Peer review of: Sexual signaling pattern correlates with habitat pattern in visually ornamented fishes (2020)



Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
Let me preface this review by saying that I still think this is a wonderful data set that I think 
deserves to be published without needing to collect more new data. The article has been 
greatly improved for readability, and the authors have done an excellent job of responding 
to my and other reviewers' concern except for my major one, that the paper as written 
advocates without evidence for the role of female evaluative biases in shaping the observed 
pattern. I hope that my critical comments are taken as constructive, as I think the paper 
can become deeper and more robust with some moderate scholarly and analytical work. 
 
The data are consistent with processing bias, which posits that stimuli are more attractive - 
they have higher hedonic value - BECAUSE they match the spatial-frequency tuning of 
females. This is testable only by measuring female neurophysiology and behavior. 
 
If anything, there is more support for the alternative, but certainly not exclusive, prediction 
that spatial-frequency matching of bright colors evolves in response to natural selection by 
fish predators detecting prey against background. This is consistent with numerous 
empirical studies and with first principles of psychophysics, while the interesting hypothesis 
of processing bias is largely untested outside humans. 
 
These two hypotheses are distinguishable only by examining the biology of receivers, and 
are by no means exhaustive. Upon rereading the ms, I was surprised not to find any 
discussion of body size or sexual size dimorphism. If male but not female body size scales 
with the spatial statistics of both male ornament and the habitat, this confounds the 
observed relationship: non-visual sources of selection like nest characteristics could explain 
variation in male body size. First-order estimates of size-habitat relationships and of 
conspicuousness to piscivorous fish (dichromats, not "color-insensitive" as I ineptly put it in 
my first review) seem like they would be easy to glean from the literature or perhaps from 
your group's existing data sets. 
 
I fear that by advocating so strongly through the lens of evolutionary aesthetics, this study 
will join the host of papers making sweeping claims about the minds of receivers purely by 
focusing on signaler biology. The hypothesis of processing bias has yet to be tested for 
spatial frequency in darters. 
 
In sum, this will be an outstanding, enduring empirical study if it presents a fair and 
agnostic discussion of alternative hypotheses, opening new ground by setting up testable 
hypotheses for signal evolution that rely on sensation and evaluation beyond the periphery. 
 
Gil Rosenthal 
 
 
 
Reviewer #2: 

Remarks to the Author: 
 
In this paper, the authors take calibrated photos of males and females from 10 different 
darter species and from various habitat types. They take a minimum of 50 photos for each 
of the sites where they collected the fish. They then ran a series of fourier transforms where 
they examined the power contained in the image as a function of cycles. So for each image, 
they calculated the slope between the power contained for each of the various cycles. The 
authors report a statistically significant relationship between the slopes that represent the 
habitat type of each species with the slopes of males, but not females. However, both of 
these are positive. 
 
The interpretation of these data is that the male secondary sex characters are tuned by 
'efficient information coding. If this is right, then it would be a pretty important paper to the 
field. However, there are a few things in here that give me pause. 
 
First, I am confused about figure 2. Figure 2 shows a photo of a male and a female darter 
and then a photo of the habitat. It illustrates how the analysis considers a square photo of 
both the fish and the habitat. The problem is that they do not represent similar areas. The 
area of the box for the male darter can't be more than 2cm x 2cm. The area of the box for 
the habitat has to be at least 8cm x 8cm. I actually think that it might be much higher than 
that. This strikes me as problematic. I assume that the 'cycles' used in the fourier transform 
actually correspond to some type of unit. I am not an expert in the fourier transform, but 
my main understanding of this is that it was originally developed for sound and is often 
used for things which vary in time. The fourier transform converts something from change 
over time to change over frequencies. But the frequencies mean something. In this case, I 
am worried that we are comparing apples and oranges because the 'bins' from one picture 
do not necessarily correspond to the same scale of magnitude for another picture. Again, 
the picture showing the very close up pattern on the side of the rainbow darter and its 
similarity to the dappling pattern in the stream make me wonder about issues surrounding 
scale. 
 
The other thing I wonder about is the intercept. What would it mean if the species and the 
habitat had similar intercepts? You can have similar slopes with wildly different intercepts. 
Does that happen here? 
 
