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Additional Editor Comments:

Thank you for submitting your manuscript to PLOS One! I apologize once again for the delay. I have now secured reviews from two qualified animal behaviorists. They both cite important theoretical and methodological concerns with your manuscript that should be fully addressed before your next submission. Below, you will find a copy of their comments.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Partly

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: Yes

Reviewer #2: I Don't Know

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3. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy

Reviewer #1: Yes

Reviewer #2: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: General

My assessment of the current paper is mainly focused on some aspects of the statistical analysis of the experimental protocol designed by the authors .

The authors investigated the combined effect of species (humans/ not-human primates) and task specificity in facial expressions (neutral, negative, positive), on conscious/ unconscious mimicry . and used minimally invasive protocols for data collection.

Standardized stimuli (2D videos of human facial expressions) and computerized data analysis (facial mimicry) showed a close connection between the emotional behavior of non human primates and that of humans. Even if the reported data and the relevant analysis and results are not totally new, further investigations may open new perspectives on the role of empathy inside evolution. Dynamic interactions between the varying facial expressions might also help communication inside and outside species, with possible interaction in the fields of human rehabilitation, as already done in various therapies with other mammals (horses, dogs..).

Statistics

I have some concern about the number of significant differences in the paper, a general effect of false positive inflation, with subsequent small sample sizes, potentially contributing to the reduced replication rate. Indeed, this topic is of current concern in several research contexts. For example, please see a recent meta analysis by Murphy et al, , Estimating the Replicability of Sports and Exercise Science Research. Sports Med. 2025 Oct;55(10):2659-2679. doi: 10.1007/s40279-025-02201-

Of course I do recognize that authors used several correction method estimators (post hoc tests, effect size, Cohen coefficients ...). But this is non-sufficient considering the limited number of landmarks/ variables used in the study, and the large numbers for statistical tests. From a mathematical point of view, an increase of statistical tests also increments the number of statistically significant values (p<0.05) but they are false positives. Which is the biological significance or practical/ clinical meaning of the current extra p values?

Reviewer #2: The paper presents interesting findings on cross-species mimicry of facial expressions from humans viewing primate stimuli; the paper generally is easy to read, and the findings are novel and interesting for the field, especially for comparative affective science. The paper requires revision, however, and adding more nuance. I made various comments on how the paper can be improved. These mainly refer to more careful reflections and assumptions, better embedding with the literature and more information in terms of the choice of statistics and descriptive stats, see below.

Before delving into each part of the paper, I would like to make a general remark, which concern the whole paper – and I believe we should all be mindful of as researchers especially when drawing assumptions of affective states in nonlinguistic species: What actually is an emotion expression in animals? Are all facial expressions in animals emotional in nature, and how do we know the expression we see is linked to the assumed affective state? Without physiology markers, what makes us sure that the expressions presented here do not comprise voluntary facial movements used for coordination (Waller et al. 2017, Neuroscience & Biobehavioral Reviews; Heesen et al., 2022 Primate Cognitive Studies)? We know certain primates have some voluntary power over their expressions (e.g., Waller et al., 2015 and several other papers). I think the field generally, including this novel paper, should apply more care in the way we phrase our work and how we use terms. Here, the authors might consider avoiding terms similar to “emotional mimicry” or “emotion expressions”, since they did not use physiological measures to support the analyses, neither for apes or humans.

Abstract

-There is a typo in the last phrase, I assume it could read as: …”suggesting that humans are able to emphasize and mirror emotions of other species.”

-I wondered: since empathy contains several “building blocks”, the term itself might not be adequate as you only looked at mimicry - which is a necessary part of it though not sufficient. A simple adjustment in phrasing is sufficient here.

Introduction

-“more egalitarian macaque species” – who are you comparing them to, to other macaque species? Please specify for clarity.

-P.4: Whether or not humans can classify primate expressions and how well they perform may be dependent on many more aspects than listed here; some studies using attention measures show they can (Kret & van Berlo, 2021) though it depends on age (children struggle at it); others using a priming design showed that humans do not pay selective attention to congruent facial expression matching a former affective scene, suggesting difficulty in cross-species perception also in adults (Heesen et al., 2024, PNAS Nexus); it may be helpful for readers if the authors better embed their current work with already existing literature on cross-species perception.

