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Among the Monarchs
 On high mountain trails in Michoacan, Mexico, I walked through forests of pine and fir, stopping to admire blossoms of purple senecio and red thistle or to listen to the cheerful buzz of a hummingbird near a stream.  An avid hiker, I was happily immersed in my surroundings, but I knew that the real prize lay ahead.  Our group rounded a bend in the trail, and there it was--a grove of oyamel fir trees, their boughs weighed down by huge clusters of resting Monarch butterflies (Danaus plexippus).
 Suddenly the sun's rays illuminated a branch, warming the butterflies to flight.  The air quickly became thick with a flutter of orange wings, creating a sound like wind blowing through the trees.  Butterflies were everywhere, sometimes landing on a hat or colorful shirt, or tickling a forearm.  After a few moments, they dispersed, only to become active again as the sun reached another cluster.  For the few incomparable hours that we shared the Monarchs' environment, we were all overwhelmed at being in the midst of one of the world's greatest natural phenomena.
 Every fall, millions of Eastern Monarchs migrate to about a dozen overwintering sites, high in the neovolcanic mountains of central Mexico.   Although Mexican farmers in the area have long known about the huge winter aggrega-tions, the migration was so shrouded in mystery that the sites were not located by scientists until the mid-seventies.
 The dazzling National Geographic article that reported the discovery stemmed from a collaboration between Fred Urquhart, a Canadian scientist, and Kenneth Brugger, an American businessman.  Brugger had encountered huge numbers of Monarchs while driving west of Mexico City; he traced the source and notified Urquhart, who hurried to the very trails where I was now walking.
 The Monarchs' remarkable journey, undertaken by a new generation to a place they have never seen, fills me with awe.  At the same time, new scientific discoveries about Monarch chemistry, flight and geographic orientation give me an appreciation of the insect as a complex, living organism.
 Robert Michael Pyle, respected natural-history writer and co-leader of our Monarch Program trip, has a similar response to the butterfly.  In Chasing Monarchs: Migrating with the Butterflies of Passage [Houghton Mifflin, 1999], Pyle describes his autumn sojourn from Canada to the Mexican border.  When asked what he had learned during two months of observing migrating Monarchs, he replies in the book's Epilogue, "I'd gained a clearer sense of how these creatures actually live their remarkable lives, as caterpillars, chrysalides, and adults--feeding, molting, moving, surviving, and, especially, always adapting.  The monarch was a real, functioning animal to me now, much more than the icon or symbol such a celebrated animal can easily become."
Geographical Distribution
 I had visited Monarch overwintering sites the previous winter, in Pacific Grove and Santa Cruz, California.  Here and in other coastal sites such as Pismo Beach, smaller numbers of Western Monarchs find shelter in groves of eucalyptus, Monterey pine and cypress.  Fog creates a cool, moist microclimate that prevents desiccation and allows butterflies to conserve energy by remaining inactive.
 It's long been assumed that the Continental Divide forms an impenetrable barrier that keeps Western Monarchs migrating to California and Eastern Monarchs to Mexico.  However, this boundary may not be absolute.  While following Western Monarchs from British Columbia to Arizona, Pyle watched for butterflies exhibiting strong directional flight.  Instead of heading west/southwest to California, many of the Monarchs he observed were pointing south/southeast--toward Mexico.  Other observers have since confirmed his observations of Western Monarchs entering Mexico.
 At the same time, Eastern Monarchs, migrating from as far north as Ontario, don't all make a beeline to central Mexico.  A Monarch tagged at Cape May, New Jersey, was found two weeks later on San Salvador, the Bahaman island where Columbus first touched land.  Most autumns, Monarchs turn up in southwest England, riding the same winds that blow North American songbirds there.
 Until the central Mexican sites were revealed, most people had a vague notion that the Gulf States and northern Mexico hosted overwintering Monarchs.  In fact, many butterflies were temporarily resting at "bivouac" sites in the Gulf or (occasionally) on offshore oil rigs, en route to Michoacan.  Some Monarch populations in Florida are non-migratory, producing several broods throughout the year.
 Monarchs are prevalent in Central and South America.  Very little research has been done to determine whether they  remain in one place or migrate to Mexico.
 Introduced to the isolated Hawaiian Islands, Monarchs breed year-round without migrating.  A rare whitish form of Monarch occurs here.
 During the nineteenth century, increased shipping trade brought Monarch stowaways to several islands in the South Pacific.  Local missionaries planted non-native milkweed, the host plant for Monarch caterpillars, inadvertently establishing resident butterfly populations.  Within Australia, introduced Monarchs perform a wet season/dry season migration.
