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  • Andy Davis

Revisiting one of my favorite Lincoln Brower papers about monarch migration

NOTE - this post has been updated since the death of Lincoln Brower in July 2018.

Hello everyone,

In this post I'm going to take you back in time to over 30 years ago, to the year 1985, and describe a scientific study done by Dr. Lincoln Brower, which I always thought was extremely important to monarch science, but yet probably hasn't been read as widely as it should. Warning - there are no pretty pictures of monarchs in this blog post - just pure science.

In case you're new to the world of monarchs and their study, I'm referring to Dr. Lincoln Brower, who had been studying monarchs for decades before he died in 2018, and was based at Sweet Briar College in Virginia, but formerly from the University of Florida. I loved Lincoln, even though I argued with him all the time, both privately and publicly, over monarch-related issues. The paper I'm referring to was published in a scientific book, of sorts, or more like a collection of studies, and it (the book) was titled "Migration: mechanisms and adaptive significance.", edited by Mary Ann Rankin. Obviously, it was a book focused on migratory animals, and Lincoln contributed a chapter/study called "New perspectives on the migration biology of the monarch butterfly, Danaus plexippus." I had found a copy of the book in a university library years ago and I've been carrying around this paper every since.

The timing of this study is important here - this was only about 10 years after the discovery of the Mexican overwintering site, so there wasn't a lot known yet about the monarch migration. Most entomologists at the time knew they migrated, but there was a lot of confusion over where they went and how they did it. Lincoln's paper appeared to be an attempt to clear up some early questions about this migration, and some of these are still ongoing mysteries 30 years later!

Rather than list these topics and questions, I'm going to paste a scanned copy of the abstract here, because I don't think the paper is available online anywhere.

In this post, I'm going to zero-in on just a couple of these questions. If you couldn't tell, this was a really meaty paper!

Let's start with question 2, about lipids, because I always thought this was the best part about the paper. Again, for those new to monarchs and their migration, monarchs consume nectar on their way south to Mexico and they transform this food into lipids which are stored internally. Basically, migrating monarchs get fat on their way to Mexico. But how do we know this? It's from studies like this one. Let me explain. In the 1970s and early 80s, we didn't really know much about how monarchs were capable, physiologically, of making this 3000+ mile journey. There had been some early research in the 1940s on monarch migration, and some again in the early 1970s, even though we didn't know where they were going back then. Maybe I'll blog about that work sometime because this early research is very cool. Anyway, this early work did indicate that monarchs gained weight during the fall migration, and most of the weight gain was in lipids. However, there was some early thought that monarchs deposited this fat before they started the migration, and this was speculated by Urquhart in his 1960 book on monarchs. Lincoln's paper pretty much put this idea to rest.

Lincoln did this by providing data on the weights of collections of monarchs he had made in the late 70s. He had been collecting monarchs at a number of places up and down the migration flyway, and for each batch of monarchs he carefully weighed the specimens (they were dead and then dried). Before they died he also extracted the volume of lipids inside them and weighed that. so he had two weights for each specimen - the dry weight, and the weight of lipids - and he had specimens from throughout the migration flyway all the way down to Mexico. Below is a figure from the paper, which I've scanned and tried to enhance. For the non-science types reading this, this is a figure composed of "histograms", which show the distribution of samples along the weight gradient.

Each row of graphs here (A through F) represents a different stage along the migration pathway, starting with the breeding area, and finishing with the overwintering site. The x axis is the range of weights observed for all samples, and the bars show you how many individuals were listed at that weight. So when reading this graph, the thing to do is to look at where the bulk of the bars lie on the x axis. For example, in the breeding sample in graph B (of which there were 230 specimens), the graph shows that the bulk of the monarchs had lipid stores weighing between 0 and 40 mg - the statistical average was 19 mg. For context, the dry weight of a typical monarch in this sample was about 161 mg. So doing the math here, the breeding monarchs had fat stores that were roughly 12% of their body weight.

Look next at the first sample from the migration (graph C), which is monarchs collected in Massachusetts, New Jersey, Kansas and Florida (north Florida - fall monarchs do fly south here). In this collection, the average lipid weight was 25mg and the average specimen weight was 171mg, or 15% of the monarch weight. So these are monarchs collected during migration, sort-of midway along the migration flyway, and they had not yet gained any significant fat deposits.

