• Andy Davis

(Sort of) New paper on monarch migration, OE, and wing size published

Greetings all. Apologies for the hiatus on this blog - my life these days is filled with diaper changes, naps and feedings, and not so much computer time!

In this post, I'm going to give a shameless plug to a recent paper done by my wife, which I was also an author on (I guess I helped enough to qualify for authorship). The paper came out a couple months ago in the journal PLoS One, and was titled "Do healthy monarchs migrate farther? Tracking natal origins of parasitized vs uninfected monarch butterflies overwintering in Mexico." Click here for the link to the online paper. I recall there was some press about this paper when it came out, although there were some important points about the paper that could be emphasized more, which I'll do here.

First, let me explain the basic elements of the project. The overall question we addressed in the paper is laid out in the title - that is, do healthy monarchs fly farther? This is a question relating to the OE parasite, which has been discussed here before, and is a huge issue with monarchs these days. For those who don't know of this parasite, you can check out my wife's website on it - www.monarchparasites.org. This parasite causes a variable amount of harm to monarchs - sometimes it kills them outright before they even eclose, but in many cases monarchs eclose even when infected (they even look normal), then they just live with the infection, although at a cost. Some of them even manage to migrate to Mexico with the infection too. This can't be easy, because we also know that monarchs with this parasite fly less well in a captive setting, based on some flight mill work we did years ago, and that I mentioned in a previous blog. So if the infection causes them to fly poorly, how is it that they can migrate 3000 miles? Read on...

The project involved capturing monarchs at the Mexican overwintering sites, then doing some high-tech chemical analyses on their wing tissue to determine where they came from. Specifically, we had some colleagues (who were also coauthors) look at their 'stable isotope' levels, which basically tells you where in the continental N. America the monarchs grew up as larvae. We specifically did this for a set of healthy monarchs and for a bunch of OE-infected ones at the overwintering sites. When we examined the isotope data from these two groups, we saw that on average, the healthy monarchs originated from areas farther north in the flyway than did the infected ones, which is pretty much what we expected. Another way to think about this is that very few of the monarchs that originated from the more northerly regions had the parasite - this means that any parasitized monarchs coming from those areas probably dropped out of the migration before they reached Mexico (i.e. they died while en route). Conversely, the infected monarchs we looked at tended to come from more southerly regions of the flyway - this is because the shorter distance did not weed out the infected ones.

This last point is very, very important for butterfly gardeners and anyone concerned with monarch conservation - basically, it means that the southern regions of the migration flyway (like Texas, Louisiana, Oklahoma and Arkansas) contribute disproportionately more OE to the monarch population each year. This is not through any fault of the people in these areas - it's just a matter of their location in proximity to the Mexican overwintering sites. Because monarchs that grow up in these southern areas don't have to travel as far to get to Mexico, then these southern monarchs with OE infections can still make the trip. So, for the folks who live in these regions, it means they need to be especially vigilent about reducing OE wherever possible.

Another interesting finding that came from this study was that we found an interesting relationship between the monarch wing size and their travel distance, as shown in the graph below, which is from the paper.

The vertical y axis on this graph shows the level of deuterium in the monarch wings, which is a proxy for latitude (i.e. where the butterfly grew up), with numbers near the top being more northerly locations. So, this graph essentially shows that larger butterflies tended to come from more northerly locations. Again, the reason for this has to do with the natural selection that goes on during the long-distance migration. The smaller monarchs that originate in the northern regions tend to die before they reach Mexico, leaving mostly larger ones from these regions. Meanwhile, small-winged monarchs that come from Texas and other southern locations can make the trip, so you end up with this neat relationship above. In fact, I believe another paper just came out from a similar study of western monarchs (link here), that essentially showed the same thing - longer travel distances leads to selection for larger wings. I'll blog on that paper soon. Combined, both of these papers show how the migration is a natural selection force that keeps migrant monarchs big.

So back to the OE results, it seems to me there has been a heck of a lot of discussion on the dplex, various facebook sites, and news articles, all speculating and talking about the size of the overwintering colonies this year - how big will they be, what the long-term trend is, etc. Invariably, these discussions usually include talk of milkweed, habitat, or something similar as a way to improve things. However, rarely do I see any mention of OE, which is unfortunate, because based on what this paper above shows, one sure way to increase the overwintering numbers is to reduce OE in the migratory cohort.

That's all for this week.

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The science of monarch butterflies

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