Integumentary Colour Allocation in the Stork Family (Ciconiidae) Reveals Short-Range Visual Cues for Species Recognition
Round 1
Reviewer 1 Report
I found this to be an interesting paper with a reasonable approach and analyses.
My suggestions for improving the paper focus on how color literature is cited.
Most critically, in the discussion, Alfred Wallace is not given credit for the species recognition hypothesis. Wallace wrote extensively on this top and his insights are a relevant today as they were when he published his work over 100 years ago. Wallace discussed the importance of signals of species identity not only in the context of mate choice but also in the context of non-sexual social groupings (just as the author emphasize). Wallace should be cited extensively. One place to find Wallace’s ideas regarding species recognition is
A.R. Wallace. Darwinism. 1889.
Dale wrote about the characteristics that are expected for traits that serve as different sorts of signals: condition, individual recognition, species recognition. The predictions of Dale are completely consistent with the findings of this paper and I recommend that these predictions be highlighted and that Dale be cited.
DALE, JAMES. "Intraspecific Variation in Coloration." Bird coloration: function and evolution (GE Hill and KJ McGraw, Editors) 2 (2006): 597-602.
Hill proposed that there are three classes of information conveyed to choosing females by color displays: condition, species identity, or sexiness. The observations from storks supports ideas proposed related to species identity and this paper should also be cited. The authors should cite this paper.
Hill, G. E. (2015). Sexiness, individual condition, and species identity: the information signaled by ornaments and assessed by choosing females. Evolutionary Biology, 42(3), 251-259.
Author Response
Ref 1
I found this to be an interesting paper with a reasonable approach and analyses.
My suggestions for improving the paper focus on how color literature is cited.
Most critically, in the discussion, Alfred Wallace is not given credit for the species recognition hypothesis. Wallace wrote extensively on this top and his insights are a relevant today as they were when he published his work over 100 years ago. Wallace discussed the importance of signals of species identity not only in the context of mate choice but also in the context of non-sexual social groupings (just as the author emphasize). Wallace should be cited extensively. One place to find Wallace’s ideas regarding species recognition is
A.R. Wallace. Darwinism. 1889.
Dale wrote about the characteristics that are expected for traits that serve as different sorts of signals: condition, individual recognition, species recognition. The predictions of Dale are completely consistent with the findings of this paper and I recommend that these predictions be highlighted and that Dale be cited.
DALE, JAMES. "Intraspecific Variation in Coloration." Bird coloration: function and evolution (GE Hill and KJ McGraw, Editors) 2 (2006): 597-602.
Hill proposed that there are three classes of information conveyed to choosing females by color displays: condition, species identity, or sexiness. The observations from storks support ideas proposed related to species identity and this paper should also be cited. The authors should cite this paper.
Hill, G. E. (2015). Sexiness, individual condition, and species identity: the information signaled by ornaments and assessed by choosing females. Evolutionary Biology, 42(3), 251-259.
Author's Reply to the Review Report:
We have added the following paragraph in the Introduction, incorporating the ideas and citations that the reviewer suggests:
“This hypothesis posits that a range of mating signals, including acoustic, olfactory and visual cues (e.g., [18]) facilitate species recognition and serve to avoid maladaptive hybridization, as first suggested by Wallace [19], and later expanded by Dale [20]. Martin et al. [21] have recently provided support for highest color divergence in closely related species at intermediate levels of breeding range-sympatry using 246 bird species belonging to 39 species with a worldwide distribution.“
Reviewer 2 Report
The authors describe a comparative analysis of the conservatism of colouration on the whole body of storks.
I find that the article rationale is correct, but it lacks a correct implementation and critical interpretation of the results and the limits of the methodology:
- There seems to be quite a lot of confusion regarding the causes and mechanisms of the evolution of colouration. Some of these confusions arise from not discussing in more detail the types of pigments that are involved. It is difficult to mix the causes of melanin use with the use of carotenoids. Not only their origin (metabolized, versus diet-related and metabolized). Also some relate to body condition and their use as a proxy for sexual selection (red carotenoids). On the other hand, Iris color is related to pterins and iris vascularization.
