By Acacia Hori (Pitzer College) and Martha Kresz Bierut (Scripps College) [edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the Claremont Colleges]
For hundreds of years, we have assumed that the tickle is a sensation that only occurs in humans. Recently, scientists have begun to question whether that is indeed the case, and their experiments have shown that tickling produces behavior analogous to human laughter in some non-human species. These researchers have suggested that laughter induced by tickling is a common trait amongst hominids, which includes chimpanzees, apes, gorillas, orangutans, and humans (Ross et al. 2009).
Tickling-induced laughter is not limited only to species within the family Hominidae. In a paper published in the journal Behavioural Brain Research in 2000, Jaak Panksepp and Jeffrey Burgdorf at Washington State University and Northwestern University suggested that rat laughter can be elicited by tickling also. When tickled by an experimenter, rats produce chirps at the same frequency as the chirps they produce during pleasurable social interactions, such as play. In their experiment, Panksepp and Burgdorf found that tickled rats made this noise significantly more frequently than non-tickled rats. The experimenters measured this sound, which is beyond the range of human hearing, by slowing the chirps down with a heterodyne bat detector. This suggests that tickle sensation and laughter are traits that occur more widely among the animal kingdom than we previously thought.
Could there be an evolutionary connection between the tickle-induced laughter of different species? Is tickling homologous or homoplastic?
Panksepp and Burgdorf, along with other colleagues, have identified a genetic basis for tickle-induced laughter in rats by selectively breeding for and against the laughter response to tickling. In their experiment, it took four generations of breeding for the offspring to “laugh significantly more” and nine generations for the offspring to “laugh significantly less” when compared to randomly bred counterparts (Panksepp et al. 2001). This means that laughter elicited by tickling is an example of what we call a “heritable trait,” or a trait that can be passed down from generation to generation through DNA. Furthermore, this suggests that tickle-induced laughter has an evolutionary basis and cannot be classified entirely as learned behavior. It also opens up the possibility that ticklish animals have a common ticklish ancestor.
This idea is further supported by evidence that humans and other mammals share brain structures and neural mechanisms involved in tickle-induced-laughter, including the reticular nuclei of the thalamus, hypothalamus, and midbrain (Panksepp and Burgdorf 2000). These homologous structures point toward a common ancestral source of the tickling-laughter mechanism.
In addition to comparing their brain structures, researchers are looking at brain chemistry to determine how related human and rodent laughters may be. They have found that a common neurotransmitter, glutamate, is necessary to trigger a laughter response to tickling. They tested this by administering different drugs in low dosages to block specific neurotransmitter receptors in the brain. The drug that worked the best to eliminate the chirping was MK-801, an antagonist of the NMDA receptor, which is sensitive to glutamate. They confirmed this result by administering glutamate directly into the rat hypothalamus and observing the laughter that resulted (Burgdorf 2008).
While nothing is certain yet about whether these high frequency chirps are homologous to human social-emotional response systems, scientists are certain that through more research, these rat chirps may reveal more about the physiology of laughter, joy, and perhaps even positive emotional consciousness in the brain.
Why would the tickle be a useful evolutionary trait?
There are a few hypotheses out there that address this question. Some think that tickle induced laughter goes back in brain evolution to a time when social interaction was mediated by the production and sensation of simple acoustic signals. Additional work implies that tickling may be an important social mechanism for many mammalian species related to joyfulness and play, but “the vocal component of this state may have diminished through negative selection in the young of many other species” (Panksepp and Burgdorf 2010). This means that many mammals may enjoy play and even tickling, but the vocal cues comparable to human laughter may be very different or absent. The negative selection of the vocal reaction to joy may have occurred in mammals that experience predation, as the noises could have attracted predators. As far as the tickle being a useful evolutionary trait, it creates a “joyful form of affective consciousness within the human brain” (Panksepp and Burgdorf 2010). The act of tickling releases endorphins and hormones that are beneficial to the mental state of humans, and its possible presence in other species points toward a similar effect.
Why does any of this matter?
The quest for the origin of laughter may seem rather inconsequential, but researchers believe that if laughter is homologous across species, its occurrence could be useful in future research in the field of medicine by providing new ways to approach phenomena such as joy and depression. In his 1998 article, Panksepp postulates that, “depressed individuals laugh and play less than normal; the elucidation of neurochemistry that promotes chirping and playfulness in rodents may help guide development of new types of antidepressants” (Panksepp 1998). This is just one example of the many ways in which a homologous animal model for joyful laughter could be useful, and a few others may include the study of human emotion in a broader sense. In summary, Panksepp acknowledges that what we know now is not concrete, but it is certainly intriguing, “We suspect that brain circuits of human laughter and the neural underpinning of rodent chirping do interconnect with brain areas that mediate positive social feelings, but the locations of those areas remain unknown. In sum, although we would be surprised if rats have a sense of humor, they certainly do appear to have a sense of fun.”
Burgdorf J, Panksepp J, Moskal JR. Frequency-modulated 50kHz ultrasonic vocalizations a tool for uncovering the molecular substrates of positive affect. Neurosci Biobehav Rev. 2010 Dec 7.
Panksepp J, Burgdorf J. Laughing rats? Playful tickling arouses high frequency ultrasonic chirping in young rodents. In: Hameroff S, Chalmers C, Kazniak A, editors. Toward a Science of Consciousness III. Cambridge, Mass: MIT Press, 1999:231–44.
Panksepp J, Burgdorf J, Gordon N. Towards a genetics of joy: breeding rats for ‘laughter’’. In: Kaszniak A, editor. Emotions, qualia, and consciousness. Singapore: World Scientific; 2001. p. 124 – 36.
Davila Ross, Marina, Michael J Owren, and Elke Zimmermann. “Reconstructing the Evolution of Laughter in Great Apes and Humans.” Current Biology 19.13 (2009): 1106-111.