• Andy Davis

Five new studies on monarch migration published together


Happy holidays blog readers,

I have some very exciting stuff to talk about today, as you can see from the blog title. This week, a "themed collection" of studies on monarchs was published together in the journal that yours truly is the editor of, Animal Migration. This is a project that has been in the works for a year now, and it is finally coming to fruition! So for today's blog entry, I'm going to provide a brief overview of a few of these articles, and boil down the science in them as best I can. I'll just focus on the first three from this collection, so as not to overwhelm the readers of this blog, but even still, this may take some time, so you'd better get a cup of coffee and settle in! Or better yet, read this post in stages. You've been warned - it's long.

The first thing you need to know is that this journal is entirely open access, which means that anyone can download the articles! So here is a link to the issue where the monarch articles are. You may want to download the articles and read them first, or read my blog summary first. I don't think the articles are extremely technical, so either is fine.

I'm going to try to break these down in the order in which they appeared in the collection.

After the brief introductory article I wrote, the first data paper is one by Micah Freedman and Hugh Dingle, from the University of California, Davis. These are the same researchers who had brought us another very interesting study from a year ago, concerning the ability of resident monarchs to switch to being migrants (see previous blog).

Their new paper was about another very exciting study with some profound results - the authors examined museum specimens of monarchs from throughout North America, and dating back over 100 years! They were interested in determining how monarch wing morphology differs across different regions of the continent, and, if it has changed over time! Think on this last bit for a second. To do this, they carefully(!) photographed over 1800 monarch specimens from a number of museums, and then used computer-based software to measure the wings of all of the monarchs. I'll paste a screenshot of their first figure, which shows the photographing setup at one of the museums they visited, then I'll paste their map that shows where all of the specimens originated from (these are where the butterfly came from, not the location of the museum!).

Note from this map that their monarch samples came from both the western population, and the eastern population, and some of these also came from overwintering areas from either population. I believe they excluded the monarchs from Florida. Also, don't get caught up in where they drew their line separating east from west - it was an arbitrary distinction.

For the wing morphology assessment, they measured the overall surface area of the forewings, to use as an index of wing size, and they also measured the elongation of the wings, as a measure of wing shape. These are pretty standard measurements these days for monarch research projects. Actually, this project was a nice extension of some previous work - from a number of previous research projects, we know that migratory populations of monarchs are bigger than non-migrant populations, and we have always assumed this was because of the selective force of the migration. That's a fancy way of saying that during the long-distance migration, the monarchs with the largest wings tend to be the most successful in reaching Mexico, and only these individuals survive to pass on their genes to the next generation. Thus, over time, the population gets larger. This has been our working theory for many years. Well, Freedman and Dingle found exactly this! Their analyses showed that monarchs in North America have been gradually getting bigger over the last 100 years! Not only that, they found that overwintering monarchs (at either eastern or western sites) tend to be larger than those collected in the breeding range, of either population. This also points to the selective force of the migration - only those with large wings survive the trip to the overwintering site. And as icing on this cake, they then demonstrated that those monarchs at the eastern overwintering sites tended to be larger than those at the western overwintering sites, which makes sense because the selective force would be stronger for the much longer, eastern migration.

There was another element of this paper that makes it interesting. The authors included results from a separate lab experiment, where they had reared monarchs on four different milkweed species, to see if this affected their eventual wing size. In fact, it did. Apparently, monarchs reared on common milkweed, Asclepias syriaca, ended up being the largest. Below is their graph demonstrating this.

The authors used these results to help explain the previous one - that monarchs have been increasing in size over time in North America. They argued that the overall breeding landscape has changed in the last 100 years, in a way that seems to support more common milkweed. If you recall, there was a non-peer reviewed study that I blogged about a few months ago, which seemed to support this conclusion - that common milkweed has increased over time. So in a nutshell, Freedman and Dingle argue that since common milkweed promotes larger monarch size, if common milkweed is becoming common-er, then you will get larger monarchs over time.

If I sound a little skeptical about this last bit, it's because I am. While I don't doubt their lab experiment data, or their analyses of it, there was actually another study in this same collection of papers that also examined how milkweed types affect wing size (I talk about this paper below), and that study did not show the same thing. Moreover, there was some other work done (by a student named Victoria Pocius) in the last couple of years that also doesn't seem to support this (link here to that paper) - she showed that adult mass did not differ from monarchs reared on 9 different milkweed types, including common milkweed. So it's not that the Freedman and Dingle data are wrong necessarily, but I wonder if there is something else going on here. Perhaps it has to do with the varying environmental conditions of the three studies; maybe monarchs grow large on common milkweed only under a specific temperature, or only when the milkweed has enough nutrients, or something like this, and maybe their experiment just happened to use the right combination of milkweeds and growing conditions. If the common milkweed really does produce the largest monarchs, we would see this same trend in all three studies. So I think the jury is still out on which milkweeds produce the largest monarchs.

