Embryonic Development of the Avian Sternum and Its Morphological Adaptations for Optimizing Locomotion

Round 1

Reviewer 1 Report

My students and I reviewed the manuscript entitled “The morphological adaptations of the sternum for optimizing locomotion to colonise land and sky”  by Eleanor M. Feneck , Sorrel R.B. Bickley , Malcolm P.O. Logan.

 

Overall we found this review paper an interesting and a relevant contribution to the literature on the adaptive radiation and developmental mechanisms of the sternum.

 

Although we found no major problems, we did find a number of smaller problems that need to be addressed in order to make this manuscript publishable.

 

Repeating issues needing correction:

 

1. “Avians” should be replaced with “birds” or “Aves” throughout when a noun is being used to described this group of animals. Avian is an adjective that should not be converted into a noun.

 

2. There are inconsistencies throughout with names of genes not being written in italics (including the References)

 

3. Scientific names are inconsistently non-italicized, and mis-spelled

 

4. “evolutionary divergent” should be “evolutionarily divergent”. this appears a number of times in the ms.

 

5. The figures are small (especially the graphs) and hard to read; suggest they be published larger

 

6. Figure letters & bird names are poorly contrasting with the background and hard to read; suggest a more contrast-y color

 

7. The figures need citations

 

Specific issues:

1. Title: since the authors address the sternum of penguins it seems that “land and sky” are not inclusive enough
2. line 9: “species” should be “lineages”
3. lines 32-35. This sentence is very long; suggest breaking it up
4. lines 46-46. This sentence is very long; suggest breaking it up
5. Results – As this is a review paper, a results section is not entirely appropriate (unless this is the journal format). Suggest: Discussion
6. lines 51 & 71: Sternal vs sternum. Suggest the authors choose one and be consistent
7. line 53. “…vertebrates where it…” suggest “vertebrates, which…”
8. line 53. “connections” suggest “joints”
9. line 53. “distal tips of the ribs” suggest “ventral ends of the ribs”
10. line 55. “distal” suggest “caudal”
11. line 61. “aid breathing” suggest “aid in respiration”
12. line 65. the m. is not italicized before the two muscle names
13. line 66. “assist the downstroke and upstroke...” seems too weak to describe the role these muscles play in flight. Suggest “that are the prime movers of the…”
14. line 75 & 77. “in chick” and “in the chick” – this appears in other places in the ms (e.g., line 233); please consistently use “in chicks” or “in the chick”
15. line 78. After “sternum” there is a period with dash over it.
16. line 80. “distal” suggest “ventral”
17. lines 89 & 101. The way figure 1 and reference 7 are cited are not the same in these two sentences.
18. lines 96-101. This sentence is very long; suggest breaking it up
19. line 109. Figure Caption: “…anterior ventral body wall…” anterior and ventral are roughly synonymous. Suggest one or the other (preferable ventral). This caption also needs a reference.
20. line 114. replace “anterior” with “ventral”
21. line 114. replace “rostral” with “cranial.” Rostral is used preferentially in the head region
22. lines 118-121. Should ectopia cordis, pectus carinatum and pectus excavatum be italicized?
23. line 121. “Sternal foramen, in which a small hole remains…” suggest, “A sternal foramen, which is a small hole remaining…”
24. line 145. “A primitive cartilaginous sternabrae … has ….” Suggest “Primitive cartilaginous sternabrae … have”
25. line 146. “Ichthyostega” is a genus and should be italicized
26. line 149. “sternabrae” are plural, so “has” should be “have”
27. line152. “attachments” suggest “joints”
28. line 154. “posterior” suggest “caudal”
29. line156. “These sternal elements which provide…” suggest “These sternal elements provide…”
30. line157. “…have additionally been proposed…” suggest “…and have been proposed…”
31. line 162. Anomodontia is not a scientific name and should not be italicized
32. line 168. Scalops Aquaticus should be Scalopus aquaticus
33. line 172. “leavers” is misspelled. Should be “levers”
34. line 181. Both Chiropterans and Pterosauria are not scientific names and should not be italicized
35. line 187. “divergently” needs to be “convergently”
36. line 190. “species” should be “group” or “taxa” or “clades” or some other non-species level
37. line 191. “undertaken” suggest a different word – e.g., “displayed”
38. line 193. “sternal size” should be “sternum size”
39. lines 195-201. The sequence here is awkward. The authors first write about ratites having a flat sternum, then the dodo and kakapo retaining a sternal keel, then the flightless cormorant without a sternal keel. Perhaps organize the paragraph and discuss flightless birds proceeding from flat sternum to keeled sternum?
40. line 199-200. “A complete absence of a sternal keel and flattened sternum is observed….” This reads as though these birds lack a keel and also a flattened sternum, where of course, they have both. Suggest “A complete absence of a sternal keel is observed…”
41. line 198. Suggest removing “famously”
42. lines 203-210 – Figure 2 caption. The caption should all be on one page. Also the bird names listed are capitalized (e.g., Lesser Auk), which is not consistent with names written elsewhere.
43. lines 211-212. “In most birds…..” I do not know of any birds where the pectoralis is not much larger than the supracoracoideus. Suggest removing “most”
44. lines 211-212. “…the former is involved in generating the power, downstroke that generates lift…” is awkward and should be rewritten
45. line 213. “…returns the wing back to into position…” makes no sense

44.line 215 “7” should be written as “seven”

45. line 278. “….as the land birds.” is awkward. Suggest rewording
46. I have a problem with referring to penguins as “flightless” (line 284, “which are also flightless”) since they are well known to “fly” underwater. To that end, it isn’t really a “surprise” that they have large keeled sternum (line 222). I would prefer to see the section on penguins better take into account that they fly underwater and that authors consider alternate terminology (e.g., “aerially flightless”) to reflect that. Related to this, penguins are labeled in Figure 3 with triangles denoting them as flightless. But of course they plot with “flying birds.”
47. line 284. “A likely reason for this difference….” I am certain this is the reason, not a “likely” reason.
48. lines 307-8. I assume that this should be header 2.4 and italicized
49. line 335. “HH equivalent stages…” does HH require an explanation and/or a citation for the audience?
50. lines 333 & 337. & 355. Emu is written as emu and Emu. See also #40 above regarding consistency
51. lines 346 & 349. Nkx2.5 is not italicized
52. lines 359-60. “..multiple different…” suggest using multiple or different.
53. line 361. “…within the ratite family.” Ratites are not a taxonomic family. Suggest “within ratites.”
54. lines 363-367. This final sentence is very long, suggest it be revised into two sentences. Also. the paper lacks a conclusion and seems to simply end with this sentence. Suggest something to wrap up the paper.

 

The Literature Cited is very inconsistent with some major problems; recommend major proofing

1. Page numbers are missing (ref #s 20, 30, 31)
2. Article titles are capitalized and should not be (ref #s 27, 32, 33, 43, 47)
3. Format inconsistent (ref #s 5, 6, 29)
4. Scientific names misspelled or incorrect (not in italics) (ref #s 30, 39, 41, 43)
5. odd misc errors (ref #s 5, 6, 29, 10)
6. Refs 25 & 26 are the same paper
7. Gene names not italicized (ref #s. 6, 7, 8, 9)

Author Response

My students and I reviewed the manuscript entitled “The morphological adaptations of the sternum for optimizing locomotion to colonise land and sky”  by Eleanor M. Feneck , Sorrel R.B. Bickley , Malcolm P.O. Logan.

Overall we found this review paper an interesting and a relevant contribution to the literature on the adaptive radiation and developmental mechanisms of the sternum.

We would like to thank reviewer 1 for highlighting the important contribution this manuscript will provide to the literature.

Although we found no major problems, we did find a number of smaller problems that need to be addressed in order to make this manuscript publishable.

We have taken all of reviewer 1’s comments on board and feel we have altered the manuscript to address the reviewers’ concerns. We would like to thank the reviewer for highlighting these issues and with the changes we have made we think the manuscript is vastly improved.

Repeating issues needing correction:

1. “Avians” should be replaced with “birds” or “Aves” throughout when a noun is being used to described this group of animals. Avian is an adjective that should not be converted into a noun.

This has now been addressed. Lines: 80, 236, 260.

 

1. There are inconsistencies throughout with names of genes not being written in italics (including the References)

All genes are now in italics whilst protein names are non-italicised. When referring to a human gene, bold is used. When referring to a mouse gene italics and non-bold is used.

 

 

1. Scientific names are inconsistently non-italicized, and mis-spelled

Scientific names have been italicised. When the non-scientific name is used, these are not in italics.

 

1. “evolutionary divergent” should be “evolutionarily divergent”. this appears a number of times in the ms.

This has been changed. Lines: 33, 66

 

1. The figures are small (especially the graphs) and hard to read; suggest they be published larger

Some of the figures have been increased in size. The figures will be enlarged in the final published version of this manuscript.

 

1. Figure letters & bird names are poorly contrasting with the background and hard to read; suggest a more contrast-y color

Figure 2 has been remade with a black background under the white letters to make the writing more visible and easier to read.

 

1. The figures need citations

The citations have been included in the text that relate to the figures. Figure 1 and 3 legends now contain the Bickely et al reference.

 

 

Specific issues:

1. Title: since the authors address the sternum of penguins it seems that “land and sky” are not inclusive enough

The title has been changed to include the penguin’s adaption to “fly” in water.

 

1. line 9: “species” should be “lineages”

This has been changed. Lines: 9

 

1. lines 32-35. This sentence is very long; suggest breaking it up

This sentence has been broken up to form two sentences.

 

1. lines 46-46. This sentence is very long; suggest breaking it up

 

1. Results – As this is a review paper, a results section is not entirely appropriate (unless this is the journal format). Suggest: Discussion

This has been changed to discussion.

 

1. lines 51 & 71: Sternal vs sternum. Suggest the authors choose one and be consistent

 

1. line 53. “…vertebrates where it…” suggest “vertebrates, which…”

where has been changed to which. Line 75.

 

1. line 53. “connections” suggest “joints”

Connections have been changed to joints. Line 75.

 

1. line 53. “distal tips of the ribs” suggest “ventral ends of the ribs”

This has been changed to “ventral ends of the ribs”. Line 75.

 

1. line 55. “distal” suggest “caudal”

Distal has been edited to caudal. Line 77.

 

1. line 61. “aid breathing” suggest “aid in respiration”

As we are discussing the role of the muscles attached to the sternum we believe “breathing” is the correct terminology used here. Breathing is the physical process, whilst respiration is the molecular process on a cellular level.

 

1. line 65. the m. is not italicized before the two muscle names

The m. has now been italicised in these lines. Line 87.

 

1. line 66. “assist the downstroke and upstroke...” seems too weak to describe the role these muscles play in flight. Suggest “that are the prime movers of the…”

This suggested change has been undertaken. Line 88.

 

 

1. line 75 & 77. “in chick” and “in the chick” – this appears in other places in the ms (e.g., line 233); please consistently use “in chicks” or “in the chick”

All “in the chicks” have now been changed to “in chicks”.

 

1. line 78. After “sternum” there is a period with dash over it.

