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  • Julie Brophy

    Mexico Trip Report
    March 18, 1997

    To: Journey North
    From: Dr. Karen Oberhauser

    Last week two U.S. research teams returned from a ten-day sojourn (February 26 - March 7) to the monarch overwintering colonies in Mexico, where we renewed our sense that the migration of the eastern North American population of monarch butterflies is the most amazing biological phenomenon in the world. Our trip was full of incredible biology, wonderful people, good weather, hard work, and a conviction that it will take many years to fully understand everything we observed.

    Sonia Altizer, Liz Goehring, Kari Guertz, Dr. Karen Oberhauser and Michelle Prysby from the University of Minnesota worked on population monitoring, disease incidence, and reproductive biology, and Dr. Lincoln Brower from the University of Florida and Sweet Briar College and Christopher O'íNeal from the University of Michigan worked on the availability and importance of nectar sources in the colonies. We were joined by Eduardo Rendon Salinas and Eneida Montesinos Patino from the University of Mexico, and Monica Missrie, an interpreter from Mexico City. We also spent a day with long-time monarch researcher, Dr. William Calvert.

    We worked at the Sierra Chincua reserve, just east of Angangueo, Michoacan. There were three distinct monarch colonies on the mountain. For reasons still not completely understood, the monarchs move to lower altitudes over the course of the winter; they were first located near the top (at an elevation of 3300 m), but the three sites that were occupied during our visit are further down the southern slope of the mountain. While we were there, we actually got to see one of the colonies more further downhill, an amazing experience! We worked in a site called Arroyo Barranca Honda (elevation 2900 m), which is only open to researchers, although another Chincua site is open to the public for the first time this year.

    This was the second visit to the sites for the Minnesota team and Chris, and the 20th (!) for Lincoln. We all had the impression that there are more monarchs at Chincua than in past years, although the cause for this increase is uncertain. It is possible that more monarchs overwintered at Chincua because of habitat destruction in other sites, which could force the monarchs to less disturbed locations. The increased numbers could also be due to a strong 1996 summer reproduction period and fall migration, and a relatively easy winter. Eneida visited several other sites in conjunction with her studies of the genetics of eastern monarchs, and her impression was that monarch numbers at these sites were also high compared to other years.

    We have not begun a complete analysis of the data we collected in Mexico, but our initial observations are interesting. We collected data on the wing condition, size and sex ratio of butterflies that were roosting (in inactive clusters on the trees), flying, and mating every day. While we have notí made a formal comparison with the condition of butterflies from last year, our impression is that the butterflies are in better condition this year. Almost every sample we collected was female-biased, with sex ratios from 60 to 70% female. This is very different from the monarch colonies in California, where the sex ratios are extremely male-biased at the end of the season. Only a small percentage of females had mated (we can tell this by gently palpating the abdomen). This also contrasts with California, where most females have mated at the same time relative to their departure from the colonies.

    As was true last year, very few of the monarchs overwintering in Mexico this year were infected with the protozoan parasite that infects most of the western monarch population and is a problem for many people who rear monarchs. Sonia'ís dissertation work focuses on testing several hypotheses for the large differences in infection levels of different populations, and determining how the parasite is transmitted from one individual toanother. This work is very relevant to recent arguments about transporting monarchs between populations. We used the scotch tape sampling method that she developed early in her work to determine whether over 2000 butterflies are infected with this parasite! This method causes no damage to the butterflies, and is now being used by researchers throughout the US.

    Lincoln and Chris are studying the importance of nectaring to overwintering monarch survival. They measured the amount of nectar available in flowers, determined the duration and amount of foraging at different flower species, and the amount of nectar in monarch crops (their crops are similar to our stomachs). While a complete understanding of the importance of different energy sources to the monarchs awaits detailed data analysis, it seems likely that most overwintering butterflies are not dependent on nectar for survival, but instead subsist primarily on stored lipid reserves. This is important, because some people have suggested that cutting trees to promote flower growth might help the butterflies. All of us are very concerned about logging in the reserves, and feel that arguments for promoting understory nectar plant growth by cutting overstory trees are largely without basis.

    We collected behavioral data on mating attempts for hundreds of pairs of monarchs. (Mating attempts as struggles between two butterflies on the ground as the male attempts to mate.) Many of these attempts last several minutes, and may even involve two males! It appears that most females try to avoid mating at this time. This could be because they will not begin laying eggs for at least the next several weeks, and that costs of mating outweigh any benefits they might receive from mating this early. Karen is working to understand what factors influence the outcome of individual mating attempts and overall female mating frequency in overwintering colonies in Mexico and California, and in summer populations.

    We brought 400 butterflies back to MN, where we are determining how mating and access to host plants affect the timing of female diapause termination. Diapause is the non-reproductive, hibernation-like state in which monarchs overwinter, and Liz'ís thesis work is to determine what environmental factors influence both the beginning and end of diapause in monarchs. While there is a great deal of mating activity in the colonies in both California and Mexico near the end of the overwintering period, most females do not have mature eggs at this time.

    In addition to doing research, we encountered many wonderful people. We met with members of one of the "ejidos" that own land on Chincua. Th impressed upon us the fact that not being able to cut timber from the protected areas represents a severe financial hardship, and they are very interested in working together to devise ways in which both the livelihood of the people and the monarch sites can be preserved. Income from tourists visiting the colonies will be important, but this is seasonal and as yet cannot compensate for lost timber dollars. We left the meeting feeling both elated with the open communication and shared interest, and still somewhat daunted by the magnitude of the problems.

    We also spent a morning visiting an elementary school in Angangueo with a group from the Science Museum of Minnesota, six U.S. schools, and the Childrenís Museum in Mexico City. We visited classrooms, talked to students and teachers, and saw the butterflies they'd made for the Journey North'ís""Journey Sout"" program (the paper butterflies made by Mexican students will actually make a journey north to US classrooms!). The highlight of this visit was playing with students in the school courtyard where they were eating a hot lunch of tortillas cooked over gas burners. We learned some Spanish and English from each other, took lots of pictures, and together marveled at the thousands of monarch butterflies overhead!

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