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  • Andy Davis

I debunked the study that tried to debunk the 2019 captive-rearing-navigation study

Hi folks,

Today I'm going to jump back into the ring with the beast that is captive rearing (of monarchs). This is an issue that is on everyone's mind these days, either because you already have hundreds of monarch caterpillars in your house right now, or, because you are fed up with seeing pictures of people rearing hundreds of monarchs in their houses! Either way, this is a post that you will want to read.

By now we all know that rearing monarchs is a very controversial topic. A lot of people engage in this practice, and they think they are helping by doing it, but on the other hand, most of the science to date has shown how captive-reared monarchs are poor substitutes for the real thing (that was actually the title of one study). This is where the controversy comes in. By my count, thus far the research has shown that 1) reared monarchs have a lower migration success rate based on tagging data, 2) they travel less far during migration, also based on tagging data, 3) their wings are paler in color (an indication of flight ability), 4) their wings are less elongated (another indication of flight potential), 5) their wings are smaller, on average, and 6) they are physically weaker.

But that's not all. Perhaps the most controversial finding regarding reared monarchs came in 2019. A study was published in the prestigious journal, PNAS, which showed how captive-reared monarchs have poor navigational senses, when they are placed in a flight simulator, as pictured above. This is a contraption that has been used a lot in monarch research over the years. Essentially you attach a monarch to the middle rod, and the monarch can flap and turn in the direction it wants to go. You can have the lid of the device be open, so they can see the sun, or you can have it be closed. Anyway, this study showed that when indoor-reared monarchs are placed in it, they don't show any directional bearing during the fall (when they should point south). But, when you put wild-caught monarchs in it (or monarchs reared outdoors), they all have an average southward direction.

This paper caused a lot of arguments, discussion, and disbelief when it came out, and these arguments are still going on. The biggest arguments I have read or heard was that 1) the contraption is not realistic, and 2) that the reared monarchs didn't have enough time to get used to the outdoors, i.e. to get their bearings. The first argument is just ridiculous. The wild monarchs all oriented in the proper direction, which proves the contraption works perfectly. It is the second argument that we'll talk about today.

A study was published a month ago that was an attempt to test this idea, i.e. that captive-reared monarchs simply need more time to get used to the outdoors, to get their bearings. A group of researchers from the University of Guelph in Ontario conducted a study that evaluated this idea using captive-reared monarchs. Here is a link to the paper, published in a journal called Conservation Physiology. In a nutshell, the researchers tested the orientation ability of a group of reared monarchs, first in their own flight simulator, and then they tracked them in the wild using special radiotracking backpacks. In the flight simulator, the reared monarchs did not show proper orientation, which confirmed the other study's findings. But, then they found that the reared monarchs did show proper orientation when released in the wild (or at least this was the claim).

This study set off its own firestorm of discussion and news stories, and I'm sure you've seen or participated in some. How many times have you seen a post or mention of the study I'm talking about? Or maybe you saw a news article touting the headline? They all say something like "Reared monarchs can regain their navigational abilities when released!" Or "Captive-reared monarchs show normal navigational ability!" These headlines seemed like a dream come true for people who were always convinced their rearing operations are saving monarchs, and for whom the science was not on their side.

But here's the thing - did anyone actually read the study, or just the headlines? For example, did you look very carefully at the way in which the researchers tracked the monarchs? Did you look at the directions the monarchs were travelling? Was it actually in the right direction?

I read it, very carefully, and I asked myself the same questions. And, once I thoroughly digested and examined the data, I saw some major problems with the methodology and with the results, that all point to one thing - this study did NOT show what it claimed. So, I submitted a "rebuttal" paper to the same journal. I've talked about these rebuttals before on this blog - they are short articles that provide counter-arguments, or alternative data, or point out problems with the study in question, and they are intended to serve as a caution to readers of the primary article. Importantly, rebuttals ARE peer-reviewed. Anyway, this rebuttal paper was just published, and here is a link.

My rebuttal basically comes down to three major points. The first, and most important point, is that the reared monarchs in this study did NOT actually fly "south", as the news articles have said. I blame a lot of this misconception on the authors here, because they kept using the very generous term "southerly" throughout their paper to describe their tracking results. However, if you closely examined their data, it is clear that all of the monarchs were heading southeast, which is NOT in the proper direction to get to Mexico from Ontario. This is extremely important, because the linchpin of their study was that the reared monarchs had displayed "natural" orientation once released - this was the gist of the title of the paper even. To make it clear, I downloaded all of their data from their supplemental file, and made my own compass figure showing the direction these reared monarchs were travelling.

