New study shows captive-reared monarchs are wimps and have poorly-suited wings for migration!
I suspect I have your attention with the title of this post! Yes, you read it correctly, and yes, I mean exactly what it says. A new scientific study was just published in the journal, Biology Letters, spearheaded by myself and some talented students here at the University of Georgia, and these were the exact findings of the study. Today I'm going to tell you all about this project. So grab a cup of coffee, strap yourself in, and be prepared to have your mind blown.
So before I begin, let me first make it clear to everyone that, as far as I'm concerned, folks are free to believe whatever they wish after reading about this study and its findings. I'm fully aware that some people in this world, when faced with evidence they do not want to hear, have a tendency to look for reasons to disbelieve said evidence. It's human nature. And, I'm also fully aware that there are certain people out there who have become convinced that rearing monarchs for release is helping to "save" them, and no amount of scientific evidence will change their minds. However, it is my hope that there are at least a few other people out there who are more open-minded, who appreciate science, and who will listen to evidence. If that's you, then read on.
As I do with each post, I'll begin by pointing folks to the journal page where the study appears, and where you can scroll down and find links to the online supplemental files that go with the study. These are important, as they include videos and a Word file with extra details of the methodology. And, I'll even add a link here where people can download my own copy of the paper!
And next, I'll also set the stage for the study, as I usually do.
Let's start with the big picture - the monarch butterfly migration. We all know that fewer and fewer monarchs have been reaching the overwintering sites in the past two decades. Last fall especially, was a brutal migration season, coming after a stellar summer too, which led to surprisingly low winter colonies. But why are monarchs having such a hard time migrating? That's the million dollar question these days. There has been lots of talk of weather problems, talk of lack of nectar sources, and I know I've talked a lot on this blog site about car strikes (don't get me started). There are indeed a lot of obstacles during the migration, which seem to be getting harder each year. But, one thing that no one has ever considered, is that the problem might really be with the monarchs themselves. In other words, is it possible that the migratory problems are because the monarchs are gradually losing their migratory ability? Is there something about monarchs themselves that has been changing over the past two decades? I'm starting to wonder about this, especially after this study.
One thing that has happened over the past 15-20 years has been a dramatic rise in releases of captive-reared monarchs into the wild population. Early on, it was mostly commercially-reared monarchs, produced by a small cottage industry that sold them for release at weddings and festive events. This industry has grown steadily over the years. No one really knows how many reared monarchs are ever released each year, but by a conservative estimate, it is easily over 200,000 per year. And, in the past decade, a growing number of citizens have taken it upon themselves to rear monarchs in their homes, for release into the wild. Again, it is difficult to estimate these numbers, but judging by the number of people in online monarch rearing groups, I would guess it is probably around 100,000 monarch releases by citizens per year, and growing.
From a scientific perspective, this cannot be good for the wild monarch population, either in the east or west. In fact, most scientists have warned people about the risks of rearing and releasing large numbers to the monarch population, especially the risks of spreading disease. There have been several blogs and warnings about this, including from the Xerces Society, Monarch Joint Venture, and even here at MonarchScience. The other big risk of this practice is that we simply don't have a lot of research on this issue, and that's always dangerous. This seems to be changing lately.
Last year, a new problem was identified with this practice - that monarchs reared indoors appear to have reduced navigational ability, when compared to wild monarchs. This evidence was based on lab experiments where monarchs were tethered to a flight-mill apparatus that tracked their migratory directions. Most of the indoor-reared monarchs (but not all) did not point south during the trials, while all of the wild monarchs did. I blogged about that study after it came out.
And keep in mind that the results of that study last year make sense, given that we already knew that reared monarchs have much lower tag-recovery rates during migration. So all of the evidence so far indicates there is something about reared monarchs that is off, when compared to wild ones. But has anyone ever stopped to wonder if these reared monarchs are as "physically fit" as wild ones? We did, which is why we conducted this study.
The main argument that people give for releasing reared monarchs (whether it be citizens or commercial breeders), is that these are helping to "boost" the monarch population. But if you think about it, in order for captive-reared monarchs to "add to the population", they need to resemble their wild cousins in exactly every way (biologically), so that they will seamlessly fit into the wild monarch population. That is, they need to behave the same, look the same, and most importantly, they need to be able to survive the brutal fall migration to Mexico (or California). This is exactly what we tested in this study. We asked, do captive-raised monarchs have the physical characteristics that we know are needed for successful migration.
