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  • Andy Davis

An exciting new study examining flight ability and genetics of eastern vs western monarchs

Greetings everyone,

Apologies for the long wait between posts. Between managing the kids during the lockdown, keeping the lab research going, and even moving houses (!), I haven't been able to keep up with this blog. But today I'm sitting at my computer for the first time in a while and I am itching to write about this new study that was just published. I'll bet you have never even heard of it too! It was quietly published, with no fanfare, no press, and in a journal that most folks except us nerdy scientists read, Molecular Ecology. But, as you'll see, this new study has some pretty significant ramifications to the ongoing conservation efforts of the western monarch population in North America.

As most readers here are aware, the smaller population west of the Rockies is not doing so well lately. Monarchs west of the Rockies have a smaller breeding range than in the east, and they migrate each winter to clustering sites along the California coastline. This is a much shorter migration than that undertaken by eastern monarchs, which travel all the way across three countries to get to their winter colonies. You should keep this in mind as I go here, since it bears on the results of this study. Anyway, the annual counts of these winter clusters in the west over the years has shown these colonies are gradually diminishing in size. This long-term trend is very much at the heart of the ongoing discussion about whether to list the monarch as "Threatened" in the United States. Recall that we are still waiting to hear if and when the USFWS will render a decision on this. Let me come back to this.

This new study was authored by an esteemed group of scientists, some which are well-known among the monarch world, including Marcus Kronforst, Jaap de Roode, Francis Villiblanca, and Amanda Pierce. The lead author is Venkat Talla, who is a postdoc in the de Roode lab at Emory University. I'll put in a link to this study here, although this may or may not be downloadable from your home. Also, it is a very dense read, meant for science nerds only!

The goal of this study was to try to sort out if western monarchs differ from eastern monarchs in any way, including in their flight ability, their wing designs, and especially in their genetic makeup. Long-time monarch research fans may recall that this question of whether eastern and western monarchs are genetically different has been one that has been asked many times before over the last 20 years. The reason it keeps getting researched again and again, is because the technology and science of genetic analyses keeps improving. Thus, as the tech improves, that allows researchers to improve upon the results of previous studies. That appears to be the case here too. Here, the scientists used the latest "genomic analyses" to ask if the two populations are genetically distinct. They also performed some very interesting tests to try to find the genes responsible for differences in flight ability!

So the researchers examined a collection of monarchs from the west, and some from the east. I'll add the figure from the paper below, which shows where the monarchs came from. Note that all of the monarchs collected came from the migratory generation of either population. This is stated explicitly in the study.

Once the monarchs were in their lab, they conducted a series of tests and comparisons on them. In one of these tests, they examined their wing morphology, that is, how big the wings were, and how elongated they were. They did this by scanning the monarchs on a flatbed scanner to get a digital image of their wings. Then they used image analysis software to extract the information on the wing area and the shape. I myself use this procedure too to measure monarch wings, and I can tell you it is about as precise as you can get for measuring fine-scale features.

Another very cool part of the study was their tests of flight performance. They wanted to know if western monarchs fly just as well as those in the east. To test this, they used a handy butterfly "flight mill" (they actually borrowed ours from UGA). I've talked about this gizmo before on this blog. Think of it as a butterfly treadmill. The monarch is glued to a rod, which spins in a circle as the monarch flies. A computer keeps track of the speed and distance as it goes around and around. Thus, with this thing you can track how "far" a monarch flies in a given flying bout. We've put monarchs on this thing and sometimes they keep flying for hours! I'll paste a youtube video below, which shows the flight mill working in our lab. It's very cool!

Lastly of course, they did their genetic analyses of the monarch specimens, specifically looking for differences between western and eastern monarchs. I'll gloss over the methodology of this part here.

OK let's get to what they found.

First, they found eastern monarchs flew longer, on average, than western monarchs! That is, their flying bouts covered more distance, and they lasted longer on the flight mill. And, by keeping track of the speed and distance covered, the researchers also calculated how much effort or power each monarch had to put out during its flight. They found that eastern monarchs tended to use less energy to fly than the western monarchs did! Collectively, these results mean that on average, eastern monarchs are "better flyers" than western monarchs - they use less energy, and they travel farther with each flying bout. This makes a lot of sense when you consider that the migration in the eastern population is about 3 times the distance of the western monarchs, so of course they would have to be "better" migrators! But, this is the first real empirical lab demonstration of this phenomenon.