Aside from the milkweed element, their paper is an excellent contribution to the collective annals of monarch science, because it makes it clear that the monarch migration and monarch wing size go hand in hand. The migration promotes large wings, and large wings are necessary for successful migration.

(ok, take a sip of coffee now, I'm moving to the next paper)

Next up, is a study that was led by Dr. Hannah Vander Zanden, an assistant professor, from the University of Florida. This paper was also exciting, because it seems to upend what we thought we knew about the fall migration of eastern monarchs!

This study focused on monarchs that winter in south Florida. We have long known that there is a non-migratory population in the southern end of Florida, or at least, there are monarchs year-round down there, and we assume this is a resident population. We also know that these monarchs are chock-full of OE. There is OE everywhere down there. In most samples, nearly 100% of the adults are infected. I know this bit about OE is a little tangential, because this paper wasn't about OE, but one of the long-standing questions about the south Florida population, is why doesn't the OE just wipe them all out? What keeps this population going if they are all sick? Apparently the answer is that this population gets a fresh infusion of healthy monarchs every fall, according to this new paper. And where these fresh monarchs come from is crazy.

The group of researchers on this study set out to determine the origins of monarchs found in the South Florida region, using a special technique called stable isotope analysis. I'm not an expert on this technique, but apparently, you can grind up a sample of the wing tissue from an adult monarch, run it through some fancy equipment, then you can tell where the specimen grew up based on its "isotopic signature". Apparently, it has to do with the isotopes in the ground, which then get into the milkweed, and then the monarch. Each region of the continent has a specific isotopic signature, and using this technique, a scientist can basically determine where an animal grew up. The researchers here used this technique to determine if the monarchs they sampled were Florida residents, or non-residents (meaning the individual came from elsewhere).

First, here is a map of their collecting sites, along with pie charts that show the initial results of their isotope analyses. Remember, all collecting was done during the winter.

One of the first things that jumps out here is that about half of the collected specimens appeared to be non-residents, or in other words, migrants. I know what you're thinking - but how do they know they were migrants? Well, the next figure will blow your mind. These isotopic analyses remember, can pinpoint (more or less) where those individuals grew up. Below is a figure that shows the predicted region where the migrant monarchs came from. The darker shading represents the area with the highest probability, based on the tissue analyses.

Crazy, right? If I interpret this figure correctly, this is saying that most of the migrant monarchs found in South Florida came from the Midwest, or the core breeding range of the eastern population. Some were even predicted to have come from Northern Texas! Note that this is a map that combines all of the locations for the migrant specimens. If you look at the article online, you'll see that there is a supplemental file you can download, which has individual maps for each of their migrant specimens.

We have always assumed that the winter destination of the eastern breeding population is the mountains of Central Mexico. But what if it isn't? What if they don't ALL travel to Mexico? And (think this through, now), what if over time, greater and greater numbers of monarchs are choosing to travel to these "alternate" winter destinations, like South Florida? Wouldn't that mean the Mexican overwintering colonies would slowly decline in size? Hmmm...

I know, this is crazy. But it's hard to deny the results from this study. To be fair, we had some inkling from past work, that was based on a different analytical technique, that the South Florida population gets an influx of migrants each year. We've always thought that these were "wayward" migrants, or those that accidentally wound up in Florida. If you think about it, there are a lot of monarchs that migrant southward down the Atlantic coast, and if they keep going south, they'll eventually arrive in Florida. And then they realize, hey, it's kind of warm down here, and hey, there's some milkweed over there... maybe I'll just start breeding. And eventually, they decide to stay. That's the scenario we thought. But this new data makes it seem like some Midwestern monarchs actually are CHOOSING to migrate to Florida. I mean, how could monarchs from the Midwest have accidentally wound up in Florida? For a monarch in the Midwest to end up in Florida means that they would need to have flown in almost the opposite direction as they should have. For those monarchs that came from Northern Texas, this is for sure. This makes it seem like it was purposeful.

To be fair, one knock on this study is the small sample size. They did not have that many specimens to play with. However, these data sure makes you think about what we think we know... I encourage you to look online at their supplemental maps and judge for yourselves.

(ok, next sip...)

Next comes a study that is close to my heart, as in, I wrote it! And before you say it, no, I did not edit and approve my own paper. We had a guest-editor for this collection (Keith Hobson), who served in this role, and he made sure this paper was adequately peer-reviewed (it was), and after careful revisions, it was accepted.

I collaborated with my colleague, Jaap de Roode, from Emory University on this. As you may recall, Jaap is one of the foremost experts on the monarch parasite, OE. Over the years, he has conducted many experiments in his lab, where he and his students rear monarchs under different conditions, parasite loads, temperatures, etc. And after most of these experiments are completed, he is left with many (deceased) adult monarch specimens, which he archives in a giant freezer. For one of those experiments conducted years ago, he had reared a bunch of monarchs on different milkweed species, and, inoculated some of them with the OE parasite. This left him with a whole bunch of perfectly preserved specimens. He and I had got to talking about those one day some time ago, and we thought it would be neat to have a closer look at those monarchs, using computer-based image analyses software. So we did!