Remarkable Adaptations
 The Monarch is unquestionably the most-recognized and beloved butterfly in North America.  Captivated by its amazing yearly migration, people from Canada to Mexico help to tag and monitor Monarchs.  Schoolchildren of all three nations excitedly follow the Monarchs' progress on Internet sites such as Journey North.
 There is much more to marvel over, when you look at the Monarch's entire life cycle from egg to larva to pupa to adult.  First, consider its history within the subfamily of tropical milkweed butterflies, Danainae.
 Milkweed butterflies are named after the milkweed plants in the family Asclepiadaceae, which feed their caterpillars.  The white, milky sap that flows through stems and leaves contains cardenolides--poisonous substances that affect heart muscle and cause vomiting.  The Monarch's host plant (Asclepias spp.) contains cardiac glycosides  in dosages that can be strong enough to kill a human.  Developing larvae are able to sequester cardenolides in their bodies without being affected by them, a condition that carries through to the adult stage.  Most preying birds, however, will have a violent reaction to the toxins and vomit or spit out the insect, then avoid Monarchs in the future.  (Two Mexican birds are exceptions: the black-headed grosbeak, which appears to be partially insensitive to the toxins, and the black-backed oriole, which has learned to strip the fat and muscles from Monarchs' bodies without ingesting high toxin levels.)
 The Monarch is the only species within this worldwide group to have expanded its range well outside of the tropics, by making an extensive seasonal migration. Scientists believe that the Monarch followed milkweed as it spread northward after the Ice Age.  Unlike the plant, however, the butterfly can't survive cold winters and must return south to wait out the cold months.
 Eastern Monarchs gravitate to the high-altitude forests of oyamel fir, relicts of once-extensive forests in Mexico.  Oyamel's scientific name, Abies religiosa, refers to the ceremonial use by ancient Toltecs, who burned the wood for incense.  Today, oyamel pitch is tapped for its use as a fire-starter.
 Monarch expert Lincoln Brower believes that a shift in spring migration patterns occurred after white settlers plowed up Midwest prairies, destroying much of the native milkweed.  Monarchs began to move eastward, where forests had been cut down.  Here the land had been opened up to the spread of milkweed, which is an early colonizer of disturbed soil.
 A similar adaptive shift took place along the California coast a century ago, when Monarchs began to roost in eucalyptus trees from Australia.  Remnants of the original Monterey pine forest still shelter some Monarchs, but the trees are scattered and subject to disease.  The butterflies' enthusiastic use of eucalyptus has created an interesting dilemma for native-plant advocates and park managers, who might prefer to remove the exotic trees.
Life Cycle
 Let's begin with the Monarch generation that performs the arduous fall migration.  Whether flying more than two thousand miles from Canada to Mexico or a much shorter distance from the Sierra to the Pacific coast, these Monarchs are up to four generations removed from last year's travellers.  Unlike most migrating birds, they have no elders to show them the way.
 How does a Monarch, weighing half a gram, negotiate such immense distances?  It's a smart flyer, riding warm air currents thousands of feet above the ground, gliding down to rest and to find nectar.  Migrating Monarchs can typically cover 50-100 miles per day. They gather in spots like Point Pelee, Ontario or Cape May, New Jersey, roosting together at night and often waiting out inclement weather.
 Urquhart, who began tagging Monarchs in 1937, once observed migrating Monarchs who were reluctant to leave their shelter on a seemingly fine morning.  A few hours later, a storm blew in, causing Urquhart to speculate that the butterflies can sense a drop in air pressure.  This cautiousness would be especially relevant at Point Pelee, where they must embark upon a long flight over Lake Erie.
 Monarchs actually gain weight during their long migration.  A constant source of nectar along their route is vital to these insects.  They're attracted to composites, such as goldenrod, aster, Joe-pye weed, blazingstar, coneflower, thistle and frostweed.  Sugar in the nectar is converted to lipids, enabling the now-fat Monarchs to survive winter without feeding.
 The butterflies need that fat, because their five-month stay can be an endurance test.  At 10,000 feet in Mexico's oyamel forests, weather can be severe.  Why have they come to such an inhospitable place?  Like California's foggy coast, the oyamel forests have traditionally provided the exact microclimate that shelters Monarchs during winter.
