Hierarchical social modularity in gorillas

Modern human societies show hierarchical social modularity (HSM) in which lower-order social units like nuclear families are nested inside increasingly larger units. It has been argued that this HSM evolved independently and after the chimpanzee–human split due to greater recognition of, and bonding between, dispersed kin. We used network modularity analysis and hierarchical clustering to quantify community structure within two western lowland gorilla populations. In both communities, we detected two hierarchically nested tiers of social structure which have not been previously quantified. Both tiers map closely to human social tiers. Genetic data from one population suggested that, as in humans, social unit membership was kin structured. The sizes of gorilla social units also showed the kind of consistent scaling ratio between social tiers observed in humans, baboons, toothed whales, and elephants. These results indicate that the hierarchical social organization observed in humans may have evolved far earlier than previously asserted and may not be a product of the social brain evolution unique to the hominin lineage.

Should the paper be seen by a specialist statistical reviewer? Yes

Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. Yes
It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria.

Do you have any ethical concerns with this paper? No
Comments to the Author This manuscript empirically tests the idea that western gorillas, like humans, some other primates (e.g. hamadryas baboons, geladas) elephants, and whales, live in a hierarchically nested, sometimes-kin-structured social organization. It addresses a very interesting question about these understudied animals, and makes good use of the extensive bai data from multiple field sites.
The introduction to the paper is extremely well written. It is clear and concise and is a pleasure to read. I have only two comments/questions about the introduction. First, I am unclear on what the justification is for claiming that HSM is 'uniquely human' if we already know that it occurs in other primates, whales, and elephants. Is this claim perhaps outdated, written before we knew that it occurred in these other taxa? Please clarify. The answer to this is important, as I think it fundamentally affects whether this paper is best suited for Proceedings B, or some other journal. Second, I would suggest adding another sentence or two on western gorilla dispersal/social structure. For example, the term 'solitaries' comes up in the methods, but the authors did not clarify earlier that many adult males live alone. It doesn't need to be much, but for the very general readership of Proceedings B, it's best not to make any assumptions about what people do or do not know about gorillas.
Beyond the introduction, the paper needs extensive editing to make the methods and results intelligible to readers in a broad range of specialties. Relatively few Proceedings B readers will have much, if any, familiarity with the modeling framework the authors have adopted, and it is not currently explained in a manner that makes it accessible to anyone who is not already intimately familiar with the analysis methods. In general, the authors need to find a way to explain the statistical modeling in a way that tells us about the biological significance. E.g., what are bifurcation heights telling us? What about bifurcation differences? Are the SR and BP association indices simply complimentary ways of analyzing the data, or is there some specific purpose to using both? What are changes in gradients/knots telling us? Those are just a few examples. The way the statistical methods-and to some degree the results-are written, they are inaccessible to anyone who does not do exactly this sort of work themselves.
I think at least part of this could be remedied by including more basic description. While I sympathize with the authors' desire to be concise, what this functionally means is that I still don't have a clear idea of what the authors are proposing this hierarchical structure looks like. How big are these second-tier associations that the authors are newly describing? How closely related, on average, are the males who make up the members of a group at this tier? What might tie together the males who are members of a group at this tier for the Mbeli population, if not genetic kinship? It doesn't help much to say that these are equivalent to e.g. elephant clans or baboon gangs if we don't know what those are (and again, most people won't). The figures help only a little, if at all, since they seem to be missing captions, and a lot of the terms used in them are never really defined. As written, the biology has gotten lost underneath the numbers.
I would be happy to read a revised version of this article and offer more specific comments once I have a clearer idea of what was done and what was found. I could not agree with the authors more that an unquestioned assumption that chimpanzees are always the best model for understanding human evolution, based only on their genetic relatedness, is extremely problematic. We need more studies like these to clarify where humans do, and do not, overlap with the many interesting characteristics of great ape social behavior.

Recommendation
Accept with minor revision (please list in comments)

Scientific importance: Is the manuscript an original and important contribution to its field? Good
General interest: Is the paper of sufficient general interest? Good Quality of the paper: Is the overall quality of the paper suitable? Good

Is the length of the paper justified? Yes
Should the paper be seen by a specialist statistical reviewer? Yes Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. Yes It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria.

Comments to the Author
In this manuscript the authors take a network-based approach to classify the hierarchical structure of western lowland gorillas. Overall, the manuscript is well written and the analyses relatively clearly presented. The authors also take care to place their work in the broader literature, and fill an important gap in our knowledge of the evolution of hierarchical social modularity (HSM) in apes (and therefore humans). Given the sophisticated approach and thorough background/context, this should be of broad interest to the readers of Proceedings B. I only have a few comments for the authors that they should address in any resubmission: 1. Overall, the description of the network methods are short and filled with jargon. It would be useful for the authors to clearly state what each approach brings to the table and, without using jargon, describe what each method does. For instance, the different clustering approaches in lines 100-110 could be explained more clearly. What does the "cluster_resolution" algorithm do? What do "multiple peaks in modularity" mean for the social structure of gorillas? Some, but not all of this detail, is present in the supplementary methods.
2. In the main text, the authors should clarify what each node in their network represents. In the main text it is unclear of each node an individual silverback (or solitary male), or if each node in the network a "group" or "solitary". I had to go to the supplementary tables to figure out that it was "group" & "solitary". For instance, in lines 130-132 they mention "silverbacks" and not "groups". Which is it? Please be consistent.
3. Lines 121-124: There is too little detail in the main methods here about the kinship coefficients and models. These details are in the SI, but should be moved here. For instance, your binarization of "related" (r>0.2) vs. "unrelated (r<0.2), should be clearly explained in the main text. The relatedness analysis/results in . What is the predictor and what is the outcome variable? What is the "membership in the higher tier modules" and what about lower tiers?
4. Lines 149-152 (and elsewhere). I don't think that Kendall's rank correlation is an appropriate statistic here since the two vectors of association indices are not independent. A better approach would be to use a Mantel's test, which accounts for the interdependence of association indices in a matrix.
5. It would be useful to include comparisons to other primates with modular societies, like golden snub-nosed monkeys, geladas (the g3 level is similar to gelada "teams"), and uakaris.
6. This is probably due to the manuscript submission system, but there were no figure captions in the manuscript or online. This made it difficult to interpret the figures.

26-Apr-2019
Dear Ms Morrison: Your manuscript has now been peer reviewed and the reviews have been assessed by an Associate Editor. The reviewers' comments (not including confidential comments to the Editor) and the comments from the Associate Editor are included at the end of this email for your reference. As you will see, the reviewers and the Editors have raised some concerns with your manuscript and we would like to invite you to revise your manuscript to address them. In particular, please focus on clarifying your statistical approach, as outlined by both reviewers, and how your paper fits in to the broader literature on hierarchical social modularity across animal species.
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Best wishes, Prof Sarah F Brosnan Editor, Proceedings B mailto: proceedingsb@royalsociety.org Associate Editor Comments to Author: I have been fortunate to receive reviews of your article from two experts in the field. Both reviewers reported that your study was strong and should be of interest to a broad audienceideal for the readership of this journal. I agree. However, both reviewers also requested that you provide greater clarity in your description of your analytical approach and your interpretation of them. I agree that this will enhance your article and make it more accessible to a wide readership and that doing so will make it a stronger contribution to this journal.
Reviewer(s)' Comments to Author: Referee: 1 Comments to the Author(s) This manuscript empirically tests the idea that western gorillas, like humans, some other primates (e.g. hamadryas baboons, geladas) elephants, and whales, live in a hierarchically nested, sometimes-kin-structured social organization. It addresses a very interesting question about these understudied animals, and makes good use of the extensive bai data from multiple field sites.
The introduction to the paper is extremely well written. It is clear and concise and is a pleasure to read. I have only two comments/questions about the introduction. First, I am unclear on what the justification is for claiming that HSM is 'uniquely human' if we already know that it occurs in other primates, whales, and elephants. Is this claim perhaps outdated, written before we knew that it occurred in these other taxa? Please clarify. The answer to this is important, as I think it fundamentally affects whether this paper is best suited for Proceedings B, or some other journal. Second, I would suggest adding another sentence or two on western gorilla dispersal/social structure. For example, the term 'solitaries' comes up in the methods, but the authors did not clarify earlier that many adult males live alone. It doesn't need to be much, but for the very general readership of Proceedings B, it's best not to make any assumptions about what people do or do not know about gorillas.
Beyond the introduction, the paper needs extensive editing to make the methods and results intelligible to readers in a broad range of specialties. Relatively few Proceedings B readers will have much, if any, familiarity with the modeling framework the authors have adopted, and it is not currently explained in a manner that makes it accessible to anyone who is not already intimately familiar with the analysis methods. In general, the authors need to find a way to explain the statistical modeling in a way that tells us about the biological significance. E.g., what are bifurcation heights telling us? What about bifurcation differences? Are the SR and BP association indices simply complimentary ways of analyzing the data, or is there some specific purpose to using both? What are changes in gradients/knots telling us? Those are just a few examples. The way the statistical methods-and to some degree the results-are written, they are inaccessible to anyone who does not do exactly this sort of work themselves.
I think at least part of this could be remedied by including more basic description. While I sympathize with the authors' desire to be concise, what this functionally means is that I still don't have a clear idea of what the authors are proposing this hierarchical structure looks like. How big are these second-tier associations that the authors are newly describing? How closely related, on average, are the males who make up the members of a group at this tier? What might tie together the males who are members of a group at this tier for the Mbeli population, if not genetic kinship? It doesn't help much to say that these are equivalent to e.g. elephant clans or baboon gangs if we don't know what those are (and again, most people won't). The figures help only a little, if at all, since they seem to be missing captions, and a lot of the terms used in them are never really defined. As written, the biology has gotten lost underneath the numbers.
I would be happy to read a revised version of this article and offer more specific comments once I have a clearer idea of what was done and what was found. I could not agree with the authors more that an unquestioned assumption that chimpanzees are always the best model for understanding human evolution, based only on their genetic relatedness, is extremely problematic. We need more studies like these to clarify where humans do, and do not, overlap with the many interesting characteristics of great ape social behavior.
Referee: 2 Comments to the Author(s) In this manuscript the authors take a network-based approach to classify the hierarchical structure of western lowland gorillas. Overall, the manuscript is well written and the analyses relatively clearly presented. The authors also take care to place their work in the broader literature, and fill an important gap in our knowledge of the evolution of hierarchical social modularity (HSM) in apes (and therefore humans). Given the sophisticated approach and thorough background/context, this should be of broad interest to the readers of Proceedings B. I only have a few comments for the authors that they should address in any resubmission: 1. Overall, the description of the network methods are short and filled with jargon. It would be useful for the authors to clearly state what each approach brings to the table and, without using jargon, describe what each method does. For instance, the different clustering approaches in lines 100-110 could be explained more clearly. What does the "cluster_resolution" algorithm do? What do "multiple peaks in modularity" mean for the social structure of gorillas? Some, but not all of this detail, is present in the supplementary methods.
2. In the main text, the authors should clarify what each node in their network represents. In the main text it is unclear of each node an individual silverback (or solitary male), or if each node in the network a "group" or "solitary". I had to go to the supplementary tables to figure out that it was "group" & "solitary". For instance, in lines 130-132 they mention "silverbacks" and not "groups". Which is it? Please be consistent.
3. Lines 121-124: There is too little detail in the main methods here about the kinship coefficients and models. These details are in the SI, but should be moved here. For instance, your binarization of "related" (r>0.2) vs. "unrelated (r<0.2), should be clearly explained in the main text. The relatedness analysis/results in Lines 193-195 are unclear. What is the predictor and what is the outcome variable? What is the "membership in the higher tier modules" and what about lower tiers?
4. Lines 149-152 (and elsewhere). I don't think that Kendall's rank correlation is an appropriate statistic here since the two vectors of association indices are not independent. A better approach would be to use a Mantel's test, which accounts for the interdependence of association indices in a matrix.