Now, look closely at graph D, which shows weights of 209 specimens from Texas and central Mexico (not the overwintering site). These are monarchs that are nearing the end of the migration. Their lipids are through the roof - the average lipid weight was 126 mg, or 73% of their body weight! So this graph makes it clear that the bulk of the lipids are gained in Texas and in northern Mexico, at the final stages of the migration. Note also how the lipid weights then drop over the course of the overwintering period (graphs E and F), which shows how the monarchs are burning this fat to sustain themselves during the wintering stage.

So these comparative graphs tell a lot about how monarchs use lipids during their life, and some of this was surprising. It looks like the idea that they deposit their fat before their migrate (like a lot of birds do), is wrong - they deposit most of their fat toward the later stages of the migration, like when they're in Texas. This is interesting, because a lot of the science coming out these days is showing how important Texas is to the monarch life cycle, and these data support that idea. From a conservation standpoint, it means that having available nectar resources here would be absolutely critical. This doesn't mean that supplying nectar for migrating monarchs in the northern parts of the flyway is not as important - it definitely still is. Adult monarchs still need to eat on a daily basis, and they probably use nectar to sustain themselves during the journey. It's just that they only begin turning that nectar into fat for storage at the later stages of the migration.

Now, let's go to question 4 - does a two-way migration occur across the Gulf of Mexico? This is something that has long been a mystery, even to this day, and in this paper, Lincoln tried to address it. For context, take a look at this map, which I lifted from somewhere on the internet. It is one of dozens you can find of the fall migration, and there are usually a series of arrows pointing to where the butterflies are going. Then, depending on the map, there are usually some questionable arrows, or dotted arrows, or whatever, pointing to a possible path across the Gulf. These dotted lines show where a hypothetical trans-Gulf route could exist.

Lincoln mostly provided speculation about this issue in the paper. He laid out some logic, which went as follows. He indicated that he had communicated with a researcher in Florida, who was said to have witnessed large numbers of fall monarchs flying out across the Gulf. Incidentally, I also have heard this from people who tag monarchs down there - they told me that they routinely see "thousands" of monarchs heading out from the beach and vanishing in the distance over the water. This is crazy to me! I'm not questioning these observations, I'm questioning what the hell those monarchs are thinking!

I think Lincoln also had a healthy dose of skepticism about this in his paper. He said the over-water distance is about 1000 km (600 miles), so if a monarch WERE to try this, it would have to be extremely efficient. He also laid out some evidence that monarchs appear to have an aversion for flying over water, so it doesn't really make sense that they would do this to begin with. Given that there are so many anecdotal reports of this behavior, even to this day, this is something that definitely needs to be resolved. If these monarchs really are heading out over the Gulf to who-knows-where, it's possible they are going to an alternative overwintering site that we aren't aware of. It's also possible that these monarchs all perish at sea, meaning that each year, millions of monarchs are lost. From a conservation standpoint, this is probably something that we have no control over - we can't really stop them from flying to where they want to go, but we do at least need to know the scale of this behavior for our population surveys and counts, etc.

In the paper, Lincoln ended this section without really answering the question (do monarchs complete a two-way migration over the Gulf?). To be fair, it could not be answered back then with the science at the time, and even now, we still can't definitively answer this question. We will have to wait for the day when the technology advances to the point where we can put miniaturized tracking devices on monarchs. Interestingly, I see that Monarch Butterfly Fund just recently stepped up to this challenge and issued a $50,000 prize for anyone who can come up with such a device - link.

Side note - in addition to the anecdotal reports of monarchs flying out over the Gulf, there is also direct evidence of monarchs stopping to rest on oil platforms in the Gulf. Years ago, a researcher named Gary Ross, undertook a series of trips to visit oil rigs during the fall migration, and he witnessed this behavior firsthand - Apparently, he had also heard of reports of thousands of monarchs landing on oil platforms, in massive clouds.

So again, no one, including me or Lincoln, is questioning if this "Gulf-flying" behavior occurs, what is being questioned is whether the monarchs doing it can survive, and/or make it to the Mexican overwintering site.

OK, I think that's all of my thoughts and musings over Lincoln's paper from 1985. There was a lot I didn't cover from his paper, which perhaps I can do at a later time. As you can see, a lot of what was brought up by it is still being investigated today. That just goes to show how much there is still to learn about the fascinating migration of this critter.


PS - after this blog was published in December 2017, Lincoln sent me a short note saying how much he appreciated it.


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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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