- P.4: I have a remark regarding the following phrase: “and hence an ability to categorize expressions into positive versus negative is plausible.” In primate behaviour, not every context may be as clear cut and thus separable on such a dichotomous scale – for example, contact-playing in primates or sexual contact in bonobos can be positive and negative at once (being close to a dominant and potentially aggressive other, while experiencing an arousing, affiliative contact); at the same time, mobbing group members or predators could combine positive and negative states at once (securing ones dominance status, dominating others, potentially being attacked by another). Such a dichomotous valence scale might be even difficult to apply to humans, because most interaction contexts are a messy pool of feelings with pleasant and unpleasant aspects – I would recommend caution in strong categorizations into positive and negative, especially when comparing across species. The authors may want to acknowledge that the phenomenon of valence is not as clear-cut as we’d like it to be, especially for animal researchers. Rather than calling expressions positive or negative, the field may benefit from more nuance moving forward, e.g., focusing on the functionality, or context in which expressions occur, rather than presuming whether the experience a good or bad state.

-P.5: Can the authors state more about which AUs they considered active in the threat related display of the primates? The display they show in the Fig. 1 looks like a mix between scream and bared teeth face, rather than open mouth threat face, see Figure 2 Parr et al 2007 (between bared teeth and scream face) and see Fig 1 c Waller et al 2016. Can the authors cite a paper that shows such a threat face expression as indicated in their Figure? Typically, threat face involves an open mouth, though teeth are not necessarily visible (see Fig 1 a and b, Overduin-de Vries et al 2016). If the expressions in the negative category were bared-teeth faces, then this needs to be discussed, because it would rather fit the “fear” than the “anger” category and it is interesting that people still attributed anger more so than fear (Fig 2, results).

Methods

-P.9: Did the authors instruct people not to talk, or have they verified this? They may have talked about the videos or said things while viewing, such that mouth movements could be affected by that – It would be good to verify.

-In the videos of the primates, were there any bystanders or objects visible? This should be mentioned somewhere.

-P. 9: The sentence was cut at the end of page.

-Could the others provide more information on what the test trial with the panthoot included?

-Did the authors ask participants whether they had any experience in watching apes, have they seen or observed them before? This is important as, like the authors mentioned, watching such videos could be affected by former training.

-Could the authors provide more information about the statistics chosen, models fitted, assumptions met within their LMMs? Use of software?

Results

-As indicated above, a summary statistics would be helpful for all ratings.

-Since the authors, in their discussion, generalize their findings across primate species, it would be beneficial to include a graph for each primate species here, such that the reader can get an insight whether the pattern holds equally across species, and across apes and monkeys in particular.

Discussion

-P.16 “That participants modulate positive mimicry as a function of the affiliation

and closeness they feel towards the expresser suggests that people may in some ways

be more “careful” when sending an affiliative signal, a smile, in response to a play-face,

than when sending an essentially antagonistic signal, a frown, in response to a threat

display.” – An alternative explanation could be that people feel more close, and therefore express more affiliative expressions. What do the authors mean exactly by “careful” and how is this explanation justified, i.e., do you have papers that support this idea, and what would it tell us if humans are more “careful” in affiliative contexts? In some way, one would expect the opposite: one should be most careful in negative contexts because those contexts bear danger and potential injuries. Also, why would you expect positive “overtures” (what is this?) to be more costly than coordinating in a potentially life-threatening conflict/fight? I would recommend adding some theoretical background and embedding the claims/conclusions/assumptions into the literature.

-At the end of the discussion, there is a random sentence on page 18 starting mid way, does it belong to the methods?

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Reviewer #1: No

Reviewer #2: No

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<p>Evolutionary Echoes of Emotion: Humans Mimic Other Primate Expressions

PONE-D-25-31307R1

Dear Dr. Hess,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication. Thank you for thoroughly addressing the reviewers' comments and concerns. Your article will be formally accepted for publication once it meets all outstanding technical requirements.

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Kind regards,

Brittany N. Florkiewicz, Ph.D.

Academic Editor

PLOS One

Additional Editor Comments (optional):

Thank you for diligently addressing all of the reviewers' comments and concerns! After reviewing your revisions, I believe you have sufficiently addressed everything. However, there is one minor issue that can be resolved during the proofing stage: the quality of the images of the example NHP facial signals is relatively low on our end. Please ensure that these images are of the highest quality when you proofread your article.