• Andy Davis

What is the optimal monarch size for successful migration?

Hello everyone,

As expected, the previous post on rearing monarchs generated a fair amount of discussion, both on this blog site and off. The gist of that post was that rearing monarchs is potentially very dangerous because of the risk of diseases and other issues. From reading the comments it seemed that one of the things that kept coming up was the point about reared monarchs maybe being smaller than normal. So this begs the question, what is the 'normal' size of monarchs? I'll address this question in this week's post, plus I'll address how wing size influences migration success, and how people can use this to figure out if 'their monarchs' can go the distance.

First, let's talk about monarch wing size. There are a few ways to measure butterfly wings, but the most traditional method is to measure the length of the forewing. A couple years ago, Tonya Van Hook and colleagues wrote a nice paper that described the best way to do this - the paper is freely available here. Below I copied one of the figures from the paper that shows how to measure the forewing length of a monarch, which is the distance (in mm) between the two red arrows.

Now that we covered how to measure wing size, let's turn to its importance. Basically, the rule of thumb is the larger the better for migratory monarchs, and we know this from a couple of scientific sources.

First, we know this from comparing different populations: back in 2010, Sonia and I worked on a paper that compared wing size of monarchs across multiple migratory and non-migratory populations (a previous post mentioned this). We looked at many hundreds of monarch specimens from the eastern population in particular, and the average wing length there was 51.4mm. The average of the non-migratory monarchs (those from Florida, Peurto Rico, etc.) was less than 48.5mm. This is about a 3mm difference - it doesn't seem like a lot to us, but in fact, for the monarch this is a huge difference. So from this we concluded if you migrate long-distance, you tend to have big wings.

Second, by comparing monarchs within the migratory generation: in any migration season, there tends to be waves of migrating monarchs passing through a given site - some come early in the season, some in the middle, and then there are the stragglers. You probably know this if you do any tagging. The stragglers are the ones that fell behind the rest of the pack because they were too slow. Sometimes this is because of a random wing tear, but it also happens if they simply have small wings to begin with. Those monarchs at the front of the pack are the ones with the best flying gear and most energy - that's why they're at the front of the race. There have been a few studies over the years that have verified this by examining the characteristics of monarchs within the front, middle and last periods within a season. One was by David Gibo back in the 90s, which showed that stragglers tend to be small and the front-runners tend to be big (I don't have the paper in front of me to know the actual numbers). Another one was a paper I did with Dara Satterfield in 2014, where we also looked at wing sizes across the early, middle and late migrants, and we found essentially the same thing. The average lengths were 51, 50 and 48mm, respectively. This paper is online and linked here.

A couple of things jump out from the two paragraphs above: 1) non-migrant monarch wings tend to be 48mm or less, and 2) the straggler migrant wings are also 48mm. So it seems like a no-brainer that 48mm or less is a key number here. So what this means is that if you are rearing monarchs and their wings all end up being 48 or less, that's a bad omen - they will most likely not survive the migration. On the other hand, if your monarchs are all 51mm or greater, then you're in business. In fact, I would argue that any rearing operations should strive to ensure that their monarchs are at least 50mm or larger - if they are not, then the monarchs may not be getting enough food, or may not have the right conditions, etc. In other words, you need to figure out what the problem is.

The other thing to point out is that the importance of wing size probably depends a lot on where you are in the flyway. If you're in the upper latitudes, like say, Minnesota, then it's probably very important. Monarchs that start migrating from here have a long way to go, so they can't have anything that slows them down (like having small wings). If you're in Texas, it may not matter so much.

One question you may be asking, is why not simply measure the monarchs at the overwintering site to know what the optimal size is? Wouldn't those represent the successful migrants? Well, yes and no. As mentioned in the previous paragraph, the overwintering population is composed of monarchs that migrated from Minnesota and Texas (and all other places in between) - so that it would contain a mix of large, long-distance champs, plus any little ones that didn't have to go as far (i.e. from Texas). In other words, the Mexico cohort would not just be large monarchs - it would have a mix.

So the take-home message for today is, wing size is crucial for migration success - the bigger the better (51mm or more). Now before I go I don't want people emailing or posting comments saying 'but I had a monarch that I tagged, UF4 5C3, or whatever, that had a wing length of 46mm, and it was recovered in Mexico'. That's like saying I have an uncle who chain-smokes (I actually do) and he doesn't have cancer (he doesn't). That monarch, and my uncle, are statistical anomalies. Is smoking bad for you? Statistically speaking, the answer is yes. Just like the statistics say that small wings are bad for monarch migration.

That's all for now.



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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs