Do migratory monarch butterflies evolve larger wings?

Yesterday the BBC reported on a study by Sonia Altizer and Andrew Davis, of the University of Georgia, purporting to demonstrate that populations of the monarch butterfly (Danaus plexippus) show differences in wing size that are correlated with whether or not the populations show migratory behavior to overwintering grounds.  Intrigued, I went to the Evolution website and read the paper, which is accepted but not yet copy-edited.  I found what seemed to be a serious problem with the interpretation—a problem that, had the BBC reporter had some expertise in evolutionary biology—could have been caught, or at least highlighted in the news report.  This underscores the recurring problem that science reporters without much formal training in science often report results without giving the proper caveats.

As you probably know (especially if you watched the NOVA program, The Incredible Journey of  the Butterflies, which aired a few days ago), some populations of monarch butterflies show bizarre and wonderful migratory abilities. Individuals from the east coast overwinter in Mexico, and those from west of the Rockies migrate to Southern California.  The migration is necessary because adult butterflies can’t tolerate cold, but also because their food plants aren’t available in winter.

But this is not a continuous movement of adults from summering grounds to wintering grounds.  The adults do fly the entire one-way journey in the fall, but it takes them several generations (each generation lasting 6-8 weeks) to get back to their summering grounds.  One of the great mysteries of monarch migration is how they’re able to return to the same summering grounds used by their great-grandparents.  What mechanism guides them in the right direction? And how did natural selection produce this directionality? We don’t know the answers to these questions.

Not all populations of monarchs are migratory.  Those in warmer areas, like southern Florida, Costa Rica, and Puerto Rica, have no impetus for migrating since the climate is tolerable and food plants continuously available. They stay put all year.

Based on this “dimorphism” among populations, Altizer and Davis  reanalyzed old data, originally collected for a study of parasitism, to test the following prediction:

We therefore predicted that monarchs from long-distance migratory populations would have larger and more elongated forewings to increase flight surface area and reduce wingtip-induced drag.

That is, individuals from migratory populations are under strong selection to fly long distances, and thus would evolve wings more suited to this task than those populations that are more sedentary.

I won’t go into all the details of this study, but here’s what they found:

1.   Wild-caught individuals from both “eastern migratory” populations (Minnesota, Georgia, and Mexican overwinterers) and “western migratory” populations (California, Utah, Nevada, Washington, and Colorado) were larger than individual from “nonmigratory” populations (Florida, Hawaii, Costa Rica, and Puerto Rico).  This verified their prediction (there were also shape differences, but I won’t discuss them here).

How do we know that these are evolved genetic differences rather than purely environmentally-induced differences in body size (correlated with wing size)? After all, we know from laboratory work that insects reared in colder temperatures grow larger than genetically identical insects reared in warmer temperatures. (I’ve done this many times, for instance, with Drosophila.) To answer this, Altizer and Davis reared three groups of butterflies under constant laboratory conditions of food and temperature.  They observed:

2.  Under these constant conditions, individuals from eastern and western migratory populations still had bigger wings than individuals from nonmigratory populations (unfortunately, they analyzed only one nonmigratory population here: that from south Florida).  From this they conclude that the differences between all populations are genetic and represent evolved adaptive differences:

Collectively, these studies suggest that the demands of long-distance flight represent an important evolutionary force operating on the physical characteristics of migratory species.

The BBC report, written by Matt Walker, echoes this conclusion in the report, titled “Supersized monarch butterflies evolved to fly far.”

These “supersized” butterflies have evolved to cope with the demands of long-distance flight.

In contrast, monarchs that live in one place all year have wings that are up to 20% smaller, report scientists in the journal Evolution. . .

Walker gives no caveats in his report. He simply blurbs the paper and gives some quotes from Altizer.  The reporter made no attempt to seek out opinions or commentary from other scientists.

Is there anything wrong with that? Well, one thing: there’s another explanation for the results, not depending on migration, that neither the paper nor Walker considers.  It is this: it has long been known that if you look at populations of insects from different areas of its range, those from colder locations tend to be larger (both developmentally and genetically) than those from warmer locations.  In other words, they conform to Bergmann’s rule, an “ecogeographic rule” that states that the body mass of an animal is positively correlated with the latitude where it lives. In other words, populations from colder areas have bigger bodies.