A second, related issue has to do with the resolution of the darter visual system. Eleanor 
Caves and others have recently published methods that focus on what types of details 
different animals can resolve with their visual systems. The high close-up in the picture with 
the darter scales makes me wonder whether the animals can see these details at all. This 
gets back to the issue of the scale of the pictures. The fish can probably resolve the large 
scale details of the habitat, but whether they can resolve the fine-scale details of the darters 
is unclear. 
 
Third, I don't understand why the authors pooled the pictures into five different habitat 
categories as opposed to using the pictures from the individual's own habitat where it was 
collected. The authors have data for each individual from its own habitat where it was 

collected. These should represent the best data for determining what makes for 'efficient 
coding'. Instead, the authors pooled the data for each of the five habitats and then looked 
at the correlations between the average slope for the habitat type and the slope for each 
individual species. This seems like it would result in a reduction in the power to detect the 
true relationship. I also wondered about the level at which this study is being performed. 
Are the slopes between the males and the habitats for individuals or for species? The 
interpretation (and figure 5) suggest species, but the text suggests individuals. I think that 
the data are sufficiently robust that the same patterns would emerge at the species level. 
This would get around the need to include site nested within species as a random effect. 
 
Here's another one that I am confused on, " 
 
Fourth, if you look at the plot between the habitat and the males, it looks like the 
relationship is driven by 3 species. The other 7 data points show not much of a pattern 
between the male and the habitat slopes. A very frequent statistical mistake is to say that 
two things are different because one of them has a statistically significant effect and the 
other does not. If you look at the 95% confidence intervals for the slopes, you can see that 
both the male and female 95% confidence intervals include the mean of the other. So this 
means that their slopes are not considered different from one another. Yes, they did a 
second analysis to try to get a sex difference, but we can see in the figures and in the stats 
for the slopes that they are both positive with one of them being more positive than the 
other. 
 
Lines 117-126 talk about the correlation between the slopes, but the numbers and 
confidence intervals do not refer to correlations. These are slopes, not correlation 
coefficients. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
First, I’d like to thank the authors for their detailed and helpful explanations on my 
comments. The revised version of the manuscript is much improved and, in my opinion, 
suitable for publication if the following minor issues were addressed. 
 
Line 120ff: For better readability, I’d move the last insertion of the sentence (“here, mean 
corresponds to effect size, pMCMC is the Bayesian equivalent of a frequentist p-value”) 
inside the other parenthesis (i.e.: “[…] 95% CI = [0.0978, 0.437], here, mean corresponds 
to effect size, pMCMC is the Bayesian equivalent of a frequentist p-value; Figure 5a, Table 
1), consistent with […]” 
 
Figure 5b: In my opinion, no regression line should be included in Figure 5b because the 
correlation is not significant. It might lead to a misleading conclusion if one is only checking 
the figures and not the text in the results section. 
 
Discussion, esp. lines 141ff: To strengthen the main finding, the authors may empathize 

that the correlation was only found for males but not females, since this sex difference only 
allows the conclusion on sensory drive and sexual signals. 
 
Line 157ff: “artists’ portraits of human faces mimic the Fourier slope of natural landscapes 
more closely than images of human faces do” -> to make the difference more clear for a 
naïve reader, I’d suggest “artists’ portraits of human faces mimic the Fourier slope of 
natural landscapes more closely than PHOTOGRAPHS of human faces do” 
 
Line 158: “Graham & Redies13 summarize evidence” -> “Graham & Redies13 summarized 
evidence” 
 
Line 174: “natural-like statistics46,59” -> both references are the same (Párraga et al) 
 
Line 175ff: “Because the foreground and the background of visual scenes are not processed 
independently, a strong difference in their spatial statistics can generate sensory 
competition60, which would further favor sexual signals that match their background.” -> I 
don’t understand this conclusion. The stronger the difference in spatial statistics the lower 
the sensory competition; competition should be higher the more similar the stimuli are. 
 
Line 180/1: “the match between color pattern and habitat is better” -> since grayscale 
images were analysed, I’d suggest to delete “color” in that sentence 
 
Line 194: “Lacking color,” -> since the female fish are not monochrome I’d rather write 
“Lacking pronounced color” or similar. 
 
Dr. Claudia Menzel, University of Koblenz-Landau, Germany