This dash has been removed.

 

1. line 80. “distal” suggest “ventral”

Distal has been changed to ventral. Line 75.

 

1. lines 89 & 101. The way figure 1 and reference 7 are cited are not the same in these two sentences.

Figure 1 and reference 7 have the same citation with the Bickely reference.

 

1. lines 96-101. This sentence is very long; suggest breaking it up

this sentence has now been broken up into two sentences.

 

1. line 109. Figure Caption: “…anterior ventral body wall…” anterior and ventral are roughly synonymous. Suggest one or the other (preferable ventral). This caption also needs a reference.

Anterior has now been removed. Reference has been included in the figure legend.

 

1. line 114. replace “anterior” with “ventral”

Anterior has now been replaced with ventral. Line 103.

 

1. line 114. replace “rostral” with “cranial.” Rostral is used preferentially in the head region

 

Cranial has been used to replace rostral. Line 159, 514, 515.

 

1. lines 118-121. Should ectopia cordis, pectus carinatum and pectus excavatum be italicized?

These have been italicised. Line 165-166.

 

1. line 121. “Sternal foramen, in which a small hole remains…” suggest, “A sternal foramen, which is a small hole remaining…”

This suggestion has been made.

 

1. line 145. “A primitive cartilaginous sternabrae … has ….” Suggest “Primitive cartilaginous sternabrae … have”

This suggestion has been edited in the text.

 

1. line 146. “Ichthyostega” is a genus and should be italicized

Ichthyostega is now italicised in the text.

 

1. line 149. “sternabrae” are plural, so “has” should be “have”

has is now changed to has. Line 209.

 

1. line152. “attachments” suggest “joints”

Joints has now been replaced attachments. Line 215.

 

1. line 154. “posterior” suggest “caudal”

Caudal is now used. Line 77

 

1. line156. “These sternal elements which provide…” suggest “These sternal elements provide…”

This comment has been addressed.

 

1. line157. “…have additionally been proposed…” suggest “…and have been proposed…”

This comment has been addressed.

 

1. line 162. Anomodontia is not a scientific name and should not be italicized

We have kept Anomodontia in italics as we think it is a scientific name. We welcome the journal editors to check this to make sure it is correct.

 

 

1. line 168. Scalops Aquaticus should be Scalopus aquaticus

This spelling mistake has been corrected. Line 227.

 

1. line 172. “leavers” is misspelled. Should be “levers”

Leavers is now changed to levers. Line 231.

 

1. line 181. Both Chiropterans and Pterosauria are not scientific names and should not be italicized

Chiropterans is now non-italicised.

 

We believe Pterosauria is the scientific name so have left this in italics.

 

 

1. line 187. “divergently” needs to be “convergently”

Divergently is now changed to convergently. Line 255.

 

1. line 190. “species” should be “group” or “taxa” or “clades” or some other non-species level

Species has been changed to taxa. Line 258.

 

1. line 191. “undertaken” suggest a different word – e.g., “displayed”

Undertaken changed to displayed. Line 260.

 

1. line 193. “sternal size” should be “sternum size”

Sternal is now changed to sternum. Line 262.

 

1. lines 195-201. The sequence here is awkward. The authors first write about ratites having a flat sternum, then the dodo and kakapo retaining a sternal keel, then the flightless cormorant without a sternal keel. Perhaps organize the paragraph and discuss flightless birds proceeding from flat sternum to keeled sternum?

 

This sentence has been edited to discuss all flightless birds with a loss of the sternal keel and then goes on to discuss the retained sternal keel in the dodo and kakapo. Figure 2 has been edited so that the galapagos cormorant is “D” and then the dodo and kakao are “E and F”. Lines 266-269.

 

1. line 199-200. “A complete absence of a sternal keel and flattened sternum is observed….” This reads as though these birds lack a keel and also a flattened sternum, where of course, they have both. Suggest “A complete absence of a sternal keel is observed…”

 

This edit has now been made.

 

1. line 198. Suggest removing “famously”

Famously has now been removed.

 

1. lines 203-210 – Figure 2 caption. The caption should all be on one page. Also the bird names listed are capitalized (e.g., Lesser Auk), which is not consistent with names written elsewhere.

The bird names are now not capitalised to make it consistent with the rest of the text.

 

1. lines 211-212. “In most birds…..” I do not know of any birds where the pectoralis is not much larger than the supracoracoideus. Suggest removing “most”

 

Most has been removed.

 

1. lines 211-212. “…the former is involved in generating the power, downstroke that generates lift…” is awkward and should be rewritten

The comma after power has been removed.

 

1. line 213. “…returns the wing back to into position…” makes no sense

This sentence has now been edited.

 

44.line 215 “7” should be written as “seven”

Seven has now been written.

 

1. line 278. “….as the land birds.” is awkward. Suggest rewording

“the” has been removed from this sentence.

 

1. I have a problem with referring to penguins as “flightless” (line 284, “which are also flightless”) since they are well known to “fly” underwater. To that end, it isn’t really a “surprise” that they have large keeled sternum (line 222). I would prefer to see the section on penguins better take into account that they fly underwater and that authors consider alternate terminology (e.g., “aerially flightless”) to reflect that. Related to this, penguins are labeled in Figure 3 with triangles denoting them as flightless. But of course they plot with “flying birds.”

The text has been changed to accommodate the penguin’s ability to fly underwater and not be able to undertake aerial flight. Figure 2 has also been edited to accommodate this change. Lines 286, 447, 458 and Figure 2 and 3 legend.

 

1. line 284. “A likely reason for this difference….” I am certain this is the reason, not a “likely” reason.

Likely has been removed from this sentence.

