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  • Andy Davis

New study tests if non-migratory monarchs can produce migratory offspring - a critical question

Hello everyone,

A very cool new study was just published, and I'm excited to give you this breakdown of it. This study addressed a topic that is of growing concern in the world of monarchs, that is, the ever-looming loss of the epic migration in North America. This is a topic I myself have discussed at length in this blog site, and there is also a growing body of research around it. Basically, researchers are starting to ask, what would happen if the monarchs lose the migration, or stop migrating? In fact this is a question that we can already tackle, at least from a research perspective, because there are already many places around the world where there are non-migratory monarch populations. These are essentially places where the migration has already been lost. This is in addition to the places like Florida, southern California, and along the Gulf shores, where there are increasing numbers of non-migratory monarchs, which are being fueled by both warmer climates and the presence of non-native milkweeds year-round.

There has been much discussion about these growing non-migratory populations, including their conservation value. I've heard some people claim that these could act as "refuges" for monarchs, in case their migration in North America gets completely wiped out. The idea here is that if the worst ever happens, then some of these non-migratory monarchs could be used to recolonize the migratory population (I'm skeptical). Similarly, there has also been much discussion about the value of the growing resident population in California; one researcher in particular (David James) has even posited that year-round milkweed could provide western monarchs with an "alternate wintering strategy," so that instead of hanging out on coastal trees for the winter, the monarchs could simply breed throughout the winter, and then, when the time is right, their offspring would re-enter the migration cohort. At the time, I had contested that claim, and the resulting debate we had over it was all published in a journal last year. I blogged about the debate we had too - link here. I'm bringing this back up again because this new paper essentially tested this very claim - if non-migratory monarchs really can produce offspring that would become migratory if the conditions are right. That is, if you took some non-migratory monarchs and brought them to the middle of the N. American breeding range in the late-summer and let them reproduce, would those offspring be migration-ready, or would they simply resemble other non-migratory monarchs? Well, this is exactly what the researchers here did!

This new study was just published in the journal, Ecology and Evolution, and it was spearheaded by members of the Kronforst lab, at the University of Chicago. This lab has been taking the lead on this topic as of late, and they have been actively studying such questions around the monarch migration, including how captive-rearing alters the migratory syndrome. Here is a link to the new paper, which I believe is online, though it is a hefty read. The two lead authors were Ayşe Tenger-Trolander and Cole R. Julick.

I just now read through the study, and I'll try to boil it all down here. Incidentally, I was a little surprised that I wasn't asked to review this paper by the journal, since it is in my wheelhouse. Anyway I was excited to read it nonetheless!

So the researchers here wanted to know if non-migratory monarchs can produce offspring that have the same migration "traits" as those produced by monarchs from a migratory population. Or, another way to say this is, can resident monarchs produce offspring that are physically and physiologically capable of migration? By the way, if this sounds a little familiar to long-time blog readers, recall that there was a slightly similar study done back in 2018, where researchers also asked a related question - do non-migratory monarchs still have the capability of becoming migratory (here is the blog post on it). The end result of that study was a clear MAYBE! That prior study showed that non-migratory monarchs (from Australia) still retain the ability to go into the migratory state of "reproductive diapause" if they experience environmental conditions suitable for migration. Recall that most monarchs in the fall migratory generation are in this state (which just means reduced reproductive development), which we think is necessary for migration. Keep this in mind going forward.

The researchers here used a non-migratory monarch population in Costa Rica for their tests, and they had obtained some adult monarchs from there and brought them to their lab in Chicago. They also obtained some adult monarchs from the local area to use for comparison (which represented the "migratory" North American population). They allowed mating to occur and provided all females with milkweed (common milkweed) for oviposition. They then reared the offspring using standard rearing procedures until they became adults. Importantly, all of this was done in pop-up tents outside. This is a very important point - there were no incubators, no greenhouses, or anything unnatural in this paper. It was all done outside, on purpose, because they wanted all of the offspring to experience the exact environmental conditions that are needed to produce a summer-breeding generation, or, a fall-migration generation.

The researchers conducted this project over two years, and in that time they had reared over 500 Costa Rica monarchs, and North American monarchs. They reared some monarchs in the summer months, and some in the late summer/early fall. The idea here was to see if the monarchs from either population show any sign of change, from the breeding season, to the migration season. For example, in the summer, N. American monarchs are usually busy breeding, and so the females are usually loaded with eggs, but then in the last generation of the summer, the adults are usually in reproductive diapause, which means the females have little to no eggs inside them. So in the migratory population, we expect to see some change from summer to fall, which indicates they are capable of "switching" to a migratory state. The important question here is, can a non-migratory population also make this switch?

From all of the monarchs produced in these rearing bouts, the researchers measured a number of traits that are important for migration, including the number of eggs inside females, as mentioned above, plus the adult wing sizes, their abdomen sizes, and their thorax size (the thorax is where the flight muscles are). They also measured the metabolic rates of the monarchs. Recall from a prior blog that migratory monarchs need to have a low metabolic rate, so as not to burn off too much energy during flight. I won't get into too much details about the procedures here for the sake of brevity.

OK, so lets get to what they found.

First, they found that the female monarchs from North America had lots of eggs (oocytes) inside them if they developed during the summer, but then fewer eggs if they developed in the fall, which was completely expected. But surprisingly, so did the Costa Rican monarchs! Yes, the offspring from non-migratory parents, showed the correct degree of reproductive diapause when they developed and experienced the late-summer conditions in the middle of North America. Recall that this is consistent with what the prior study from 2018 had found too, so to be fair, this was not new. I'll paste a screenshot below showing the figure on this.

Here's the new bit: they found that North American monarchs from the fall generation had larger wings than those in the summer generation, which is also expected, given that most monarchs from this population develop into the "super generation" in the fall. But, the Costa Rican monarchs did NOT show this. There was a minor increase in wing size of the fall generation from that population, but it was not statistically significant. I'm pasting a screenshot of one figure below, which shows this finding.

From my read, this finding here about wing sizes is a critical new piece of evidence. It means that the offspring from non-migratory parents did not fully develop into the migratory super-generation, even though they appeared to go into reproductive diapause. This means that even if they tried to undertake the long-distance fall migration, they probably would not be successful with small wings. And, based on my read of this figure, those monarchs are about the size of a typical Florida monarch.

Next, let's talk about the metabolic rates. Here the findings were a little odd, to me at least. Recall that I mentioned that the super-generation in North America usually has a low metabolic rate, and this is something that has been shown in a prior study from a student here at Georgia. In the current paper, the authors found that N. American monarchs showed no difference in rates from the breeding cohort to the migratory cohort. Meanwhile, the non-migratory Costa Rican monarchs showed an elevated metabolic rate in the fall generation. The authors interpreted this to mean that the non-migratory monarchs would not be capable of migration if their metabolism was so high. I think this is correct, although I am puzzled at why the N. American monarchs did not show a drop in metabolism, as was found in the prior study.

OK so the last thing they talked about was the size of the wing muscles (the thorax size). They found that the female monarchs in N. America increased their flight muscles in the fall, as would be expected, though the males did not. But, neither the males or females from Costa Rica did this. So this was more evidence that these monarchs failed to show the proper development into the super-generation.

So based on all of the data and evidence gathered, the researchers concluded that monarchs from non-migratory populations still show some traits that resemble their migratory ancestors, though not all. In other words, if you were to take monarchs from south Florida and transport them to Michigan in late-summer, they would not be able to produce offspring that would be capable of migration. Apparently, the loss of migration in these populations has changed the monarchs too much, so that they no longer remember how to do it.

To me, this finding also justifies my own stance in the ongoing debates about the importance of year-round milkweed in North America. I've been arguing why these non-native milkweeds are dangerous to the North American population. These places promote year-round breeding, and growing pockets of non-migratory monarchs. In other words, places where monarchs no longer migrate. The findings from this new study indicate that the offspring from these monarchs would never be capable of becoming migratory, even if they experienced all of the appropriate environmental conditions. What this means is, that once monarchs enter a non-migratory population, they don't change back - it's a one-way street.

Everyone who has been active in promoting year-round breeding within the United States should take note of this study.

OK, that about does it for this post. Kudos to the authors on a fine study.


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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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