Play is rich and fascinating; it is also strange and puzzling. It is playing all kinds of tricks with seriously-minded thinkers and researchers. Play is easy to recognize in children below one year of age, yet professors at the zenith of their play-research career are struggling to work out a simple and useful definition of it. It leaves psychologists, anthropologists and philosophers bewildered by the labyrinth of forms among humans of all ages and cultures, luring them into digging deep into one aspect while so many others evade them. And biologists fare no better. Here is their paradox: evolutionary biologists, who, by definition, investigate the adaptive functions of behavior, define play behavior as “having no obvious function”.

With all the diversity in form and variation in function, what keeps “play” connected? The puzzle is made even more challenging by the fact that the variety of play really expands in two realms that are only connected through a rather narrow isthmus. For animal ethologists, the richness of play resides in the plethora of forms across animal species, with us humans being just one case among thousands of others. And for human psychologists, the play variability extends across the variety of child, youth, adolescent and adult forms of gamboling, games and grotesques, reaching into sports, arts, rituals and politics. Of these play forms, there are just a few that we share with other species – rough-and-tumble, for instance, or juvenile object play.

One dimension of variability that is common both for mammalian play in general and specifically for human play, is the continuum between solitary and social activities. In between the cases of purely solitary playing (such as a lone weasel gamboling in fresh snow) and elaborated social play (such as structured rough-and-tumble play bouts described in the article by Heesen and colleagues, there are various degrees of play (non-)coordination in groups of mammals. Thus, young calves or foals engage in non-contact, yet parallel playful scampering; piglet play stretches from solitary runs-jumps-pivots through isolated and fleeting head-knock contacts to committed pair-wise shoveling duels.

Social play has attracted far more research interest than solitary play – understandably so, as we humans are an extremely social species. One particularly vivid research field focuses on highly structured and rule-based forms of play, and seeks to uncover in it evidence for high cognitive and communicative abilities, either in non-human animals or in very young children. Thus, Pellis and Pellis devote their article to a look at how the balance between competition and cooperation is achieved and maintained during play fighting bouts by various rodent and pig species. The various species employ different strategies to strike the competition-cooperation balance: restraint in executing the competitive tactics, not taking advantage after a competitive success, or honoring signals of submission. Notwithstanding these differences in how it is achieved, the competition-cooperation dynamics make it necessary for the animal to monitor continuously both their own actions and those of their partner.

Heesen and colleagues make the case that animal rough-and-tumble play requires a very high level of coordination because the play partners need to agree on opening, maintaining, and closing a play bout. In a brief comparative overview, the authors review evidence that while managing cooperatively the play bouts, various mammalian and bird species employ cognitive and communicative means (e.g., mutual responsiveness and role reversal) that belong to the building blocks of the human “shared intentionality” as it was defined by Tomasello and Moll in 2010.

Thus, structured animal social play may be a window through which we can reveal how the specific human “cognition for interaction” came into existence. Earlier, Bekoff suggested that social play requires the play partners to behave “fairly”, i.e. not to take too much advantage at the expense of the other, which could be one of the building blocks for the evolution of morality.

The articles in the special issue of Learning & Behavior dedicated to The Evolutionary and Psychological Significance of Play reflect a general trend in comparative play research: complex forms of social play get most research and theoretical attention. Researchers hope that the study of the highly elaborated play cases will advance our understanding of social cognition, communication, emotional coordination or even morality. This “top-down” approach (no derogation here) begins with some very complex play skills and then looks for their evolutionary and ontogenetic roots.

Let us here take, for a while, the opposite bottom-up approach (as for example summarized by de Waal and Ferrari) starting with the simplest form of play – the solitary locomotor-rotational play, such as a young monkey leaping on and bouncing off a substrate where it cannot hold or a calf kicking its hind feet sidewise during a boisterous gambol,  thus rotating its body asymmetrically along the spine. Describing limb flexions and torso rotations might seem a boring task. In reality, watching animals as they jump and scamper, frolic and gambol, slide and swing, roll and rotate is most amusing, and, at the scientific level, taking cognition as an embodied phenomenon is gaining currency.

The surprising fact is that there is a fascinating similarity between the space-time structure of locomotor play, the communicative dynamics of bodily social play, and the mental engagement in human cognitive and verbal games. In all these realms of playing, there is the same pattern of three crucial aspects. Firstly, the animal puts itself temporarily in an “as-if” play Umwelt that somehow mirrors the “serious” Umwelt but is free of the pragmatic evolutionary-fitness load. The second aspect is that play is performed as a series of repetitive actions limited to a small subsection in the vast multidimensional space offered by the as-if world. And finally, the repetitive stream of playing abounds in unpredictable variations that are invited through deliberate lack of control by the playing animal(s), as I noted with colleagues some time ago.

Here are some examples:

• Locomotor play: a weasel in Europe emerges from a hole to find snow cover for the first time in life. The unreal substance incites her to frolic wildly, jumping, pivoting and twisting her long body in all kinds of wriggling shapes. No dignity, no elegance, no efficiency, no care.
• Social play: two young Norway rats play fight, attacking and counterattacking in a 3D-melée that includes many probabilistic role reversals and a lot of excited 50-kHz squeals. A successful attack may result in the winner pinning the other to the ground with its forepaws. But once the play partner is pinned down, the on-top animal does a maneuver that gives up the hard-won control – it switches to standing on its supine partner with all four of its paws, thus making its position vulnerable to an easy counterattack.
• And finally, cognitive play: U.S. parents gang up with a somewhat unruly and rather unpredictable breed of ‘elves-on-a-shelf’ to stage ongoing night-to-night series of pre-Christmas performances for their kids. Yesterday’s post by Cindy Clark explained this in detail.

Do you see the pattern? I think it is quite obvious, but that begs the next question – where does the pattern come from? How come play is so deeply similar across the various mammalian species and across the bodily, social and mental realms?

My guess (nothing more at the moment) is that what is working behind the scenes is a phylogenetically old mammalian capacity which I hereby call the ‘play engine’. This label deliberately refers to Stephen Levinson’s influential concept of the ‘interaction engine.’ The interaction engine, according to Levinson, is an ensemble of cognitive skills and motivational predispositions that enable us humans to get into, maintain, and get out of a devoted communication space.

Similarly, the putative play engine is an assemblage of the cognitive ability to enter and leave the as-if Umwelt, the motivational proclivity to visit this world with gusto and the emotional capacity for the fun emotion that enjoys the rollercoaster-like switches between loss and regain of control about what is happening, as shown in the work of Panksepp, me and colleagues, and Trezza and colleagues.

Interestingly, there is also an ontogenetic parallelism between the two engines: pre-verbal human infants master the interaction engine as effortlessly as very young mammals handle the as-if world of play. But then there is a difference: the interaction engine was seemingly assembled into a saleable product just recently in the Homo sapiens lineage. In contrast, the play engine is an old mammalian invention that had originally been utilized for bodily gamboling in 3D space, before being drawn into the even richer hyperspace of social tumbling and finally exploding in the meta-space of human culture and language.

The capacity to enter, roam and leave the playful mode of existence is widespread, if not universal, among mammalian species. At the same time, the as-if Umwelts of various species are vastly diverse. Even closely related species often contrast strikingly in their way of playing. For instance, rats are avid play-fighters while mice are devoted solitary pouncers. The extreme variety in the semblance of play (and, as far as we can say, in its ultimate functions) indicates that while the core engine remains conserved, there has been a wide diversification in the mammalian phylogeny of the adaptive machines driven by it.

Often the play engine has been coupled with other capacities, such as with exploration in object play, or yoked for a new purpose, such as to mitigate aggression among adults. In humans, the derivative forms of play are manifold, including, among others, the rough-and-tumble, role playing, pretend play, symbolic play, imaginative play and strategic games discussed in the special issue.

What I propose is that in each of those fancy gadgets, there is the old good play motor that drives it. And what I argue is that besides investigating all the sophisticated functions of the products, it is important to also understand the workhorse that drives them. Psychologists are more and more aware of the central role of play in human development and educationists are concerned about the shrinking physical and social space for free child play. And yet, the physical locomotor-rotation play is primarily valued just for its anti-obesity and physical fitness function. That is important, for sure, but it is not the whole story. The engine in a machine is not just for gas-guzzling. We need to understand the engine and take proper care of it. What use will be a machine if the driver chokes the engine by putting too much gas on it? And is the engine maintained properly so that it will kick on reliably even in bitter frost conditions?

On the submission deadline day, I am returning to the text, with the intent to write a nice coda. I enter the para-real world of competing and collaborating play theories. Here I see the new beast: the idea that all the diverse and advanced forms of mammalian and human play are highly transfigured clones of the phylogenetically ancient bodily gamboling. I pounce on it, trying to seize with both arms. It kicks with several of its protean buts, escapes and grimaces from an inviting distance. It is great fun, anyway. I will keep trying to pin it down, for a while.