 

1. lines 307-8. I assume that this should be header 2.4 and italicized

Yes, this is a title header.

 

1. line 335. “HH equivalent stages…” does HH require an explanation and/or a citation for the audience?

Hamburger-hamilton stages (HH) has now been explained in the text.

 

1. lines 333 & 337. & 355. Emu is written as emu and Emu. See also #40 above regarding consistency

All mention of emu mid-sentence has been changed to not be capitalised.

 

1. lines 346 & 349. Nkx2.5 is not italicized

Nkx1.5 has now been edited to be italicised throughout the manuscript.

 

1. lines 359-60. “..multiple different…” suggest using multiple or different.

Multiple has been removed, different remains.

 

1. line 361. “…within the ratite family.” Ratites are not a taxonomic family. Suggest “within ratites.”

Sentence has been changed to within ratites. Line 571.

 

1. lines 363-367. This final sentence is very long, suggest it be revised into two sentences. Also. the paper lacks a conclusion and seems to simply end with this sentence. Suggest something to wrap up the paper.

We have modified the final paragraph to extend our conclusion. We have broken the last sentence up and expanded it. These changes can be found between lines 642-649.

The Literature Cited is very inconsistent with some major problems; recommend major proofing

1. Page numbers are missing (ref #s 20, 30, 31)
2. Article titles are capitalized and should not be (ref #s 27, 32, 33, 43, 47)
3. Format inconsistent (ref #s 5, 6, 29)
4. Scientific names misspelled or incorrect (not in italics) (ref #s 30, 39, 41, 43)
5. odd misc errors (ref #s 5, 6, 29, 10)
6. Refs 25 & 26 are the same paper
7. Gene names not italicized (ref #s. 6, 7, 8, 9)

 

The reference list and citations have been edited and corrected and are now more consistent.

 

Reviewer 2 Report

The authors have presented a review of sternum development, morphology and evolution and how this relates to major habitat transitions within tetrapod vertebrates. As the attachment site for very important locomotor muscles and a key part of the ventilation mechanisms of many vertebrates, the sternum is an important piece of the vertebrate body plan, but one which has not necessarily received the attention it deserves. I therefore believe this review has the potential to be an important addition to the literature. However, in its current form I believe it is not suitable for publication and I suggest major revisions.

My main comment is that the authors do not seem to have done enough to survey the anatomical and palaeontological literature. This paper is a review, and yet whole paragraphs may contain only a single reference to previous work. Also, not all citations appear to be relevant/correct, making it difficult to assess how well the authors have reviewed the literature. I suggest the authors double check their reference numbering.

The paper does not really address ‘transitions’ in the evolution of the sternum, as the authors only consider one side of each transition. For example, the authors spend a great deal of time discussing the sterna of modern birds, but literature on sternal evolution and development in fossil birds (and non-avian dinosaurs) is ignored. Similarly, the authors discuss the earliest fossil evidence of the sternum in tetrapods, but there is little or no discussion of where the first sternum may have come from or why (it unlikely first arose due to the pressures of terrestrial locomotion since the earliest tetrapods like Ichthyostega were predominantly aquatic).

More time and space should be devoted to the literature on bats and pterosaurs, and how our knowledge of these groups compares to birds. While it is true that birds have received most of the research attention bats and pterosaurs – the other two clades of vertebrates to have evolved powered flight – are only mentioned briefly. A review such as this would be an excellent opportunity to go over what is and is not known regarding sternal evolution in these clades in more detail and how it relates to the acquisition of powered flight and locomotor diversity.

Although the authors discuss differences in sternal anatomy between different tetrapod clades, the only sterna figured or illustrated are of birds. I know collection or specimen access is likely complicated because of COVID-19, but there are plenty of other data sources available online (e.g., Morphosource for 3D models) which could be used to illustrate taxonomic or ecological variation in sternum morphology.

Taxonomic names are often incorrectly italicized/capitalized. Genus names (e.g., Ichthyostega) should be in italics, but clades names (e.g. Pterosauria) should not. Please go through and correct these.

Author Response

The authors have presented a review of sternum development, morphology and evolution and how this relates to major habitat transitions within tetrapod vertebrates. As the attachment site for very important locomotor muscles and a key part of the ventilation mechanisms of many vertebrates, the sternum is an important piece of the vertebrate body plan, but one which has not necessarily received the attention it deserves. I therefore believe this review has the potential to be an important addition to the literature. However, in its current form I believe it is not suitable for publication and I suggest major revisions.

 

My main comment is that the authors do not seem to have done enough to survey the anatomical and palaeontological literature. This paper is a review, and yet whole paragraphs may contain only a single reference to previous work. Also, not all citations appear to be relevant/correct, making it difficult to assess how well the authors have reviewed the literature. I suggest the authors double check their reference numbering.

 

The referencing numbering has been checked and corrected.

We have deliberately aimed the focus of our review on recent work in avians on sternum adaptations and reduction of the wing programme. We have touched on other work to contextualise our area of focus. The reality is that there is not a wealth of literature on sternum evolution and adaptation in other animals.

 

The paper does not really address ‘transitions’ _in the evolution of the sternum, as the authors only consider one side of each transition. For example, the authors spend a great deal of time discussing the sterna of modern birds, but literature on sternal evolution and development in fossil birds (and non-avian dinosaurs) is ignored. Similarly, the authors discuss the earliest fossil evidence of the sternum in tetrapods, but there is little or no discussion of where the first sternum may have come from or why (it unlikely first arose due to the pressures of terrestrial locomotion since the earliest tetrapods like Ichthyostega were predominantly aquatic).

 

Unfortunately, there are very few studies that focus on the evolution and development of the sternum. We do mention that due to the cartilaginous nature of early sterna, samples have not been preserved and there is therefore a gap in the early sternum fossil record. We have chosen to focus on papers that discuss development of the avian sternum, and we decided to focus on this as birds show a variety of sternal adaptions that interestingly correlate to locomotion.

 

 

More time and space should be devoted to the literature on bats and pterosaurs, and how our knowledge of these groups compares to birds. While it is true that birds have received most of the research attention bats and pterosaurs – _the other two clades of vertebrates to have evolved powered flight – _are only mentioned briefly. A review such as this would be an excellent opportunity to go over what is and is not known regarding sternal evolution in these clades in more detail and how it relates to the acquisition of powered flight and locomotor diversity.

 

We have taken the advice of reviewer 4 and added literature on studies that mention the sternum in pterosaurs. This includes the studies Geist 2013 and Frey 2003. Unfortunately, there is a limited range of studies that describe the sterna of bats and pterosaurs and no studies that specifically focus on the sternum.

 

 

 

Although the authors discuss differences in sternal anatomy between different tetrapod clades, the only sterna figured or illustrated are of birds. I know collection or specimen access is likely complicated because of COVID-19, but there are plenty of other data sources available online (e.g., Morphosource for 3D models) which could be used to illustrate taxonomic or ecological variation in sternum morphology.

These images are used so the reader can understand the diversity of the sternal keel in a variety of birds. We thank the reviewer for directing us towards the morphosource datasets. We carefully analysed the database and the majority of images are of bird sterna. There is one pterosauria model and, even though this is interesting, we feel with only one sample available it is challenging to draw firm conclusions. We have now included a description of the sternum morphology within pterosauria. Lines 194-201.

The Pterosaur sternum image on Morphosource appears to be protected by copyright and not free for us to use in our figure.

 

 

Taxonomic names are often incorrectly italicized/capitalized. Genus names (e.g., Ichthyostega) should be in italics, but clades names (e.g. Pterosauria) should not. Please go through and correct these.

 

The italics have now been corrected.

Reviewer 3 Report

Unfortunately, in such a short period I cannot provide a detailed review, it requires a lot of work with literature. Therefore, I will only express general considerations about the article.

 

The problem raised by the authors is really very important. However, the content of the paper does not correspond much to the high-sounding title. Seeing it, the reader expects to get an idea of how the sternum functionally evolved during the adaptation land and air conditions in different groups of vertebrates. By the way, in the frames of the serious approach to the topic, one cannot but say about the sternum state in mammals that have secondarily returned to the water (there is not a word about this). The matter is that a consideration of the trends observed in aquatic mammals in comparison with the Devonian tetrapods would allow a better understanding of what was the role played by the formation of the sternum at entering land.

 

However, all this has little to do with this manuscript, since tetrapods land conquest, like other groups of powered flight tetrapods, are only superficially mentioned in the introductory part of the article. The main text deals only with the adaptive role of the sternum in birds.

In fact, the article discusses two issues:

1. Morphometric characteristics of the sternum in birds with different degrees of flight specialization;
2. Genetic mechanisms of inhibition of the development of the sternum and forelimb in birds that have lost their flight.

I believe it is appropriate to reflect this real content of the article in its title.

 

As for the section on morphological adaptations, it deals with three morphometric parameters: keel height, sternum length and width. The technique of this analysis raises many questions, since it is not specified how the data were obtained and how the measurements were taken. If the data were taken from the literature, then what is the confidence that all authors of the studied sources took measurements in the same way? In case this question has been tested, this is required to be noted. Maybe the authors of this manuscript measured something themselves? Anyway, the measurement technique should be indicated or a link to an article where it is described in detail needs to be given.

 

It is very controversial to use the thorax length parameter (by the way, how is it measured?) as a dimensional variable to convert the analyzed measurements to dimensionless quantities. Is this indicator taken because it is easy to measure with only the sternum or an unassembled skeleton available? It is highly questionable that there is no correlation between the thorax length and the length of the sternum. The absence of such a correlation must be proved. Generally, as a dimensional variable, it would be better to take a parameter that is as distant as possible from the structure under study in order to avoid internal correlations within the thorax. At the same time, this parameter should be minimally subject to adaptive changes. Finding such a structure, indeed easily measurable, in a bird body is a really big problem.  

 

This search could have been avoided by dividing the width of the sternum by its length. This index would allow assessing the expansion of the sternum in various birds. Such a parameter could be represented on the graph with the ratio of the keel height to the length of the sternum (or better to the length of the keel itself!) on the other axis.

 

As far as the keel is concerned, its functional role strongly depends on whether it is high and / or extended. The keel may have the shape of a "high" protrusion (like in hummingbird, fig 2) and be very short, or it may be relatively low but elongated along the entire sternum (like in corvids) or be both high and long (like in parrots). This heavily determines whether it works as a place for relocation of the fibers of m. pectoralis which changes the arm of the muscle force application in the shoulder joint, or as a platform for expanded attachment of m. pectoralis and / or m. supracoracoideus. It is also important that the hypertrophy m. pectoralis with occupation of a broad area on the sternum (usually on the keel) and the hypertrophy of m. supracoracoideus are completely different flight adaptations. Only on the basis of the total area of the muscles attachment without differentiation into the pectoral and supracoracoid zones, it is impossible to determine the type of specialization manifested in the sternum enlargement. The tasks to be solved can be absolutely different– to confront gravity in horizontal flight, to confront gravity when hovering with a vertically oriented body, or to resist drag in underwater flight.

