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  • Andy Davis

How the monarch got its spots: an overview of the new research



Hello blog readers new and old,


Welcome once again to another interesting post about the latest and greatest research around the critter that we all know and love. Today's post is all about some very exciting research from my own lab, which has been in the works for about 2 years now. The study has just been published, and if you can't tell, I'm super-pumped about it. This just might be the most exciting project I've ever worked on, because the findings not only uncover something previously unknown about monarchs, but they also have some real-world application for the aerospace industry! Sit back and let me tell you about it...


Before getting too far, let me put in a link to the actual paper itself, published in the science journal, PLoS One (which is freely viewable) - link here. You may see some media coverage of this paper too, or hear about it when it makes the rounds on social media. This project was a collaboration between myself and some mechanical engineers (yes really) from New Mexico Tech, who study, and build, flying machines like drones or even spacecraft! My colleague in this endeavor is Dr. Mostafa Hassanalian, who has been lately all over the news for another of his projects, where he built a flying drone out of bird feathers (google "bird drone"). Anyway, his lab is a leader in the engineering field of "bioinspiration", where research is conducted on flying animals, to learn how they have overcome the challenges posed by flight, and then these findings are used to help design better aircraft. Cool right? Every time I zoom with his people or see photos of them working, I see his lab which looks like something out of Tony Stark's garage - gizmos and contraptions everywhere, wires, computers, machines. It's very high-tech and futuristic. In fact, I once did a blog (link here) describing some of this "bioinspiration" work when he and I were in the early stages of this collaboration.


Anyway, for a number of years, this lab has studied the migrations and flight patterns of birds, like albatrosses, seagulls, and even penguins (i.e. how they swim), to learn their secrets. Through this work, Mostafa has discovered that the colors on animal wings (or even flippers) can actually have aerodynamic effects on the flight performance, especially if the colors are black or white. Black pigmentation, simply because of physics, tends to heat up faster than white color, when in the sun. If the black color is on the top surface of the wing, then this actually creates a mini-zone of heated air just above the wing surface, which reduces drag. This black surface then, actually improves flight efficiency! Their team has actually built model wings and put them in wind tunnels to prove this. So it seems that a lot of bird species have wings that are specifically colored in a way that improves their flying! This is something that up until now, no one has ever considered, even going back to the days of Darwin.


So a couple of years ago, their team started looking into the flight ability of monarchs, and this is where our two research paths crossed. I have also studied how the wing colors of monarchs is related to their migration success, although in a slightly different way. My prior work showed that the shade of the orange colors of monarchs is positively correlated with their flight ability. Anyway, their team read some of this work and began looking into the monarchs further. At some point we connected and then began this collaboration. It has been a fun journey so far - they have been teaching me about the finer points of aerodynamics, and I'm teaching them about monarch migration and their biology. Together we feel there is so much we can learn from the monarchs, which when you think about it, are the world's best flying machine. This critter, which weighs less than a paper clip, can traverse thousands of miles in two months, using just a smidge of energy!


So the initial goal of this first paper was to determine if the amount of black on the monarch wings was important for the migration, because of the prior research on birds. In fact, we actually did not plan to study the monarch spots at all. We were initially looking to see if the black pigment on monarchs affects their migration. Going into this we thought that we would see darker monarchs being more successful (spoiler - this was not the case).


Anyway, the entire paper was based on precise digital measurements of monarch wing images (not live monarchs at all). And, there were two parts to the study. In the first part, we examined some archived monarch wing images I have been accumulating over the years. Basically, between myself and my collaborators, and over the last 20 years, I now have thousands of images of monarchs on my computer, from various older projects, and they mostly look like this...



These are computer scans of live monarchs (yes really, on a flatbed scanner!) from a prior project years ago. I have thousands of these. I think of these images as "digital museum specimens," because they can still tell us a lot, and can be used for many future projects to come. Anyway, we did a deep dive into my computer archives and pulled out very specific collections of monarchs for comparison. We had images of monarchs from the breeding period, some from the migration period, and then some from the overwintering sites in Mexico. We figured that the monarchs that were in Mexico represented the "successful" migrants, since they had obviously made it there. Those from the breeding period obviously hadn't started the migration yet. In total there were about 375 monarch wings for this part.


We then performed some very precise, fine-scale computer measurements of these specimens, to determine how much black was on each wing, in terms of the overall percentage of the wing area. We also computed how much orange there was, and then finally how much was white. See the images below of each of these steps. The idea was to see if the "successful" migrants had more black on their wings than the other groups, which we figured would make sense, based on some of Mostafa's work on bird flights. We figured that if black was important, then the monarchs with more black would be more represented in Mexico, because it provides an aerodynamic advantage, over those with less black (or so we thought).



We were wrong. It turned out that the monarchs in Mexico had slightly LESS black on their wings than those that had not yet started the journey, or even those that were midway along the journey. About 3% less. And then, it turned out that they also had about 3% more WHITE on their wings. In other words, having more slightly white pigment on your wings leads to greater migration success. Who knew? See the charts below, which make it clear.



