A handy summary of all research on captive-reared monarchs
That's right, get ready to strap in for another patented, all-encompassing, uber-informative, list from MonarchScience. The last time I did one of these I summarized all of the research around tropical milkweed, and what it does to monarchs. I think that ended up being quite useful to folks, so I figured I would try it again, this time on captive rearing! This is a topic that needs no introduction to monarch aficionados, and it is one that is fraught with misinformation, emotion, and controversy. I'm talking about the practice of raising monarchs from wild-collected eggs or larvae (in any captive environment), and releasing them into the wild. In some circles this is called "head-starting", others call it "protective rearing", but whatever you call it, know that this practice has issues. Moreover, these issues become magnified when the rearing numbers increase. Anyway, over the last 5-7 years this issue has been studied by many labs and scientists, with each focusing on different elements. Today I'm going to summarize all of these studies, so that this will end up being the ultimate and definitive guide to the research.
I'm going to organize this list by topic, and list each study that falls within that topic, including adding links to the papers. In some cases, certain studies will be listed more than once, if they contain relevant information to multiple topics. Then at the end, I'll come up with a handy bullet point summary of everything, for the lazy readers.
Tagging and Migration Success
I'm starting with this topic because it is probably the biggest point of contention with captive-rearing. I'm referring to the practice of putting numbered stickers on monarchs and letting them go during the fall migration - the idea here is that if those same monarchs are found at wintering colonies, we can confirm that they were successful. A lot of people (more so these days) are tagging monarchs that they themselves reared in captivity, and a few people have had "their" reared monarchs found in Mexico (or California). For some people, this signifies that the reared monarchs are just as good as wild ones, but that is overly-simplistic. Let's take a look at the actual science...
In 2015, a study was published by citizen-scientist extraordinaire, Gayle Steffy, from Pennsylvania, which was a summary of her own tagging results (link here for the paper). For 18 years, she had been rearing monarchs in captivity and releasing them with tags, plus, she had been capturing and tagging wild migrants too. In her study she reported how many of the reared and wild monarchs had been recovered in Mexico. She reports that out of ~11,000 wild monarchs tagged, 56 were recovered in Mexico (0.5%), and out of ~3000 reared monarchs, only 2 were recovered (0.065%). In other words, the migration success rate of reared monarchs was 8 times less than that of wild monarchs.
Another study that falls into this category is a paper focused on monarchs that were studied in Arizona, and where tag recoveries were reported. This was spearheaded by Gayle Morris and colleagues and published in the Journal of the Lepidopterists' Society (link here). From this location, monarchs apparently can travel either to Mexico or to California during the fall, since this appears to be a middle point between the west and east (for monarchs anyway). They reported that out of 7,800 wild monarchs, 24 were recovered in either wintering destination (0.31%). They also had tagged and released 4,279 "farmed" monarchs (their wording), which I presume to be from the butterfly farm that is run by one of the coauthors. Of those, 5 were recovered at a winter colony (0.11%). So in this case, the reared monarchs had a migration success rate that was about one third of the wild monarchs' rate.
I should put in some plugs here for the work of David James, who had spearheaded a multi-year study of monarch tagging in the Pacific Northwest. He had published this work in the Journal of the Lepidopterists Society (link here). This work involved releasing a lot (13,000) of tagged, captive-reared monarchs, and tracking their movements. He reports that a number of the reared monarchs did reach the wintering colonies in California, meanwhile none of the 900 wild monarchs were recovered. While I do not doubt these facts, I put less emphasis on this study here simply because it is based on the very short migration of western monarchs - this migration is only a third of the distance of those in the main eastern subpopulation.
This is a topic that goes hand in hand with the previous one, because one of the biggest (see what I did there?) indicators of migration success for monarchs is the size of their wings. Bigger is better, and there is ample evidence for this, much of which has been covered before in this blog (link here for summaries). And, there are a number of studies that have reported how reared monarchs compared to wild ones, including the Steffy study from above. In her paper, she reports that average wing length of the reared monarchs was smaller than the wild ones, though she doesn't actually report the overall means. I'll put in a nice figure from the supplemental files of that paper though, which shows how wing length is typically measured (the white line).
Another paper that reports wing size data of wild vs reared monarchs is one from the Davis lab, published in the journal, Biology Letters in 2020 (link). In that study, monarchs had been scanned and their wings were measured with computer software. Of the wild monarchs (n=41), their average forewing area was about 900mm2. Meanwhile, the average area for monarchs reared indoors in summer (n=42) was 885mm2. Another group that was reared in fall-like conditions (n=41) had an average wing area of 880mm2. Importantly, these differences were not statistically significant, but they do match the pattern shown by Steffy above.
The next paper to discuss is another from the Davis lab, and it was just published earlier this year in the journal, Insects (link). It was actually a short rebuttal paper that was focused on western monarchs, and it was written to challenge some assertions made by David James. In an earlier study, James had argued that monarchs reared in California during winter would be able to join the migratory cohort when they become adults. In my paper I had pointed out that the monarchs reared by his colleagues (citizen scientists, who had collected wild eggs and reared them in their homes) had an average wing length of 47.8mm, which was smaller than the average of all prior reports of wild monarchs in the west (50.8mm). In fact, the wing length of the reared monarchs more closely matched that of other non-migratory monarch populations around the world (47.4mm). I had argued that this evidence suggests that winter-reared monarchs likely do not become migrants.