 

As for the part devoted to the genetic basis of sternum reduction, I can say little here without being an expert. However, as a reader I have not formed a holistic picture. I can see that certain genetic mechanisms have been shown in individual species, and it is not clear how widely they can be extended to other species, and even more so to groups for which parallel independent flight loss is assumed. To avoid this uncertainty, I would suggest making a table for all the data so that it becomes clear for which groups they are rightfully applicable, for which they are hypothetically applicable, and for which they are unlikely to be applicable.

Author Response

Unfortunately, in such a short period I cannot provide a detailed review, it requires a lot of work with literature. Therefore, I will only express general considerations about the article.

 

The problem raised by the authors is really very important. However, the content of the paper does not correspond much to the high-sounding title. Seeing it, the reader expects to get an idea of how the sternum functionally evolved during the adaptation land and air conditions in different groups of vertebrates. By the way, in the frames of the serious approach to the topic, one cannot but say about the sternum state in mammals that have secondarily returned to the water (there is not a word about this).

We feel that changing the title would conflict with the other reviewers’ comments.

 

“in vertebrates” is not included in the title.

 

The reviewer makes an interesting point about mammals that have returned to water. We have carried out a literature search and disappointingly the literature is very limited on this topic. However, we did come across some older references relating to the “broad and flat” nature of the sternum in aquatic mammals such as whales and these references have been included along with a statement referring to these animals being hindlimb dominant. Lines 289-293. Reference 37 and 38 have been included that refer to the new literature that has been added on aquatic mammalian sternum.

 

The matter is that a consideration of the trends observed in aquatic mammals in comparison with the Devonian tetrapods would allow a better understanding of what was the role played by the formation of the sternum at entering land.

 

However, all this has little to do with this manuscript, since tetrapods land conquest, like other groups of powered flight tetrapods, are only superficially mentioned in the introductory part of the article. The main text deals only with the adaptive role of the sternum in birds.

In fact, the article discusses two issues:

1. Morphometric characteristics of the sternum in birds with different degrees of flight specialization;
2. Genetic mechanisms of inhibition of the development of the sternum and forelimb in birds that have lost their flight.

 

We have focused on the adaption of the sternum of birds as this is where much of the literature and research studies around sternum adaption is directed and where the most significant changes across this group related to locomotion are discussed in the literature.

 

I believe it is appropriate to reflect this real content of the article in its title.

 

As for the section on morphological adaptations, it deals with three morphometric parameters: keel height, sternum length and width. The technique of this analysis raises many questions, since it is not specified how the data were obtained and how the measurements were taken. If the data were taken from the literature, then what is the confidence that all authors of the studied sources took measurements in the same way? In case this question has been tested, this is required to be noted. Maybe the authors of this manuscript measured something themselves? Anyway, the measurement technique should be indicated or a link to an article where it is described in detail needs to be given. 2

 

The details of measurements have been referenced in the text and the figure legends. These details can be found in the open accessed paper. The measurements attempt to normalise measurements across a group of animals that vary in size. These measurements were carried out by one group and is based on one study that has been per reviewed. Due to these reasons, we are confident that the measurements were taken in the same way.

 

 

It is very controversial to use the thorax length parameter (by the way, how is it measured?) as a dimensional variable to convert the analyzed measurements to dimensionless quantities. Is this indicator taken because it is easy to measure with only the sternum or an unassembled skeleton available? It is highly questionable that there is no correlation between the thorax length and the length of the sternum. The absence of such a correlation must be proved. Generally, as a dimensional variable, it would be better to take a parameter that is as distant as possible from the structure under study in order to avoid internal correlations within the thorax. At the same time, this parameter should be minimally subject to adaptive changes. Finding such a structure, indeed easily measurable, in a bird body is a really big problem.

This search could have been avoided by dividing the width of the sternum by its length. This index would allow assessing the expansion of the sternum in various birds. Such a parameter could be represented on the graph with the ratio of the keel height to the length of the sternum (or better to the length of the keel itself!) on the other axis.

As far as the keel is concerned, its functional role strongly depends on whether it is high and / or extended. The keel may have the shape of a "high" protrusion (like in hummingbird, fig 2) and be very short, or it may be relatively low but elongated along the entire sternum (like in corvids) or be both high and long (like in parrots). This heavily determines whether it works as a place for relocation of the fibers of m. pectoralis which changes the arm of the muscle force application in the shoulder joint, or as a platform for expanded attachment of m. pectoralis and / or m. supracoracoideus. It is also important that the hypertrophy m. pectoralis with occupation of a broad area on the sternum (usually on the keel) and the hypertrophy of m. supracoracoideus are completely different flight adaptations. Only on the basis of the total area of the muscles attachment without differentiation into the pectoral and supracoracoid zones, it is impossible to determine the type of specialization manifested in the sternum enlargement. The tasks to be solved can be absolutely different– to confront gravity in horizontal flight, to confront gravity when hovering with a vertically oriented body, or to resist drag in underwater flight.

 

We have added a sentence that discusses the hypothesised movement of the origin of the m. supracoracoideus in the adaption of a sternal keel in parrots to expand the importance of the muscle attachment onto the varying locations of the sternum. Lines 283-285. The addition of reference 36 referring to the importance of muscle attachment onto the sternum in the adaption of a sternal keel has been included.

 

As for the part devoted to the genetic basis of sternum reduction, I can say little here without being an expert. However, as a reader I have not formed a holistic picture. I can see that certain genetic mechanisms have been shown in individual species, and it is not clear how widely they can be extended to other species, and even more so to groups for which parallel independent flight loss is assumed. To avoid this uncertainty, I would suggest making a table for all the data so that it becomes clear for which groups they are rightfully applicable, for which they are hypothetically applicable, and for which they are unlikely to be applicable.