The classic explanation of this “rule” involves mammals: if you’re living in a colder climate, it’s adaptive to have a larger mass, for the ratio of heat produced (proportional to the cube of a linear dimension, in other words body mass) to heat lost through radiation (proportional to the square of a linear dimension, in other words body surface area) is lower for larger animals.  That is, it’s easier to stay warm if you’re bigger.  Now this explanation holds only for warm-blooded animals (homeotherms), but we now know that the “rule” is also obeyed by many cold-blooded animals (poikilotherms).  I’ve spent a lot of my career documenting this in Drosophila, and it’s clear that, regardless of the species, populations from colder areas evolve larger size.  Why this is so in poikilotherms, who don’t produce body heat to keep warm, is an intriguing but unanswered question. But the phenomenon is real.

The apparent problem with Altizer and Davis’s result is this: all the “nonmigratory” populations live in warmer areas than do the “migratory” populations.  Therefore, we expect nonmigratory individuals to be smaller than migratory individuals (i.e., have smaller wings), even if there were no difference in migration behavior. (This is aside from the fact that the authors draw sweeping conclusions about genetic differences from comparing only two migratory populations with only a single nonmigratory population.)

Now the authors don’t discuss this potential problem, which I think is serious.  The reviewers of the Evolution paper should have caught it.  Nor does the BBC highlight it.   My verdict on the paper: it’s intriguing but nowhere near conclusive, and should have been reviewed more thoroughly.

I may be wrong in this conclusion, and perhaps the authors will point out my error. And of course further work may show that they’re correct about a correlation between migration and wing size.  But in the meantime, it highlights an apparent breakdown in not only reviewing papers (which has grown more cursory with the exponentially increasing submissions reflecting both the existence of more scientists and the greater pressure on scientists to publish more), but also in the tendency of science reports to avoid looking too hard at research that produces interesting conclusions.


Altizer, S., and A. K. Davis. 2010. Populations of monarch butterflies with different migratory behaviors show difference in wing morphology. Evolution, in press.


  1. Richard Harrison
    Posted January 28, 2010 at 8:29 am | Permalink

    If you liked that, you’ll love this:

    Note, the article is by… Matt Walker…

  2. JScarry
    Posted January 28, 2010 at 8:39 am | Permalink

    Is it really true that, “all the “nonmigratory” populations live in warmer areas than do the “migratory” populations.”

    The weather is colder on average where the migratory butterflies live, but what is the average temperature when they are actually there?

    Winters in Mexico are fairly warm and summers in the Midwest are downright sweltering. It wouldn’t surprise me if the temperature difference between where the migratory butterflies are living and breeding and the ones with permanent populations was less than 5°F.

  3. NewEnglandBob
    Posted January 28, 2010 at 8:46 am | Permalink

    Nice analysis, Jerry and quite unpretentious and gracious in your final paragraph.

    The Creotards and IDiots will unlikely understand the following though:

    if you’re living in a colder climate, it’s adaptive to have a larger mass, for the ratio of heat produced (proportional to the cube of a linear dimension, in other words body mass) to heat lost through radiation (proportional to the square of a linear dimension, in other words body surface area) is lower for larger animals.

  4. Confused
    Posted January 28, 2010 at 8:53 am | Permalink

    While the BBC do tend to be terribly sloppy with their science journalism, I have to speak up and say that this is really really not the responsibility of the journalist to pick up on. Popular press is not a peer review process. I think it’s absurd to expect science journalists to even read the papers they are describing, much less to critically appraise them. Usually these stories are picked up from press releases, and if the limits of the study haven’t been flagged up by either the press release or the researcher, that is their fault, not the journalist.

    Now if the journalist can be shown to have glossed over limitations that a scientist has clearly stated, that’s another matter; and it’s good practise to get an independent opinion on a piece, although not always practical.

    Science journalism is generally pretty dire, a fact not helped that most of them are BAs instead of BScs, but even in a perfect world where all science journalists have a university science education, expecting the journalist to bring the science criticism to a piece, rather than a specialist in that area (either the author or an independent scientist), is just unreasonable.

