Monarchs? Where Do They Go? How Do They Get There?
Monarch butterflies, Danaus plexippus, migrate from the Northern US and Southern Canada to Central Mexico beginning in late August, then migrate north once the weather gets warm. It's a journey of up to 2000 miles, and an individual monarch butterfly doesn't live long enough to make the full journey. 1 Monarch butterflies only live two to six weeks once they metamorphose into butterflies.
Interestingly, however, while most generations of monarchs born in the spring and summer live two to six weeks, the last generation of monarchs that emerge in late summer undergo arrested development called diapause, brought on by less daylight. Missing a reproductive chemical called juvenile hormone, these monarchs can live seven months or more. During this time they migrate to Mexico, hibernate, then begin the flight back north, all before reproducing. But even this longer living generation of butterflies will not finish the trip, nor, most likely, will their shorter lived progeny. It will usually take one or two more generations of monarchs to complete the trip to North America from Mexico.
Scientists have long investigated how it is that these monarchs in successive generations can travel thousands of miles and manage to navigate the route so precisely that they often overwinter in the same tree year after year. Previous scientific research revealed that, like other insects, monarchs use the sun for navigation. More research showed that as the sun moves across the sky, the butterflies also use circadian clocks to adjust their route and maintain a southerly course. Then, as years passed scientists identified genes which control various circadian clock functions, and evidence from their research suggested that these genes could reside in the brain.
Public domain photo from US Fish and Wildlife, via Wikimedia Commons.
Where The Clock Lies
However new science last week brought an unexpected turn in the path of monarch circadian clock research. Science published a paper by monarch butterfly researchers at the University of Massachusetts Medical School who have long been studying monarch circadian clocks. Steven Reppert's lab showed that circadian clocks which control the monarchs ability to navigate the long migration reside not in the brain but in the antennae.
Following up on research done in the 1960's the scientists studied the influence of butterfly antennas on navigation. They found that butterflies without their antennas were unable to navigate a southerly course, nor could butterflies navigate whose antennae researchers blocked from light. They found that neither eyesight nor smell influenced navigational ability which was solely determined by light interacting with the antennae.
The current research, building on all the previous research, shows that circadian clock in the antennae are necessary for navigation, whereas the brain circadian clock may have different or complementary purposes. The paper, "Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies," was published in Science last week.2 3
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1 Monarchs hatch from eggs after four days to become caterpillars for about two weeks. They then enter the pupa or chrysalis stage which lasts about two weeks before molting and undergoing metamophorsis to mature butterflies. They can then flit around as butterflies for two to eight weeks before dying.
2 Christine Merlin, Robert J. Gegear, Steven M. Reppert*, "Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies" Science 25 September 2009: Vol. 325. no. 5948, pp. 1700 - 1704 DOI: 10.1126/science.1176221
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