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  • Andy Davis

New study just published describing why female monarchs are better fliers

Before I begin this week, let me first apologize for the personal horn-tooting. The paper I'm talking about in this post is one that I worked on with an undergraduate here at the University of Georgia. This was a paper that was in the pipeline for a couple of years and it's finally out this month, in the Journal of Insects, an online journal. If you'd like to read it, it can be downloaded here - link.

We're both pretty happy with the final product. It was a short but interesting project that fills an important gap in our knowledge of this amazing insect.

So the gist of this paper was that it was a comparison of male and female monarchs, specifically, of their body parts. Essentially, we wanted to know if the sexes differ in any way (except for size, which we already know - males are slightly bigger). The reason we wanted to know this has to do with their migration. Remember the previous discussion about how males outnumber females during migration? Well, this has nothing to do with that! This is a completely different issue, which I explain below.

For a few years now, there has been a quiet accumulation of evidence indicating that for some reason, female monarchs seem to be better flyers than males. Gayle Steffy's paper that came out a couple months ago showed this - of all the monarchs she tagged over the years (11,000+) in Pennsylvania, the females had a higher tag recovery rate than males. This was also found in Virginia from tagging activities led by Larry Brindza, which was subsequently published in the Journal of the Lepidopterists Society.

The other evidence comes from work with captive monarchs in our lab. Specifically, in the past few years we've done a couple of projects where we put monarchs on a 'flight mill'. This is essentially a butterfly treadmill. The monarch is attached to a rod that spins around and the butterfly just flies around the track for as long as it can. I'll blog about this gizmo at a later date because it's pretty cool. Anyway we've put a lot of monarchs on this thing and when we looked at all of the flight data, it turned out that females usually flew for longer bouts than did males.

So it looks like females are better at reaching Mexico, and they fly better than males (and these are both related issues). Why is this? Here's where our new paper comes in. We thought that one way to examine this question would be to see if there are any inherent differences in the body parts that are important for flight. So, we took some older (dead) specimens from a previous project, which had been stored in our freezer, and we essentially just dissected them. We (or at least my student) took apart all of the different bits of the butterflies, like in the picture below, then we weighed each of the parts on a super-sensitive scale. We also figued out how large the wings were, using some computer-based methods that allowed us to measure their surface area.

We then took each of the body part weights and did some calculations to arrive at some metrics that are commonly used to evaluate flight potential in butterflies. In other words, we looked at the parts of the body that are important for promoting flight. We first figured out the relative size of the flight muscles - this was a no-brainer. Butterflies with bigger flight muscles are better fliers, so we thought maybe females have larger miuscles than males. Since all of the flight muscles are housed in the thorax of the butterfly (the large part in the middle of the picture above), we compared if female thoraxes are larger than males, relative to their total size. We were surprised to find out they weren't. In both sexes, the thorax accounted for about 35% of the body weight.

Next we figured out the degree of 'wing loading' for each butterfly, which is an index that estimates how much weight the wings have to carry. Lower scores in this measurement indicate the butterfly has an easier time flying. Here, we found that females had a lower average wing loading score than males. That means females expend less energy flying than do males. This was perhaps the biggest 'smoking gun' of the study, and it explains why females can fly for longer bouts on our flight mill.

There were a few other measurements we made that I won't go into here, but there was one final one that made a lot of sense. We calculated the actual thickness of each wing, which is an indirect measure of wing 'toughness'. We were surprised to find that females had thicker wings, on average, than did males. This means that females would be less likely to damage their wings during migration, which would be a huge advantage over males. Having damaged wings during migration would be a death sentence to any monarch. Wing damage slows monarchs down and they end up becomming stragglers, then eventually falling out of the migration altogether - there is evidence of this that I'll save for another blog. Actually, all you have to do is visit the winter colonies in Mexico and look at the monarchs down there - they are all pristine - not a scratch on any of them - meaning the ones with any wing damage don't survive the trip.

The cool thing about this last result (risk of damage) in our study is that it's actually consistent with what people have seen during tagging activities. In one of the chapters of the 2004 monarch book (The Monarch Butterfly: Biology and Conservation), Borland et al. described their fall tagging activites in Texas and Minnesota, and in this chapter they specifically said that they found far fewer females with damaged wings than males. It's always nice to have lab results match up with field observations!

So in the end our paper concluded that female monarchs have certain inherent physical characteristics that give them an aerodynamic and energetic advantage over males, and these go a long way to explain why females are more successful in the migration.

That's all for this week. Feel free to check out the paper online at the journal website - it's freely downloadable: Also, look for some press and media coverage on this paper soon - our university just sent out a press release - link here

The science of monarch butterflies

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

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