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  • Andy Davis

New study: caterpillar coloration linked to local weather, but not geography

Hello everyone,

Thanks for tuning in to this blogsite to hear about the latest goings-on in the world of monarch research!

Today I have an interesting new study to tell you about, straight from the lab of yours-truly. It is a new paper that examines coloration in monarch caterpillars from across North America! And the best part about this new study (I think) is that we were actually re-testing the findings from a prior study that I had worked on 17 years ago! This is kind of a rare thing in science, and it was certainly a first for me.

Let me set the stage for you first. Take a look at the monarch caterpillar in this picture. This picture was taken by Ginie Abney Anthony, who is a member of one of my favorite Facebook groups, Serendipity (a group that promotes science and supporting nature by non-interference). She kindly supplied this photo which we used in the paper to describe our methodology, which I'll get to later. Anyway, notice the alternating black and white and yellow stripes of the caterpillar, which are characteristic of this species, and which everyone knows. You may also know that the size of the black stripes of monarch caterpillars has some plasticity (variation). Some caterpillars can have really thick black stripes and some have very thin stripes. Sometimes this can seem quite random, but in fact, there is some science to why this happens, and a lot of it has to do with the temperature of their rearing environment. In a nutshell, caterpillars that grow up in a cooler environment tend to have more black (thicker black stripes), and those growing up in a warm environment tend to have less black, or at least that's what we've found in lab studies.

This idea has been tested and confirmed in a few studies where monarch larvae were reared in temperature-controlled chambers. In each study, the results were the same - larvae from cold temperatures are always darker than those from warm temperatures. We think this has to do with their need to regulate their body temperature, and the simple fact that darker colors more readily absorb more solar energy. Think of how a black leather car seat gets piping hot if the car is parked in the sun, while a white seat, not so much. Apparently, monarch caterpillars somehow sense the temperature around them early in their lives and then somehow their skin color responds accordingly! It's an evolutionary adaptation that allows monarchs to live in a very wide range of locations, regions and habitats! If you think about it, monarchs can successfully live in pretty much every state in the U.S. and most provinces in Canada. How many other butterflies can do this? Side note - see for yourself with this prior blog.

Anyway, so this caterpillar color thing is pretty well known and we scientists have studied it a lot, at least in a lab. In fact, 17 years ago, I was on a team that studied this, but in that study, we were testing how the color-temperature thing differs between monarchs from the western U.S. and those from the eastern U.S. (and also to those from Florida). At that time, the prevailing notion was that the western monarchs were a completely separate population, and therefore "distinct" from the east. We now know that this is not the case, but anyway, back then, there was a fair amount of research done to compare and contrast the biology of western and eastern monarchs, including that study. Anyway, the results of that study pretty conclusively showed that western monarch caterpillars tend to have less black on their skin. At the time, this seemed to make sense, because we do know that there is greater solar exposure in the west than the east. In fact, a more recent study from 2021 actually confirmed this about the solar exposure in the west. So, it all seemed to make sense that western monarchs would tend to be lighter in color, so as not to become overheated in such a warm environment.

That all made sense, until now.

This new study (link here) re-examined this idea, that western monarchs would be different than the eastern ones, but with a twist - we examined photos of wild monarchs submitted by people to iNaturalist! Sidebar here- if you don't know about this website yet, you should check it out ( It is a platform where anyone from anywhere around the world can submit photos and sightings of any creature or species, and then the photos get placed on a map, geotagged, dated, etc. This allows people and scientists to then use these sightings and photos for research purposes, as we did here. I should point out that the "we" in this case was a talented undergraduate student in my lab, Christian Deneka, who did a lot of the work on this project.

What "we" did for this project was to go to iNaturalist and download and examine photos of monarch caterpillars from 4 states in the west, and 6 states in the east, totaling over 500 individual photos. A map of where these photos were taken is below, which is from the paper. Before looking too closely at these states, know that these were chosen beforehand by us as "representative" of monarchs in either the west or the east. I know there could be arguments made for which state is more "representative" than which, but in the end it didn't really matter anyway, as I'll point out later. In the map the red dots are "eastern" and blue dots are "western" monarchs. Also, note that we considered monarchs from Colorado to be from the east, since most of the actual sightings were from points east of the Rockies in that state.

I'll point out here that we made sure that all photos were of caterpillars in a wild setting (i.e. not captive-reared). This was usually obvious from the photos. So, once we had the photos on our computer, we then set out to measure how much black was on each caterpillar. For this, we used some fancy image analysis software I have in my lab. This is a computer program where you can measure the amount of area of anything in an image. In this case, we wanted to know how much black pigment was on the caterpillar in question. I'll put a screenshot of a figure from the paper next, which shows the general procedure, and using the same caterpillar photo.

The right side of the figure shows pictures of monarchs in a lab setting, where I had measured their black stripes using this procedure. As you can see, this approach allows us to get a percentage number for every caterpillar, and this is what we did with these 500+ iNaturalist photos.

So, once we had all of these caterpillar images measured, we next gathered some data about the weather where these caterpillars lived. That's right, it is possible to collect this information from the many thousands of weather stations around the country. Using the latitude and longitude coordinates of the photos, plus the date of the photos, we (or really, my colleague, Nate Nibbelink) were able to download the temperature from the 3 nearest weather stations around the place where the photo was taken! This was very cool, and for us (Nate), this was the first time we had tried this and it worked. We did all of this because we expected this temperature to be an important variable, because of the aforementioned lab studies.

So now you know how the data was collected, so let's get to what we found.

First and most importantly, when we compared the average level of "melanism" (% black) between all western and eastern monarchs, we found absolutely no difference! This was quite surprising, since it goes against what I had found in our prior lab study 17 years ago (that western monarchs had less black). More on this later.

Second, when we examined the temperature data we found that it showed a correlation with the amount of black on caterpillars, and in the expected direction. That is, in areas where the local temperature was cool, the caterpillars were more dark, and vice-versa. This at least fits with the prior lab studies on temperature. If you think about it, this also means that the coloration of monarch caterpillars is affected by the weather!

I'll next put a table in here that shows a breakdown of how the caterpillar color varied by state, and this was also a surprise, but also consistent with the prior geographic finding. We had a variety of states represented in our dataset, from warm ones (Arizona, Georgia) to cool ones (Minnesota, Washington), and, the caterpillars were not overall darker in the cooler states and vice versa! In fact, there was little discernable difference across states, or at least none that made sense based on their temperature. All of this tells us that temperature alone is not the only factor that affects monarch coloration.

Finally, I'll add another screenshot of a graph which shows how the melanism level did in fact change across the growing season - monarch caterpillars tended to become darker later in the summer and into the fall months, and this was true across the board. This certainly is consistent with the local temperature finding, since the temperatures drop later in the season.

Getting back to the east-west thing, recall that the prior study I had worked on 17 years ago did in fact show that western monarchs were different than eastern, at least in terms of skin color, and when the caterpillars were grown in a lab. But in this latest study, we found that this is not the case, when looking at wild monarchs. So what's going on? Well, our best guess is that the prior lab study (or any lab study) may not have truly replicated the lives of monarchs in the real world. In the lab, all of the monarchs are grown in tubs (with milkweed leaves), and the tubs are placed in incubators (think of it like a fridge) with a set temperature. Meanwhile in nature, monarchs grow on a milkweed plant. In the lab setting, the monarchs are grown at the temperature we set in the chamber, but perhaps in the real world, monarchs themselves can change their degree of solar exposure simply by hiding under a milkweed leaf for part of the day. In other words, real-world monarchs may have some degree of behavioral control over their own body temperature! If this were the case, then perhaps monarchs in the west simply adjust their solar exposure on their own. Anyway, this is one idea.

So the bottom line from this new study is that the degree of black on caterpillars is certainly affected by the local weather and season (temperature, anyway), but that does not translate to overall differences between colder and warmer states, nor to overall differences in western vs. eastern monarchs. In other words, there is not really anything special about western monarchs - i.e. they are not a distinct population, and they really just seem like an offshoot of the main, eastern cohort.

OK, I think I'm going to leave it there for you to ponder. The next time you see a monarch caterpillar, I bet you'll look a lot more closely at its stripe pattern!



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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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