Tickling Rats in the Name of Science
What the neuroscience of play teaches us about the evolution of morality.
Last summer I obsessively read a substantial portion of the last 40 years of the neuroscience of play behavior in rats. Surprisingly, studying rats play teaches us a lot about human development, and even morality.
This interest stemmed from Jordan Peterson’s Biblical Lecture Series. In this excerpt from the third lecture, God and the Hierarchy of Authority, Peterson discusses Jaak Panksepp’s pioneering research on the neurocircuitry of play.
One claim in particular stood out to me: if you pair a large and small rat together to play-wrestle, the smaller rat will refuse to play unless the larger rat lets it win at least 30% of the time. I read all of Panksepp’s original research on rat play, and wasn’t able to find evidence for this. It is true that rats which are about 10% larger win about 70% of the time, and sometimes let subordinate rats win. However, I couldn’t find anything about the subordinate rats refusing to play if they weren’t winning enough.
The research is decades old - perhaps Peterson misremembered this fact. But if my search wasn’t thorough enough, and you know of the original source, please send it my way. In any case, this deep-dive inspired the following article on mammalian play and emergent morality (slightly modified from its original appearance in Psychology Today).
Tickling Rats in the Name of Science
On a temperate, controlled laboratory-environment day in the late 1990s, Jaak Panksepp, one of the pioneers of affective neuroscience, puts on his lab coat and gets to work. He is not conducting brain scans, but rather, conducting “vigorous manual stimulation of the ventral body surface” of the rat (Panksepp & Burgdorf, 1999). This is scientific-speak for, “I tickled the rat’s belly with my finger.” He takes out his recording device, and notes “50 kHz ultrasonic vocalizations as indices of positive affect” (Panksepp & Burgdorf, 2000). This is scientific-speak for, “the rats laugh when I tickle them.”
Believe it or not, this was groundbreaking scientific research.
Laughter and Play in Rats
Rats, like primates, make various sounds associated with various behaviors or emotions. They scream when they are scared, they laugh when they play, and they vocalize during sex (Knutson et al., 2002). Rats, however, vocalize ultrasonically: we cannot hear them with our human ears. We did not know this until the 1960s, when clever scientists using ultrasonic recording devices to study insects decided to use them on small mammals, too (Pye & Flinn, 1964; Sewell, 1967).
Rats not only like to laugh and be tickled, but to wrestle during play. Panksepp (1981) studied this phenomenon scientifically through observational coding of “pinning” in rats. A rat was pinned when it wound up in the “usually improbable posture of having their dorsal surface to the ground, with another animal hovering above in a ‘dominance’ stance.”
The Evolution of Play
This research is a big deal, because it shows us that play, laughter, and joy, are evolutionarily ancient behaviors and emotions in mammals (Panksepp & Burgdorf, 2003). We’ve known for a long time that primates laugh and play like humans do (Loizos, 1967). Humans and chimpanzees, however, diverged on the evolutionary tree about six million years ago, whereas humans and rats diverged about ninety-six million years ago (Nei et al., 2001).
Play is a developmentally critical phenomenon in humans, and these findings indicate that this has been the case in mammals since at least the age of the dinosaurs.
Play Is Critical for Social Development
From an early age, play is important in the development of motor skills and musculature in both humans and rats (Byers & Walker, 1995; Trawick-Smith, 2014). Even more important than the effects of physical play, which can be practiced alone, are the effects of social play on social development. This too, is true not only in humans, but also in rats.
Just as children possess an inherent drive for social play, especially when cooped up inside for too long, rats deprived of socialization will be more motivated to play, and play for longer when reintroduced to peers (Panksepp & Beatty, 1980). If deprived of socialization for an extended period of time during critical windows of adolescent brain development, the orbitofrontal and medial prefrontal cortices, brain regions critical for social decision-making, will not develop properly (Bell et al., 2010). Conversely, if these brain regions are lesioned, rats will exhibit aggressive and antisocial behavior, indicating maldevelopment of social cognition (Bell et al., 2009).
Rough-and-tumble play, in particular, is important for children identifying the limits of their body, and in learning to self-handicap their strength so as not to cause others pain or harm (Pellegrini, 2002). This function appears to have evolved in rats, too.
When rats play-wrestle, the dominant rat will pin the subordinate rat about 70% of the time (Panksepp et al., 1984). The dominant rat is usually about 10% larger, though interestingly, this occurs even if the rats are initially the same size. Eventually, one will come out as dominant, and begin to grow (Panksepp et al., 1984). Despite winning 70% of the time, during play (but not during genuine aggression), dominant rats assume postures that make it easier for subordinates to flip them over (Pellis et al., 2005). Just like humans, they self-handicap.
This self-handicapping is hypothesized to be related to reciprocity. Rats who are injured by a playmate are much less likely to play again with that particular partner (Panksepp et al., 1984). Interestingly, rats with lesioned cortical brain regions can sustain this self-handicapping reciprocal play, but rats with lesioned vocal cords cannot (Pellis et al., 2010; Kisko et al., 2015). They need to communicate to play fair, but whatever brain regions govern this communication are subcortical and more evolutionarily ancient.
Play Makes Us Ethical
These findings indicate that reciprocity of play, famously identified by Jean Piaget (1932) as the central catalyst of children’s moral development, may have first emerged in our earliest mammalian ancestors.
Piaget observed that children began to construct a sense of morality and fairness through shared games. Children around the ages of 5-8 learn to play cooperatively, and can detect violations of an implicit rule structure, even before the cognitive capacity necessary to explicitly state the rules of a game emerge around the ages of 8-12 (Piaget, 1932).
In this sense, though humans are unique in being able to explicitly codify moral laws and rules for cooperative play, concepts of fairness may still exist in animals without such cognitive capacities, even in those as far removed from us as rats. (And certainly in other primates: for a great example of how Capuchin monkeys respond to violations of fairness, see this excerpt from Frans de Waal’s TED talk).
This sense of fairness, as well as our social intelligence and our capacity for empathy, joy, and laughter, all have their roots in our 96+ million year old shared lineage of social mammals who engage in reciprocal play. Studying the play behavior of rats, and even tickling them in the name of science, shows us what it means to be human.
References
Bell, H. C., McCaffrey, D. R., Forgie, M. L., Kolb, B., & Pellis, S. M. (2009). The role of the medial prefrontal cortex in the play fighting of rats. Behavioral Neuroscience, 123(6), 1158. https://doi.org/10.1037/a0017617
Bell, H. C., Pellis, S. M., & Kolb, B. (2010). Juvenile peer play experience and the development
of the orbitofrontal and medial prefrontal cortices. Behavioral Brain Research, 207, 7-
13. https://doi.org/10.1016/j.bbr.2009.09.029
Byers, J. A., & Walker, C. (1995). Refining the motor training hypothesis for the evolution of play. The American Naturalist, 146(1), 25-40.
Kisko, T. M., Euston, D. R., & Pellis, S. M. (2015). Are 50-khz calls used as play signals in the playful interactions of rats? III. The effects of devocalization on play with unfamiliar partners as juveniles and as adults. Behavioral Processes, 113, 113–121. https://doi.org/10.1016/j.beproc.2015.01.016
Knutson, B., Burgdorf, J., & Panksepp, J. (2002). Ultrasonic vocalizations as indices of affective states in rats. Psychological Bulletin, 128(6), 961–977. https://doi.org/10.1037/0033-2909.128.6.961
Loizos, C. (1967). Play Behaviour in Higher Primates: a Review. In D. Morris (Ed.), Primate Ethology (pp. 176–218). AldineTransaction.
