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However, if natural selection has preserved two expressions, probably their adaptive value differs.<\/jats:p><\/jats:sec><jats:sec><jats:title>Materials and Methods<\/jats:title><jats:p>We collected video data on play interactions in two lowland gorilla groups (<jats:italic>N<\/jats:italic>\u2009=\u200921; <jats:styled-content style=\"fixed-case\"><jats:italic>Gorilla gorilla gorilla<\/jats:italic><\/jats:styled-content>) housed at La Vall\u00e9e des Singes and the ZooParc de Beauval (France). Lacking a tool tailored for gorillas during this study, we analyzed facial action\u2010unit activation via chimpFACS and OpenFace.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>We found that PF and FPF activated partly different action units as it occurs for chimpanzees and humans' PF\/FPF. We detected the rapid replication (Rapid Facial Mimicry [RFM]) of either PF or FPF that was associated with longer play sessions. Not\u2010mimicked PF was linked to increased play session variability (different types of play patterns) measured via the Shannon Index, whereas not\u2010mimicked FPF was associated with increased play asymmetry (imbalance between offensive\/defensive patterns) measured via the Play Asymmetry Index.<\/jats:p><\/jats:sec><jats:sec><jats:title>Discussion<\/jats:title><jats:p>Lowland gorillas may use PF to manage sessions that are more complex in terms of pattern types and FPF\u2014a more salient signal\u2014to prevent misunderstandings when the session is imbalanced. RFM of both expressions may favor the prolongation of play sessions by increasing player synchronization and possibly emotional sharing. Our study opens the door to further comparative studies on playful expressions in humans and other primates as a way to fine\u2010tune possible emotional communication and delineate potential evolutionary roots of Hominidae facial communication.<\/jats:p><\/jats:sec>","DOI":"10.1002\/ajpa.70061","type":"journal-article","created":{"date-parts":[[2025,5,8]],"date-time":"2025-05-08T07:05:00Z","timestamp":1746687900000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Functional and Morphological Differences in the Play Face and Full Play Face in Lowland Gorillas, a Hominid Species: Implications for the Evolutionary Roots of Smile and Laugh Face"],"prefix":"10.1002","volume":"187","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-7093-0025","authenticated-orcid":false,"given":"Giada","family":"Cordoni","sequence":"first","affiliation":[{"name":"Department of Life Sciences and System Biology University of Torino  Turin 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