New study confirms captive-rearing hinders monarch navigational ability after eclosion, but wait...
Wow, this makes three blog entries in one month - there must be some cool new stuff going on in the world of monarch research. Well, this one today is definitely cool, but in a weird kinda way.
A new monarch study just popped up on the pre-print journal, BioRxive. You may recall, I discussed this interesting new type of "journal" before. This is where scientists can upload their draft manuscripts before they go through peer-review. This does not mean they are unworthy, or necessarily flawed, but these may have bits or elements in them that may not end up in a final version when it does go through full peer-review.
This new study is from the lab of Dr. Ryan Norris at the University of Guelph in Canada, and was spearheaded by PhD student, Alana Wilcox. Here is a direct link to the pre-print. This new study has a very interesting title, "Captive-reared migratory monarch butterflies show natural orientation when released in the wild ". Now, before I get started, let me say that I just read this paper from front to back, and even the supplemental material. I have some doubts about whether this title is accurate. I don't question the results necessarily, but whether this is the right title and take-home message of the paper. Let's circle back to this later.
You may remember last year, there was a bombshell of a study that came out that showed (or appeared to show) that captive-reared monarchs have trouble navigating properly. It was from the Kronforst lab at Chicago, and spearheaded by his PhD student, Ayse Tenger-Trolander. That study set off a firestorm of social-media chaos, plus lots of news coverage. It seemed like everyone and their dog had read that paper, and, had something to say about it! I know I did! If you recall, that study had tested if captive-reared monarchs could orient properly when attached to a directional flight mill/simulator. The reared monarchs showed reduced orientation ability compared to wild-caught migrants, which all showed proper directional flight. But, as a lot of people pointed out later, there are indeed some reared monarchs that successfully reach the Mexico, and/or California winter colonies. So there is a bit of a conundrum here. How can some reared monarchs successfully migrate if their navigational ability is hindered after eclosion? Could it be that they are indeed impaired at first, but they then regain their navigation later?
This new study seems to come in right about here, and looks to be designed as a bit of a test of the 2019 study, but using some slightly different methods.
The researchers report on the results of two experiments in this paper. In the first one, it looks like they conducted their own navigational flight mill trials. Apparently they have their own version of the Tenger-Trolander et al. gizmo in their lab at Guelph. Below is a simplified diagram from their paper. Like the one from the other study, this thing is essentially a washer-sized apparatus with an upright tube, and you attach a monarch to a rod in the middle of the tube. The monarch pulls on the rod in the direction it "wants" to go in, and the device records these pulls. When you put migrating monarchs in these things, they should pull in the direction of their migration pathway, or that's the idea.
The Guelph researchers reported they reared monarchs in their lab for this study. I looked over these methods, and they basically did what I would have done. They apparently gathered wild monarchs from nearby locations in Ontario, then had them mate in their lab to lay eggs. They then reared these eggs to adulthood in their lab. From these adults, they "flew" 54 monarchs on this device. Their results showed that only 33% of the monarchs displayed "proper" migratory orientation, that is, an overall southward direction. If you look at the figure provided here, showing the directional tendency of all the monarchs (averaged), it's pretty much all over the place. This circle represents a compass, and each dot on the outer ring represents the direction that a monarch wanted to fly. Keep in mind this test was done during the fall migration (Sept), when the monarchs should have wanted to go south, or southwest from this location in Ontario.
As you can see, only a small proportion of monarchs in this group appeared to want to fly in the proper direction, which is where the 33% comes from. So, this experiment appeared to confirm that captive-reared monarchs do have impaired navigational ability, just as the 2019 study showed. But, there were a few monarchs that did want to fly in the proper direction (south), and this is also consistent with the prior study, which found similar results.
Now, here's where it gets interesting...
The researchers next conducted a "radio-tracking" study of reared monarchs, to see how they do "in the wild". How cool is this? I put the words radio-tracking in quotations here because it was not what you're probably thinking of. They didn't have GPS tags or anything like that - that technology is not available yet (although I have been reading about new developments on this from the lab of Andre Green...). Dr. Norris' lab has been involved with something called the MOTUS network, which is a system that allows researchers to track migrating and flying animals, including birds, bats, and even monarchs! The way this system works is, you place a tiny device on your critter in question and it emits a signal. Each animal gets a different frequency. And, at specific places throughout the U.S. and Canada, there are special radio-towers that are constantly listening for these signals. When the animal in question comes within a certain distance (I can't recall the exact distance), the signal is "detected" and then you can tell where the animal is going if you place all of the "detections" on a map. The beauty of this system is that you don't have to actually catch the animals to get the data. That's the case with some other tracking devices out there today, and it makes it very hard (do you know how hard it is to recapture the same bird again from one year to the next?).
Anyway, it looks like Norris' team attached transmitters to two groups of reared monarchs (about 40 in each), and then let them go from their location in Guelph. They then "listened" for the signals throughout the network. And...bingo, they got some! A lot, in fact. They got signals from 97% of the reared monarchs in locations away from the release site. They touted this number a lot. They also report that, based on these locations, these reared monarchs appeared to have an overall "normal" directional flight pattern. Below is a map from the paper.
The asterisks show the two release sites, and the lines show the directions the monarchs went after release. All of the little dots on this map shows the locations where these tracking towers are. For comparison to the flight mill experiment, the researchers also created another compass with these tracked directions. They report that these directions all appear to be uniform, which implies actual "navigation", as opposed to random orientations as seen in the other compass. From this evidence, the researchers conclude that the captive-reared monarchs, while being clearly impaired at first, must be able to regain their navigational ability when exposed to natural conditions. Hence, the nature of their title.
OK, here's where my two cents come in. I must admit, I'm a little on the fence about this claim implied in the title. I'm not doubting the experimental results or the really cool tracking results, but there is one critical thing missing from this radio-tracking part - an experimental control group. Unless I missed something in reading this paper, the authors apparently only tracked reared monarchs. They did not track any wild-caught migrants, which would have been the only true control here. The Tenger-Trolander paper had done exactly that - used a control group of wild migrants to compare with their reared monarchs. This was important for the 2019 project because it demonstrated that the device works perfectly to illustrate which direction real migrants want to fly in. In this new paper, it seems like this is a critical oversight because a control group would have told us exactly which direction the "real" monarchs want to go from this location in North America. To illustrate this issue, take a look at the map above, picture this region of the flyway in your mind, and look at where those monarchs are heading. Yes, southEAST. What? Shouldn't they be going southwest? Below I've pasted a simple map I made with the release location and the mean direction of those reared monarchs.
I suppose it's possible that this is normal for monarchs in Ontario to travel in that direction first, and then begin flying in a more southwesterly direction later. There are certainly lots of tagged monarchs from that region which end up along the Atlantic coast, or even Florida. But, this is where a control group would have helped. If the researchers had just tracked a bunch of wild migrants, we'd know for sure if those reared monarchs were indeed following the "normal" migration path for this region. Since this direction seems off, it is also possible that these reared monarchs did not actually regain their navigational ability after all. Perhaps their sense of direction was off. Or, perhaps the weight of these new radio-tags was actually too much for them to carry, and it pulled them to the left as they flew! Who knows? Again, a control group of wild migrants was critical here for sorting all of this out.
So, I think my main critique of this new project is in the interpretation of the results. The authors touted the directional nature of those monarchs a lot in this paper, and even went so far as to say the monarchs had "regained" their navigational ability once released into the real world. As I said, I'm on the fence. They may be right, or they may not be.
There is one more thing that I didn't mention that really, really, REALLY complicates this story. The authors write that half of these monarchs had been reared on neonicitinoid-treated milkweed! I know, crazy, right? The authors report that these monarchs had been reared as part of a separate study on the effects of neonics on monarch navigation. This does happen in science, and it's happened to me before too, where you use the same critters for more than one project. However, I'm not sure I would have done so here. They state in the methods, almost in passing, that there were no effects of the neonicitinoids in that separate study, and so they used the treated monarchs for this one! They indicate that the results from the neonic study will be forthcoming! So reading between the lines a bit here, it sounds like that other study found no effects of neonicitinoids on monarchs... Wow, I'll be very interested in reading those results when they come out!
So, before you go sharing the paper with your rearing friends, and making claims that this new study "invalidates" the other one, keep this last bit in mind. That is, if you wish to hold up this study as your "proof", then you also must accept the neonicitinoid part (as being valid) as well.
Let me see if I can sum all this up now. We now have a new study that still needs to go through peer review. Right now it shows that captive-reared monarchs cannot properly orient when hooked up to a flight-mill apparatus. It also shows that they do show a directional flight when released in the wild. But, that flight direction may or may not be "normal", and, half of the monarchs had been fed neonicitinoid-treated milkweed.
And finally, earlier I mentioned that the title of this paper seemed off. Here is what I would have used: "Captive-reared monarchs show reduced orientation ability on a flight mill, but show semi-directional tendency in the field." This seems more accurate to me.
That's all for now. Cheers.
Direct link to this blog entry: