How does the diversity of play across species complicate identifying common characteristics, and which comparative measures could support valid cross-taxa comparison?

Why does play diversity make common characteristics hard to pin down?

Play takes many forms, and the same label can cover very different actions. In rats, social play involves competing to pin a partner and attack its neck, while in hamsters it starts as head attacks in early puberty and later shifts to the flanks [3]. These are both called social play, but the movements and targets differ, so a simple behavioral checklist would not capture what is shared. Similarly, playful expressions in primates and carnivores involve open-mouth faces, but the specific muscle activations vary, and only detailed coding revealed that carnivore expressions are likely homologues of primate ones [5]. Without such coding, the commonality would be missed.

This diversity means that a characteristic that looks universal—like rough-and-tumble play—may vary in its expression across species and cultures [1]. The challenge is to distinguish superficial differences from deeper commonalities, which requires measures that can see past the surface.

Which comparative measures can support valid cross-taxa comparison?

One promising approach is to compare the neurobiological mechanisms underlying play. In rats and hamsters, play deprivation alters dendritic morphology in mPFC neurons and impairs flexible, context-dependent behavior in adulthood, suggesting the animals may have converged on a similar function for social play [3]. By measuring brain changes and behavioral outcomes, researchers can test whether play serves a common function even when the observable actions differ. This kind of comparison moves beyond description to test shared mechanisms.

Another measure is developmental timing and function. The hamster play transition from juvenile to adult aggression corresponds with engagement of neural ensembles controlling aggression [3], showing that play can be a stage in a developmental sequence. Comparing such sequences across species can reveal whether play serves similar roles in development. For vocal babbling, researchers have proposed a unifying definition to facilitate cross-taxa comparison [4], and a similar effort may be needed for play: a definition that focuses on function or context rather than specific movements.

What is still uncertain about comparing play across species?

It is not yet clear which measures are most valid across all taxa. Some studies focus on sex differences in rough-and-tumble play across species and cultures [1], while others examine personality dimensions that include play [2], but these approaches use different frameworks. The babbling paper proposes a unifying definition for vocalization [4], but whether a similar definition can capture all play is an open question. Moreover, playful expressions may have a polyphyletic history—arising independently in different lineages [5]—which complicates the search for a single common characteristic. The most robust comparisons will likely combine multiple measures: behavior, neurobiology, development, and social function, interpreted with caution about what each can tell us.

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