For the sake of thoroughness, I'll paste the main figure from the paper below, which shows the actual data on the flight distance, duration, and power. Each of the dots is a single monarch. Note that the dots of the eastern monarchs (black) are mostly higher in the graphs of flight duration and distance, and lower for the graph of power. The horizontal lines in each graph are the averages for each group.

Next, they found that eastern monarchs had larger wings on average, than the western monarchs. This also makes sense - large wing size is critical for long distance migration, in monarchs, as well as in birds. Longer migrations in the east leads to larger wings.

But, here's the really, really crazy finding - there was no genetic difference found between western and eastern monarchs! The authors, who are all experts in this type of research, appeared to use every available tool at their disposal to find any shred of genetic difference between the two populations, and they found absolutely none. And, this evidence builds on a number of prior studies conducted over the past 20 years (using outdated techniques), which have also failed to find evidence of genetic differences. So they concluded that eastern and western monarchs are not distinct at all! They surmised that there must be enough "gene flow" between the two populations, and intermingling, to ensure that they all have the same genes. That's a fancy way to say that there must be some monarchs that travel from one population and mix with the monarchs from the other, and this must happen often enough so that their genetic makeup is all the same, across all of North America.

But wait - there were actually real population differences in their wing structure and their flight abilities! But yet, the monarchs themselves were basically genetically the same, meaning they were all from the same population. This is getting really weird. If you think on this for a minute, you start to see that these findings are implying that a monarch from Pennsylvania is genetically the same as a monarch in Oregon, yet for some reason, the Pennsylvania monarch grows up to be bigger, and a better flyer, than the Oregon monarch.

The authors of this study made this same implication toward the end of their paper - that these results imply that it is something about the environment where the monarchs grow up, and not their genes, that makes monarchs from the east and west really different. Another way to think about this is with this hypothetical scenario - you could (theoretically) take a monarch egg from Pennsylvania, transport it to Oregon, and somehow that egg will develop into an Oregon-looking monarch, with smaller wings and less powerful flight! Meanwhile, the siblings of the Pennsylvania egg will all develop into eastern-looking monarchs! Crazy! It looks like there is something about the conditions and the environment the monarchs experience during development that "program them" to be who they are supposed to be.

Speaking of programming, now imagine a scenario where monarch eggs are reared indoors, out of their natural environment. Do they develop into the monarchs that they are supposed to be? I'll leave that thought hanging for folks to chew on.

If you think about it, these new results have some very very profound implications for the butterfly breeding and shipping business, where monarchs are shipped between states for releases at wedding and festive events. There has always been a ban on shipping monarchs between eastern and western states, because of this idea that these two populations are "distinct". Based on these new findings, it could be argued that since there is no genetic difference, then there is no reason to keep this ban. Although keep in mind that this scenario ignores the fact that the OE prevalence of eastern and western monarchs is indeed quite different, so you don't want to be shipping monarchs around for this reason alone. And if the monarch becomes federally listed, I doubt this practice will be allowed to continue anyway.

Finally, this study also raises some interesting questions about the conservation of western monarchs in the U.S. Here in the east, monarchs are doing pretty well, despite the small overwintering colonies in Mexico. Their numbers this summer have been down a little, but in the past twenty years, their summer numbers have shown no overall decline. Meanwhile, the western monarchs are in more dire trouble, which raises an interesting conundrum - should the whole monarch species in the U.S. be listed as threatened, if one (small) population within its range is declining? The Endangered Species Act does not have any way to list only certain populations of a species, thus the entire species must be listed, which comes with a plethora of ramifications for homeowners, butterfly breeders, home-rearers, etc. And then, what if that declining population is now shown to be indistinct genetically, from the larger population? Does it still warrant federal protection then? Is it even a population at all?

These are some big questions, to which I, nor anyone else, has answers to.

Kudos to the authors for this fine study, which greatly expands our knowledge on this bug, while opening up more questions!


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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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