relationship to human laughter. The morning after, I came to the lab and asked my undergraduate assistant at the time to “come tickle some rats with me.”
Over the ensuing years Panksepp and his research assistants systematically conducted study after study on rat laughter, revealing a striking overlap between the functional and expressive characteristics of this chirping response in young rodents and laughter in young human children. To elicit laughter in his rat pups, Panksepp used a technique that he called “heterospecific hand play,” which is essentially just jargon for tickling.
Rats are particularly ticklish, it seems, in their nape area, which is also where juveniles target their own play activities such as pinning behavior [when one rat pins another on its back]. Panksepp soon found that the most ticklish rats—which, empirically, means simply those rats that emitted the most frequent, robust and reliable 50-kHz chirps in human hands—were also the most naturally playful individuals among the rat subjects. And he discovered that inducing laughter in young rats promoted bonding: tickled rats would actively seek out specific human hands that had made them laugh previously. In addition, and as would be expected in humans, certain aversive environmental stimuli dramatically reduced the occurrence of laughter among rodent subjects.
For example, even when tickling stimulation was kept constant, chirping diminished significantly when the rat pups got a whiff of cat odor, when they were very hungry or when they were exposed to unpleasant bright lights during tickling. Panksepp also discovered that adult females were more receptive to tickling than males, but in general it was difficult to induce tickling in adult animals “unless they have been tickled abundantly when young.” Finally, when rat pups were given the choice between two different adults—one that still spontaneously chirped a lot and one that did not—they spent substantially more time with the apparently happier grown-up rat.
Perhaps not surprisingly, Panksepp has encountered an unfortunate resistance to his interpretation of this body of findings, especially among his scientific colleagues. Yet he protests:
We have tried to negate our view over and over, and have failed to do so. Accordingly, we feel justified in cautiously advancing and empirically cultivating the theoretical possibility that there is some kind of an ancestral relationship between the playful chirps of juvenile rats and infantile human laughter.
Now, Panksepp would be the first to acknowledge that his findings do not imply that rats have a “sense of humor,” only that there appear to be evolutionary contiguities between laughter in human children during rough-and-tumble play and the expression of similar vocalizations in young rats. A sense of humor—especially adult humor—requires cognitive mechanisms that may or may not be present in other species. He does suggest, however, that this may be an empirically falsifiable question: “If a cat … had been a persistently troublesome feature of a rat's life, might that rat show a few happy chirps if something bad happened to its nemesis? Would a rat chirp if the cat fell into a trap or was whisked up into the air by its tail? We would not recommend such mean-spirited experiments to be conducted but would encourage anyone who wishes to go in that direction to find more benign ways to evaluate those issues.”
Differences between laughing “systems” among mammals are reflected by cross-species structural differences in brain regions as well as in vocal architecture. In the same issue of Behavioural Brain Research, neuropsychologist Martin Meyer and his colleagues describe these differences in rich detail. Although brain-imaging studies of human participants watching funny cartoons or listening to jokes reveal the activation of evolutionarily ancient structures such as the amygdala and nucleus accumbens, more recently evolved, “higher-order” structures are also activated, including distributed regions of the frontal cortex. So although nonhuman primates laugh, human humor seems also to involve more specialized cognitive networks that are unshared by other species.