Evolution Icon Evolution
Intelligent Design Icon Intelligent Design

For Evolution, Monarch Butterfly Migration Is a Mystery

Eric Cassell
Photo: Monarch butterfly, by liz west from Boxborough, MA [CC BY 2.0 ], via Wikimedia Commons.

Editor’s note: The following is an excerpt from the newly released book, Animal Algorithms: Evolution and the Mysterious Origin of Ingenious Instincts, from Discovery Institute Press.

Every year an estimated 100–200 million monarch butterflies (Danaus plexippus) migrate two thousand to three thousand miles between the United States/Canada and Mexico.1 While there are other populations of monarchs, including in western North America, South America, the Caribbean, and Australia, the population in eastern North America is the best known because of its amazing migration. During the migration, the butterflies lay their eggs on milkweed, where they then go through the larval and pupal stages. Milkweed is the only plant that provides food for the developing larvae. The butterflies are thus dependent on milkweed during their migratory route through the US.2

It typically takes up to three generations of butterflies to make the complete journey.3 This means that the navigation information is genetically programmed. One of the unique aspects of the migration of the monarchs in eastern North America is that during their summer stay in Canada they occupy close to 400,000 square miles, while during their overwintering hibernation in Mexico they occupy less than half a square mile. As noted previously, they often migrate back to the same tree that their ancestor butterflies departed from in a mountainous region in Mexico. That means they must have an extremely accurate method of navigation to locate such a small target.

Navigating by Sun Compass

Monarchs navigate using a sun compass, and as previously described, this includes time compensation to account for the movement of the sun.4 The circadian clock used in the process is embedded within the butterfly’s antennae.5 The sun’s azimuth position is detected through the butterfly’s compound eyes.6 Researchers are only just beginning to decode the biological information required for these amazing feats. The genome of monarch butterflies has been decoded, including the genes related to the neurobiology and physical systems used for migration.7 Comparisons of migratory monarch genomes with the genomes of non-migratory monarchs has revealed that more than five hundred genes are involved in migratory behavior.8

A neuronal model has been proposed to explain how the time-compensated sun compass functions by integrating the azimuth-position information with the butterfly’s internal circadian clock.9 The theoretical model also explains how monarchs are able to maintain the southwest course in the fall, as well as the northeast course on the return migration in the spring. Further research is required to determine whether the model is correct. However, the model does help to underscore the programming complexity required of such a system and behavior. The model is also a good example of how engineering can be applied to analyzing complex programmed animal behavior. 

Origin of the Information

At the same time, while this mechanism may explain how the system works, it does not explain where the information came from that defines the course flown by the monarchs, or the overall control and decision making. There exists no evolutionary model that satisfactorily explains its origin. That by itself does not prove that gradual evolution didn’t produce such programming, but the lack of such a model should at least give the open-minded pause for reflection. 

Monarchs are also able to continue migrating accurately when the sky is overcast and the sun compass is not available. This is possible since they use a magnetic compass as a backup source of navigation. They do this by sensing the inclination angle of the magnetic field to determine latitude.10 The origin of the programming that allows for this also has not been explained in evolutionary terms.

A Decision Tree

Logically, we can approach the question as a decision tree of two possibilities. One, the satisfactory explanation for how such complex algorithms may have blindly evolved simply hasn’t been found yet, despite considerable efforts to uncover such a process. Or two, no such process exists. 

How to proceed? One is to take a never-give-up, never-doubt approach. But that isn’t how successful science is generally done. While there is a place for doggedness, science is ultimately about following the evidence, and the historical sciences, including origins science, are about seeking out the best explanation given the available evidence. If geologists proposed a series of purely natural, evolutionary explanations for the arrangement of rocks known as Stonehenge, and each succeeding explanation collapsed under scrutiny, over decades of investigation and conjecture one could insist that “absence of evidence is not evidence of absence.” Or one could take the many failed attempts as a possible indication that Stonehenge was not formed by any blind, evolutionary process, and open oneself up to other possible causal explanations. 

The point isn’t that this must be the case with the migratory feats of monarch butterflies. The point is that it’s illogical to never be open to the possibility that something other than blind evolution engineered a complex programmed animal behavior that remains stubbornly inexplicable in such evolutionary terms. Whether we have already learned enough about monarch butterflies to abandon blind evolution as an explanation for their navigational prowess is a question we will take up later.

Notes

  1. Richard A. Holland, Martin Wikelski, and David S. Wilcove, “How and Why Insects Migrate,” Science 313 (August 2006): 794. 
  2. “Random Sample,” Science 343, no. 6171 (February 7, 2014): 584, https://doi.org/ 10.1126/science.343.6171.583-c. As noted, the observed population of monarchs in Mexico has decreased significantly in recent years. It appears this is likely caused by a decrease in the availability of milkweed.
  3. Michelle J. Solensky, “Overview of Monarch Migration,” in The Monarch Butterfly: Biology and Conservation, eds. Karen S. Oberhauser and Michelle J. Solensky (Ithaca, NY: Cornell University Press, 2004), 81.
  4. Sandra M. Perez and Orley R. Taylor, “A Sun Compass in Monarch Butterflies,” Nature 387 (May 1997): 29.
  5. Christine Merlin, Robert J. Gegear, and Steven M. Reppert, “Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies,” Science 325 (September 2009): 1700.
  6. A good description of the mechanisms involved in monarch navigation is contained in the paper by Steven M. Reppert, Patrick A. Guerra, and Christine Merlin, “Neurobiology of Monarch Butterfly Migration,” Annual Review of Entomology 61 (2016): 25–42.
  7. Shuai Zhan et al., “The Monarch Butterfly Genome Yields Insights into Long-Distance Migration,” Cell 147 (November 2011): 1171–1185; Shuai Zhan et al., “The Genetics of Monarch Butterfly Migration and Warning Colouration,” Nature 314 (October 2014): 317–321, https://doi.org/10.1038/nature13812.
  8. Reppert et al., “Neurobiology of Monarch Butterfly Migration,” 37.
  9. Eli Shlizerman et al., “Neural Integration Underlying a Time-Compensated Sun Compass in the Migratory Monarch Butterfly,” Cell Reports 15, no. 4 (April 26, 2016): 683–691, https://doi.org/10.1016/j.celrep.2016.03.057.
  10. Reppert et al., “Neurobiology of Monarch Butterfly Migration,” 16.