Recommendation
Accept with minor revision (please list in comments)

General interest: Is the paper of sufficient general interest? Excellent
Quality of the paper: Is the overall quality of the paper suitable? Excellent

Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. Yes
It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria.

Comments to the Author
The authors have done a reasonably satisfactory job of addressing both my concerns, and the concerns of reviewer #2. Their clarifications do help, though I think it would be useful to move some of the information in the supplementary materials to the main text-I am thinking especially of Figure S2 and its accompanying information, which are an important part of the reason this paper is of broad interest.
If the authors are given another chance to revise, then I would also encourage them to work on clarifying their figure captions. These are still lacking much, if any, biology, and largely do not address the all-important question of what the figure means. Thanks to the clarifications in the text it is now easier to figure out, but a one-sentence description of the biology in each of their figure captions would go a long way. More specific axis titles might also help-e.g. distance from what? Probability of what? It should not be necessary to hunt through the text to figure this out.
As I mentioned in my comments to the editor during the first round of reviews, I believe that this paper should also be evaluated by someone with greater expertise in the statistical techniques the authors are using. If reviewer #2 has such expertise, wonderful, but I myself do not feel qualified to say with any authority that the clustering and network modularity analyses were handled appropriately.
That said, this is an interesting paper with important ramifications for our understanding of a phenomenon-HSM-that has received much attention in the cognition and evolution literatures. I thus feel that Proceedings B is an appropriate home for it, and would recommend acceptance.

