5 Things You Didn't Know About the Monarch Butterfly Migration

Some monarch butterflies don't migrate

The monarchs are best known for their incredible, long distance migration from as far north as Canada to their wintering grounds in Mexico. But did you know these North American monarch butterflies are the only ones that migrate?

Monarch butterflies (Danaus plexippus) also live in Central and South America, in the Caribbean, in Australia, and even in parts of Europe and New Guinea. But all of these monarchs are sedentary, meaning they stay in one place and don't migrate.

Scientists have long hypothesized that the North American migratory monarchs were descended from a sedentary population, and that this one group of butterflies developed the ability to migrate. But a recent genetic study suggests the opposite may be true.

Researchers at the University of Chicago mapped the monarch genome, and believe they have pinpointed the gene responsible for migratory behavior in the North American butterflies. The scientists compared over 500 genes in both migratory and non-migratory monarch butterflies, and discovered just one gene that is consistently different in the two populations of monarchs. A gene known as collagen IV α-1, which is involved in the formation and function of flight muscles, is expressed at greatly reduced levels in the migratory monarchs. These butterflies consume less oxygen and have lower metabolic rates during flights, making them more efficient fliers. They're better equipped for long distance travel than their sedentary cousins. Non-migratory monarchs, according to the researchers, fly faster and harder, which is good for short-term flight but not for a journey of several thousand miles.

The University of Chicago team also used this genetic analysis to look at the monarch's ancestry, and concluded that the species actually originated with the migratory population in North America. They believe the monarchs dispersed across the oceans thousands of years ago, and each new population lost its migratory behavior independently.

_Sources:

• _{Monarch Butterfly, Danaus plexippus Linnaeus, by Andrei Sourakov, University of Florida IFAS Extension. Accessed online June 8, 2015.}
• _{Genetic secrets of the monarch butterfly revealed, University of Chicago Medicine, October 2, 2014. Accessed online June 8, 2015.}

Volunteers collected most of the data that taught us about monarch migration

Volunteers – ordinary citizens with an interest in butterflies – have contributed much of the data that helped scientists learn how and when monarchs migrate in North America. In the 1940's, zoologist Frederick Urquhart developed a method of tagging monarch butterflies by affixing a small adhesive label to the wing. Urquhart hoped that by marking the butterflies, he would have a way to track their travels. He and his wife Nora tagged thousands of butterflies, but soon realized they would need much more help to tag enough butterflies to provide useful data.

In 1952, the Urquharts enlisted their first citizen scientists, volunteers who helped label and release thousands of monarch butterflies. People who found tagged butterflies were asked to send their finds to Urquhart, with details on when and where the monarchs were found. Each year, they recruited more volunteers, who in turn tagged more butterflies, and slowly, Frederick Urquhart began to map the migratory paths the monarchs followed in the fall. But where were the butterflies going?

Finally, in 1975, a man named Ken Brugger called the Urquharts from Mexico to report the most important sighting to date. Millions of monarch butterflies were gathered in a forest in central Mexico. Several decades of data collected by volunteers had led the Urquharts to the previously unknown wintering grounds of the monarch butterflies.

While several tagging projects continue today, there's also a new citizen science project that's aimed at helping scientists learn how and when the monarchs return in the spring. Through Journey North, a web-based study, volunteers report the location and date of their first monarch sightings in the spring and summer months.

Are you interested in volunteering to collect data on the monarch migration in your area? Find out more: Volunteer With a Monarch Citizen Science Project.

_Sources:

• _{Dr. Fred Urquhart – In Memoriam, Monarch Watch, University of Kansas. Accessed online June 8, 2015.}
• _{Tagging Monarchs, Monarch Watch, University of Kansas. Accessed online June 8, 2015.}
• _{The fall migration flyways of monarch butterflies in eastern North America revealed by citizen scientists, Elizabeth Howard and Andrew K. Davis, Journal of Insect Conservation, 2008. (PDF) Accessed online June 8, 2015.}
• _{Documenting the Spring Movements of Monarch Butterflies with Journey North, a Citizen Science Program, by Elizabeth Howard and Andrew K. Davis. In Monarch Butterfly Biology & Conservation, by Karen Suzanne Oberhauser and Michelle J. Solensk.}

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How Monarchs Know When to Migrate

By Debbie Hadley

Monarchs navigate using both a solar and a magnetic compass

The discovery of where the monarch butterflies went each winter immediately raised a new question: how does a butterfly find its way to a remote forest, thousands of miles away, if it's never been there before?

In 2009, a team of scientists at the University of Massachusetts unraveled part of this mystery when they showed how a monarch butterfly uses its antennae to follow the sun. For decades, scientists believed the monarchs must be following the sun to find their way south, and that the butterflies were adjusting their direction as the sun moved across the sky from horizon to horizon.

Insect antennae had long been understood to serve as receptors for chemical and tactile cues. But the UMass researchers suspected they might play a role in how the monarchs processed light cues when migrating, too. The scientists put monarch butterflies in a flight simulator, and removed the antennae from one group of butterflies. While the butterflies with antennae flew southwest, as usual, the monarchs sans antennae went wildly off course.

The team then investigated the circadian clock in the monarch's brain – the molecular cycles that respond to changes in sunlight between night and day – and found it was still functioning normally, even after the removal of the butterfly's antennae. The antennae seemed to interpret light cues independent of the brain.

To confirm this hypothesis, the researchers again divided monarchs into two groups. For the control group, they coated the antennae with clear enamel that would still allow light to penetrate. For the test or variable group, they used black enamel paint, effectively blocking the light signals from reaching the antennae. As predicted, the monarchs with dysfunctional antennae flew in random directions, while those that could still detect light with their antennae stayed the course.

But there had to be more to it than simply following the sun, because even on extremely overcast days, the monarchs continued to fly southwest without fail. Could monarch butterflies also be following the Earth's magnetic field? The UMass researchers decided to investigate this possibility, and in 2014, they published the results of their study.

This time, the scientists put monarch butterflies in flight simulators with artificial magnetic fields, so they could control the inclination. The butterflies flew in their usual southerly direction, until the researchers reversed the magnetic inclination – then the butterflies did an about face and flew north.

One last experiment confirmed that this magnetic compass was light dependent. The scientists used special filters to control the wavelengths of light in the flight simulator. When the monarchs were exposed to light in the ultraviolet A/blue spectral range (380nm to 420nm), they remained on their southerly course. Light in the wavelength range above 420nm made the monarchs fly in circles.

_Source:

• _{Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies, Christine Merlin, Robert J. Gegear, And Steven M. Reppert, Science 25 September 2009: Vol. 325. Accessed online June 8, 2015.}
• _{Butterfly 'GPS' found in antennae, by Judith Burns, BBC News, September 25, 2009. Accessed online June 8, 2015.}
• _{Scientists show that monarch butterflies employ a magnetic compass during migration, by Jim Fessenden, UMass Medical Schools, June 24, 2014. Accessed online June 8, 2015.}

Migrating monarchs can travel as far as 400 miles per day by soaring

Thanks to decades of tagging records and observations by monarch researchers and enthusiasts, we know quite a bit about how monarchs manage such a long fall migration.

In March 2001, a tagged butterfly was recovered in Mexico and reported to Frederick Urquhart. Urquhart checked his database and discovered this hearty male monarch (tag #40056) was originally tagged on Grand Manan Island, New Brunswick, Canada, in August of 2000. This individual flew a record 2,750 miles, and was the first butterfly tagged in this area of Canada that was confirmed to complete the journey to Mexico.

How does a monarch fly such an incredible distance on such delicate wings? Migrating monarchs are experts at soaring, letting the prevailing tailwinds and southward cold fronts push them along for hundreds of miles. Rather than expend energy flapping their wings, they coast on the air currents, correcting their direction as needed. Glider plane pilots have reported sharing the skies with monarchs at an altitude as high as 11,000 feet.

When conditions are ideal for soaring, migrating monarchs may stay in the air for up to 12 hours per day, covering distances of up to 200-400 miles.

_Sources:

• _{"Monarch Butterfly, Danaus plexippus L. (Lepidoptera: Danaidae)," by Thomas C. Emmel and Andrei Sourakov, University of Florida. Encyclopedia of Entomology, 2^nd edition, edited by John L. Capinera.}
• _{Monarch Tag & Release, Virginia Living Museum website. Accessed online June 8, 2015.}
• _{Longest Monarch Migration – The Record Flight, Journey North. Accessed online June 8, 2015.}

Monarch butterflies gain body fat while migrating

One would think that a creature that flies several thousand miles would expend a good deal of energy in doing so, and therefore arrive at the finish line considerably lighter than when it began its journey, right? Not so for the monarch butterfly. Monarchs actually gain weight during their long migration south, and arrive in Mexico looking rather plump.

A monarch must arrive at the Mexico wintering habitat with enough body fat to make it through the winter. Once settled into the oyumel forest, the monarch will remain quiescent for 4-5 months. Other than a rare, brief flight to drink water or a little nectar, the monarch spends the winter tucked in with millions of other butterflies, resting and waiting for spring.

So how does a monarch butterfly gain weight during a flight of over 2,000 miles? By conserving energy and feeding as much as possible along the way. A research team led by Lincoln P. Brower, a world-renowned monarch expert, has studied how monarchs fuel themselves for migration and overwintering.

As adults, monarchs drink flower nectar, which is essentially sugar, and convert it into lipid, which provides more energy per weight than sugar. But lipid loading doesn't start with adulthood. Monarch caterpillars feed constantly, and accumulate small stores of energy that largely survive pupation. A newly emerged butterfly already has some initial energy stores upon which to build. The migrant monarchs build their energy reserves even faster, since they are in a state of reproductive diapause and aren't expending energy on mating and breeding.

Migratory monarchs bulk up before they begin their journey south, but they also make frequent stops to feed along the way. Fall nectar sources are extremely important to their migration success, but they aren't particularly picky about where they feed. In the eastern U.S., any meadow or field in bloom will function as a fueling station for migrating monarchs.

Brower and his colleagues have noted that the conservation of nectar plants in Texas and northern Mexico may be crucial to sustaining the monarch migration. The butterflies gather in this region in large numbers, feeding heartily to increase their lipid stores before completing the final leg of the migration.

_Sources:

• _{"Monarch Butterfly, Danaus plexippus L. (Lepidoptera: Danaidae)," by Thomas C. Emmel and Andrei Sourakov, University of Florida. Encyclopedia of Entomology, 2^nd edition, edited by John L. Capinera.}
• _{Fueling the fall migration of the monarch butterfly, Lincoln P. Brower, Linda S. Fink, and Peter Walford, Integrative and Comparative Biology, Vol. 46, 2006. Accessed online June 8, 2015.}