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  • Andy Davis

Our warming climate is making monarchs burn more energy during migration

Hello blog readers,

Thanks as always for tuning in. Today I'm going to tell you about a brand new study that demonstrates exactly what the blog title states - that the ever-increasing temperatures we are experiencing year after year are actually making migrating monarchs have to burn more energy to get to their winter destination! Who knew, right? In fact, this is something that very few researchers are looking into, i.e. how climate warming affects the physiology of animals. Anyway, this study was just published last month, and, it made the cover of the scientific journal it was published in - see the photo below.

The paper (link here) was authored by a crew from the University of Cincinnati, i.e. the lab of Patrick Guerra. Chip Taylor was also a coauthor, I think because there was some tagging data included. Full disclosure, I was one of the reviewers of this manuscript in its early phase, and so I had provided some feedback at one point to help improve the study. I do recall being very impressed with this paper, and I see that the published version is equally impressive. The only downside is that it is very, very technical, and hard to penetrate for the average monarch enthusiast. So, I'll try to break it down here as best I can.

Before I start, I first need to tell you some basic insect physiology. Insects are ectothermic animals, which means their body temperature is highly dependent on the external air temperature - when the temperature goes up, the insect temperature goes up, which then makes all of their internal processes work. Conversely, if the temperature is very low, insects don't do much moving around, and their internal processes slow down too. All of these bodily processes (like digestion, blood pumping, muscle function) take energy to work, and this energy can be calculated by measuring the rate at which the insects are "breathing," or in other words, their metabolism. So if it is warm outside, an insect's metabolism will be fast, and if it is cold, it will be slow. But, what happens when the outside temperature gets warmer and warmer - does this continue to increase the insect metabolism, so that they are burning "too much" energy? That was the basic question of this study. The authors (rightly) reasoned that if the temperatures during the fall are getting warmer, this could cause the monarchs to have to burn more energy.

To answer this question, the authors used a combination of some archived monarch tagging data, with some environmental (weather station) data, and then some nifty benchtop tests, and finally some very sophisticated computer modelling. The methods section of the paper is a blur of numbers, formulas, graphs and maps, and you have to read carefully. From my own read the tagging data seemed to be a combination of MonarchWatch records, plus some historic records from the files of Fred Urquhart, since some of the records go back to the 1960s. The authors combed through these records to identify the exact places where monarchs were captured, and then where they went (i.e. which direction). They then extracted weather data from the nearest places along the trajectory, to use in the computer models. I'll throw in a screenshot below from the paper that shows these tag-recovery records, and the lines used to delineate the monarch migration trajectories.

The bench experiments involved measuring the oxygen consumption of monarchs, as an indicator of their metabolism. With this info, plus the monarchs' weight, you can calculate how much energy they would be burning while sitting still. This would be how much they burn while stopped for the night at a roost, for example. The authors also assumed that this would be the same rate of energy consumption of a "soaring" monarch, since they aren't flapping. Then they figured out how much energy would be burned during actual flapping flight (which is about 30% more). Since we know from prior work by David Gibo that monarchs spend about 85% of the time soaring and 15% flapping, they could then come up with estimates for the entire migration, including the time spent roosting, gliding or flapping.

Now the big part was to figure out how much energy is being spent each year, going back to the 1960s. This was done using computer modeling and archived weather data, while taking into account the temperatures along the routes of the monarchs, their travel time, and even their rates of flapping and gliding. Not only that, the authors also estimated how much energy would be spent in the future during the migration with global warming!

The results from this entire (tedious) work was that the authors confirmed that yes, the monarchs appear to be burning more energy during migration in modern times compared to the energy used 50 years ago. And, it looks like they will burn even more in the years to come. The below graph shows the energy burned per day by a migrating monarch, over the timeframe of this study, and including the future. Each of the points on the graph are actual estimates of the metabolism of a single monarch in the year indicated!

In the graph above, each month has a separate energy estimate, and you can see that the monarchs typically burn more energy during August, simply because it is warmer. Next is September, and then October, again because of basic ectotherm physiology. You can also see that in the future, the highest increase in metabolism is, and will be, felt during September, which is the peak migration time for monarchs.

If you squint at this graph, you'll also see something that should be pointed out too - the actual increases in energy expenditure over time are very small. The red points on the graph do show an increase over time, but it is not a huge increase. In fact, in the results section, the authors also point out that the difference between the historic estimates and today is about 6% in the September numbers. So, while these are clearly increases, it should be noted that they are small. Yes, this increase will make monarchs burn more energy, but will it kill them? Don't know.

I don't mean to detract from the importance of this study, since it is a first of its kind (to show how climate change impacts energy during migration). But, I did point out to the authors during the review that this metabolism increase does not necessarily spell doom for the migrating monarchs. I see they did add some text in the discussion about this too. What I'm getting at is that the climate-driven increases in metabolism could end up being yet another "natural selection" force acting on the monarchs during this strenuous journey. There are already so many selective forces during the journey anyway. In this case, those monarchs that happen to start the journey with a "naturally" higher metabolism may end up being the ones that are successful, (because they can deal with the higher temperatures) while those with lower metabolisms may not be successful, and would not pass on their genes. Thus over time you could end up with a population filled with high-metabolism monarchs that are "adapted" to the climate change effect. Or alternatively, monarchs may simply shift their migration timing in the first place, so that they aren't migrating during the high temperatures! We're already seeing this on the Atlantic coast, with migrants showing up later and later at Cape May, NJ. So my point is that monarchs are adaptable creatures, and they may find ways to deal with these changes.

However, given that this is such an energetically taxing journey, filled with stressors and mortality along the way, really, we don't want anything to add to this energetic cost if we can help it. So, I guess this means that we need to stop global warming to help the monarch migration. No biggie, right?

I think this about covers the paper. Kudos to the authors for a fine study. And thanks to the readers here for tuning in to this blog, and hearing about the science around our favorite butterfly!


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The science of monarch butterflies

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

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