Captive-reared monarch wings are "flimsier" than wild ones

Hi blog readers -

Todays post is about an unpublished project from the lab of yours truly, which deserves to be shared, even though it is not (yet) published in a scientific journal. I usually don't do this, but the topic of captive-rearing keeps coming up in the world of monarchs, and there seem to be (still) people out there who think this is helping, despite all of the evidence to the contrary. Well, get ready to see some more evidence today!

So let me start by saying this project was completed by an undergrad working in my lab a few years ago, Farran Smith, who has now graduated and moved on to other career pursuits. She was the one who had performed many of the tests I'll describe below, which required dutiful attention to detail! Kudos to her for all of the hard work.

Now, you're probably wondering, how in the world can you test "flimsiness" in monarch wings? Well, I'll tell you, it involves breaking them, on purpose! Don't worry, these were already-deceased specimens from prior research projects. Like many monarch researchers, I have lots of these in storage, and for this project, we pulled out a few hundred to test. Specifically, we pulled out some that had been reared in captivity, and some that had been caught in the wild.

The idea behind this project was to test how well the captive-reared monarchs can develop their bodily tissues, and importantly, how strong their wings are. Wing strength is a key component of the fall migration, because prior research tells us that if monarchs incur any damage to their wings during the migration, they are not likely to survive the trip. It's all about aerial efficiency, and those (even tiny) wing tears can slow them down. In fact, if you look at monarchs at the wintering colonies in Mexico, 99% of them are pristine, with no major wing tears.

So, for the tests we removed one of the forewings from each monarch. Remember, they were dead already! This then was secured in a vice, as shown below. Next, we attached a clamp to the top of the wing, which was itself attached to a force meter. Some felt fabric was used to keep the clamp from damaging the wing itself. The top clamp was then moved upward until the wing tore, at which point the force meter recorded the number (in Newtons). This then was the "force required to break the wing," or the "tear-resistance."

So with this procedure, Farran then went through and systematically tested each monarch, recording its tear-resistance. We also knew what sex they were, the overall size of the wing (from prior measurements), and of course, whether they had been reared in captivity or wild-caught. After all of this data had been collected, we ran some statistical tests to determine which wings were stronger.

You guessed it, the wild monarchs had stronger wings, and this was statistically significant. Below is a graph showing the difference. The bars here show the average out of a couple hundred wings tested.

In plain terms, the data showed that it took an average of 3.4 Newtons of force to tear wild monarch wings, but only 2.4 Newtons to tear captive-reared monarch wings. And, this effect was regardless of the size or sex of the monarchs. Put another way, captive-reared monarch wings are 30% weaker than wild monarch wings.

Even though this finding is not yet published, I can tell you that from my knowledge of the scientific literature, this discovery is new to science. This is the first-ever documented evidence that rearing an insect in captivity can result in less-developed bodily tissues.

OK, so why is this? Why exactly are captive-reared monarchs so flimsy? To be honest, I'm not sure, and this may require some more investigation even before this project can be published. The monarchs reared in captivity had been given plenty of fresh milkweed, kept free of disease (we checked), and given every opportunity to develop soundly. Like with other findings about captive-rearing, it might simply be a case of natural selection working in the wild, but not in captivity. In other words, the "flimsier" monarchs in the wild simply perish before even making it to the adult stage, so that only those strong monarchs become adults. Meanwhile, in captivity, all monarchs survive, even the wusses.

So, what does this mean for captive-reared monarchs that actually are released? I know there are people doing this, and who are probably reading this right now. To those people, keep in mind that if those monarchs' wings are not strong enough, it means that they are more likely to suffer damage during the fall migration. Remember, the migration is tough and rigorous, and monarchs need to be able to fly around obstacles, flap against strong winds, maneuver around tree limbs at night, and even bump up against each other at night when roosting. All of these things can lead to wear and tear (literally) on their wings. And, recall that even small wing damage incurred during the fall migration is a death sentence.

Given the recent study showing how the fall migration is diminishing year after year (see here), this discovery highlights how releases of captive-reared monarchs is the worst-possible thing we can be doing to help, and in fact, if thousands of people are doing this, it may even be one of the very reasons for the migration decline!

Please stop releasing captive-reared monarchs. I'm not sure how else to say this.

That's all for now.

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Direct link to this blog entry:

https://www.monarchscience.org/single-post/captive-reared-monarch-wings-are-flimsier-than-wild-ones

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