I'm back at my computer already! That's because a brand new study was just published that everyone who cares about monarchs needs to read, or at least be aware of. As you can see from the title of this blog post, the new study focused on the increasingly controversial topic of captive rearing of monarchs. Today I'm going to do my best to tell you about this study.
This new study, published in the journal, Proceedings of the Royal Society (link here for the abstract) was conducted by the same researchers who had done the now-infamous study of captive-rearing from last year, which had been published in the Proceedings of the National Academy of Sciences - Ayşe Tenger-Trolander and Marcus R. Kronforst. I had blogged about that one at the time. Recall that that study had examined how the "navigational ability" of monarchs is affected by the conditions in which they eclose. The authors had strapped monarchs into a nifty flight simulator and recorded which direction they oriented. They found that monarchs purchased from a commercial breeder did not orient south during the migration, and that wild monarchs brought into short-term captivity even did not seem to fly south. Meanwhile, wild monarchs reared in outdoor conditions all flew south on this device. To jog your memory, the picture of the device is below.
At the time, this study had created a bit of an uproar within the monarch community. I recall loads of discussions about it in online forums, and even we scientists got involved. This paper got dissected from top to bottom, with everyone examining it carefully for any shred of evidence that their findings may not be true! Some people indeed found potential issues, and were adamant that the study was flawed. One prominent scientist even wrote a rebuttal paper that pointed out one potential limitation with that study - that the indoor monarchs were not really reared "indoors", but in an environmental chamber that maintained a constant daylength. As we all know, in the wild, daylength is not constant, but gradually changes (shortens) throughout the fall, and it was pointed out that this may be important for triggering a truly migratory state in monarchs. And, since most indoor-rearing involves placing containers next to a window, some people (and some scientists) argued that these results did not apply to their own indoor rearing operation, since a simple window would provide the necessary changing daylength.
Well hang on to your butts - this new study proves that argument to be wrong.
In this latest project, Tenger-Trolander and Kronforst appeared to take these rebuttals to heart, and it prompted them to go back and essentially re-do the entire experiment over again, but this time, they did not use an environmental chamber. They reared their monarchs like everyone else does - in a greenhouse, or in a building next to a window.
They began by collecting 20 wild adult monarchs from nearby areas around Chicago, and they also purchased 20 more adults from the same commercial butterfly breeder that had supplied them in the last experiment. They allowed those monarchs to mate (not between the two groups!) and lay eggs on milkweed plants (common milkweed) outdoors near their lab. They then placed the eggs into three different rearing treatments. One group was reared outdoors, one group was reared in their lab next to a south-facing window, and the last group was reared in a greenhouse. Note that the commercial group was only reared outdoors. Importantly, I see that all of the rearing was conducted in the fall, so that every monarch would be exposed to the appropriate conditions for the migratory generation. And importantly, all of the monarchs experienced the crucial declining daylength!
Once all larvae had emerged as adults in these treatments, they allowed them 4 days to fly around in cages within their respective conditions (indoors, greenhouse, outdoors). Then, they tested each one on the flight simulator apparatus pictured above, just as they had done in the 2019 study. Recall that this device records the directionality of the monarch flight - even though the monarch is strapped into this thing, it can turn and swivel so that the researchers can tell which direction it "wants" to fly in. Any monarch from the Chicago area should "want" to fly in a southward direction in the fall.
Here is what they found.
In total, they tested 83 monarchs, and some of these were tested multiple times (this is important for later). Of the wild monarchs that were reared completely outdoors, nearly all of them oriented in a proper southward direction. Of the commercial monarchs (that had been reared outdoors), there was little overall direction - some oriented north, some west, etc. But importantly, a small fraction of them did show proper southward orientation. I note that this was pointed out repeatedly in the paper. I'll paste below a figure from the paper that shows these two findings. These circles represent a compass, and each line is a monarch's preferred direction. Note that the red line shows the overall average direction of each group. The length of the red line shows the relative strength of that directionality.
Incidentally, there are two videos of monarchs "flying" on this device that are freely available to watch, which have been uploaded to the article's supplemental material section - https://royalsocietypublishing.org/doi/suppl/10.1098/rspb.2020.1326. There is one video of a wild monarch that is showing proper southward flight. The other video shows a monarch with no apparent directionality. Note how it aimlessly spins on the rod! This one monarch apparently was reared in a greenhouse.
Now, that brings us to the meat of the study - for the adult monarchs that had been reared indoors, either in a greenhouse, or simply next to a window, they did not show consistent southward orientation. In 43% of the cases, those monarchs wanted to fly north! Some of them did want to fly in a sort-of-southward heading, but overall, the mean direction of the group was WEST! Below is a figure from the paper showing the orientations of these indoor-reared monarchs. Specifically, you should compare the top circle in this figure with the circle from the figure above that shows the wild outdoor group. Note how few monarchs from the wild outdoor group above wanted to fly north. Remember, all of these monarchs came from the same source parents, they were reared in the same way and the only thing different was that one group was reared outdoors, and one was reared indoors.
Lastly, there was another very important finding of this study that bears mentioning here. One of the other notable arguments against the prior study was that the monarchs may not have been allowed "enough time outdoors to develop their migration sense". Some people speculated that maybe these monarchs showed poor navigation now, but maybe once released into the wild, they will simply re-learn it. In fact, this argument was even tested in a (very lacking) study that compared flight distances of reared vs wild monarchs. That study (I blogged about it here) appeared to show that reared monarchs would re-learn how to be migrants once released. However, that study suffered greatly from a lack of a control group, and the authors failed to point out that their reared monarchs actually were migrating in the wrong direction. I note that that study is still in a pre-print form and not yet even peer-reviewed.
Anyway, despite the problems with that study, this argument has persisted. So, Tenger-Trolander and Kronforst actually went about to test it! They figured that if the migratory state becomes more ingrained the longer an adult monarch stays outside, then those monarchs that had been tested later in the study should have a more pronounced southerly directionality. That was not the case. The authors found no correlation between time spent outdoors (as an adult) and navigation sense.
Since I'm getting close to the end I'm going to list a summary of the key findings from this new study here:
- monarchs reared from eggs indoors next to a window do not have the same orientation sense as those reared completely outdoors
- a small fraction of monarchs reared from commercially-bred adults did have proper navigational sense, but the majority were "navigationally-challenged"
- reared adults that spend time outdoors do not re-gain their navigational sense
Before I sign off, there was a rather important section toward the end of the paper where the authors speculated on the effects of the massive number of monarchs being reared and released these days. While they pointed out that rearing can have some educational and scientific value, they also suggested that these releases could be screwing up the genetics of the whole monarch population. This is an area where I know that Kronforst and his lab are real experts. So that I don't get it wrong, I'm going to paste their statement here:
"Non-migratory alleles (i.e. genes) could persist in the genetic background of a migratory individual. Releasing these commercial individuals may result in more monarchs in Mexican overwintering grounds in the short term, but have unintended consequences on their genetics in the long term. Additionally, the introduction of non-migratory alleles into the wild population may actually increase the number of individuals that breed year-round in the southern USA, which has implications for the increased transmission of the monarch parasite OE."
Andy here again - What they are saying here is that it is possible that even if one of your reared monarchs is recovered in Mexico, it may not have a completely normal genetic code, and in fact, it may be carrying recessive "non-migratory" genes. When that migrant passes on its genes to the next generation, it also passes those ones. Over many years, you could get a build-up of these "bad" genes within the population, to the point where fewer and fewer monarchs actually have the "correct" genetic code for migration. This is certainly something that needs to be investigated in the future, and I hope that Dr. Kronforst and his people tackle it, since they seem to have the expertise to do so.
OK, a lot to ponder here, so I'll leave it at that.
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