 When lengthening days of late winter trigger a hormonal response,  the Monarchs finally reach sexual maturity.  In Mexico, they move lower on the mountainsides to open, sunny areas, where mating is top priority.  The male aggressively pursues a female, carrying her to a sheltered spot in the foliage, where mating can last for several hours.  Both may mate with several partners during the spring migration.
 A gravid (pregnant) female may fly hundreds of miles north from Michoacan, seeking milkweed on which to lay her eggs before she dies.  Western Monarchs may locate milkweed along the coast or some fifty miles inland, in California's Central Valley.
From Egg to Pupa
 The female lays eggs singly on the underside of young, tender milkweed leaves, which may be just poking out of the ground in early spring.  Carrying up to 500 eggs, she looks for clumps of milkweed to ensure enough food for the developing larvae.  In the Midwest, the closely-related sand vine (Cynanchum laeve) may also be used as a larval host plant.  The Monarch's elongated, yellowish eggs are exquisitely patterned in honeycomb-like ridges.
 In about five days, tiny larvae emerge and eat the protein-rich eggshell.  They then embark on a two-to-three-week eating binge, molting five times (including final molt) to accommodate a body that increases its weight by 3,000 times.  Bold horizontal stripes of yellow, ivory and black advertise their unpalatability to predators.  If attacked, they may curl up and drop to the ground, staying inactive for a few moments.  Some insects prey on the larvae successfully: ambush bugs, some stink bugs and praying mantids eat them, while tachinid flies and braconid wasps parasitize them.
 When the larva reaches its final size (almost three inches), it may wander restlessly in search of a sheltered place where it can pupate. Likely spots are the shady undersides of tree branches, fenceposts, eaves and windowsills.
 The smooth, rounded chrysalis hangs like a celadon ornament, subtly colored in shades of blue-green with flecks of gold.  A day or so before the butterfly ecloses, the pupal case becomes transparent, revealing the orange and black wings inside.  On a fine-weather day, the adult butterfly will climb out of its split case and pump fluid from its swollen abdomen to expand its wings—an extremely vulnerable period lasting an hour or so.  It's common for the Monarch to remain quietly sheltered until the following morning.
 Amazingly, this butterfly will continue the next leg of the migration, flying north in search of new milkweed and laying eggs for the next generation.  The Monarchs who emerge from chrysalides in New England and Canada may be the great-great-grandchildren of those who left Mexico five months earlier.  When autumn days grow short, they will head south to overwinter in Mexico.  Western Monarchs undertake a shorter spring migration, dispersing across California and sometimes ending up as far north as eastern Washington and southern Idaho.  The source of summer Monarchs in British Columbia is uncertain.
Studying the Monarch
 Tagging.  For over six decades, tagging Monarchs has been the primary means of gaining information about their migration.  Its limitation, of course, is the pitifully small return it yields.  Nonetheless, some important findings have emerged.  Volunteers for the Monarch Program, based in San Diego, monitor tagged butterflies at winter sites.  They have found that Monarchs move frequently among sites, rather than staying in one place all season.
 David Marriott, Director of the Monarch Program, has had great success in retrieving tagged butterflies at the Mexican sites.  He has developed a strong relationship with the local people while studying the Monarchs and bringing in tour groups.  By paying for recoveries, he managed to locate more tags in 1998-99 than in all of the previous sixty years combined.
 Chemical Analysis.  While tagging is still an important research tool, some scientists are making huge leaps in understanding the Monarchs' biology and complex migration.  A 1993 study by S. B. Malcolm, B. J. Cockrell and Brower compared chemical "fingerprints" of Eastern Monarchs at different stages of migration.  They analyzed the insects' toxicity, correlating levels to milkweed species growing in various areas.  As expected, they found that most fall migrants, coming from the North and Midwest, had fed on common milkweed (Asclepias syriaca).
 This fingerprint held true at overwintering sites and at early stops in the Gulf during the spring migration.  Butterflies caught further north in June, however, had clearly fed on more-toxic Southern species, such as antelope-horn milkweed (A. viridis), thus establishing them as a second generation.  Although the study puts to rest a century-old  "single-sweep" remigration theory, Brower doesn't doubt that some individuals do make the trip back--some tattered Monarchs have been spotted as far north as Long Island and New Jersey, as early as mid-May.
 Isotopes in Rainwater.  One of the most exciting developments, which has implications far beyond the study of Monarchs, involves the measurement of hydrogen isotopes in the butterflies' bodies.  This chemical "tag" indicates the region of the continent where the butterfly grew through its larval stage.