    • Colin
      Posted January 28, 2010 at 9:26 am | Permalink

      “I think it’s absurd to expect science journalists to even read the papers they are describing, much less to critically appraise them.”

      Wow! I completely disagree on this point. I’ve had some media coverage for a few research projects and in each case it was very clear from interviews and articles that the reporters read the papers. I couldn’t imagine that they would do even a passable job otherwise. The process was 1) read the press release; 2) contact me for the original article; and 3) interview myself and sometimes others. The better scientifically informed the reporter, the better the end result of course.

      • Posted January 28, 2010 at 9:41 am | Permalink

        Science journalists will often not have the expertise to critically evaluate a paper (which they should read regardless). That’s why they need to interview other scientists, who do have the expertise, to get their views on the paper. And the other scientists interviewed should be independent of the paper’s authors, and perhaps even be known critics. Carl Zimmer does a nice job of doing exactly this in his report in today’s NY Times on the dinosaur color paper:

      • Posted January 28, 2010 at 10:38 am | Permalink

        At least the reporter did not go to creationists for comments to achieve a “balanced view”.

        Consulting other scientists for their comments is indeed the best approach, and we might expect the better news organisations to keep to that standard. But since that takes time and money, one can see how that step can get skipped.

    • qbsmd
      Posted January 28, 2010 at 11:07 am | Permalink

      If all you expect journalists do is pick up press releases, why do they exist? That function could be outsourced to Google for a lot less money.

  5. Gary
    Posted January 28, 2010 at 9:32 am | Permalink

    You can find the NOVA episode here:

    If I’m reading this correctly, the paper was submitted, reviewed, accepted, and then reported upon. I guess I’m agreeing with “Confused” here. The reporter was able to accurately report the conclusion of the already reviewed and accepted paper. I don’t find fault with the reporter.

    Interestingly enough, the result of that report was that the paper captured your attention, and you reviewed it yourself. In doing so, you found what you believe to be an inaccurate conclusion. Hopefully the same happened for other experts, and some discussion will come about as a result.

  6. Posted January 28, 2010 at 1:24 pm | Permalink

    Surely the point isn’t that the reporter, or any one particular reporter, is at fault, but that if this is the way science journalism is being done then it’s not being done well. Surely that’s the point that matters.

  7. Posted January 28, 2010 at 5:38 pm | Permalink

    But are we asking the journalist to do what the peer-reviewers didn’t? Why shouldn’t the reporter assume that someone else would have poked holes? His job is to render the paper intelligible to non-specialists. Otherwise, he’d be a scientist, not a science reporter.

  8. Torbjörn Larsson, OM
    Posted January 28, 2010 at 7:34 pm | Permalink

    We would expect a science reporter to be well versed in science. (Not all of them obviously, on-the-job training should be permissible.) This problem would IMHO been a tough catch though.

    But of course that is not how the market works. If you want to sell papers, an increasingly difficult work, you have to choose between values of sensationalism and honesty. The former gains more on the mass market.

    Btw, today I learned about “Bergmann’s rule”, but not from here:

    Better food makes high-latitude animals bigger – New solution to 163-year old puzzle?

    To answer that question, Ho along with colleagues Steven Pennings from the University of Houston and Thomas Carefoot from the University of British Columbia, devised a series of lab experiments. They raised several groups of juvenile planthoppers on a diet of cordgrass, which was collected from high to low latitudes. Ho and his team then measured the body sizes of the planthopppers when they reached maturity. They found that the planthoppers that fed the high-latitude grass grew larger than those fed low latitude grass.

    The researchers performed similar experiments using two other plant-eating species—grasshoppers and sea snails. “All three species grew better when fed plants from high versus low latitudes,” Ho said. “These results showed part of the explanation for Bergmann’s rule could be that plants from high latitudes are better food than plants from low latitudes.” Although this explanation applies only to herbivores, Ho explained that predators might also grow larger as a consequence of eating larger herbivores.

