Review of RSOS-240632

In this intriguing study of communicative correlates of object use in social interactions among infant and juvenile chimpanzees, the authors examined whether the artificiality of such objects influenced (a) facial expressions, (b) vocalisations, and (c) head orientations of signalers towards their social partners. They report a number of associations, including some age-based interactions, on these variables. I think this study is a creative and interesting way of looking at the available data, but I had a number of unanswered questions after reading the paper. In particular, I think the authors could have presented the rationale more clearly, and there were aspects of the coding scheme that I found difficult to understand. There were also some minor typographical or expressive matters that I list below. On balance, though, assuming the questions raised, below, can be adequately addressed, this paper has the potential to significantly add to our understanding of the impact of object features on communicative behaviours in chimpanzees, and I agree with the authors that the pattern of findings opens up important new conceptual pathways for considering ecological/contextual effects on communicative behaviour in a good-sized sample of humans’ nearest living relatives.

1. The introduction is full of sentences like the following: ‘. . . the use of artificial objects, such as certain types of toys, is known to increase the production of vocalisations and facial displays. . . ‘ (Lines 57-58). Increased, relative to what? Absent a baseline, the literature summary is uninterpretable, without the reader consulting the cited literature. Again, on Lines 65-66: ‘. . . object properties impacted child’s [sic] behaviour . . . .’—which object properties? Which behaviours were impacted? How were they impacted? There is a paragraph (Lines 99-114) that is almost completely uninterpretable for lack of clear description of the cited sources and their relevance to the present study. Thus, for this reason, I found the introductory material to be poorly presented, which is surprising, given the seniority and outstanding expertise of some of the co-authors. In my opinion, under no circumstances should the reader of Paper A feel compelled to have to consult Papers B through n just to understand the rationale for the study described in Paper A.
2. How can chimpanzees not have a facial expression? The authors’ approach to coding facial expressions implies that chimpanzees can lack facial expressions, but how can that even be? Is a relaxed, neutral facial expression, not a facial expression? I feel like I am missing some key information about how the authors managed to define the absence of facial expressions, which seems so crucial to the analyses. It is obvious that vocalisations (or calls, more generally) can be absent, but it is not obvious (to me, anyway) how facial expressions can be absent, and because one of the headline findings seemingly depends on facial expressions being absent, I think the authors should exhaustively provide descriptions of all facial expression types included in the analyses, along with a clear description of what a lack of a facial expression looks like.

Minor Issues
3. Why use one-tailed tests in a study with a novel design (Lines 397-409)? As far as I am aware, the novelty of these analyses is part of the rationale for the study, therefore, on what body of findings were directional hypotheses justified? Surely, it cannot be that previous studies of Western children in laboratories or institutionalised great apes could be used to predict the outcome of the present analyses, can it? Even if the authors disagree with this, on what basis is it justified to ignore significant results in the tail opposite to the research hypothesis? In the three instances of one-tailed analyses reported in this section, all of them involved comparisons of natural with artificial objects. To the extent that the present study is novel is the degree to which the use of one-tailed tests seems to me to be inappropriate. Therefore, I think a more detailed justification of the use of one-tailed tests is warranted. With the same research design, I would have gone with two-tailed tests; from the low probabilities, no conclusions would have changed.
4. Lines 69-71: please re-visit the phrasing of this sentence; I found it awkward.
5. Line 112: errant comma between ‘that’ and ‘material’.
6. Line 141: ‘differ from those’ is more formal.
7. Lines180-182: something is missing in this sentence—maybe the subspecific designation for P. t. troglodytes?
8. Line 194: should be ‘fixtures’.
9. Line 204: Altmann (1974) referred to the technique as ‘focal-animal sampling’ not animal focal sampling.
10. Line 206: ‘focal’ is an adjective: focal animal or focal subject.
11. Line 243 and elsewhere: ‘s/s’ is not a standard definition. I think the authors mean to indicate ‘seconds’ but I cannot find any use of s/s to indicate seconds with a quick internet search. The standard SI unit for seconds is either ‘s’ or ‘sec’, according to my quick search.
12. Line 265: ‘in accordance with’, not *’in accordance to’.
13. Line 271: degree symbol misplaced to centre of character field, rather than top.
14. Line 508: ‘a higher state’ or ‘higher states’?

Additional Considerations
15. Why were gestures not considered? Object-mediated or object-accompanying gestures seem to me to be a fertile area for discovery.
16. There is a substantial literature on protolanguage (e.g., Arbib, Corballis, others) that the authors mention very briefly in their conclusion; I thought that this section could have been expanded quite a bit, really, insofar as the present findings could address some of the assumptions these authors have made about the capabilities of the last common ancestor of humans and the other great apes, and therefore the evolutionary timing of the onset of ‘proto-languages’.
17. Why weren’t vocalisation and facial expression types analysed by type across natural and artificial objects?

In conclusion, none of the questions or points that I raise seem especially difficult to address, although I do think they should be addressed. No new data need be collected, and no new analyses need be conducted, unless the authors want to pursue gestures (Point 15) or the fine-grained analyses of vocalisation and facial expression types that I mention in my final point (No. 17). Therefore, I am recommending a minor revision.