Habitat Loss in Mexico Threatens the Monarch Butterfly Winter Migration:

An Interview with Lincoln Brower

From the Environmental Review Newsletter Volume One Number Six, June 1994


Lincoln Brower is a professor in the Zoology Department at the University of Florida. He received the Ph.D. in 1953 from Princeton University studying with Charles Remington. Professor Brower has numerous refereed publications on the natural history and biology of monarch butterflies.

ER: Professor Brower, what is so remarkable about the monarch butterfly?

LB: Monarch butterflies have an incredible migration in the fall. It was discovered in the late 1800s that there are beautiful over-wintering clusters on the Monterey Peninsula in California. The butterflies migrate there in the fall, overwinter for several months, and in the spring fly eastward to breed in the Sierras. Their great grandchildren come back the next fall to overwinter. Originally nobody had a clue where they went in the spring. The migration in California is now well known as are the over-wintering sites. But the really big fall migrations of monarchs occurred in the midwestern states all the way to the Atlantic coast.

ER: What's the northern limit of the monarch's range?

LB: The northern limit of the range is defined by the distribution of the milkweed plants upon which they lay their eggs and which the caterpillars eat. Milkweeds peter out in southern Canada. Fall migrations of monarch butterflies had been noted going back to the 1850s. Periodically there were enormous fall migrations and over the years an English biologist, C.B. Williams, who kept summarizing the data, didn't have a clue where they went. In the late 1880s monarchs were discovered clustering in trees along the Gulf Coast in Florida near Apalachacola (near Tallahassee). This was seized upon as the over-wintering site of the huge fall migrations in the eastern U.S. But unlike California where the butterflies kept coming back to the same trees, they might be in Apalachacola one year and might not be there the next.

     During the fall, monarchs migrate but they stop flying towards dusk and they come down in trees. The trees they land on can be virtually anywhere along the southern migration route, which is from Maine all the way to North Dakota and then south to Texas and Florida, an enormous area. The early scientists confused these overnight clusters with true overwintering clusters. Nobody really went about studying the migration systematically.

     Professor Fred Urquhart (a biologist at the University of Toronto) got interested in tagging monarchs to see if he could trace their movements. Butterfly tagging became wildly popular with school teachers and amateurs all over the country east of the Rocky Mountains, and to some extent also in California, beginning in the 1950s. Over the years they marked tens of thousands of monarchs; Urquhart would issue little gummed labels to put on the front wing of the monarchs.  Each tag had a number and said "send back to the University of Toronto Zoology." So by plotting the points of release and recapture on maps, Urquhart established that monarchs were for the most part flying in a southwesterly direction.  They seemed to be headed toward Texas. Then they just disappeared, presumably across the border.

ER: At what time - about late fall?

LB: The fall migration begins at the end of August. Where they are breeding at the end of the summer is from approximately the latitude of Virginia, west to the Dakotas, and then north to Maine, in the Toronto area, and especially in the Great Lakes region.  The Great Lakes region and the Toronto area seem to be the major centers of breeding for the last summer generation.

By the end of the summer there will have been several (three to five depending on the weather) monarch generations produced. They build their population up to enormous numbers - millions of them.  Nobody knows how many.

     Urquhart's evidence traced the fall migration towards Texas and occasionally there would be migrations through Florida.  Where did they go?  What happened to them?

In the early 1960s I wrote two papers hypothesizing that many monarchs just fly south, scatter and breed, and that a new generation flies back. So up to 1973, no one had a clue about where most monarchs over-winter.  In February 1973, an American salesman in Mexico - Kenneth Brugger - read one of Urquhart's papers, called Urquhart and they started working together. Brugger traveled in Mexico a lot and a year later he went through a storm in the mountains west of Mexico City, and pelting out of the sky with hail, monarch butterflies were falling out of the sky. That was the first clue to where they might overwinter.  

     Then on January 2, 1975, Brugger and his wife discovered the first over-wintering site of the monarchs. Brugger's research had been supported by Urquhart and the National Geographic Society, and in the August of 1976 issue of the National Geographic magazine, the discovery was announced to the world. Finally, it was realized that tens of millions of monarch butterflies migrate each fall into Mexico from the United States.

     On Urquhart's first site visit a branch broke, hundreds of  butterflies rained down on top of him, and one of them was tagged. That proved that the monarchs were migrating  from the United States all the way to Mexico. I wrote to Urquhart after he made the discovery; he would not share the location of the sites with colleagues so we could not pursue our experiments on bird predation. He would not tell us (or anyone else) where these sites were. The National Geographic Society and Urquhart established a policy not to divulge the location of the sites for fear that they would be damaged.