Nei, M., Xu, P., & Glazko, G. (2001). Estimation of divergence times from multiprotein sequences for a few mammalian species and several distantly related organisms. Proceedings of the National Academy of Sciences, 98(5), 2497-2502. https://doi.org/10.1073/pnas.051611498
Panksepp, J., & Beatty, W. W. (1980). Social deprivation and play in rats. Behavioral and Neural Biology, 30(2), 197–206. https://doi.org/10.1016/s0163-1047(80)91077-8
Panksepp, J. (1981). The Ontogeny of Play in Rats, Developmental Psychobiology, 14(4). https://doi.org/10.1002/dev.420140405
Panksepp, J., Siviy, S., & Normansell, L. (1984). The psychobiology of play: Theoretical and methodological perspectives. Neuroscience and Biobehavioral Reviews, 8(4), 465–492. https://doi.org/10.1016/0149-7634(84)90005-8
Panksepp, J., & Burgdorf, J. (1999). Laughing Rats? Playful Tickling Arouses High-Frequency Ultrasonic Chirping in Young Rodents. In Hameroff, S., Chalmers, C., & Kazniak, A. (Eds.), Toward a Science of Consciousness III (pp. 231-244). MIT Press.
Panksepp, J., & Burgdorf, J. (2000). 50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables. Behavioral Brain Research, 115(1). https://doi.org/10.1016/S0166-4328(00)00238-2
Panksepp, J., & Burgdorf, J. (2003). “Laughing” rats and the evolutionary antecedents of human joy? Physiology & Behavior, 79(3). https://doi.org/10.1016/S0031-9384(03)00159-8
Pellegrini, A. D. (2002). Rough-and-tumble play from childhood through adolescence: Development and possible functions. In Smith, P. K., & Hart, C. H. (Eds.), Blackwell Handbook of Childhood Social Development (pp. 438-453). Blackwell Publishing.
Pellis, S. M., Pellis, V. C., & Foroud, A. (2005). Play Fighting: Aggression, Affiliation, and the Development of Nuanced Social Skills. In R. E. Tremblay, W. W. Hartup, & J. Archer (Eds.), Developmental Origins of Aggression (pp. 47–62). The Guilford Press.
Pellis, S. M., Pellis, V. C., & Reinhart, C. J. (2010). The Evolution of Social Play. In C. Worthman, P. Plotsky, D. Schechter, & C. Cummings (Eds.), Formative Experiences: The Interaction of Caregiving, Culture, and Developmental Psychobiology (pp. 404-431). Cambridge: Cambridge University Press.
Peterson, J. (2017, June 6). Lecture: Biblical Series III: God and the Hierarchy of Authority [Video]. YouTube.
Piaget, J. (1932). The Moral Judgment of the Child.
Pye, J. D., & Flinn, M. (1964). Equipment for detecting animal ultrasound. Ultrasonics, 2(1). https://doi.org/10.1016/0041-624X(64)90334-8
Sewell, G. (1967). Ultrasound in Adult Rodents. Nature 215(512). https://doi.org/10.1038/215512a0
Trawick-Smith, J. (2014). The physical play and motor development of young children: A review of literature and implications for practice. Center for Early Childhood Education, Eastern Connecticut State University.
Tickling Rats in the Name of Science
Great write-up! And I just wondered... Assuming that we first learn to appreciate moral norms in pre-verbal form, are we doomed to fail to capture them in verbal form? It seems to me that the conflict brewing over the past couple of centuries that could be seen as coming to a head between Marxist and Capitalist ideologies is precisely around the question of how to put into a propositional form that is well reasoned and understandable (and shared) the notion of "fair play", such that in a naturally competitive environment, the value of applying meritocratic principles is upheld, while at the same time the value of then not assuming the best (or natural!) outcome is for the winner to take it all *all the time*, is also integrated into the propositional rules. We have seemingly settled on a very crummy and poorly functioning version of central government deciding to use taxation as the implementation of "30% of the time" rule, by which some part of the earnings of the more capable are given to the less capable; but by formalizing it this way, and leaving the decision to a bureaucratic apparatus rather than the people (or rats) "at play", it creates an unsavory incentive structure that, as we can observe now, fosters a victimhood ideology or cult, in which the person with the strongest claim of persistent oppression gets to extract concessions from the community, at the risk of damaging the social fabric. Would love to read more on this topic!!