Anyway, so there are at least three different studies that I can think of that all show the same thing - reared monarch wings are usually smaller than wild monarch wings. I won't get into why this is, but the important thing here is since bigger monarchs tend to be better at migrating, the small reared monarchs would not be as good. Note that this is exactly what we are already seeing with the tagging data.
This next topic is one that most people may already know about, since the first paper describing this issue made such a big splash back in 2019. However, I've noticed there has been a heck of a lot of misinformation shared about this topic, so this will hopefully clear things up.
In 2019, a very big, splashy paper made headlines - it was from the lab of Markus Kronforst from U. of Chicago. It was spearheaded by a graduate student at the time, Ayse Tenger-Trolander. Here is a link. The paper reported on a series of experiments where wild and reared monarchs had been hooked up to a nifty orientation chamber, which is like a clothes-washer sized barrel where a monarch hangs in the middle from a rod. The monarch can flap and spin in any direction it wants to fly in, and this direction is recorded by the apparatus. See prior blogs about this paper for more info. Side note - a lot of people had criticized this study for using this device, saying it was "artificial" so therefore the results aren't valid. That's just plain stupid, as you'll see next.
Anyway, the researchers placed groups of wild, migratory monarchs on this device, and the vast majority of them demonstrated that they wanted to fly in a southerly direction, exactly as they should. This is why the aforementioned argument is ridiculous - if the contraption is so artificial, why did the wild monarchs perform perfectly on it? Anyway, they also placed groups of indoor-raised monarchs on this device (also during the fall migration season), and those monarchs didn't really choose any direction to fly in. The researchers concluded that the indoor rearing somehow interferes with the proper (internal) development of their navigation senses.
Another point of contention with this study is that people didn't think their "indoor-reared" monarchs were appropriate for comparison, since they had been reared in an incubator, and, because they had not been allowed any time outside to "regain" their navigational senses. Well, these arguments were both proved wrong in the follow-up study by the same authors (link here). In this next paper, the researchers essentially repeated the entire initial study, but used both incubator reared monarchs, plus some that were reared indoors next to a window. The results didn't change at all - reared monarchs simply did not choose the proper orientation, while wild monarchs did perfectly. Moreover, in this study the authors also kept the monarchs in outdoor flight cages prior to testing. Some were kept for a few days and some for more. They found that the length of time left outdoors before testing had nothing to do with their ability to properly orient.
I would be remiss to not mention a followup study that was published in the journal, Conservation Physiology, by Wilcox et al (link here). This group of researchers from the University of Guelph had attempted to replicate the Tenger-Trolander study in their lab, but they had specifically tried to determine if captive-reared monarchs could "regain" their navigational ability once released. They had released some reared monarchs during one fall migration, and these were outfitted with small backpack transmitters, which could be tracked by an array of cell-tower-like devices scattered across the eastern seaboard. They reported that all of their reared monarchs had indeed been tracked going in a uniform direction (which the authors generously referred to as "southerly"), and they claimed that this proved that reared monarchs do indeed regain their navigational abilities.
If you can't tell, I myself doubted this claim, and I had actually submitted a rebuttal paper that refuted this evidence (link here). In my paper I demonstrated that the reared monarchs in their study were actually all flying in a southeast direction, which is the wrong direction. Based on their own data, none in fact were flying "southerly." Also, the backpacks in the study weighed about 50% of the weight of the monarchs. So the evidence presented in that study was pretty thin to say the least. So anyway, if anyone tries to claim that the Wilcox paper soundly "debunks" the navigational evidence, they are either uninformed or simply lying.
Bottom line here is that the majority of evidence indicates that reared monarchs are not as good at orienting (or navigating) during the fall migration.
OK, this is a topic that I just recently blogged about. It is certainly something that goes hand in hand with migration success. I actually only know of one study that had compared actual data on flight itself (like flight speed, distance, power, etc.), and it was an unpublished graduate student thesis from UGA. In the previous blog I described this project. In a nutshell, this student had placed wild and reared monarchs on our custom-made flight mill, where they can fly around in circles while our computer tracks their speeds and effort. She had found that reared monarchs (either reared in summer conditions, or fall conditions) have a lower average flight speed than wild migrants. She also found that reared monarchs fly with lower flight power (or effort).
So, this study demonstrated that reared monarchs are not as good at flying as wild monarchs are.
This topic dovetails nicely into the previous one, because the results are basically consistent. Basically, the 2020 Biology Letters paper from the Davis lab had done a nifty test of the physical "strength" of monarchs, using the device pictured in the video below. This video describes the study in question, and was also included in the blog entry I wrote at the time (link). It was a small wooden perch connected to a force gauge - individual monarchs were lowered onto the perch so their legs could grab it, and then the person gently lifted upward until the monarch let go of the perch. The gauge recorded the force required for them to let go. Think of it like holding onto a bucket while someone pours water into it - the point at which you can't hold on anymore would be like your own physical strength measurement.
Anyway, for this study, we had tested the strength of groups of reared monarchs (reared in summer conditions or fall conditions), and compared their readings to some that were wild-caught. I'll put a quote from the study here, which shows the results - "the average force required to release the perch in wild monarchs was 0.36 Newtons, which was approximately 38% greater than the average for monarchs in our indoor room (0.27 N), and 80% greater than those reared in our incubator (0.20 N)."
Bottom line here - reared monarchs were not as physically strong as wild monarchs. The thing I appreciated about this study was that this result is quite unambiguous, and leaves no room for doubt.
This is an interesting topic which most folks probably have never thought about before - I'm talking about how monarchs can learn! If you think about it, during their fall migration they must have to learn which flowers to visit to get the most nectar, or which trees provide the best roosting spots, etc. Even with their tiny brains, they still must need to learn some things as they go. Believe it or not, recently a scientist actually tried to determine if reared monarchs were just as capable of learning as wild monarchs. This paper was published in the journal, Insects, by one author, Robert J. Gegear, from the University of Massachusetts (link here). I had written a blog also of this study (link here for that).
In a nutshell, this study involved teaching monarchs to choose different color cards, and for certain colors he gave the monarchs a sugar reward. He was asking if the monarchs could learn to associate the key colors with the reward (short answer was yes). Importantly though, he had tested some wild monarchs and some captive reared monarchs (from Chip Taylor's lab) using this approach. He found that reared female monarchs scored very low in one of the memory tests, though this was not true for males. Meanwhile, wild males and females apparently did fine.
The bottom line here is that the reared (female) monarchs were not as smart as wild female monarchs. Reared male monarchs seemed to be ok.
Stress and stress reactions
OK, last topic to discuss, and it is one of my personal lines of inquiry in my lab, so the work here is all my own. I guess you could call this a shameless plug for my research (!).
In 2020, I published a solo paper that described my efforts to determine if human handling stresses monarchs (link here). Sidebar - I'm including this paper in this list of captive rearing studies, because human handling is typically something that goes along with captive rearing. In most indoor rearing setups, there is always some handling involved, either in the daily cleaning of containers, or even simply moving containers between tables. Anyway, in my lab I often test how insects and other critters (even spiders) become stressed by various anthropogenic pressures, and I actually do this by measuring their heartrate! Yes, monarchs have hearts that beat rhythmically, and their rate goes up if they become scared. For this study in question I had conducted a series of tests across each of the larval, pupal, and adult life stages, to determine if brief periods of handling affects their heart rate.
The results of that study showed that brief handling does cause caterpillar heart rates to increase by about 20%. Also, gentle jostling of pupae caused extreme spikes in their heart rate (yes, the heart beats during the pupal stage too!). But curiously, the adult monarchs were not stressed by the brief handling.
The bottom line here is that human handling leads to temporary bouts of stress to the developing stages of monarchs. And keep in mind that such bouts of stress would likely be repeated daily during captive-rearing scenarios.
OK, so let me sum up everything here into some handy bullet points for sharing purposes. If you skipped over everything and just read these, that's ok, but if you want to know more about any of these bullets, just go back and click through to the published studies themselves.
Based on the studies included in this list, the collective scientific evidence shows:
- captive reared monarchs (in the eastern subpopulation) have lower migration success rates than wild monarchs, by anywhere from one third lower to 8 times lower. In the west, this may or may not be the case.
- captive reared monarchs have smaller wing sizes than wild monarchs, which does not do them any favors during the fall migration
- captive reared monarchs have poorer navigational ability than wild monarchs
- captive reared monarchs fly less well than wild monarchs do - they fly slower and with less effort
- captive reared monarchs are physically weaker than wild monarchs, by anywhere from 40-80% weaker
- there is some evidence that captive reared monarchs are not as smart as wild monarchs
- the daily handling, jostling and moving of containers that comes with captive rearing can lead to repeated bouts of stress in caterpillars and pupae. The long-term effect of this repeated stress is not known.
OK, so lastly, let me throw in a thought about all of this, because by now you are probably wondering why do all of these studies show (basically) the same thing - that captive reared monarchs are so much worse than the real thing? While others may have their own theory, mine is that captivity essentially bypasses natural selection. In the wild, only the strongest, biggest, smartest, etc, survive to reach adulthood, while most of the wimps, rejects, etc. would be killed or eaten long before that. This is the way mother nature has designed these creatures - there is a reason why females lay hundreds of eggs, and most of these are not intended to reach adulthood. Only the best and brightest live to pass on their genes. Meanwhile, in the captive scenario, all of the immature monarchs (or most anyway) live to reach adulthood, even the wimps and losers who were never supposed to make it. This is why just about every comparison we make between captive-reared and wild monarchs shows these differences - because we are comparing a whole bunch of wimps to a bunch of superstars.
I'll leave you all with this one final thought/question - what is the effect of releasing hundreds of thousands or even millions of wimps and rejects (annually, over many years) into the wild monarch population? Food for thought for sure.
That's all for now.
Direct link to this blog entry: