When Monarchs take flight

Ryan Baker-Branstetter

It's that time of year again.  No, not time to start making spring break plans.  Monarch butterfly overwintering season!  Two weeks ago, I went down to Pismo State Beach and feasted my eyes on thousands of butterflies covering a eucalyptus grove.  There were an estimated 25,000 butterflies there last month during the peak of the season and "only" about 10,000 when I visited.  I wanted to know more and promptly went to the most reliable and important resource on this planet: Wikipedia.  My interest was piqued, so I actually did the research to make this an interesting and hopefully entertaining blog post.

Monarch butterflies at an overwintering site in Mexico

Monarch butterflies (Danaus plexippus) undergo one of the most incredible migrations on Earth.  During the summer, they can be found throughout the northern US, when temperatures are tolerable for this fragile species.  Once fall arrives and the temperature starts to drop, they begin a journey that can span thousands of miles.  There are two general migration patterns that are followed, separated by the continental divide.  Butterflies on the western side of the divide travel to the California coast or down to Baja California, while those on the eastern side winter in Florida and Central Mexico.

 Monarch butterfly migration map

After spending the winter in more favorable conditions, it's time to start the journey north.  If a species that weighs between .27 to .75 grams traveling thousands of miles wasn't amazing enough, here's where it gets really interesting.  It usually takes at least two generations of spring and summer butterflies to reach their original range.  Monarch butterflies "return" to specific geographic areas that they have never actually been that are usually hundreds to thousands of miles away.  Wow.  The next big question is how.

Monarch butterflies use up to three specific systems to navigate their way: sun compass, circadian clock, and possibly magnetic compass.  Primary compass sense is usually attributed to the sun compass, but it needs further navigational aid to orient itself properly as the season changes since the sun is a moving target, but the butterfly is trying to navigate to a fixed point.  Scattered sunlight creates a polarization pattern and gradient of skylight that is received by the butterfly eye for orientation. 

The circadian clock is used to compensate for changing amounts of daylight and is regulated at an mRNA and protein level in the cells of the core clock components.  Research has demonstrated that these structure are located in the antennae (Merlin et al.,2009).  The butterfly's antennae were covered with black paint and then exposed to light conditions similar to Fall.  The butterflies were then flown and the direction that they took was recorded.  Butterflies that were not covered with paint oriented towards the south/south-west, while those with covered antennae moved in a west/north-west direction, demonstrating that the antennae was important in orientation based on changing day lengths.

Sunlight signal transduction pathway in Monarch butterflies

Use of a magnetic compass is another sense that has been implicated in navigation as well. Experimentally generated magnetic fields can induce disorientation.  Interestingly, flight behavior is also disrupted when flown under experimentally cloudy skies, which would seem to be the most useful time to utilize this sense. It has been hypothesized that since his experiment was performed using Plexiglass, which blocks specific light wavelengths, magnetic sense may be mediated by light exposure, but the exact mechanism is still under investigation.

Further questions still stand about the exact mechanisms Monarchs use for navigation. For instance, does the Monarch have an actual "map sense" or do they primarily rely on environmental cues?  Do social interactions have an effect on migration?  What determines the end point of the southward migration? 

Recently published research (Guerra et al., 2013) sheds some light on the effect of environmental factors.  Summer Monarch butterflies in Massachusetts experimentally exposed to colder temperatures that imitated spring-migration signals in Mexico.  They started to migrate northward, even when under daylight conditions that imitated fall-like conditions.  The authors suggested that a strongly temperature dependent system would be probable since temperature is such an influential factor in over-wintering survival.  Overwintering sites typically have low enough temperatures to keep metabolic demands low, but not so cold to cause freezing.  This strong temperature-dependent mechanism has potential for disruption of normal migration patterns due to global warming.

Flight of the Butterflies trailer - 2012 documentary

The yearly journey of the Monarch butterfly is truly astounding.  Generations of butterflies return to the same over-wintering sites year after year without using any learned behaviors.  The utilization of environmental cues by multiple senses is essential to the navigation of Monarch butterfly,and disruption to these environmental cues by global warming could have harmful effects.  If you have the time, you should definitely travel down to Pismo Beach to witness the amazing collection of Monarchs that amass every year before they leave on their perilous journey north.

References

Goehring, L. and K.S. Oberhauser. 2002. Effects of photoperiod, temperature, and host plant age on induction of reproductive diapause and development time in Danaus plexippus. Ecological Entomology 27:674–685.

Guerra, P.A. and S.M. Reppert. 2013. Coldness triggers northward flight in remigrant monarch butterflies. Current Biology 23:1-5.

Heinze, S. and S.M. Reppert. 2012.  Anatomical basis of sun compass navigation I: the general layout of the monarch butterfly brain. The Journal of Comparative Neurology 520:1599-1628.

Merlin, C., R.J. Gegear, and S.M. Reppert. 2009. Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies. Science 325:1700-1704.

Reppert, S.M., R.J. Gegear, and C. Merlin. 2010. Navigational mechanisms of migrating monarch butterflies. Trends in Neurosciences 33:399-406.

Image References

K. Kahler.  Migratory patterns of Monarch Butterflies in the United States.  Retrieved from http://www.bigsurcalifornia.org/monarchs.html.

Mexico Today.  Monarch Butterfly Migration Attracts Tourists to Michoacan, Mexico.  Retrieved from http://mexicotoday.org/natural-wonders/monarch-butterfly-migration-attracts-tourists-michoac%C3%A1n-mexico

Richard Ellis. Thousands of Monarch butterflies roost on a branch. Retrieved from http://www.richardellisblog.com/?p=109.

SK Films.  Flight of the Butterflies - Official Theatrical Trailer.  Retrieved from http://www.flightofthebutterflies.com/home/.