OK, so I know what you're thinking - the difference between the breeding and wintering monarch spot sizes (3%) is not a lot. And, it is nearly imperceptible to the human eye, but, take a look at these two pictures below (which were both taken by my mother!). The one on the left is typical of a monarch in the summer, and the one on the right is what they look like when in Mexico.



Can you see it now? Cool right? Actually, when you really look close at pictures of overwintering monarchs, and if you know what to look for, it really becomes obvious. In fact, it is hard to un-see it, once you do see it. I suspect that this pattern has largely gone unnoticed for decades.


So I also know what you're thinking now - no, we don't think the monarch spots actually change color during the fall migration. We think this is more of a natural selection thing, whereby all of the monarchs begin the migration (those with big spots, small spots, or orange spots), but, then only those with the biggest, whitest spots successfully make the trip. Those with small, or orange spots tend to drop out.


OK, so the findings from this first part of our project caused us to change direction a bit, since we realized that the white color seems more important than the black. So then we decided to look at this question from an evolutionary standpoint, and to look more closely at the close relatives of monarchs, i.e. those that do not migrate. There are a number of similar-looking species and subspecies of Danaids in the Americas, and they, along with monarchs, all probably evolved from a similar-looking common ancestor. We figured that if white spots are important for migration, then the migratory monarch would have evolved with larger, whiter spots. Conversely, those butterflies with no migration would never have this selection force, and their spots would not evolve to be bigger.


We once again dug into some archived images of butterflies for this part. It was one of my colleagues, actually, Paola Barriga, who had the images we needed. She had done a project a few years ago where she had spent some time in museums, photographing butterfly specimens of various species. Here she is doing that. She is a co-author on this study, because her images were so useful to this project.



For that project, Paola had photographed a number of monarch relatives, which I've shown below in this list, along with images of them. When you look close you can tell that these all have the same basic wing design, with orange/reddish backgrounds, black edges, and some spots along the edge. That's a sign that they are all related, and all evolved from a common ancestor.

Once we had all of the butterfly images we needed, we once again used computer image analysis to measure what we wanted. In this case, we measured the actual size of the marginal spots on the butterfly forewings, as shown in the images below. Actually, I'm using the royal "we" here, but in reality it was a super-student named Christina Vu who did this, and she is a co-author of the paper. Each spot was digitally traced and "we" then measured their surface area, and calculated how much of the wing surface is covered by these spots (i.e. the percentage). I'm quite sure that this is the first time anyone has done such detailed measurements of butterfly spots before!



So, once we had measured the size of the spots for all of the monarchs, plus each of the New World Danaid butterflies, we then looked to see which butterfly species (or subspecies) had the largest spots. And, you guessed it, the monarchs did. The graph below shows the (average) size of the marginal spots of each butterfly, relative to its wing area. The dark green bars are the monarchs, broken down into their breeding, migration and wintering stages. The light green bar are the "Southern Monarch" which is a related species in South America, and it has a partial migration. The rest of the bars are species that have no migration.



Once again, if you look at the numbers, these differences don't seem that great, even though there is a statistical trend here. Basically, these numbers tell us that migratory monarchs tend to have spots that cover about 2.5% of their wing surface, while in non-migratory species they cover about 1 to 2 percent. These are ridiculously small differences, at least to us humans. For monarchs though, this seemingly minor difference may be all they need to improve on their aerodynamic performance. Keep in mind that they travel thousands of miles in their journey, flying for 10 hours a day. Any small positive boost they can get could be the difference between migration success and failure.


So this part of the paper basically confirmed the first part, and confirms that spots are somehow important for the migration. Cool.


OK, so now you're wondering what in the world do these spots actually do? Well, we have a working theory, which is that the spots are acting to reduce the aerodynamic drag of the wing, as the butterfly flies through the air. The position of the spots are such that they are arranged in a repeating black and white pattern, almost like zebra stripes, and all along the trailing edge of the wing (i.e. when monarchs are soaring). Since we know that the black portions are heating up under the sun, but the white spots are not, this would create a confluence of mixed air above the wing, or micro-eddies, if you will, that rise up. This rising air along the trailing edge of the wing may be just enough to enhance the flight efficiency. This is our working theory, and it is based on some verified findings from Mostafa's lab.


Anyway, we have some experiments planned to confirm this hypothesis, using some wind tunnels, monarch wings, and even some robotic, flapping monarchs! Stay tuned for this. In fact, we plan to continue studying monarchs and their wings in the tears to come, as a way to not only learn about how these insects are able to make such a tremendous journey, but also, as a way to learn how to improve on man-made aerial vehicle design. Think about the implications for drone technology, for example, if it turns out that their flight efficiency can be improved by simply adding some white (and black) paint to their wings. This right here, is the essence of the "bioinspiration" field.


So, I think this just about covers the story behind this new study about monarch wings and "how the monarch got its spots." It's the migration itself that "gave" the monarchs its spots! Cool right?


Thanks for reading, and remember, if you wish to comment on this study, or to participate in more discussions of it, this blog will be posted in the Facebook group, "The Thoughtful Monarch" (https://www.facebook.com/groups/565065511941624). If you have an interest in the science around monarch butterflies, feel free to join.


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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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