Officially, the stated goal of the project was to determine if the OE parasite causes any substantial effect to the wing morphology of monarchs. A secondary goal was to evaluate the effect of milkweed species on the wing morphology. We focused our efforts on examining three aspects of the wing:

1) the overall wing size, for obvious reasons (recall the first study...)

2) the wing color - many previous studies have shown how the redness of the wing is a predictor of migration success

3) the weight of the wing in relation to its size - we wanted to see if OE reduces the wing mass, or makes it less "sturdy"

As you can see, we were looking to see if the OE parasite substantially affects the development of the very thing the monarchs need for optimal migration - their wings! We suspected it does, because prior work has shown how OE prevalence becomes lower along the fall migratory flyway. This suggests that OE-infected monarchs are dropping out of the migration along the way. But why? Are they dying? Or, as we tested here, maybe their wings simply aren't up to the task. Maybe they're too small, as indicated above, or maybe they are not red enough, etc., etc.

To do this project, we first defrosted all of the specimens from the prior experiment. There were nearly 150 in total. We next removed one forewing from each specimen, then scanned it with a standard flatbed scanner (see below). This made a digital version of the wing, which we could then measure with our software, in much the same way as did Freedman and Dingle above. We then weighed each wing with a very, very sensitive digital scale. Fun fact - most monarch forewings are about 10-15 grams! Also, because larger wings will be heavier than smaller wings, we calculated the mass per unit area for each wing. This means that we accounted for differences in wing size.

We next performed some basic statistics that tested for differences (in each of the three variables above) between infected and uninfected monarchs, and between monarchs that were reared on 7 different milkweed species, including common milkweed.

Here's what we found:

- There was no statistical effect of OE on wing size (surprising!) - that means the infected monarchs were NOT smaller

- There was no effect of OE on wing color - that means infected monarchs were no less red

- There was a clear effect of OE on wing mass, per unit area. Infected monarch wings weighed less. Below is a figure showing this.

We used the word "Density" here to describe the wing mass measurement. This is essentially the weight of the wing, after accounting for its size. Another way to think about this is the "thickness" of the wing. So, this figure shows that OE-infected monarchs tend to have thinner wings than healthy monarchs. This difference was statistically significant and it was about a 6% difference. In other words, OE infection seems to impede the normal development of monarch wings during metamorphosis. This makes sense, considered that OE undergoes the heaviest replication (and therefore, tissue damage) during metamorphosis. By the time the butterflies emerge as adults, most of the damage to the tissue has been done, and all you have left is the inert spores covering the abdomen and other body parts.

So this last result got us thinking, does having thinner wings make them more prone to damage? So we tested this very question using a new gizmo in my lab (love my gizmos!). I set up a force meter apparatus which allowed us to gauge how much force it takes to cause a rip in the wing. Below is a picture.

You'll see that the lower part of the wing is secured in a bench vice, so it can't move. The blue thing is the force gauge which is attached to the tip of the wing. Basically, the force gauge gets lifted up until the wing tears, and then I can read how much force this took from the onscreen readout of the device.

So, we did this test on about half of the original specimens. AND, we also did this on a collection of monarchs that had recently died, because we wanted to be sure that our readings were not an artifact of the storage of the specimens. Again, here is a graph below showing these results.

We were glad to see that the results were basically the same whether we used the stored specimens or fresh ones, which means that the effect of storage on the wings was not an issue. If you compare the two graphs, you can see that the fresh specimens required more force to tear than did the stored ones, which does make sense.

OK, that about covers the parasite study. Let me finish with some thoughts about what these studies can do for our conservation efforts.

If you're an astute reader of this blog site, and/or if you keep up with the current scientific research on the monarch (not just what you read on Facebook), you know that one of the biggest problems monarchs face is successfully reaching their overwintering destinations. This means that to really "help" the monarchs, we need to focus efforts on conserving and protecting the migratory phenomenon. The studies I just outlined can help with this. Collectively, these studies each highlight a different element of the migration that needs to be considered for conservation efforts. For example, if large wings are critical for successful migration, efforts should be made to identify habitats and conditions that will promote this trait for the fall generation. The isotope study makes it clear that we should be considering other winter destinations of the eastern population, and perhaps even working to identify what causes some monarchs to choose these alternate locations. The parasite study demonstrates that OE infections can reduce migration success, so anthropogenic activities that exacerbate infections should be curtailed.

As you can see, the results from these studies should go a long way in improving our ability to protect and promote this migration. But they also should serve as a reminder, that we really still have a lot left to learn about it too!

OK, whew. Thanks for sticking with me til the end here. And please do download those papers from the journal and take a look at them.

Cheers

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Direct link to this blog entry:

http://akdavis6.wixsite.com/monarchscience/single-post/2018/12/19/Five-new-studies-on-monarch-migration-published-together

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The science of monarch butterflies

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