Recommendation
Accept as is

General interest: Is the paper of sufficient general interest? Acceptable
Quality of the paper: Is the overall quality of the paper suitable? Good

Do you have any concerns about statistical analyses in this paper? If so, please specify them explicitly in your report. No
It is a condition of publication that authors make their supporting data, code and materials available -either as supplementary material or hosted in an external repository. Please rate, if applicable, the supporting data on the following criteria. Decision letter (RSPB-2019-0681.R1)

14-Jun-2019
Dear Ms Morrison I am pleased to inform you that your Review manuscript RSPB-2019-0681.R1 entitled "Hierarchical Social Modularity in Gorillas" has been accepted for publication in Proceedings B, pending some minor revisions. In particular, please consider moving the supplemental information Reviewer 1 highlights to the main body of your paper, clarify your Figure captions as per the reviewer's comments, and make your R code accessible. You can find the additional specific requests in the reviewers' and AE's comments below. Because the schedule for publication is very tight, it is a condition of publication that you submit the revised version of your manuscript within 7 days. If you do not think you will be able to meet this date please let me know immediately.
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If you wish to submit your data to Dryad (http://datadryad.org/) and have not already done so you can submit your data via this link http://datadryad.org/submit?journalID=RSPB&manu=RSPB-2019-0681.R1 which will take you to your unique entry in the Dryad repository.
If you have already submitted your data to dryad you can make any necessary revisions to your dataset by following the above link. Comments to the Author(s) some of the information in the supplementary materials to the main text-I am thinking especially of Figure S2 and its accompanying information, which are an important part of the reason this paper is of broad interest.
If the authors are given another chance to revise, then I would also encourage them to work on clarifying their figure captions. These are still lacking much, if any, biology, and largely do not address the all-important question of what the figure means. Thanks to the clarifications in the text it is now easier to figure out, but a one-sentence description of the biology in each of their figure captions would go a long way. More specific axis titles might also help-e.g. distance from what? Probability of what? It should not be necessary to hunt through the text to figure this out.
As I mentioned in my comments to the editor during the first round of reviews, I believe that this paper should also be evaluated by someone with greater expertise in the statistical techniques the authors are using. If reviewer #2 has such expertise, wonderful, but I myself do not feel qualified to say with any authority that the clustering and network modularity analyses were handled appropriately.
That said, this is an interesting paper with important ramifications for our understanding of a phenomenon-HSM-that has received much attention in the cognition and evolution literatures. I thus feel that Proceedings B is an appropriate home for it, and would recommend acceptance.

20-Jun-2019
Dear Ms Morrison I am pleased to inform you that your manuscript entitled "Hierarchical Social Modularity in Gorillas" has been accepted for publication in Proceedings B.
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Thank you for your fine contribution. On behalf of the Editors of the Proceedings B, we look forward to your continued contributions to the Journal.

Associate Editor
Comments to Author: I have been fortunate to receive reviews of your article from two experts in the field. Both reviewers reported that your study was strong and should be of interest to a broad audience -ideal for the readership of this journal. I agree. However, both reviewers also requested that you provide greater clarity in your description of your analytical approach and your interpretation of them. I agree that this will enhance your article and make it more accessible to a wide readership and that doing so will make it a stronger contribution to this journal. Extensive additions to the methods section to make them more accessible, and clarification within the results section

Referee: 1
Comments to the Author(s) This manuscript empirically tests the idea that western gorillas, like humans, some other primates (e.g. hamadryas baboons, geladas) elephants, and whales, live in a hierarchically nested, sometimes-kin-structured social organization. It addresses a very interesting question about these understudied animals, and makes good use of the extensive bai data from multiple field sites.
The introduction to the paper is extremely well written. It is clear and concise and is a pleasure to read. I have only two comments/questions about the introduction. First, I am unclear on what the justification is for claiming that HSM is 'uniquely human' if we already know that it occurs in other primates, whales, and elephants. Is this claim perhaps outdated, written before we knew that it occurred in these other taxa? Please clarify. The answer to this is important, as I think it fundamentally affects whether this paper is best suited for Proceedings B, or some other journal. Hierarchical social modularity is not unique to humans, but rather what has been suggested in the past is that the form of hierarchical social modularity observed in humans is qualitatively different to that in other species. Humans show a greater number of levels of social structure and cooperation between large numbers of unrelated individuals. This was thought to be evolutionarily distinct from HSM in other species, as no other apes showed the behaviour, suggesting it evolved separately and was thought to have been enabled by brain expansion during hominin evolution. In the sentence: "An extreme version of the social brain narrative is that human HSM is a unique product of hominin brain evolution" We mean to communicate how human HSM was thought to be unique from HSM in other species, but can see how that can easily be misinterpreted, so have removed the word unique and added "distinct from HSM observed in other primates" (line 41-42) Second, I would suggest adding another sentence or two on western gorilla dispersal/social structure. For example, the term 'solitaries' comes up in the methods, but the authors did not clarify earlier that many adult males live alone. It doesn't need to be much, but for the very general readership of Proceedings B, it's best not to make any assumptions about what people do or do not know about gorillas.

Extra information on gorilla social structure and dispersion added in lines 55-58
Beyond the introduction, the paper needs extensive editing to make the methods and results intelligible to readers in a broad range of specialties. Relatively few Proceedings B readers