- And some of the character changes are between different pigments, others just result from the lack of deposition of one pigment and the resulting color is either white or the other underlying pigment or red, due to vascularization. Some discussion how this can have an effect on the general conclusion is needed.
- Regarding the conservatism, some specific examples could help on your discussion. Do melanin deposition on flight feathers tips and areas more exposed to UV is transversal to all species?
- I would like also to understand if the percentages of cover were taken into account in the models. If not, discuss if this has an impact on the model or not.
In more detail, I include some specific comments:
line 34
"they comprise all-white (e.g., the wood stork Mycteria americana)"
Mycteria americana has the whole primaries, secondaries and tertiaries black. Please rephrase the sentence.
Line 51
In this paragraph I would also add the role of melanin to increase resistance to damage in specific feathers such as flight feathers.
Figure 1
It is important in the Figure 1 to add the Initials for each character that is used in Figure 2 & 3. Also, these initials do not link directly to the sections identified in figure 1. I think that all Initials and names should be discriminated in figure 1. As it is, it is very confusing.
Fig 2 & 3
I can't see why the two color status are not in the same figure, by shrinking the tree branches. Actually, if you manage to include the bird in figure 1, deleting the two stork photos, it would make the whole figure more informative.
Table 1
I can't understand the order of the body areas. It is not by order of body location (not even divided by frontal/ventral) and also it is not ordered by log-likelihood, what I thought could be more informative.
Table 2
I would suggest to merge this with table 1. Just add two columns for the degree of conservation and % of the body surface and if ordered by log-likelihood it could be more informative.
Line 115
"with the ER models being the better fitted"
First, only in line 131 you tell exactly what is a ER model. Second, exactly which alternative models were fitted. Third, how did you concluded this was the better fitted? Which summary statistics were used? Probably they should be stated.
Line 117
AUTEUR is a method, that uses reversible jump Markov chain Monte Carlo machinery. The phrase as it is is not very clear.
Line 120
The log-likelihood classes are all positive values, but the table only has negative values. What exactly is right?
Line 178
I would emphasize that you are only talking about transversal or shared selective pressures. I disagree that color divergence only emerges when selective pressures do not operate. Actually, you contradict yourself when you say later that some parts diverge due to selective pressures due to species recognition. There are just other kinds of selective pressures (e.g. for species recognition in overlapping distributions of sister species) that also are operating. I would suggest to clearly discriminate between niche selective pressures, due to the use of similar habitats, transversal to many stork species, that is what I think you refer to with your first phrase and not to overall selective pressures.
Line 212-226
There seems to be a lot of confusion regarding what the "species isolation hypothesis" is. This only states that differences in plumage color between species (not genders!) arose to decrease the probability of hybridization. It is not a type of sexual selection. Please revise this paragraph altogether.
Author Response
Ref 2
The authors describe a comparative analysis of the conservatism of colouration on the whole body of storks.
I find that the article rationale is correct, but it lacks a correct implementation and critical interpretation of the results and the limits of the methodology:
- There seems to be quite a lot of confusion regarding the causes and mechanisms of the evolution of colouration. Some of these confusions arise from not discussing in more detail the types of pigments that are involved. It is difficult to mix the causes of melanin use with the use of carotenoids. Not only their origin (metabolized, versus diet-related and metabolized). Also some relate to body condition and their use as a proxy for sexual selection (red carotenoids). On the other hand, Iris color is related to pterins and iris vascularization.
Paragraph modified as follows:
Pigments involved in iris coloration include melanins, carotenoids and pteridins [5,6]. In fact, even though there are no stork species sexually dichromatic in relation to plumage coloration, the two species of Ephippiorhynchus are sexually dichromatic for just iris coloration [6]. Many stork species have bare portions of the head, or even the whole head and neck. Sometimes the exposed skin is colored, either by melanin, carotenoids, or by the haemoglobin in circulating blood through skin vessels [7]. The metabolic pathways for each of these pigments differs, as well as their signaling potential. Melanins and haemoglobin are endogenously synthesized by the birds, whereas carotenoids are necessarily ingested from the diet [8]. Carotenoids may be linked to foraging efficiency and also to body condition and thus individual quality [9]. Therefore, carotenoid-dependent coloration is more often related to sexual selection than melanin-dependent coloration, which in turn is largely responsible for composing cryptic patterns [10,11].
- Regarding the conservatism, some specific examples could help on your discussion. Do melanin deposition on flight feathers tips and areas more exposed to UV is transversal to all species?
Modified and expanded:
The conservativism of a phenotypic character can be defined as its maintenance in a group of species or phylogenetically related lineages. Conservatism is maintained by a selective pressure, related to niche, over a functional character [45]. Regarding color of the dorsal area and belly, we suggest that most of its surface accomplish an adaptive function, and this is the reason for a selective maintenance of overall coloration and thus, conservativism. For instance, remiges and rectrices may be coloured black, even in species with overall white coloration, such as the white stork, because melanin is known to provide structural resistance to mechanical breakage and abrasive wear [46,47]. On the other hand, the color of smaller body regions may be less important for functional adaptation and could diverge due to the mechanism of character displacement driven by the need for species recognition.
- I would like also to understand if the percentages of cover were taken into account in the models. If not, discuss if this has an impact on the model or not.
The models have only tested the degree of conservatism of the colour character of each area. The percentage of body cover has only been considered when characters have been classified following the degree of conservativism. The areas with low conservativism sum a low percentage of body cover.
In more detail, I include some specific comments:
line 34
"they comprise all-white (e.g., the wood stork Mycteria americana)"
Mycteria americana has the whole primaries, secondaries and tertiaries black. Please rephrase the sentence.
We have added the word “almost” in order to avoid confusion. Thanks for pointing this out.
Line 51
In this paragraph I would also add the role of melanin to increase resistance to damage in specific feathers such as flight feathers.
Added in discussion, along with two new references:
For instance, remiges and rectrices may be coloured black, even in species with overall white coloration, such as the white stork, because melanin is known to provide structural resistance to mechanical breakage and abrasive wear [46,47].
Figure 1
It is important in the Figure 1 to add the Initials for each character that is used in Figure 2 & 3. Also, these initials do not link directly to the sections identified in figure 1. I think that all Initials and names should be discriminated in figure 1. As it is, it is very confusing.
We have reworded the caption to clarify this. Figure 1 is based in Yezarinac & Weatherhead (1995), and includes the same areas but recalculated using body proportions typical of Ciconidae. We have also measured the cover of each area used in our analysis. We have linked both in the caption providing specific cover percentage when needed:
Regions of the stork body scored for coloration. The percentage of the surface of each region has been averaged following Yezerinac and Weatherhead 1995. The dotted red line separates frontal from rear areas of the body. This figure is based in the areas of the mentioned authors, and recalculated using Withe Stork proportions, thus, relating this figure with selected areas of figures 2 and 3: Bill(B) correspond with Bill. Head (H) corresponds with Chin, crown, and nape. Iris (I) with eye. Neck (N) includes part of breast and dorsal area of the neck (It is not represented in figure 1 but calculated percentage of cover is 4%. Facial skin (F) corresponds with Chin without eye in the cases of facila skin is present. Ventral (V) with belly and part of breast. Remiges (R), Dorsum (D) and Tail were measured as 50% of cover. Finally, Legs (L) corresponds with legs on this figure.
Fig 2 & 3
I can't see why the two color status are not in the same figure, by shrinking the tree branches. Actually, if you manage to include the bird in figure 1, deleting the two stork photos, it would make the whole figure more informative.
We tried to merge the two figures, but the amalgamation of colour squares made difficult the interpretation of a single figure. In addition, using two figures we neatly separate the color variability of frontal versus not frontal characters. One figure results very colourful, whereas the other is not.
Table 1
I can't understand the order of the body areas. It is not by order of body location (not even divided by frontal/ventral) and also it is not ordered by log-likelihood, what I thought could be more informative.
This table is not ordered following any criteria and it is not our aim. It is only the results of log. Likelihood. This information is available in table 2, where the are ordered by conservativism degree and percentage of cover.
Table 2
I would suggest to merge this with table 1. Just add two columns for the degree of conservation and % of the body surface and if ordered by log-likelihood it could be more informative.
We agree, but we want to show the summed percentage of each cluster of conservatism degree. This allow to clearly see how non conserved areas or medium conserved areas sum a low percentage of cover.