Take a good look at this figure. Each of the blue lines represents the direction of one of the captive-reared monarchs, after release. Are they heading in a natural direction? Do you think "southerly" is an appropriate characterization?

You might also wonder, well, what is the proper direction from this location? I'm glad you asked! We actually have a lot of science on this already. For my rebuttal, I went through all of the research that had ever been done for the last 40 years and tracked down each paper which had examined flight directions of monarchs from this region. I found a lot. In fact, this has been studied very well, using a variety of methods. Researchers have watched migrating monarchs fly across the sky, they have captured migrants and then released them to track their vanishing bearings, they've placed migrants in their own flight simulators, and there is even lots of data from tagging studies. Anyway, when you look at the directions that monarchs were travelling across all of these studies, it is clear that the "proper" direction for migration in this region of the world is southwest. I made a big collage of the different studies in the paper, and I'll paste it below. The first compass (B) again shows the directions of the captive-reared monarchs. The rest are findings from other published studies.

If you find yourself looking very closely at figure G, let me explain something about it. This compass shows the "estimated" directions of tagged monarchs from the southern Ontario region, which I inferred by looking at the MonarchWatch tagging maps, which are online. But keep in mind that these "directions" are based on drawing a line between point of release and point of recovery, which is very rudimentary, and may or may not represent the actual bearing the monarch took. So, this evidence from tagging is NOT as good as the direct observations of flying monarchs, or even the evidence from flight simulators. I added that figure in only for completeness.

OK, so that's the navigation problem. The second main point I made was related to the transmitters the authors used to track the released monarchs. I'll paste a screenshot of these below. You can see they were glued to the belly of the monarchs in the study. These are a special type of radiotransmitter that emits a unique frequency. There are special radio towers set up around the eastern seaboard that can listen for these, and whenever one of these transmitters comes near, the tower reports that location and time to the researchers. These things work great for tracking birds, and bats - I know because some of this research is published in my own journal, Animal Migration. But, I don't think these are suited yet for tracking monarchs, at least not until the technology improves.

Here's the thing about the transmitters that's not obvious from this picture - they weigh 200mg, which is half the weight of a typical monarch! This would be like strapping a microwave on your back and then going for a jog. To me, this is just too much. In the world of animal migration research, we have a rule of thumb that any tracking device needs to weigh less than 5% of the weight of the animal! So really, how could these monarchs fly "normally" with these things?

The last point I made in the rebuttal was that the researchers did not have a control group in the study. This was just plain sloppy. This is basic high school science - if you want to know how any treatment is affecting a test group, you have to also have a control group for comparison. The authors should have collected a bunch of wild monarchs as a control group, as every other study does, including the original PNAS paper. If the control monarchs also flew in a southeast direction, then I would be more convinced of this new evidence.

I really don't know what was up with those reared monarchs, but I do know that this study was not ready for prime time. I ended my rebuttal by stating that this study is preliminary at best, and the authors should have collected more data before going public with it. And, I also indicated it is reckless for them to be promoting the practice of captive-rearing, as a way to bolster the monarch population. When every other research study to date shows that reared monarchs are lesser versions of wild monarchs, do we really want to populate the continent with more of these?

For completeness, I'll also give you a link to a short counter-rebuttal that was submitted by the authors, in case you want to see what they said about each of my points. I read it and saw that they were mostly just reiterating the same bits of information in the original study. And, in the title of their rebuttal they still insist their monarchs flew "southward"!

Let me end here with a thought or two about sharing this blog post. I'm sure that the people who are already against home-rearing will share this post, but these aren't really the people who need to hear it. It's the people who have hundreds of caterpillars in their home right now, who really need to read this. So, this post really needs to be shared within the dozens (hundreds?) of Facebook groups that are popping up that are devoted to home-rearing. But, there a big problem with these groups - the administrators control the information that gets seen, and, they very well might just delete this post so their followers won't see it. If you see this happen - my advice would be to leave that group, and join one that is based on science and evidence.

That's all for now.


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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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