We examined 4 physical traits that are either known or suspected to be crucial for migration success, and we asked, are captive-reared monarchs just as good as wild ones in these four traits. These traits were: 1) the physical strength of monarchs (really!), 2) their wing size, 3) their wing color (how red they were, which we know is important), and finally, 4) how elongated their wings were.
Before getting to how we did this, let me go over the monarch groups we used for comparison, which were the study subjects for the project.
There were two groups of monarchs that had been reared in our lab late last summer. There were 40 monarchs in one group and 40 in the other group. In one group, all of the monarchs had been reared in containers on our lab bench, which was next to a large glass window. We also have overhead lights in the rearing room. And, the temperature of that room was set to a warm summer range (28C). Monarchs in the other group had been reared in one of our nifty environmental chambers. These things are sort of like high-tech fridges, with programmable temperatures and lighting. Here, we programmed the chamber to reproduce the conditions for late-summer/early fall. That is, the daylength was set to gradually get shorter, and the temperature was set to low, fall-like temperature. If you want to know more details about the rearing, you can view the online Word document in the supplemental files at the journal website. So essentially we had one group of monarchs that experienced "summer-like" conditions, and one group that experienced "fall-like" conditions.
The monarchs in each rearing group were reared using our standard procedures, including feeding with cuttings of greenhouse-grown milkweed, containers cleaned daily, gloves always worn, etc. They remained in these conditions until they eclosed into adults. All adults were checked for OE and were not infected.
Our "control" group were 40 monarchs that I captured last Sept during the fall migration through our area. These were monarchs hand-netted on campus or nearby, then brought to the lab. There's not much else to tell about this group - they were randomly-caught, wild migratory monarchs.
Below is a pictorial display of each "rearing" treatment, to give you a sense for how these treatment groups differed.
Now, let's get to the measurements, starting with what I think is the coolest one - measuring their physical strength.
This was a procedure that I had seen done with beetles and I've always wanted to try it with monarchs. Essentially, I came up with a way to measure the "gripping strength" of monarchs - that is, how much force they use to grip onto something. I have a benchtop force gauge in my lab and I attached a simple rod onto it that will support a butterfly. This gizmo is pictured at the top of the blog. The way it works is you hold a butterfly next to the rod and let it grip onto it with all of its legs. Then you pull up on the butterfly. Naturally, it tends to want to hang on, and it eventually releases its grip of the rod. The force gauge registers how much "force" was required to pull off the monarch, measured in Newtons.
I'm going to post a youtube video below that we put together to demonstrate this procedure, as well as some of the others we did for this project. Many thanks to UGA student, Annabel Prince for her efforts in putting this video together!
If you think about it, this measurement of "grip strength" is actually quite relevant for migrating monarchs. Think of all of the times when migrants would need to hold tight to tree branches during high winds and in roost trees! If they fall during these storms they may wind up spending the night on the ground, where they are at risk from rodent predators or scavenging insects. Also, this grip strength assay can be considered a rough proxy for the overall strength of the insect (we know this from beetle studies). Thus, a monarch with greater grip strength probably is stronger in other areas, like "flapping strength."
Alright, now let's get to the results.
So for this project recall that we had measured grip strength on all of the reared monarchs, and all of the wild ones for comparison. Here is what we found:
- Captive-reared monarchs from our summer group were 30% weaker than wild monarchs, and those reared in our fall-like incubator were really weaker, by about 80%!
Next, let's talk about the measurements of wings. In the video above, I described how we "scanned" the monarchs to get a digital rendition of their wings. I know this looks and sounds weird, but believe me, this does not hurt them! We've been scanning monarchs for years now, and in fact, most other monarch researchers use digital scans to measure wings these days. It provides a much more comprehensive assessment of wing features. The other neat thing about this method is that the scanner provides uniform lighting on all specimens equally, so that the colors of the wings can be visualized and measured using special computer software.
Using this software, we first measured the surface area of the right forewing of each reared and wild monarch for comparison, to compare overall body size among the groups. Here is what we found:
- Captive-reared monarchs were slightly smaller than wild ones, although this trend was not statistically significant.
