Hi blog readers,
Today, I'm bringing us back to the contentious issue of captive-rearing of monarchs! This is a topic that never fails to elicit comments, emotions and arguments, likely because so many people out there think they are helping monarchs by doing this, and they don't want to hear anything different. Recall though, that there is already ample research on this issue, which all points to the same conclusion - that the monarchs reared in captivity tend to have a variety of deficiencies when it comes to migration (see the prior blog with a summary of the research). And, recall that there is now even research showing how the collective migration of the N. American monarch population is failing. So you can see how the releases of hundreds of thousands (probably millions, really) of home-reared monarchs is the absolute worst thing people can be doing to conserve monarchs.
With all of that said, now let me tell you about a little-known study that everyone should be aware of, because it actually helps to answer one especially contentious bit of this ongoing discussion - that is, whether these negative issues can be "fixed" by simply rearing monarchs outside. This idea has come to the forefront in recent years because of some research from the Kronforst lab in Chicago, which had demonstrated that indoor-reared monarchs have problems navigating, compared to wild monarchs. Similarly, some of the other research on this topic has also been based on indoor-reared monarchs. So, this has led many to believe that the actual problems stem from the monarchs being reared indoors, where they are often kept in artificial conditions (lighting, air-conditioning, or whatever). Meanwhile, monarchs reared outdoors would be exposed to natural conditions, or so the idea goes. What people fail to consider though, is that we still don't really know what it is about the captive-rearing that is causing the migration deficiencies. It may or may not simply be "exposure to natural conditions." For all we know, it could be something related to human touch, or even the chemical smell of the containers being used, or something no one has thought about yet.
The study in question was part of a masters thesis that was conducted by Caroline Aikins at the University of Georgia. The completed thesis itself can be downloaded from the UGA library (link). The first chapter of the thesis was already published in a scientific journal, and it focused on monarch oviposition (link here to read about it). The second chapter is the one I'll talk about today, which was focused on monarch fall migration. It has not yet been published. In it, Caroline had been trying to figure out if migrating monarchs actually "follow each other" during the trip, and for this, she had used a number of different sets of monarchs, including those that had been reared under different conditions. This is the part I'll focus on today. Keep in mind this was just one small part of that project. you can read the thesis itself to learn about the rest.
Importantly, as part of this project, she had used some lab-raised monarchs in captivity, which were the grand-offspring of wild monarchs. These are what some would call "captive-bred" monarchs, because they were derived from monarchs that had mated and laid eggs in captivity. The cool thing here though is that she took those "captive-bred" offspring and reared some of these indoors, and some of them outdoors. The outdoor larvae were placed on potted swamp milkweed plants, which were covered with mesh (see picture). The indoor larvae were reared in plastic containers (6 per container) and fed cuttings of swamp milkweed daily.
Next, Caroline had a third group, which were "wild-caught" larvae. For this, she had collected some wild 4th-instar caterpillars from swamp milkweed in pollinator gardens near campus, and then placed those into shoebox-sized plastic containers with mesh lids. These containers were placed on a table outdoors, and the larvae were fed cuttings of swamp milkweed daily until the monarchs eclosed.
I should also mention that these experiments took place in early fall, so that the outdoor monarchs in each treatment were exposed to the natural environmental conditions that wild monarchs would also get.
Finally, Caroline had collected some wild adult migrants for comparison. These were netted as they were migrating through the area during the fall migration at the time of the research.
So to recap, there were 4 groups of monarchs in this research: 1) captive-bred, reared indoors, 2) captive-bred, reared outdoors, 3) wild larvae, reared outdoors, and 4) wild adult monarchs.
Caroline had conducted a suite of experiments on these monarchs when they became adults, but the thing I'll focus on for now is their wing color. Once each monarch had eclosed, Caroline had scanned them with a flatbed scanner, which allows for very detailed computer-assisted measurements of their wings, including measuring how "orange" they are. This is an important trait for monarch migration, and one where there is a fair amount of research already. In a nutshell, the shade of the orange color of a monarch is a very good predictor of how well it will migrate. The redder the color, the better for migration. Don't ask why, because we still haven't figured that out yet. To illustrate, below is a composite image of four monarch "scans" along with a color bar showing the range of "redness" that we typically see across monarchs. Look close and you'll see that the monarchs on the left are more "red" than those on the right.
Now let's get to the results.
So, below are two graphs that I made, based on her results (I replicated what she had presented in her thesis). These are graphs of the average wing hue score (redness score) across each of the four groups, and I've configured the graphs to show the redder values to be higher on the graphs. The males and females are separated because there are some inherent differences in color between the sexes. But, note that the graphs are basically the same pattern for each sex.
I hope you stared long and hard at these graphs, because the implications here are pretty big. The wild-caught migrants were the most red, which is what we've found in many other studies. These are what you could call, the "gold standard." Importantly, none of the larvae reared outdoors had this amount of redness, either the captive-bred or the wild larvae. In other words, rearing the larvae outside did not replicate the truly wild condition.
At the time, I recall Caroline being quite surprised at finding this, because the prevailing notion was that the reared monarchs simply needed to be exposed to the elements, so that they would be exposed to everything that a wild monarch would also encounter. But that is not what she found. Monarchs reared outdoors were paler than truly wild monarchs. Recall that paleness is an indicator of poor migration ability in monarchs.
To this day, it is still unclear why the outdoor-reared larvae failed to match the wild monarchs. Recall there were two rearing approaches used - rearing in a mesh netting on a potted plant, or rearing in a container with a mesh lid. In the former case, there was little to no human exposure, and in the latter, the larvae were fed cuttings of milkweed daily, with some minor human exposure. So it may not simply be a case of "human handling."
If you carefully read the prior research on captive-rearing, especially the work on navigation, it becomes clear how little we know about the suite of conditions needed by monarch larvae to properly develop into capable migrants. The Kronforst lab research really demonstrated this - that work showed how sensitive the monarchs are, and how delicate their internal compass is to perturbations during development. Similarly, this masters project demonstrated something similar - that the development of wing colors are also sensitive to changes during early life. All in all, these both show how little we really know about how artificial conditions affect monarchs, and, their ability to effectively migrate.
I guess I should end with a take-home message here, although I assume you can figure it out. The bottom line is that there is no substitute for mother nature.
That's all for now.
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