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Thank you for returning the manuscript promptly. It looks good to go.
Thank you for suitably addressing the reviewers' concerns in your revised manuscript. You paper is essentially accepted, however I have attached an annotated pdf with a number of edits and suggestions for typos, grammar and clarity. Note that I edited your manuscript across two computers so that those attributed to "Reviewer" are also mine. Address these and double check the manuscript closely. I look forward to your returned manuscript.
I have received three reviews that range from Minor to Major Revisions. I disagree with Reviewer 3 who questions the appropriateness of this study for PeerJ, and thus look forward to your revisions in response to the reviewers. I have marked this manuscript as Major Revisions in accordance with the majority opinion, and provide you with the opportunity to revise and respond to the specific and general critiques of the reviewers. I encourage you to take the reviews as an indication of the reception of the paper.
Reviewer 1 provides an number of targeted questions regarding the statistical analysis.
Reviewer 2 has important questions regarding the overall scope of the implications for evolutionary issues and potentially additional studies that may be relevant in the discussion.
Reviewer 2 & 3 have suggestions for additional data or analyses that could have been conducted. Please revise or address why these data were not included and potential implications.
Again, I look forward to receiving you revised manuscript.
Best,
Phil
This is a very well-written, and concise, manuscript. I have no critiques regarding basic reporting.
I have a couple of minor quibbles regarding experimental design. First, as the authors state in the discussion, the chain leading from limb length to stride length, stride frequency, and ultimately COT has more to do with effective limb length (and hence posture) than with morphological limb length per se. I understand that the Treadscan system doesn't provide joint kinematics, and approve of how the authors handled this potential confound in their discussion. However, I think the explanation would be stronger if they provided a figure illustrating their "triangle" argument about the determinants of stride length. I know these kind of figures abound in the literature (I'm thinking of Fig 7 Reynolds's 1987 paper, "Stride length and its determinants in humans, early hominids, primates, and mammals"), but putting one in here nonetheless would make things clearer to the reader.
Second, I was curious why the authors didn't test for covariate-by-factor interactions in their GLM analysis (i.e., line by body mass, or speed by body mass, or even line by speed by body mass). In other words, does the relationship between the gait variables and body mass differ between lines, or between speeds, or between lines at different speeds? If these covariate interactions are significant, it would preclude from performing simple tests of the main effects.
Finally - really minor - how was stride length calculated if the animals were on a treadmill and, presumably, not exhibiting any net forward movement of the center of mass? Was it calculated from belt speed and stride duration? Please elaborate.
All of the results appear valid and the interpretations sound. Aside from my minor point about checking for covariate interactions (see above) I see nothing the authors could fix.
The article fails to cite basic key texts on the relationship between lower limb length and locomotion, as it pertains to evolution. In fact, the authors state that within- population studies on differences in limb length have not been successful in teasing out how selection pressures might work, yet multiple papers have been successful, namely work by Patricia Kramer and Karen Steudel-Numbers (who control for mass and vary limb lengths), as well important experimental work by Ted Garland (who also has specific lines of mice with changes in morphology (as well as physiology)). The failure to even cite these papers, and to suggest that important work on infraspecific variation on locomotion hasn't been done is misleading to potential readers of this paper.
The relevance of this experiment seems to be located in the idea that changes in limb length have the potential to change fitness; in fact the authors argue that this is what gives this heavily controlled sample an advantage over other samples of convenience, yet there are no tests here of reproduction or fitness so it seems as if the experiment is incomplete.
It is not clear why the authors failed to weigh the limbs after sacrificing the animals. If the limbs are different lengths and total body mass is the same, doesn’t this mean that the limbs are either different masses or different shapes, which would then suggest that they have different inertial properties? Given the variation in swing phases, knowing more about the shape of the limbs might aid the authors in uncovering what is happening throughout the gait cycle.
It is also not clear why the authors scanned the limbs instead of just dissecting them to measure the bones directly.
Given that speed was constant, and stride frequency is speed divided by stride length, why does stride length increase by 10% but stride frequency decrease by 7%?
It doesn't seem as if this part of the larger set of experiments really does what the introduction and conclusion hopes it will. Either the context needs to be narrowed to exactly what you think can be solved with kinematics, or you need to loop this part of the experiment in with the physiology and breeding studies to get at some of the larger issues at play.
Reported beautifully. Well written paper
Great. Perfect use of controls and experimental group. What we all strive for in locomotor studies. Greater range of locomotor variables encouraged.
Great. Exactly what I expected from the title.
Gait changes in a line of mice artificially selected for longer limbs
I thank the editor for the opportunity to review for PeerJ. This was such an enjoyable paper to read, and I am glad this work will soon enter the scientific literature. Overall, I have very few comments. I think this work is creative, well-done, and adds a great deal to how we think about morphological variables and locomotor performance. That being said, I found the article to be somewhat uninteresting. The authors only collected the bare minimum of locomotor gait variables, where a more complete suite was certainly possible and I think should be encouraged.
The authors describe only spatiotemporal gait variables, and of them only three were significant between mice stains, and of those three two of those variables (stride duration and stride frequency) are directly related to each other. The authors go out of their way to explain why kinematic variables could not be collected, but as a reviewer I would want the authors to at least try and present this data and then explain to the readers the limitations of the kinematic analysis rather than not providing data at all. The explanation for how these mice deal with these strange proportions would be incredibly valuable to our understanding of the evolution of increased limb length. Perhaps, the inclusion of gait sequence data would be more interesting. Do the presence of Lateral sequence lateral couplet increase with limb length? This would be very interesting. Related to this accommodation question, surely the forelimb of these mice also accommodate in some way, or did the mice behave like sifaka walking quadrupedally (Granatosky et al., 2016)? This would be great if the authors could address this potential mismatch between the forelimbs and the hind limbs.
In the end, I’m not sure what my opinion is of this article. I really have no specific problems. I think it is well done, reads easy, and is solid, but I it also appears that the authors did the bare minimum to get an article published. I believe this article is worthy of publication, but without the inclusion of other gait variables, perhaps a lower impact factor journal may be more appropriate.
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