 This is how it works:  Isotopes are forms of the same element that have a different atomic weight.  The hydrogen isotope known as deuterium is heavy; when combined with oxygen, it forms deuterium oxide, or "heavy water."  When storms blow in off the ocean, this heavy water falls first; further inland, rainwater contains less deuterium.  Also affected by temperature and altitude, deuterium levels remain constant in the plants growing in each region.  Caterpillars feeding on these plants absorb the same level of deuterium and retain their chemical signature as adult butterflies.
 Keith Hobson and Len Wassenaar studied songbird migration in Saskatchewan before turning their attention to isotope analysis of Monarchs.  The study, cosponsored by Environment Canada and Monarch Watch, revealed two important facts: Monarchs from various regions intermingle at Mexican sites (good news in case one site is destroyed); and over half of the overwintering Monarchs derive from the heavily-agricultural Midwest.
 This is cause for concern because of pesticide use, destruction of milkweed, and the use of transgenic (genetically-altered) plants.  These plants may be altered to be highly resistant to weed-killing herbicides, or to contain a gene for the organic pesticide, B.t. (Bacillus thuringiensis).  Recent headlines about Monarch caterpillars' sensitivity to pollen from B.t.-engineered corn sent alarm bells ringing throughout the scientific community.
 Sun Compass and Magnetic Field.  How do Monarchs orient themselves toward their overwintering sites?  Sandra Perez, Orley Taylor and Rudolf Jander hypothesized that the butterflies navigate by the sun, and that on cloudy days, Monarchs resort to a back-up plan: their sensitivity to the earth's magnetic field.  They conducted two separate experiments with fall-migrating Monarchs, the first involving a six-hour clock shift to gauge their subsequent angle of flight, and the second exposing them to a strong magnetic field prior to release.  In each experiment, half of a group of detained migrants received the treatment, while the other half was considered a "sham" control group; a third group consisted of naturally migrating Monarchs who flew through the area.
 Butterflies who had been kept in darkness until midday thought it was six hours earlier in the day; they consistently misjudged the sun's position and flew off in a west-northwest direction.   When released, the sham control group flew in a southwesterly direction like the migrants happening through, proving that captivity had not affected their sun compass.
 Like other migratory animals, Monarchs have crystals of magnetite in their bodies.  In an effort to find out whether the earth's magnetic field might play a role in migration, the scientists gathered a fresh batch of butterflies, then treated half of them magnetically.  Results were dramatic--while untreated captives and natural migrants headed uniformly southwest, the treated Monarchs were totally disoriented on release and scattered randomly, proving that they are indeed sensitive to the earth's magnetic field.
 Aerodynamics of Flight.  Twenty years ago, David Gibo, a University of Toronto zoology professor, became fascinated with butterfly migration after he became a glider pilot.  Watching Monarchs in flight, Gibo realized that they were extremely effective at using thermal updrafts to  save energy while covering long distances.  He began to amass data from observers across North America, hoping to discover if Monarchs change flight tactics during their long migration.  He established "Tactics and Vectors," an extensive (and entertaining) website to correlate the data [see www.erin.utoronto.ca/~w3gibo/].
 Gibo is looking for volunteers along the migratory pathway to provide crucial information to Tactics and Vectors.  You needn't be a glider pilot; just stand in an open area for a few hours and report Monarchs' flight style (flapping, gliding, soaring), altitude and direction of flight, along with wind and weather conditions.  Because other migrants, such as dragonflies and Painted Ladies, are likely to fly by, this data is included in the study, too.  [David Gibo, Insect Behavior Group, Dept. of Zoology, Univ. of Toronto at Mississauga, Erindale College, Ontario, Canada L5L 1C6; dgibo@credit.erin.utoronto.ca.]
 Monitoring Larvae.  Volunteers in the U.S. and Canada can also contribute to the Monarch Larval Monitoring Project, a long-term study directed by Michelle Prysby and Karen Oberhauser, with a grant from Xerces Society.  Choosing a wild milkweed stand close to home, participants examine it each week during summer, noting numbers of eggs and larvae, condition of milkweed, weather and other factors.  Prysby and Oberhauser hope to piece together a better understanding of the ecology of milkweed stands across North America--information that will greatly aid in conservation efforts.
 In conjunction with the study, Oberhauser and Kristen Kuda produced A Field Guide to Monarch Caterpillars, a nifty booklet that illustrates the larvae at each developmental stage, or instar ($7 ppd., payable to Univ. of Minn. Monarch Fund).  For information on classroom materials or the project, contact Karen Oberhauser, Dept. of Ecology, Evolution and Behavior, Univ. of Minnesota, 1987 Upper Buford Circle, St. Paul, MN 55108; oberh001@tc.umn.edu.
An Endangered Phenomenon
 What does the future hold for beleaguered Monarchs on both sides of the Continental Divide?  Brower calls the Monarch "a hardy and fertile breed"that is able to rebuild populations after catastrophic weather such as snowstorms.  He's less optimistic about the yearly migration, calling it an "endangered phenomenon"--a term that he and Pyle came up with independently.  Pyle and colleagues listed the migration as a "threatened phenomenon" in the IUCN Invertebrate Red Data Book in 1984.
 Logging proceeds at a frightening pace in and around Mexico's oyamel fir forests, opening up the forest canopy and altering the microclimate.  A year ago, due to drought and human disturbance such as burning, Monarchs left the sites early.  Many early-arriving females found little milkweed available in the Gulf states--a disaster which meant smaller summer populations further north.
 In 1986, the Mexican government decreed that several sites, including the popular El Rosario, were to be preserved for Monarch habitat.  However, it did not compensate the landowners, who are mostly poor families belonging to cooperatives called ejidos.  While providing some benefit to the region, ecotourism is seasonal and the large numbers of visitors are further degrading the fragile ecosystem.
 There are problems north of the border, as well.  Development pressures severely impact habitat for all wildlife, and create special stress for migrating birds and Monarchs, who must replenish energy along their route.  Several Western sites are fragile, due to mismanagement, coastal development and loss of milkweed.
 In Ontario, milkweed is considered a noxious weed, and landowners can be forced to remove it.  Pesticide and herbicide use makes much of the U.S. farm belt an inhospitable place for Monarchs.
 Western Monarchs are especially susceptible to a protozoan disease which spreads through exposure to spores.  The two populations should not be frivolously moved from one place to another, whether by innocent exchanges between amateurs or by ill-advised releases at special events or weddings.
 Brower believes that Western populations may be historically unstable, periodically becoming extinct and requiring replenishment from Mexico.  Should this be true, both migratory phenomena are at risk.
 A 1997 conference in Morelia, Mexico, brought together representatives from Canada, the U.S. and Mexico.  Government officials, conservationists, businessmen and rural ejiditarios engaged in several days of dialogue.  Did anything concrete emerge from the ambitious project?  Said one participant, "After the initial frustration, I realized that these amazingly diverse people were actually engaging in a two-way dialogue about preserving the Monarch sites." Proceedings from the conference are currently being published.
How to Help
 As we walked out from the oyamel forest, I asked Robert Pyle what we can all do to help preserve the yearly Monarch migration.  "Conserve or plant new stands of native milkweed, and fight roadside spraying," he replied.  He also suggested planting nectar-rich flowers that bloom in fall, such as aster, sedum and goldenrod; supporting organizations like Monarch Watch and Monarch Program; and getting involved in tagging or some other volunteer activity.  Although we both live in the Monarch-poor Pacific Northwest, he invited me to join him for fall tagging in eastern Washington.  I'm hooked; I'll be there!
Organizations:
Friends of the Monarchs, P.O. Box 51683,
  Pacific Grove, CA 93950; (831) 375-0982.
Journey North, 125 N. First Street, Minne-
  apolis, MN 55401.  www.learner.org/jnorth
La Cruz Habitat Protection Project, c/o Bob
  Small, 260 Mather St., Oakland, CA 94611;
  (510) 658-6758; danaus@pacbell.net.
Midwest Monarch Project, 3116 Harbor Drive S.E.,
  Rochester, MN 55904.
Monarch Butterfly Sanctuary Foundation, c/o Karen
  Oberhauser, 2078 Skillman Ave., Roseville, MN 55113.
Monarch Migration Association of North America, c/o
  Dick Walton, Cape May Bird Observatory, PO Box 3,
  Cape May Point, NJ 08212.
The Monarch Program, P.O. Box 178671, San Diego, CA
  92177. (800) 60MONARCH. Excellent monthly newsletter.
Monarch Watch, Dept. of Entomology, Univ. of Kansas,
  Lawrence, KS 66045; (785) 864-4441; monarch@ukans.edu;
  www.MonarchWatch.org.  Season summary: $3 ppd.
Western Monarch Migration Project, c/o Dan Hilburn,
  Oregon Dept. of Agriculture, 635 Capitol St. NE, Salem,
  OR 97310-0110; (503) 986-4663.

Copyright 1999 by Claire Hagen Dole.  All rights reserved.  Please request permission to reprint articles.
 

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