 

We would like to thank reviewer 3 for their suggestion of adding a table but we feel this would add confusion

Reviewer 4 Report

This is an interesting paper summarizing the present day knowledge of the sternum formation in ontogeny and phylogeny of tetrapods, with special emphasis to its diversity and function in birds. Although i find different part of the paper to be largely independent from each other (e.g. adaptive morphology/evo-devo part), i still think that the paper merits publication, as it would be useful to combine this information at one place. I have few notes on the ms. First, the paper requires significant edition, both technical and stylistic (e.g. bird groups are sometimes called species, and so on), including the reference section (many references are incomplete, some doubled).

As the paper deals with avian sternum in particular, it is worth mentiong that many relevant literature sources are missed. The authors definitevely should consider the following paper in their review, and references within it: Mayr, G. 2017. Pectoral girdle morphology of Mesozoic birds and the evolution of the avian supracoracoideus muscle. Journal of Ornithology 158:859-867. Additionally, as hummingbirds are extensively discussed, the following reference is needed: Zusi, R. L. 2013. Introduction to the skeleton of hummingbirds (Aves: Apodiformes, Trochilidae) in functional and phylogenetic contexts. Ornithological monographs:1-94. I would also recommend considering a paper on the function of the forelimb in parrots, a group of birds which also have an unusually large m. supracoracoideus, apparently used in supporting veritcal flight (Razmadze, D., A. A. Panyutina, and N. V. Zelenkov. 2018. Anatomy of the forelimb musculature and ligaments of Psittacus erithacus (Aves: Psittaciformes). Journal of Anatomy 233:496-530.)

Importantly, the datasets for the avian sternum variation must be added as a supplementary information.

 

Author Response

This is an interesting paper summarizing the present day knowledge of the sternum formation in ontogeny and phylogeny of tetrapods, with special emphasis to its diversity and function in birds. Although i find different part of the paper to be largely independent from each other (e.g. adaptive morphology/evo-devo part), i still think that the paper merits publication, as it would be useful to combine this information at one place. I have few notes on the ms. First, the paper requires significant edition, both technical and stylistic (e.g. bird groups are sometimes called species, and so on), including the reference section (many references are incomplete, some doubled).

We would like to thank the reviewer for highlighting the benefits our review paper would provide to the research community. We believe we have undertaken significant adaptions to the text to address the issues outlined by reviewer 2.

As the paper deals with avian sternum in particular, it is worth mentiong that many relevant literature sources are missed.

The manuscript has been updated to include more relevant literature. Including Mayr 2017, Zusi 2013 and Razmadze 2018.

The authors definitevely should consider the following paper in their review, and references within it: Mayr, G. 2017. Pectoral girdle morphology of Mesozoic birds and the evolution of the avian supracoracoideus muscle. Journal of Ornithology 158:859-867.

Mayr 2017 has now been added into the manuscript. Reference 28.

Additionally, as hummingbirds are extensively discussed, the following reference is needed: Zusi, R. L. 2013. Introduction to the skeleton of hummingbirds (Aves: Apodiformes, Trochilidae) in functional and phylogenetic contexts. Ornithological monographs:1-94.

The reference Zusi 2013 has now been included. Reference 32.

I would also recommend considering a paper on the function of the forelimb in parrots, a group of birds which also have an unusually large m. supracoracoideus, apparently used in supporting veritcal flight (Razmadze, D., A. A. Panyutina, and N. V. Zelenkov. 2018. Anatomy of the forelimb musculature and ligaments of Psittacus erithacus (Aves: Psittaciformes). Journal of Anatomy 233:496-530.)

This reference has been included. Reference 36.

Importantly, the datasets for the avian sternum variation must be added as a supplementary information.

All of the datasets of the avian sternum variations are referenced and cited throughout the text and are readily available through open access to everyone.

Round 2

Reviewer 3 Report

Unfortunately, the changes which were made by the authors are far from major revision.

In the title, they added word 'water' which does not at all clarify the subject of the research. It would be more appropriate to add 'as exemplified by birds'. The claim of the authors that “in vertebrates” is not included in the title, is an abuse because their "colonise land" comes far beyond birds which only are really treated in the manuscript.

As to the measurements of the avian sternum and its keel, which are the true subject for the main line of consideration of the authors, all this have been already published in the earlier paper entitled "Regulatory modulation of the T-box gene Tbx5 links development, evolution, and adaptation of the sternum". Therein, all the same inferences were presented (only less detailed). Thus, they are not new in the current submission.

Since the current manuscript is submitted as a Review, it could be regarded as a valuable scientific contribution, in my opinion, if at least one of the two following lines of consideration was covered comprehensively. The first one could be an in-depth functional analysis of the sternum and its keel in diverse birds, including analysis of respective muscular attachments based on a great amount of data which were already published by numerous authors. The second one could be a complete overview of the literature on the sternal ontogeny and function.

The authors did not follow the first line as soon as they made no attempt to approach even elementary biomechanics of the sternal apparatus in flying and swimming birds. The basis for this analysis could be taken from R. McN. Alexander's " Animal Mechanics " 1968 (see chapters on flight and swimming). An elementary analysis of static forces for birds suggested by James Gray (1944) is not even mentioned.

Neither did authors follow the second line as can be seen from great gaps in their reference list which fails to include a number of fundamental studies of their predecessors, e.g.:

1. Parker WK. 1868. A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata. R. Hardwicke
2. Kravetz, LP. 1905. Entwicklungsgeschichte des Sternum und des Episternalapparatus der Saugetiere. Bull. Soc. Imper. Natur. Moscow., 1 (5): 1-59
3. Eggeling HV. 1906. Clavicula, Praeclavium, Halsrippen und Manubrium sterni. Anat Anz.;29:99-110.
4. Hanson FB. 1919 The development of the sternum in Sus scrofa. The Anatomical Record. Sep;17(1):1-23.
5. Klima MI. 1978; Comparison of early development of sternum and clavicle in striped dolphin and in humpback whale. The scientific reports of the whales research institute. 30:253-69.
6. Klima M. 1987. Introduction and Review of the Literature. Early Development of the Shoulder Girdle and Sternum in Marsupials (Mammalia: Metatheria). 1-2.
7. Cave AJ. 1970. Observations on the monotreme interclavicle. Journal of Zoology. 160(3):297-312.

It could seem impossible, but the authors don't even cite one of the papers which directly concerns their narrow subject, i.e. the avian sternum:

Zheng X, Wang X, O'connor J, Zhou Z. 2012. Insight into the early evolution of the avian sternum from juvenile enantiornithines. Nature Communications. Oct 9;3(1):1-8.

Therefore, the manuscript under review, in my opinion, is not a mature scientific paper but a kind of a rough draft.

Author Response

Reviewer 3:

 

Unfortunately, the changes which were made by the authors are far from major revision.

In the title, they added word 'water' which does not at all clarify the subject of the research. It would be more appropriate to add 'as exemplified by birds'. The claim of the authors that “in vertebrates” is not included in the title, is an abuse because their "colonise land" comes far beyond birds which only are really treated in the manuscript.

We can now see that the original title of our manuscript was misleading.

We have changed this title to “Embryonic development of the avian sternum and its morphological adaptations for optimizing locomotion” to make the objective of our manuscript clear.

As to the measurements of the avian sternum and its keel, which are the true subject for the main line of consideration of the authors, all this have been already published in the earlier paper entitled "Regulatory modulation of the T-box gene Tbx5 links development, evolution, and adaptation of the sternum". Therein, all the same inferences were presented (only less detailed). Thus, they are not new in the current submission.

We agree that this work is not new but we include this reference as this is one of the main papers we are reviewing. We wanted to highlight the genetic developmental elements that have importance in sternum development and adaption and unfortunately there are not many studies that focus on this. 

Since the current manuscript is submitted as a Review, it could be regarded as a valuable scientific contribution, in my opinion, if at least one of the two following lines of consideration was covered comprehensively. The first one could be an in-depth functional analysis of the sternum and its keel in diverse birds, including analysis of respective muscular attachments based on a great amount of data which were already published by numerous authors. The second one could be a complete overview of the literature on the sternal ontogeny and function.

The authors did not follow the first line as soon as they made no attempt to approach even elementary biomechanics of the sternal apparatus in flying and swimming birds. The basis for this analysis could be taken from R. McN. Alexander's " Animal Mechanics " 1968 (see chapters on flight and swimming). An elementary analysis of static forces for birds suggested by James Gray (1944) is not even mentioned.

Neither did authors follow the second line as can be seen from great gaps in their reference list which fails to include a number of fundamental studies of their predecessors, e.g.:

1. Parker WK. 1868. A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata. R. Hardwicke
2. Kravetz, LP. 1905. Entwicklungsgeschichte des Sternum und des Episternalapparatus der Saugetiere. Bull. Soc. Imper. Natur. Moscow., 1 (5): 1-59
3. Eggeling HV. 1906. Clavicula, Praeclavium, Halsrippen und Manubrium sterni. Anat Anz.;29:99-110.
4. Hanson FB. 1919 The development of the sternum in Sus scrofa. The Anatomical Record. Sep;17(1):1-23.
5. Klima MI. 1978; Comparison of early development of sternum and clavicle in striped dolphin and in humpback whale. The scientific reports of the whales research institute. 30:253-69.
6. Klima M. 1987. Introduction and Review of the Literature. Early Development of the Shoulder Girdle and Sternum in Marsupials (Mammalia: Metatheria). 1-2.
7. Cave AJ. 1970. Observations on the monotreme interclavicle. Journal of Zoology. 160(3):297-312.

 

Our review provides a summary of the developmental and morphological adaptations of the sternum, with a focus on avians. We would like to thank the reviewer for recommending these references, but we find that many of these references are beyond the scope of what we aim to focus on in our review. We therefore do not consider these references appropriate to be included in our manuscript.

It could seem impossible, but the authors don't even cite one of the papers which directly concerns their narrow subject, i.e. the avian sternum:

Zheng X, Wang X, O'connor J, Zhou Z. 2012. Insight into the early evolution of the avian sternum from juvenile enantiornithines. Nature Communications. Oct 9;3(1):1-8.

This reference has now been included in the manuscript. The manuscript now includes an expansion of the early development of the sternum in ornithuromorphs and the comparison to enathiornithes.

Reviewer 4 Report

The paper seems ok for me, though some minor edits still may be required (e.g., see line 228). I would like to note, however, that a large portion of the presented information, i.e. that on the development of the sternum, is completely not reflected in the title, and I would advise reconsidering the title  accordingly. This would, additionally, attract readers interested in developmental aspects of the sternum. 

Author Response

The paper seems ok for me, though some minor edits still may be required (e.g., see line 228). I would like to note, however, that a large portion of the presented information, i.e. that on the development of the sternum, is completely not reflected in the title, and I would advise reconsidering the title  accordingly. This would, additionally, attract readers interested in developmental aspects of the sternum. 

As we have detailed above, we have now changed the title of our review to: Embryonic development of the avian sternum and its morphological adaptations for optimizing locomotion

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.