Next, we measured the shade of orange on the monarch wings. If you recall from previous studies of monarchs, we know that the color of their wings is a great predictor of migration ability. This has been shown in lab experiments and even with wild monarchs. Basically, the redder the color the better. Keep in mind that we don't really know why this is - the orange pigment certainly does not convey any aerodynamic advantage. It may be that those monarchs that can synthesize redder pigments are simply better in all things, including flying. So when we compared the orange color of the reared versus wild monarchs, here is what we found:
- Captive-reared monarchs in both rearing groups were significantly paler than wild monarchs, meaning they would not be as good at migrating.
Finally, we used the computer software to measure the wing shape, or elongation of the forewings. With monarchs, we know that migratory populations tend to have more elongated forewings than do non-migrant populations. This same phenomenon can be seen with birds too. Basically, elongated wings are more aerodynamic. So, when we compared the degree of elongation among the reared vs wild monarchs, here is what we found:
- Captive-reared monarchs in both groups were much less elongated than wild monarchs. In fact, the reared monarch wings had average elongation values that matched those from non-migratory populations around the world.
Collectively, these results demonstrate that captive-reared monarchs were weaker and had more poorly-suited wing features than wild migrants. This result was the same whether we reared monarchs inside next to a window, or if they were reared in conditions that mimicked late-summer conditions - the conditions that the migratory generation experiences.
Now, let's talk about why. This is something that is going to take some time and more study to figure out, which we will do here at UGA for sure. Clearly, there is something off about monarchs raised indoors. Our study showed this, as well as the Tenger-Trolander et al 2019 study. Our study showed there is something about the rearing that reduces proper wing and muscle development in monarchs. Their study showed that rearing interferes with navigational development. Given that we found this same pattern in monarchs reared under two very different scenarios, then logically, it seems like the issue has nothing to do with the lighting, or temperature, or humidity, or anything like that. It must have something to do with the rearing practice itself. I proposed two explanations in the published paper, which I'll describe next.
If you recall from last week's blog, one of my other projects that was just published was about how handling causes stress to developing monarchs. Given this, it may be possible that the stress of repeated handling and disturbance is somehow interfering with proper development of wings, muscles and neurons, etc., in the adult butterflies. Even though the handling bouts are brief during rearing, they still would cause temporary energy drains, which if happening every day, could gradually diminish the energy needed for these important bits to grow. This idea makes sense in our study, because even though both rearing groups experienced different environments, they each went through the same handling (feeding, cleaning, etc) procedures daily.
A second, equally-plausible explanation, is that the monarchs in the rearing treatments are sub-standard because they avoided the element of natural selection. In the wild, fewer than 5% of all larvae ever reach adulthood. Then during migration, poorly-suited adult monarchs probably perish along the way. If you think about it, the monarchs we used in our control group had first survived to become adults, and then survived the migration long enough to reach us here in Georgia. In other words, these individuals were the cream of the crop of the monarch population. Meanwhile, the captive-reared monarchs had faced no natural adversity growing up at all. They were reared from eggs and fed daily cuttings of fresh milkweed. There was no "natural selection" at all on these monarchs - in fact, they ALL survived, even those that were small and weak. See where this is going? In the wild, mother nature ensures that only the very best monarchs get to pass on their genes to the next generation. Captive-rearing ensures that all of the weaklings survive.
Finally, let me end by telling you what questions I have now after completing this project.
We know that some captive-reared monarchs do reach their overwintering destinations, both in the east, and in the west, but we also know that the vast majority do not. What happens to those that do not? Do they die, or do they survive and pass on their (substandard) genes? I think this is one of the biggest questions I have, and so should everyone else. Years ago, it was probably safe to assume that they died, because it wasn't possible for monarchs to exist in the US during January. These days, failed migrants can easily still persist at locations in the southern US. If so it is entirely possible that these monarchs are indeed passing on their genes. Imagine if this happens year after year, and with greater and greater numbers. Are we genetically watering down the population with monarchs that can't migrate?
In a time when the migration is suffering, should we be flooding the population with monarchs that are poorly-built for migration? Could it be that rearing itself is responsible for the demise of the monarch migration?
That's all for now.
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