     At that point it became a challenge for me and my students to locate the overwintering sites of these butterflies. I knew a young post-doctoral investigator at the University of Massachusetts by the name of William Calvert. Calvert was a native Texan, an entomologist, and he had done a lot of traveling in Mexico with friends. So Calvert and I sat down, got out some old Geological Society of America maps. We read Urquhart's articles over carefully and in them there were two clues about the location of the site: it was above 10,000 feet of altitude, and it was in the state of Michoacán. With that information all we had to do was to block out the areas on the map above 10,000 feet in the state of Michoacán; this told us pretty much where the butterflies were. So in December 1976, Calvert took off from Austin and on New Years Eve he called me from Mexico saying that he had discovered the butterflies on December 29. It took him three days from leaving Texas to find the site, which we designated as "site alpha."

     We mounted a second expedition in January 1977 to site alpha. As we walked in we encountered Professor Urquhart and his wife along with the Bruggers tagging butterflies. All hell subsequently broke loose and the international press had a feeding frenzy.  But this enormous aggregation of butterflies was one of the most beautiful and remarkable things one can see in the whole world. I have seen several of the world wonders and this certainly needs to be added to the list.

     Here was a perfect laboratory experiment to test predation in the wild: an enormous aggregation of insects which represents an incredible potential food supply for predators, yet it appeared that very few butterflies were being killed. So when we went to the site, one of our major objectives was to see if butterflies were being eaten by birds - and they were, by the tens of thousands.

     Over the years with my students at Amherst College, we determined that out of about fourteen or fifteen potential bird predators in the site area, only two are feeding in any numbers on the
monarchs. One was an oriole related to the Baltimore Oriole, and the other was a grosbeak related to the Rose-breasted Grosbeak.

     In 1979-80 Calvert, Carolyn Nelson, my students and I put 100 nets in the colony and we measured the number of butterflies killed per day by the birds. They were killing up to 35,000 butterflies a day! Over the wintering season, from November until the end of March, the birds were killing well over a million butterflies at site alpha.

     Another one of my students, Linda Fink, and I then figured out that the way the birds are doing it is related to the fact that monarchs are not all toxic. Some are poisonous and some are not, depending upon which milkweeds they had eaten in their caterpillar stage back in the U.S.A. and Canada. The orioles peck the butterflies, taste them, and only eat the ones that are lacking the poison. And the grosbeaks appear to be somewhat insensitive to the poison. That was an interesting evolutionary dynamic. The butterflies could not possibly over-winter there in such huge numbers if they weren't protected, but their chemical protection as we showed, is not perfect and some of the birds have breached it.
     With my students at Amherst College we did experiments  which proved that monarchs derive toxic substances from the milkweed plant, and that is what makes them poisonous to birds. The monarchs absorb and store those poisons.  When they eat the leaves, the poisons are taken in and digested, and they end up stored in the tissue of the caterpillar.  And these poisons are carried over into the adult butterfly.  This is rather amazing, that they are able to store poisons that are strong enough to kill a human. That's what makes monarchs poisonous to birds.
     In the course of raising monarchs on these plants in 1955, I said, "Well, I'll just test this theory of how toxic these plants are."  So I tasted the exudate of a milkweed plant and it made me violently queasy. A very small amount of the latex was incredibly bitter. And in 1973 and 1974, on sabbatical leave, I joined up working with James Seiber at the University of California at Davis, to  isolate, identify and separate poisons that occur naturally in milkweeds. Together with Carolyn Nelson, we discovered that different species of milkweeds have different arrays of these plant poisons in them. They are called "cardiac glycosides" because of their effects on vertebrate hearts.

     We also discovered that some milkweeds lack the poisons, and different species of milkweeds had different constellations of cardiac glycosides in them.  Since the butterflies store the intact poison from the milkweeds, if you extract the poisons from an individual butterfly, you can tell which species of milkweed it ate.

Milkweeds in North America are botanically extremely interesting because they'have undergone an explosive evolution to 108 species. The biogeography of these 108 species, together with the fact that they contain different cardiac glycosides, raised the possibility that instead of putting tags on the butterflies and getting a return of about 1 in 10,000 butterflies tagged, the butterflies were being tagged naturally by the plants they ate.

     So our hypothesis  was that we could use these chemical tags (or "fingerprints") to trace the movements of the butterflies. The results of the chemical analysis indicated that monarchs from Maine all the way across Southern Canada to North Dakota and down to Virginia, migrate to the over-wintering sites in Mexico. They spend the entire winter there, and then they come back at the end of March and in early April through Texas to the Gulf Coast where they lay their eggs on the southern milkweeds which have just begun to put up their new spring shoots.

     The exodus from the overwintering sites in Mexico is triggered by the Spring Equinox, (March 21) and by the end of the first week in April, they're pretty much gone from Mexico. These butterflies head back very rapidly to the United States.
There has always been a debate as to whether these overwintered butterflies in the spring sweep gradually up towards Canada or whether they breed and the new generation works its way up. So the idea of successive broods versus a big general single sweep was never resolved.

     What we found was by looking at the chemical  fingerprints, that the butterflies in the fall migration predominantly ate the northeastern milkweed Asclepias syriaca. The fingerprints of the butterflies at the over-wintering sites in Mexico are predominately Asclepias syriaca. The fingerprints of the butterflies collected from Texas through Florida during March and April are the same - predominantly syriaca. And we then made a collection of butterflies from North Dakota to Massachusetts in June and fingerprinted these. If the migration occurred in a single sweep, the monarchs would still have that Asclepias syriaca fingerprint, but they're not. They're completely different. They're the fingerprint of monarchs which have fed on the plants that are spring milkweeds from Texas to Florida. So we showed that the spring migration into the U.S.A. and Canada is a two generation process - the butterflies that go to Mexico in the fall get back into the United States in the spring, but they don't get very far. They get only to the Gulf Coast. But now milkweeds are there, so there's this cornucopia of food for their caterpillars. It gets hot during April in the South and the butterflies develop very rapidly. The new generation carries the monarchs back up to the Great Lakes states and then into Canada. There are a variable number of generations produced over the summer. In a warm summer, there'll be maybe one or two extra generations of monarchs produced. On average, it takes them about thirty-five days to kick off a generation.

     Perhaps the most incredible and fascinating aspect of the monarch biology - is that the butterflies get back to Canada, breed, and produce a new generation, another generation and still another generation in some years. So it's the grandchildren or great grandchildren of the butterflies that flew south the year before that are now making the trip back to Mexico in the following fall. This proves that the migration is a genetically inherited pattern of behavior that enables them to find their way back to exactly the same over-wintering sites in Mexico.

ER: How do they navigate when they migrate?

LB: Nobody knows. There are lots of hypotheses, but there's no evidence to support any of them. I think what is needed is the ability to track monarchs with radio tags-technology not yet available to us. There are so many monarchs produced that we don't have a clue what proportion actually survives and makes it to Mexico.

     Then the other aspect of monarch butterfly biology that I've gotten deeply involved in over the years is what I call the Achilles heel of the monarch. The monarch is one of the most abundant butterflies in the world. As a species it's widely distributed - it occurs in the Caribbean and South America, and it's even gotten to Australia and across the Pacific islands during historical times. There's no danger of the monarch becoming extinct. What is endangered is the biological phenomenon of the fall migration in eastern North America. And the problem is that monarchs go to an area that's about thirty by fifty kilometers in extent in Mexico, and there are half a dozen mountain ranges there that have fir forests at the higher altitudes in those mountains. And there are only a few of these forests still intact in Mexico, and that's the only place that they can go to survive during the winter.

     The problem is deforestation in Mexico.  Deforestation is accelerating and if it is not brought under control - if these areas are not effectively converted to national park status, comparable to national park protection in the United States, the endangered phenomenon will be a non-existent phenomenon, in my opinion, within fifteen to twenty years.

ER: Because of deforestation.

LB: Encroachment is going on right now at a furious rate, and exponential population growth is occurring in Mexico.  Maybe we have a window of time here, and that's what I'm hoping. It's our challenge as educators to get out to these people and make sure they realize what the consequences of exponential growth are. And I'm working very hard to try to save monarchs.  

ER: What are the prospects for saving this habitat for the wintering butterflies?

LB: The area is poor, the primary income resource is the forest. The clue is to get the economic power of the developed countries to subsidize these areas with alternative economies. The butterflies are a tourist attraction now and thousands of people are  visiting one of the sites which is open to the public.

ER: There is more money to be made in tourism than in cutting down the trees, I would think.

LB: Well, it's potentially a permanent, long term income as opposed to the exploitative short term benefits derived from cutting the forest. The other thing is that what they don't seem to realize is that these are forest islands surrounded by very arid land; from October through May it's arid there, and most of the water supply for the people who live in the towns around these mountainous areas is captured by the intact forest on those mountains.  Their water supply, the quality of their life, is utterly dependent on the integrity of that fir forest ecosystem, as are the monarchs. I am hopeful, but time is running out.

Copyright 1994 Environmental Review