    “We don’t think that this is the only explanation for Bergmann’s rule,” Ho added. “But we do think that studies of Bergmann’s rule should consider ecological interactions in addition to mechanisms based on physiological responses to temperature.”

    It’s not known why the higher-latitude plants might be more nutritious. But research in Pennings’s lab at the University of Houston offers a clue. Pennings has shown that plants at low latitudes suffer more damage from herbivores than those at higher latitudes. Ho and Pennings suggest that perhaps lower nutrition and increased chemical defenses are a response to higher pressure from herbivores. [My bold.]

    Well, that is one hypothesis at least. 😮

    • whyevolutionistrue
      Posted January 28, 2010 at 8:27 pm | Permalink

      This work by Ho et al. is interesting, but can’t explain all the data. In flies (and other poikilotherms), the individuals from high latitudes are larger even when all individuals are grown in the same environment. Thus there is a genetically based cline in body size. Ho’s hypothesis is that the cline is only developmental: due to different quality of food at different latitudes.

  9. Kevin
    Posted January 30, 2010 at 2:56 am | Permalink

    They don’t produce body heat expressly to keep warm but they do use their muscles which warm them and, warm or cold blooded, a larger animal still retains that heat better.

  10. Sonia Altizer
    Posted January 30, 2010 at 7:48 am | Permalink

    As a co-author of the paper in question, I found this commentary and criticisms relating to Bergmann’s rule quite interesting. However, I would like to briefly note two points that were overlooked in the blog critique. First, even though winter temperatures are much colder in North America than at lower latitudes, during the summer months when peak monarch breeding activity occurs over much of the US, that is probably not the case. So as noted earlier by one commenter, it is probably more biologically relevant to consider the range of temperatures monarchs from each population actually experience throughout their annual life cycle. Second, the actual paper referenced here placed equal emphasis on differences in wing shape among migratory and non-migratory populations (namely that migratory monarchs on average had more elongated, or angular wings), whereas the commentary here focused on results for size alone. I think each of these points are worth noting, together with the interesting criticisms raised in this commentary.

    • whyevolutionistrue
      Posted January 30, 2010 at 9:38 am | Permalink

      Yes, but my criticism was that this was a POTENTIALLY confounding factor that was simply ignored, as it is in your response here. Waving your hands doesn’t make this problem disappear, which will be addressed properly only by looking at geographic variation among only one migration morph.

      • Posted February 19, 2010 at 9:54 am | Permalink

        This was going to be a separate comment, but maybe more appropriate to put here.

        “…which will be addressed properly only by looking at geographic variation among only one migration morph.”

        I don’t think this is the *only* solution to the challenge of distinguishing between selection for flying ability vs. cold tolerance. Furthermore, how would this disentangle Bergman’s rule from selection for long distance flight? It seems like you would need to do a more complete factorial study: warm and cold in both migrant and sedentary populations?

        I don’t see how the wing-shape data is “hand waving”. If one could nail down structural characteristics expected to differ under cold-adaptation vs. flight adaptation (wing shape, body surface:volume ratio, etc.), that evidence could be pretty informative in pointing out the ultimate cause(s) of wing size and shape. Without knowing what other data exists, I think the wing morphology results are a step in the right direction… that is, if they are consistent with adaptation for long-distance flight, and not just a response to increased size.

        Am I missing something here?

  11. D. Reid Wiseman
    Posted February 1, 2010 at 10:58 am | Permalink

    We often are ensnared by the Post hoc ergo propter hoc fallacy .. confusing correlation with causation. Should we not adopt the Null Hypothesis that infers chance is fully operational, id est, there are no cause and effect relationships between the independent variable … exempli gratia … latitude, nectar volume and the dependent variable … wing size and structure? Have the original researchers made a Type II inferential
    error and rejected a True Null by declaring there is a cause and effect relationship between latitude-length-of-migratory route and wing size and morphology? Perhaps there is correlation between the density of mitochondria-per-wing muscle cell and migratory length?

  12. Andrew N
    Posted February 8, 2010 at 6:10 am | Permalink

    Hey Jerry, this post got me thinking…could this “rule” occur in lakes? Between benthic vs limnetic fish if the temperature difference is significant?

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