Line 115
"with the ER models being the better fitted"
First, only in line 131 you tell exactly what is a ER model. Second, exactly which alternative models were fitted. Third, how did you concluded this was the better fitted? Which summary statistics were used? Probably they should be stated.
How we have concluded ER models are the better fitted was already explained in the text:
Models were fitted using different Bayesian methods included in the R Geiger package 2.0 (Pennell et al. 2014). These methods include the Accommodating Uncertainty in Trait Evolution Using R (AUTEUR) following Eastman et al. (2011), including reversible jump Markov chain Monte Carlo machinery (Greeen, 1995) to test multivariate models of various complexities
Other models tested were SYM and ARD
We recognize the paragraph may be confusing and thus, we have added the mentioned information about other modes
As recommended in Münkemüller et al. (2012), Geiger models, based on maximum likelihood, were constructed to test the conservativism of characters in relation with phylogeny, with the ER (equal rate transitions) model being the better fitted. Other models tested were SYM (Symmetric transitions are equal) and ARD (all rated different model). Models were fitted using different Bayesian methods included in the R Geiger package 2.0 (Pennell et al. 2014). These methods include the Accommodating Uncertainty in Trait Evolution Using R (AUTEUR) following Eastman et al. (2011), including reversible jump Markov chain Monte Carlo machinery (Greeen, 1995) to test multivariate models of various complexities.
Line 117
AUTEUR is a method, that uses reversible jump Markov chain Monte Carlo machinery. The phrase as it is is not very clear.
Our sentence reads as follows, and we believe it is consistent with the reviewer’s proposal:
These methods include the Accommodating Uncertainty in Trait Evolution Using R (AUTEUR) following Eastman et al. (2011), including reversible jump Markov chain Monte Carlo machinery (Greeen, 1995) to test multivariate models of various complexities.
Line 120
The log-likelihood classes are all positive values, but the table only has negative values. What exactly is right?
It is true, sorry for the mistake and thanks for pointing it out. We have fixed it
: log-L ≤- 30 low; -30 < log-L ≤ -20 medium; -20<log-L ≤-10 high; log-L >-10 very high
Line 178
I would emphasize that you are only talking about transversal or shared selective pressures. I disagree that color divergence only emerges when selective pressures do not operate. Actually, you contradict yourself when you say later that some parts diverge due to selective pressures due to species recognition. There are just other kinds of selective pressures (e.g. for species recognition in overlapping distributions of sister species) that also are operating. I would suggest to clearly discriminate between niche selective pressures, due to the use of similar habitats, transversal to many stork species, that is what I think you refer to with your first phrase and not to overall selective pressures.
Reworded as follows:
The conservativism of a phenotypic character can be defined as its maintenance in a group of species or phylogenetically related lineages. Conservatism is maintained by a selective pressure, related to niche, over a functional character [45]. Regarding color of the dorsal area and belly, we suggest that most of its surface accomplish an adaptive function, and this is the reason for a selective maintenance of overall coloration and thus, conservativism. For instance, remiges and rectrices may be coloured black, even in species with overall white coloration, such as the white stork, because melanin is known to provide structural resistance to mechanical breakage and abrasive wear [46,47]. On the other hand, the color of smaller body regions may be less important for functional adaptation and could diverge due to the mechanism of character displacement driven by the need for species recognition.
Line 212-226
There seems to be a lot of confusion regarding what the "species isolation hypothesis" is. This only states that differences in plumage color between species (not genders!) arose to decrease the probability of hybridization. It is not a type of sexual selection. Please revise this paragraph altogether.
Reworded
The “species isolation hypothesis” should apply to species at high risk of finding the wrong mate because they easily encounter both conspecifics and members of other species with similar phenotypes. The majority of the storks are highly social and tend to forage sympatrically with other stork species or other large wading birds, exhibit delayed plumage maturation and delayed reproduction [53]. In this regard, it may be advantageous for an individual, whether sexually mature or not, to recognize not only compatible mates, but also conspecifics and their different ages to decide, for instance, to join a flock about to move among foraging patches and/or roosting sites, or even departing on migration [54].
Round 2
Reviewer 1 Report
The authors did a commendable job dealing with previous comments. I have no further suggestions for improvement.
